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fall 1997 logo1997 MRS Fall Meeting & Exhibit

December 1 - 5, 1997 | Boston
Meeting Chairs:
 Harry A. Atwater, Peter F. Green, Dean W. Face, A. Lindsay Greer 

Symposium JJ—Nondestructive Characterization of Materials in Aging Systems



Jan Achenbach Robert Crane, Northwestern Univ Air Force Wright Laboratory
Pierre Khuri-Yakub Theodore Matikas, Stanford Univ Univ of Dayton
Surendra Shah, Northwestern Univ 

Symposium Support 

  • Wright Laboratory, Materials Directorate

Proceedings published as Volume 503 
of the Materials Research Society 
Symposium Proceedings Series.

* Invited paper

Chair: George A. Alers 
Monday Morning, December 1, 1997 
Fairfax B (S)

8:30 AM *JJ1.1 
EMERGING TECHNOLOGIES FOR NDE OF AGING AIRCRAFT STRUCTURES. Robert E. Green, Jr. and B. Boro Djordjevic, Center for Nondestructive Evaluation, The Johns Hopkins University, Baltimore MD.

This paper will describe several emerging technologies affording new opportunities for the nondestructive evaluation of grains aircraft structures. Among the topics to be discussed are non-contact ultrasound, including lasers, air-coupled, EMATs, hybrids and materials process control and defect detection; laser based acoustic emission source identification; full-field double-pulse holographic imaging of defects in metals and composites; ultrasonic evaluation of fatigue damage in aircraft aluminum alloys and adhesive bond quality; x-ray diffraction topographic imaging of the quality of nickel based alloy single crystal turbine blades and identification of stray grains.

9:00 AM JJ1.2 
PAINT AS A CORROSION SENSOR; A COMPARISON OF THE SENSITIVITY OF DIFFERENT COATING SYSTEMS. J. Zhang, G. S. Frankel, Fontana Corrosion Center, Dept. of Materials Science and Engineering, The Ohio State University, Columbus, OH.

The detection of localized corrosion in a large complex structure such as an airplane is critical, but very difficult. In order to be practical, a corrosion sensor must have access to the whole surface of a structure and enough sensitivity to detect the initial stage of corrosion damage. For a painted structure, it is possible for the organic coating itself to serve as a corrosion sensor since it covers the entire surface. The objective of this work is to develop an indicative coating system that will change in an easily recognizable and quantifiable fashion when corrosion occurs in the underlying or nearby metal. The approach is to sense the pH increase associated with the cathodic reduction reaction that accompanies the oxidative corrosion reaction, since the cathodic reaction occurs nearer to accessible surfaces in the case of localized corrosion processes such as crevice corrosion. In this study, various pH-sensing chemicals have been mixed with different organic matrices. Both color-change and fluorescing compounds have been used. Phenolphthalein is a common pH indicator that changes from colorless to red when the pH exceeds 10. The compound 7-hydroxycoumarin fluoresces under UV radiation when the pH exceeds 8. The critical pH values of indicating compounds can change when they mixed into an organic matrix, and a titration technique was used to determine these values. Coating systems consisting of an indicating layer with a non-indicating top-coat were studied. The sensitivity of these coating systems for detection of cathodic reactions associated with corrosion was determined by applying constant cathodic current densities and measuring the charge at which color change or fluorescence was detected. The coating systems are sensitive to a charge corresponding to a hemispherical pit with depth on the order of 5-15 microns. Electrochemical Impedance Spectroscopy was also performed after different immersion times in 1 M NaCl to test the influence of the indicating compound addition on the coating corrosion protectiveness, and to correlate impedance change with color change as the coating degrades with time.

9:15 AM JJ1.3 
PRACTICAL OPTICAL CHARACTERIZATION OF FRETTED SURFACES. Steven C. Gustafson, Jaideep R. Mahta, University of Dayton, Dayton, OH; and George N. Frantziskonis, University of Arizona, Tucson, AZ.

Noninal smooth surfaces subjected to repeated contact may develop characteristic fretted textures. An important example concerns fretted surfaces that may develop on disk dovetails that secure fan and turbien blades in jet engines. These surfaces may form sites for crack nucleation and subsequent disk and engine failure. Thus a practical and reliable method for the nondestructive characterization of such fretted surfaces is critically needed. This paper reports experimental results and analyses on the practical use of laser scattering to quantify the degree of disk dovetail surface fretting. Ideally this optical characterization should be rapid, consistent and insensitive to measurement setup variations.

9:30 AM JJ1.4 
ELECTROMAGNETIC ACOUSTIC RESONANCE TO ASSESS CREEP DAMAGE IN 2.25Cr-1Mo STEEL. Masahiko Hirao, Hirotsugu Ogi, Faculty of Engr Sci, Osaka Univ, Toyonaka, JAPAN; Toshihiro Ohtani, Advanced Materials Lab, Ebara Res Co Ltd, Fujisawa, JAPAN; Tomohiro Morishita, Dept of Mech Engr, Akashi College of Tech, Akashi, JAPAN.

Electromagnetic acoustic resonance (EMAR) is an emarging technique for measuring the ultrasonic velocity and attenuation of metalic samples. It incoporates the electromagnetic acoustic transducer (EMAT) in the resonant spectroscopy technique, realizing a noncontact and highly accurate measurements. In this study, the EMAR was applied to detect the creep damage in 2.25Cr-1Mo steel, which is an important material for the fossil power plants. Samples were crept in air at several stresses for 650 deg after being annealed. Reference samples were attached to see the temperature effect only. We observed the increase of the attenuation for the 2-10 MHz range as the creep advanced, while the reference samples showed nearly no change. Plotting the attenuation versus the elongation up to 10 %, the data from different stresses fell on a single curve, indicating that the damaging mechanism is independent of the stress of this range. Experimental results were interpreted, comparing with the destructive observations of grain shape, carbide precipitaion, and void/microcracks as well as the change in the magnetic properties of the metal.

9:45 AM JJ1.5 
ULTRASONIC QUANTIFICATION OF CORRODED SURFACES. William M. Mullins, TMCI, Dayton, OH; and S. Sathish, University of Dayton, Dayton, OH.

The surface damage introduced by general corrosion attack of surfaces is considered deleterious to long term structural integrity. As a result, the quantification of this damage represents an interest in the NDE community. In this document, the chemical kinetics of general attack are used to model the morphology of the surface as a function of time of exposure. Time-of-flight ultrasonic data for corroded surfaces are presented which appear to agree with the model predictions. The experimental results are critically reviewed with respect to the practical limitations of the ultrasonic experiments. In conclusion, the implications of the experimental limitations of field measurements of general attack corrosion damage are discussed.

10:00 AM JJ1.6 
THERMAL WAVE IMAGING FOR CHARACTERIZING STRUCTURES IN AGING AIRCRAFT. Xioyan Han, L.D. Favro and R.L. Thomas, Dept of Physics & Institute for Manufacturing Research, Wayne State University, Detroit, MI.

We present examples of the application of thermal wave imaging to the detection of structural defects in aging aircraft. Examples will include the imaging of bonded and disbonded internal doublers. Another class of examples will consist of quantitative corrosion thinning measurements made form thermal wave images of aircraft fuselages and wing skins. In all cases, we will show images taken in hanger facilities, aircraft maintenance facilities and on airstrips, demonstrating the utility of the system form practical applications.

10:45 AM JJ1.7 
ON THE ROLE OF GRAIN NOISE IN EDDY CURRENT INSPECTION OF TITANIUM. Mark Blodgett, Metals, Ceramics, and NDE Division, Wright Laboratory, Wright-Patterson Air Force Base, Dayton, OH; Peter B. Nagy, Department of Aerospace Engineering and Engineering Mechanics, University of Cincinnati, Cincinnati, OH.

Elastic anisotropy of single crystals plays an important role in ultrasonic materials characterization of polycrystalline materials. Microscopically homogeneous but randomly oriented individual grains make up a macroscopically isotropic but slightly inhomogeneous medium which produces incoherent wave scattering commonly called grain noise. While grain noise has an obvious adverse, often prohibitive, effect on ultrasonic flaw detection, it can be also exploited for ultrasonic characterization of the grain structure. We have found that the much more rare electric anisotropy exhibited by noncubic crystallographic classes can play a very similar role in electromagnetic characterization of polycrystalline metals. The resulting macroscopic inhomogeneity of conductivity produces an electrical equivalent of the ultrasonic grain noise. In contrast with elastic properties, the electric conductivity is completely isotropic in cubic crystals which constitute the majority of polycrystalline metals. On the other hand, less common materials of hexagonal symmetry can exhibit electrical anisotropy with significant difference in conductivity between the basal plane and normal to it. The only metal of great industrial importance that crystallizes in hexagonal symmetry is titanium, which has been extensively used in the aerospace industry for more than twenty years and continues to find further applications in other industries requiring low specific weight and high strength and low corrosion rate at elevated temperatures. We present experimental results illustrating how electric grain noise can reduce flaw detectability on one side and can be exploited for microstructural characterization on the other side during eddy current inspection of titanium and its alloys.

11:15 AM JJ1.8 
ELECTRONIC X-RAY SYSTEMS FOR FIELD AIRCRAFT INSPECTION. Harold Berger, Thomas S. Jones, Industrial Quality, Inc., Gaithersburg, MD; and George Cevallos, U.S. Air Force, Kelly AFB, TX.

Technical and economic evaluations of electronic, digital x-ray imaging systems are continuing beyond the preliminary evaluations completed early this year. The project objective is to evaluate electronic, digital x-ray techniques as possible replacements for the use of x-ray film at Air Force field aircraft maintenance facilities. Incentives for replacing film in aircraft inspection facilities include reduced costs for continuing supplies and, most important, elimination of the serious environmental problems associated with the use and disposal of film processing chemicals. Additional incentives for digital systems include advantages for electronic data storage, retrieval and processing. Experimental evaluations included systems that make use of flat panel imagers, storage phosphors, scintillator-camera systems and the reverse geometry method. The technical evaluations were conducted with a test piece that included typical honeycomb structures, both aluminum and composite skin, with faults such as entrapped water, crushed core and foreign objects, and quantitative measurements with line pair gages and ASTM E 1025 image quality indicators on 1/4 inch aluminum. Several electronic x-ray systems were found to meet many of the technical requirements in terms of exposure sensitivity, ease of use and image quality, and to offer system costs within the probable acceptance range for implementation at many of the higher film use field operations. The easiest electronic system for field implementation was judged to be the storage phosphor approach because its use would be very similar to that of film but without most of the recurring costs and without the environmental problems for use and disposal of film processing chemicals. The evaluation project is continuing.

11:30 AM JJ1.9 

It is well known that Nuclear Magnetic Resonance specroscopy may be used in solids as an nondestructive method for investigation of internal tension, hardening or crystal fields' changes arised by defects, deformation, irradiation. In particular it is possible to measure some parameters of internal stress-strain tensor or to control internal strength state of solids. Due to the necessity of study the mechanical properties and in connection with different problems of nuclear reactor's blanket material science NMR-investigation of metals has special significance. Therefore it is necessary to use massive metal samples in order to have the possibility of comparison with mechanical tests or electron microscope investigation of one and the same samples. The thickness of samples are selected from the range 100-300 mkm. But application of pulsed NMR-methods in metals is difficult because of skin effect i.e. different values of radio frequency amplitude inside the sample. The older technique of Continuous Wave NMR was used for massive metals for a long time, but nowadays its signal to noise parameters is not enough. So in this work an approach using the two-dimensional 2D-NMR spectroscopy was developed. In this method a sample to be placed in static magnetic field with gradient. Gradient,field and the normal to the sample surface are parallel. Division of absorption spectra of each layer of sample must increase with increasing of gradient. But for a good division it is necessary to use a very high gradient.If such gradients are unattainable in addition to gradient a special sequences of pulses must used. In connection with the corresponding special sequences of radio frequency pulses and its superoperator's presentation were obtained. That method with appropriate modifications may be used in another solids too.

Chair: Robert E. Green 
Monday Afternoon, December 1, 1997 
Fairfax B (S)

1:30 PM *JJ2.1 
APPLICATION OF HIGH FREQUENCY DIELECTRIC ANALYSIS TO THE STUDY OF AGEING IN ADHESIVE BONDED STRUCTURES. Richard A Pethrick, Sadanand B Joshi, David Hayward, Zhi-Cheng Li, Dept of Pure & Applied Chemistry, Univ of Strathclyde, Thomas Graham Building, Glasgow, UNITED KINGDOM; Steven T. Halliday, William M. Banks, Dept of Mechanical Engineering, Univ of Strathclyde, James Weir Building, Glasgow, UNITED KINGDOM; Ray Gilmore, Lawrence W Yates, Structural Materials Centre, Defence Research Agency, Farnborough, Hants, UNITED KINGDOM.

The ageing processes which occur within adhesive bonded structures are complex. A considerable amount is known about the way in which loss of mechanical strength can be interpreted in terms of changes in the properties of the adhesive and stability of the interface between the adhesive and substrate. High frequency dielectric methods allow identification in new joints of changes in the cross-section and disbonds or regions of poor adhesion. On exposure to moisture and elevated temperatures water enters the bond line and is distributed between voids, matrix and the interface. The dielectric method allows quantification of the distribution of water within the joint. Preliminary studies of the ageing of typical adhesive bonded formulations will be presented. The data may be interpreted in terms of plasticisation of the epoxy resin matrix, creep of the matrix material, and conversion of the oxide to hydroxide corrosion of the resin substrate interface. The effect of various pretreatments and surface preparation of aluminium - epoxy adhesive bonds will be discussed. Mechanical testing of aged joints indicates a decrease in the shear strength and fracture strength and these changes appear to correlate with the variation seen in the dielectric properties of the joints. The dielectric method is capable of providing unique and useful data on the state of joints and has potential to be developed as an NDT method. The method does, however, have limitations and it is best seen as a complement to more conventional methods such as ultrasonic and radiographic imaging.

2:00 PM JJ2.2 
EARLY FATIGUE DAMAGE IN CARBON FIBER COMPOSITES, OBSERVED BY ELECTRICAL RESISTANCE MEASUREMENT. Shoukai wang, Xiaoping Shui, Xuli Fu and D.D.L. Chung, Composite Materials Research Laboratory, State University of New York at Buffalo, Buffalo, NY.

Early fatigue damage during the first tenth (or less) of the fatigue life was observed in carbon fiber composites by DC electrical resistance measurement. The damage was most severe in the first loading cycle and the incremental damage in each subsequent cycle diminished cycle by cycle. For the continouse carbon fiber carbon-matrix composite, the resistance increased irreversibly during early fatigue due to matrix damage and possibly fiber fracture as well. For the short carbon fiber polymer-matrix and cement-matrix composites, the resistance decreased irreversibly during early fatigue due to matrix damage near the junction of adjacent fibers and the resulting increase in the chance that adjacent fibers touched one another.

2:15 PM JJ2.3 
FIBER BREAKAGE IN POLYMER-MATRIX COMPOSITE DURING STATIC AND FATIGUE LOADING, OBSERVED BY ELECTRICAL RESISTANCE MEASUREMENT. Xiaojun Wang and D.D.L. Chung, Composite Materials Research Laboratory, State University of New York at Buffalo, Buffalo, NY.

By measuring the electrical resistance of a continuous unidirectional carbon fiber epoxy-matrix composite along the fiber direction during loading in thid direction, fiber breakage was progressively monitored in real time. Fiber breakage occurred in spurts involving 1000 fibers or more. It started at about half of the failure strain during static tensile loading and at about half of the fatigue life during tension-tension fatigue testing. Immediately before static failure, 35% of the fibers were broken. Immediately before fatigue failure, 18% of the fibers were broken. The fiber breakage was accompanied by decrease in modulus.

2:30 PM JJ2.4 
STRATEGIES FOR NDE OF FIBER REINFORCED POLYMER STRUCTURAL COMPONENTS. John C. Duke, Jr., Engineering Science & Mechanics and Materials Science & Engineering; J. J. Lesko, Engineering Science & Mechanics; R. E. Weyers, Civil Engineering; Virginia Tech, Blacksburg, VA.

This paper will discuss strategies for selecting and developing nondestructive evaluation (NDE) methodologies for fiber reinforced polymer composite structural components to be used in civil infrastructure applications. Both conventional as well as innovative evaluation approaches will be addressed. Actual experience with NDE of hybrid carbon and glass fiber reinforced vinyl ester pultruded beams that are installed in a bridge carrying automotive traffic will be included.

3:15 PM *JJ2.5 
EPOXY CURE MONITORING WITH AN INTERDIGITATED GATE ELECTRODE FIELD-EFFECT TRANSISTOR. Edward S. Kolesar, Texas Christian University, Department of Engineering, Fort Worth, TX; John M. Wiseman, SRT Electro-Optics, Birmingham, AL.

An interdigitated gate electrode field-effect transistor (IGEFET) has been designed, fabricated and utilized to monitor the cure of a common epoxy. The IGEFET sensor consists of an interdigitated gate electrode structure which is coupled to the gate of a conventional metal-oxide-semiconductor field-effect transistor (MOSFET). The epoxy was deposited on the interdigitated gate electrode and the IGEFET¹s electrical performance was observed as the epoxy cured. The cross-linking chemical reaction during epoxy cure caused electrical impedance changes that were quantified when the IGEFET was operated with a periodic voltage pulse signal. Charge transferred through the chemically-active epoxy is manifested as a temporally-dependent potential applied to the MOSFET gate electrode. By operating the MOSFET as a linear amplifier, a potential corresponding to the Fourier transform of the IGEFET¹s time-domain response at specific time increments was computed. The resulting epoxy cure spectra were compared to a common baseline formed by computing the difference spectra involving the IGEFET¹s pulse excitation signal and the signal corresponding to the epoxy¹s chemical state at a specific instant of time. The abundance of difference spectral components yields valuable information for determining the chemical state of the epoxy¹s cure.

3:45 PM JJ2.6 
MEASUREMENT OF THE MATERIAL PROPERTIES OF THIN LAYERED STRUCTURES. Marty Jones, EWI, Columbus, OH; Richard Stiffler, RC Stiffler & Associates, Pittsburgh, PA; Bahram Farahbakhsh, EWI, Columbus, OH.

Layered structures are found in many aircraft. Characterization of the material properties of the layers can indicate the integrity of the bondlines not only at original manufacture but also during aging of the aircraft. In this paper, ultrasonic waveforms transmitted through and reflected from various layered structures are compared to waveforms generated through a computer model. The model waveforms are generated through convolution of the incident ultrasonic waveform with the transfer function of the layered material and then the inverse Fourier Transform is performed. The transfer function of the layered structure incorporates variables such as layer thickness, density, attenuation, and wavespeed. Comparison of theory and experiment enables characterization of the properties of the structure such as its density, moduli, thickness, and attenuation. Data are shown for three thin structure types: single layer, coatings, and layered bonded materials.

4:00 PM *JJ2.7 
COHERENT X-RAY SCATTER IMAGING FOR AIRCRAFT-CORROSION DETECTION. Jerel Smith, Cary Pincus, Advanced Research and Applications Corp. (ARACOR), Sunnyvale, CA.

A technology, based on the Compton/Rayleigh backscatter ratio, is being developed for the detection of subsurface corrosion in aircraft. This technique, called COREX, detects the presence of corrosion products by the change in the average atomic number of the materials present (low-Z materials exhibit little coherent scatter). Results will be shown for scans of corrosion samples made on a current Air-Force sponsored evaluation program.

4:30 PM JJ2.8 

The service life of many military aircraft is being extended far beyond original design intents. When the C-130 is eventually retired, for example, it will have been in service for 79 years, well beyond its planned life expectancy of 40 years. Similarly, the line of KC-135s is presently expected to remain operational for 86 years, and the venerable B-52 an astounding 94 years! Not only are inventories of parts in short supply, but it may be necessary to acquire parts no one ever expected to replace. The first step in any reprocurement activity is the creation of a data package that can submitted to suppliers for bid. If no CAD model exists, which is likely with older parts, the demands of modern electronic commerce dictate that one be created. If the original alloy is no longer obtainable, which is also likely with older parts, the demands of functional equivalence dictate that a suitable substitute be found and certified. Creating a CAD model of an existing part and/or modeling its performance with candidate replacement materials is referred to as ìreverse engineering.î Computed tomography (CT) offers an ideal way to obtain dimensional and material data critical to reverse engineering efforts. Industrial CT systems have progressed to the point where they can nondestructively measure part dimensions at an accuracy competitive with coordinate measuring machines and a speed competitive with laser scanners. Of the existing methods for obtaining part coordinates, only CT can dimension interior surfaces. Moreover, only CT has the ability to densitometrically quantify internal defectsóa key consideration for computer-aided engineering activities. The use of CT to help create data packages for resupply efforts will be described and examples presented.

Chair: John S. Popovics 
Tuesday Morning, December 2, 1997 
Independence West (S)

8:30 AM *JJ3.1/MM4.1 
DURABLE CONCRETE STRUCTTURES, A PROVEN REALITY OR STILL A DREAM? Carsten Henriksen, IN SITU S.A., LUXEMBOURG; and Claus G. Petersen, German Instruments A/S, Copenhagen, DENMARK.

Durability of reinforced concrete structures today is a hot topic in the concrete industry. The new trend is to specify a service life or 100 years, or more, by means of ``High Performance Concrete'' based on specialized theoretical service life calculations. Quite naturally, such calculations are founded on laboratory methods that reflect the desired, ideal ``lab-crete'' condition. The actual concrete produced on-site, the ``real-crete'', is however, receiving less attention - if any. The paradox is further emphasized by the fact that most durability problems relate to inadequacies created during the construction phase. Today, reliable and well proven in-situ test methods for essential durability aspects of the structure itself are available on the market. However, in general have not been used with a great deal of hesitation, meaning that it is still uncertain if a structure made of high performance concrete - when handed over to the client - in fact is fulfilling the stipulated service life requirements. In essence, the modern concrete world is facing a situation where no serious means of proving the actual durability of the structure is applied. On the other hand, the best intentions are exercised in design, in the choice of materials, in motivation of the workmanship or checking it, and in conducting careful quality assurance programs. Are the well-known mistakes made 25 years ago in relation to durability going to be repeated in the years to come, just in a much bigger scale? The paper presents in-situ methods related to essential durability properties of the structure itself. The methods are for use in the planning stage, during construction and after the construction has been completed. Furthermore, the paper speculates over the reasons for general lack of acceptance of the methods in the concrete industry and suggest means of improving the acceptance/use of the methods.

9:00 AM JJ3.2/MM4.2 
INTEGRATED FIBER OPTIC SENSORS FOR NONDESTRUCTIVE CHARACTERIZATION OF CONCRETE STRUCTURES. Farhad Ansari, Smart Sensors & NDT Laboratory, Department of Civil & Environmental Engineering, New Jersey Institute of Technology, Newark, NJ.

In-service performance of structures depends on consistent monitoring of condition for early scheduling of repair and retrofit operations. Early detection of large displacements and cracks m concrete elements will result in increased safety and considerable savings in rehabilitation costs. Optical fiber sensors are emerging as superior nondestructive means for evaluating the condition of concrete structures In contrast to Ousting nondestructive evaluation techniques, optical fibers are able to detect minute variations in structural conditions through remote measurements. It is possible to monitor the initiation and progress of various mechanical or environmentally induced perturbations in concrete elements by way of fully integrated optical fiber sensors. This article is intended for a brief introduction into the theories, principles, and applications of fiber optic sensors as they pertain to civil engineering applications. Most of the civil engineering related research in this field have been focused in applications to concrete. This is due to the geometry adaptability of optical fibers and ease by which they can be embedded within concrete elements. However, due to the fact that the transduction mechanism in optical fibers is invariant of the materials employed, the principles introduced here also correspond to other structural materials. The only application related differences among various materials pertain to sensitivity and choice of optical fiber sensor type.

9:15 AM JJ3.3/MM4.3 
A MULTIPLEXED OPTICAL FIBER SENSOR SYSTEM FOR DISTRIBUTED MEASUREMENT OF STRUCTURAL STRAINS. Farhad Ansari, and Zhongyu Chen, Smart Sensors & NDT Laboratory, Department of Civil & Environmental Engineering, New Jersey Institute of Technology, Newark, NJ.

Structurally integrated optical fiber sensors form the basis for smart structure technology. Over the past decade a variety of sensor configurations have been developed for measurement of strains and deformations in structures. Strains and deformations alter the refractive index and the geometry of the optical fiber material. These changes perturb the intensity, phase, and polarization of the light-wave propagating along the probing fiber. The optical perturbations are detected for the determination of strain. The research presented here describes the development of a new optical fiber sensor system for measurement of structural strains based on white light interferometry. An optical switch provides for multiplexing of strain signals from various locations in the structure. Redundant Bragg grating type fiber optic sensors as well as strain gauges were employed for comparison and verification of strain signals as measured by the new system. The system provides capability for distributed sensing of strains in large structures.

9:30 AM JJ3.4/MM4.4 
FAILURE LIFETIME PREDICTION OF CRACKED CONCRETE STRUCTURES. Fabrizio Barpi, Silvio Valente, Dept. of Structural Engineering, Politecnico di Torino, ITALY; Francesco Chille', ENEL-CRIS, Milano, ITALY; Lorenzo Imperato, ISMES, Bergamo, ITALY.

When subjected to high tensile stresses, brittle and disordered materials (concrete, rock, ceramics, fiber reinforced composites, etc.) undergo strain-softening, which is localised in a narrow band (process zone), outside which the material behaves linearly. According to cohesive crack model (Barenblatt, Dugdale, Hillerborg), the process zone can be represented as an extended portion of the real crack, called fictitious crack, where the material, albeit damaged, can still transfer stresses. This model can explain the size-effect on many structural responses in both Mode I [1] and Mixed-mode [2, 3]. Using the finite element method in Mixed-mode problems, a remeshing technique is applied at each crack growth step. Under high level sustained load, creep outside the process zone can be neglected compared to creep in the process zone. In this paper the creep law has been determined experimentally, through a serie of direct tensile tests [4]. The same creep law has been used to simulate 30 three point bending tests (Mode I) and 3 laboratory tests on notched gravity dam models (Mixed-mode, 1:40 scale [5]). Numerical results were found to be in good agreement with experimental results. The load vs. C.M.O.D. (Crack Mouth Opening Displacement) curves obtained during static tests can be assumed as a valid envelope criterion for creep fracture in Mode I problems (direct tensile tests and three point bending tests). In the Mixed-mode problems analysed creep fracture occurred before the static envelope was reached.

9:45 AM JJ3.5/MM4.5 
NON-DESTRUCTIVE EVALUATION OF THE INTERFACE IN REINFORCED CONCRETE USING PHASE MEASUREMENT INTERFEROMETRY. Masoud Ghandehari, Northwestern Univ, Dept of Civil Engineering, Evanston, IL; Sridhar Krishnaswamy, Northwestern Univ, Dept of Mechanical Engineering, Evanston, IL; Surendra Shah, Northwestern Univ, Dept of Civil Engineering, Evanston, IL.

Studying the behavior of steel and concrete as a composite is of fundamental importance to the understanding of the cracking of reinforced concrete structures. In this article, a technique leading toward the development of a constitutive model for the interaction of steel and concrete is described. Experiments are based on pull-out specimens, where the shear stress and the two displacement components at the interface are measured. Phase Measurement Interferometry is used for accurate surface displacement measurement, and crack growth detection. The normal stress is then deduced using the measured crack length and crack opening displacements, along with a fracture mechanics based numerical simulation.

10:30 AM *JJ3.6/MM4.6 
CONTINUOUS AIRPORT PAVEMENT DEFLECTION MEASUREMENTS USING A ROLLING DYNAMIC DEFLECTOMETER (RDD). Kenneth H. Stokoe, II, James A. Bay, Michael McNerney, and B. Frank McCullough, Department of Civil Engineering, University of Texas at Austin, Austin, TX.

Nondestructive testing of airport pavement plays an important role in the management of the runway infrastructure. A new technique for continuous profiling of pavements, called the Rolling Dynamic Deflectometer (RDD) has been developed. The RDD is a large truck on which a servo-hydraulic vibrator is mounted. The vibrator is used to apply large vertical dynamic loads to the pavement. The resulting dynamic displacements are measured with rolling sensors. A description of the RDD and procedures used to analyze RDD data are discussed in this paper. The results of continuous RDD profiling of taxiways and runways at the Dallas-Forth Worth Airport are presented. These results show that continuous stiffness profiles of displacement per given load of the pavements can be used to characterize: 1. the pavement stiffness and its longitudinal variation; 2. the location of transverse cracks and joints; 3. the efficiency of transverse cracks and joints; 4. the efficiency of longitudinal joints; and 5. the lateral variation in average mid-span stiffness. Three significant benefits of continuous RDD profiles which are clearly shown are: 1. softer versus stiffer areas are clearly delineated, 2. the variation in joint efficiency is readily identified, and 3. variations with time are readily identified by repeat measurements over time.

11:00 AM JJ3.7/MM4.7 
THE LEACHING OF THE REACTIVE POWDER CONCRETE: THE RESULTS ON THE TRANSFER PROPERTIES. Veronique Matte, Bouygues, Direction Scientifique, Challenger, St-Quentin-en-Yvelines, FRANCE; Micheline Moranville, Laboratoire de Mecanique et de Technologie, Cachan, FRANCE.

The Reactive Powder Concrete is a new cementitious material which exhibits a very low porosity and a high compressive strength. In order to study its viability concerning the storage of the nuclear wastes, the RPC is submitted to a water leaching test. The pH is buffered to 7 and the water is often renewed to maintain it deionised. The concentration of the ions released by the the material is measured and expressed as a function of time. The modifications of the transfer properties after leaching are also studied (porosity and diffusion). Concerning the diffusion aspect, two cases are studied: 
- The diffusion of an external ion without interaction with the cement paste (tritium); 
- The diffusion of the ions coming from the cement paste. This permits the understanding of the chemical equilibriums that occur during the leaching process. Thus, it could be possible to model the leaching process and predict the RPC's changes due to the leaching for 300 years or more. On the other hand, the modification of the pore distribution due to the leaching process is studied by MIP (Mercury Intrusion Porosimetry), but also by using more powerful techniques like proton NMR, BET (sorption-desorption of water vapour), SAXS (Small Angle X-Ray Scattering) and SANS (Small Amgle Neutron Scattering). In this way, the investigation of the pore structure under 100 angstroms is possible.

11:15 AM JJ3.8/MM4.8 
FATIGUE DAMAGE MONITORING IN CONCRETE. Kolluru Subramaniam, John S. Popovics, Surendra P. Shah, Center for Advanced Cement Based Materials, Northwestern University, Evanston, IL.

Rigid airport pavement structures suffer damage from multi-axial high magnitude cyclic stresses resulting from passing heavy aircraft. It is of interest to model the response of plain portland cement concrete to such loading conditions; thus, the sensitive detection and characterization of such damage during the loading process is important. Low strain vibrational resonance frequency measurement offers direct information concerning the global, apparent elastic moduli of the material and preliminary results have shown that such measurements are sensitive to the presence of damage in concrete.

The work reported here includes the theoretical foundation and experimental results of a non-destructive technique, based on vibrational resonance measurement, applied to monitor damage imparted to end-mounted hollow concrete cylinders subjected to monotonic and cyclic torsional (bi-axial) loads. An introduction to the concepts of vibration testing and details of the mechanical and vibrational test procedures employed are given first. The most significant vibrational modes, of all of the possible modes set up within the specimen, are identified. The frequency value of these significant modes in the concrete specimen are experimentally obtained throughout a controlled monotonic testing procedure to failure. Distinctions in the frequency values of the various excited resonance modes are noted. Moreover, effects of the two damage types (monotonic and cyclic) on the frequency values of the modes are studied. Conclusion concerning the applicability of vibrational resonance techniques for monitoring imparted damage in concrete are finally drawn.

11:45 AM JJ3.9/MM4.9 
Abstract Withdrawn.

Chair: Jan D. Achenbach 
Tuesday Afternoon, December 2, 1997 
Fairfax B (S)

1:30 PM *JJ4.1 
THE FUTURE OF NDE APPLICATIONS TO RPV EMBRITTLEMENT MEASUREMENTS. Michael G. Vassilaros, U.S. Nuclear Regulatory Commission, Washington, DC.

Abstract Not Available

2:00 PM JJ4.2 
MEASUREMENT CHALLENGES ASSOCIATED WITH IRRADIATED REACTOR COMPONENTS. Steven R. Doctor, Pacific Northwest National Laboratory, Imaging and NDE Group, Richland, WA.

Nuclear reactor components are known to become embrittled as reactors age. The reactor pressure vessel and the reactor internals are subjected to the highest radiation fields and thus, age the fastest. Some boiling water reactors are experiencing extensive cracking of the core shroud and some pressurized water reactors are experiencing a reduction in their fracture toughness. This paper provides details of reactor pressure vessels materials, fabrication, and construction practices. It identifies the challenges that are associated with the inspection and measurement of changes in material properties associated with these components. The inspection environment, surface conditions and component designs are also discussed.

2:15 PM JJ4.3 
THE NANOSTRUCTURES AND MICROMECHANICS OF IRRADIATION EMBRITTLEMENT: IMPLICATIONS TO NON DESTRUCTIVE EVALUATION (NDE) METHODS. G. Robert Odette, Dept of Mechanical and Environmental Engineering, University of California, Santa Barbara, Santa Barbara, CA.

Irradiation embrittlement of RPV steels, typically characterized by toughness-indexed transition temperature shifts, is primarily due to a complex array of ultrafine (< 1 nm) scale coherent copper rich and manganese-nickel rich precipitates, point defect cluster-solute complexes and alloy compound phases containing carbon, nitrogen and phosphorous. Hardening caused by these irradiation induced features increases (shifts) the temperature where internal tensile stresses in the highly plastic region near the tip of a crack reaches a critical high level over a critical volume that are required to trigger cleavage. The balance of the features depends on a synergistic combination of irradiation flux, fluence and temperature, as well as alloy composition and heat treatment history. For a given population of nanofeatures, the amount of hardening depends on the pre-irradiation microstructure. Likewise, the shifts caused by a specified level of hardening depend on the unirradiated properties of the alloy. The need to accurately characterize changes in properties, rather than absolute values, and the non-uniqueness of the structure-property relations, must be recognized in efforts to develop NDE methods. The most promising NDE approaches would appear to involve anelastic measurements that produce large amplitude dislocation motion, perhaps, coupled with methods that are sensitive to alloy resistivity. If successfully developed and demonstrated in field applications, volumetric vessel measurements with such techniques, coupled with a broader framework of information and selected direct sampling measurements, could usefully supplement established embrittlement prediction methods.

2:30 PM JJ4.4 
THE RIDDLE OF NDE FOR EMBRITTLEMENT DETECTION . Michael Blaszkiewicz, Westinghouse Electric Corporation, Science and Technology Center, Pittsburgh, PA.

The ability to nondestructively determine the level of irradiation induced degradation in nuclear reactor pressure vessels (RPVs) would enhance the integrity assessment currently used by the nuclear industry. Presently, destructive testing of Charpy specimens from surveillance capsules is used to approximate the RPV upper shelf energy and the ductile-to-brittle transition temperature, and approved models and guidelines are used to determine the state of embrittlement. However, these models and surveillance programs do not always provide enough accurate information to support decisions for premature RPV life termination, life continuation to license expiration, or life renewal and extension by means of annealing. Effective nondestructive techniques would extend the usefulness of the surveillance material by reducing the amount of material used for destructive studies, and ultimately by allowing tests to be performed directly on the RPV. Nondestructive techniques, ranging from electrical resistivity to hyperfine interactions, have been, and continue to be, explored for use in embrittlement assessment. The current states of these various techniques are discussed, and future directions for research are suggested.

2:45 PM JJ4.5 
ULTRASONIC ATTENUATION DETECTION OF NEUTRON IRRADIATION EMBRITTLEMENT OF NUCLEAR REACTOR STEEL. Allen L. Hiser, Jr. and Robert E. Green, Jr., Center for Nondestructive Evaluation, The Johns Hopkins University, Baltimore, MD.

The Nuclear Regulatory Commission is responsible for establishing limits on Nuclear Reactor Pressure Vessel (RPV) material fracture toughness. As the RPVs age the mechanical properties change due to irradiation embrittlement causing increases in strength of the RPV steel and decreases in fracture toughness. Of particular concern is the fracture toughness of the steel during normal operating conditions of temperature and pressure. Research is underway to assess the capability of ultrasonic attenuation to provide a direct nondestructive evaluation of the state of neutron irradiation embrittlement of nuclear reactor steel. The goal of this effort is to provide a means for direct measurement of reactor pressure vessel fracture toughness and possibly develop a basis for replace of surveillance programs and generic equations currently utilized for calculating the toughness of reactor pressure vessels.

3:30 PM JJ4.6 

This paper will discuss the effects of neutron irradiation and other types of degradation on the magnetic properties of steel structural materials used in nuclear primary systems such as pressure vessels and pipework. The results can be interpreted in terms of well-tested existing models of the magnetization processes. It is found that the properties of the aged structural materials were dependent on the level of damage incurred in the materials as a result of service exposure.

3:45 PM *JJ4.7 
ULTRASONIC NDE OF RADIATION DAMAGE. B.R. Tittmann, Penn State University, University Park, PA.

Irradiation embrittlement is a form of radiation damage which has been characterized in terms of the role of dislocations. Specifically, irradiation is thought to cause point defects which increase the number of pinning sites of dislocations on the dislocation network This in turn has been shown to affect the velocity and anelastic attenuation or internal friction of ultrasonic waves traveling through the damaged material. A large body of knowledge exists on the use of anelasticity in the investigation of radiation damage. This paper attempts to review the basic concepts of the interaction between the ultrasonic waves and dislocations. In this context the role of high amplitude ultrasound in causing the dislocations to break away from point defect pinning sites is described. This break away causes a significant amplitude dependence of the ultrasonic attenuation. Examples from previous research results are given to illustrate these effects. The possible benefits and limitations of this approach to evaluate embrittled materials are given in the context of the non-destructive evaluation of reactor vessels.

4:15 PM JJ4.8 

This paper summarizes research advances in ultrasonic and magnetic technologies for materials characterization. Included are discussions of the possibility that one can characterize structure, such as grain size and dislocations, properties, such as fracture toughness, and degradation mechanisms, such as creep, fatigue and hydrogen attack. Throughout the discussion, careful attention is given to differentiating those quantities which are fundamentally related to the nondestructive evaluation response, and those which are related by empirical correlations. Future opportunity and challenges in developing NDE techniques for measuring irradiation embrittlement in aging reactor components will be discussed.

Chair: Eric N. Landis 
Wednesday Morning, December 3, 1997 
Fairfax B (S)

8:30 AM JJ5.1 
ENERGY-MICROCRACK GROWTH MEASUREMENTS FOR MORTAR CYLINDERS IN COMPRESSION. Edwin Nagy and Eric Landis, University of Maine, Dept of Civil and Environmental Engineering, Orono, ME.

We are studying digitized microtomographic data of axially loaded mortar cylinders to measure microcrack growth as a function of external work of load. Microtomography, like medical tomography, produces three-dimensional x-ray absorptivity data, only with micron resolution. Using a series of common image processing techniques, we are able to convert a stack of 2-d gray-scale image tomographic slices to three dimensional representations of internal structure, as well as make measurements of internal crack surface area. The experiments described here were conducted using a specially designed in situ loading apparatus from which specimen load and displacement could be monitored while specimens were being scanned. A series of five to six scans were made of each specimen at varying loads. Based on the internal crack areas measured at different load levels, a quasi-R-Curve type of analysis can be made. That is, plots of non recoverable work of load versus internal crack surface area can be constructed from the data. Preliminary results of this three dimensional analysis confirm previous findings that the crack growth resistance increases with crack length. What differs from previous analysis is that present analysis considers multiple cracks with varying degrees of tortuosity.

9:00 AM JJ5.2 

The stock of reinforced concrete structures has been rapidly growing throughout the world during the past 30-40 years. As the service life of most of the structure is less than this period, a rapid increasing need for repair of the structures or replacement has been experienced. This trend will no doubt be accelerated in the future. As it is well known, the service life of existing structures (or of repairs) can be extended by proper preventative maintenance, involving only minor costs compared to the alternatives, repair or replacement. The cost of preventative maintenance, repair or replacement of a given stock of infrastructure always exceed the available economic resources. An optimal use of available funding is obviously required, i.e. to spend minimum economic resources to keep the structures in operational shape within a certain time frame in such a manner that the correct remedial activities are executed at the right time. To establish a rational decision foundation for how, when and where to maintain, repair or replace structures the cause(s) and the extend of the damage(s) have to be established through inspection and testing of the structures(s). Also the future rate of deterioration has to be evaluated based on the testing information, taking into account the environmental loads and the present state of the structure(s). The paper describes service life modeling for the evaluation of the technical and economical need of maintenance/replacement of reinforced concrete structures. Furthermore, the paper describe the state-of-the-art in-situ testing methods for establishment of the rational decision foundation concerning the cause(s) and the extend of the damage(s) of the structures.

9:15 AM JJ5.3 

This investigation involves the identification and use of a novel type of fiber optic sensors in the nondestructive testing and monitoring the deformation behavior of critical sections of the structural concrete elements and transforming them into smart systems. Deformation behavior was studied of high performance concrete continuous composite beams reinforced with prestressed prisms and instrumented with Bragg Grating fiber optic sensors. Such elements would be useful as components of continuous bridge decks where prevention of cracking in the negative moment regions is essential in maintaining the integrity of a bridge. Use of the Bragg Grating sensors would enable on-liner, real time monitoring of existing constructed concrete structures. Thirteen Simply supported and four continuous beams were tested to failure. These beams had clear spans of 2743 mm (9 ft). High performance concrete with compressive strength, fc , in excess of 90 MPa (13,000 psi) was used for both the precast prestressed prisms and the situ-cast main beams. Experimental results were compared with the theoretical evaluations obtained from nonlinear analysis. The research results indicate that Bragg grating fiber optic sensors are ideal for monitoring on-line long-term deformation behavior of old and new structures thereby giving ample warning of deterioration and failure.

9:30 AM JJ5.4 
DEVELOPMENT OF A CAPACITOR PROBE TO DETECT SUBSURFACE DETERIORATION IN CONCRETE. Imad L. Al-Qadi, Department of Civil Engineering; Sedki M. Riad, Bradley Department of Electrical Engineering; Brian K. Diefenderfer, Department of Civil Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA.

Portland cement concrete (PCC) structures deteriorate with age and need to be maintained regularly and rehabilitated occasionally. Thus, civil engineers have shifted their focus from construction of new facilities towards preservation through maintenance and rehabilitation. Early detection of deterioration in PCC (e.g., alkali-silica reaction, freeze-thaw damage, and/or chloride presence) can lead to significant reductions in maintenance costs. However, it is often too late to perform low-cost preventative maintenance by the time deterioration becomes evident. By developing techniques that would enable civil engineers to evaluate PCC structures and detect subsurface deterioration at early stages, optimization of life-cycle costs of PCC constructed facilities and minimization of disturbance to facility users can be achieved. Quantitative performance characteristics of PCC structures need to be assessed periodically. Nondestructive evaluation (NDE) methods are potentially one of the most useful techniques ever developed for assessing constructed facilities. PCC can be rapidly and nondestructively (and noninvasively) be evaluated by electrically characterizing itsí complex dielectric constant. The real part of the dielectric constant depicts the velocity of electromagnetic waves in PCC. The imaginary part, termed the loss factor, describes the conductivity of PCC and the attenuation of electromagnetic waves. Dielectric properties of PCC have been investigated in a laboratory setting using a Parallel-Plate Capacitor (developed at Virginia Tech) operating in the frequency range of 0.1-40.1 MHz. This capacitor set-up consists of two horizontal-parallel plates with an adjustable separation for insertion of a dielectric specimen (PCC). The capacitor was used successfully to measure chlorides diffused into PCC. While useful in research and to measure dielectric properties of PCC accurately, this approach is not practical for field implementation. A new capacitor probe is currently under development and consists of two plates, located within the same horizontal plane, for placement upon the PCC structure to be tested. PCC specimens of varying composition and w/c ratios will be characterized using the new probe and the results will be compared to those obtained from the existing capacitor method.

9:45 AM JJ5.5 
NDE OF TENDON DUCTS IN CONCRETE USING 3D-SAFT. Martin Krause, Herbert Wiggenhauser, Federal Institute for Materials Research and Testing (BAM), Berlin, GERMANY; Wolfgang Müller, Volker Schmitz, Fraunhofer Institute for Nondestructive Testing (IZFP), Saarbrücken, GERMANY.

Prestressed or post-tensioned concrete structures are vulnerable to hazardous corrosion if the tendon ducts are not fully grouted. Also, compaction faults or honeycombing reduce the concrete strength. Advanced ultrasound testing techniques and data analysis have been utilized to evaluate nondestructively ducts in concrete elements. A larger number of measurements must be taken to perform a three dimensional reconstruction analysis (3D-SAFT) of the concrete volume under investigation. A laser doppler vibrometer has been used as an ultrasound receiver to automatically collect data. With this method ducts could be investigated which are not accessible by other methods such as radiography or impact-echo. The encouraging results on several specimen were accompanied by simulations to optimize the experimental conditions. The influence of pore size and distribution and of reinforcements is still under investigation.

10:30 AM JJ5.6 
A BASIS FOR THE NONDESTRUCTIVE CHARACTERIZATION OF USED ROPES OF NYLON 6. Hao Jiang, Wright Patterson Airforce Base, Dayton, OH; Mark Davis,Mark Davis R.K. Eby, The University of Akron, Department of Polymer Science, Akron, OH; Peter Arsenovic, NASA, Goddard Space Fight Center, Greenbelt, MD.

Physical properties and structural parameters have been measured for ropes of nylon 6 as a function of the number of use operations. The fractional content of the /alpha crystal form, sound velocity, birefringence, tensile strength and length all increase systematically and significantly with increasing numbers of use operations. The fractional content of the /gamma crystal form and fiber diameter decrease with use. These trends indicate that the measurement of such properties and structural parameters, especially the length, could be used as a basis for establishing a nondestructive characterization method to predict the remaining service life of nylon 6 ropes.

10:45 AM JJ5.7 
NON-INVASIVE MONITORING OF ACUTE INTRACRANIAL MASS LESIONS USING ULTRASONIC FINGERPRINTING. Peter B. Nagy, Department of Aerospace Engineering and Engineering Mechanics, University of Cincinnati, Cincinnati, OH; William S. Rosenberg, Lawrence M. Stankovits, Department of Neurosurgery, University of Cincinnati, Cincinnati, OH.

Potentially dangerous neurological changes in shock-trauma patients are currently monitored by computer-aided X-ray tomography, which is prohibitively expensive and even dangerous for long-term, e.g., comatose, patients. Only low-frequency diffuse ultrasonic inspection is feasible through the skull so that the details are irreversible lost in the essentially random scattering process. In order to overcome this inherent limitation, we adapted a continuous computer-controlled ultrasonic monitoring system based on the Ultrasonic Fingerprinting method originally developed for materials characterization purposes in the nuclear, civil engineering, and aerospace industries. An ultrasonic detector directed at the general area of interest can be used to record and repeatedly update the personal signature of the patient, which is then used as an ultrasonic fingerprint. Any abrupt change in this signature indicates the immediate need for further investigation by CT or other sophisticated diagnostic tools. Experimental studies were conducted on both a human skull/gelatin phantom and 5 intact human cadavers. To determine signal instabilities, the differentials between the baseline and a 1 hour series of intracranial ultrasonic fingerprints were analyzed. Only baseline corrected deviations that exceeded the 1 hour system instability were considered significant. Thermal changes produced minor baseline corrected deviations, which were eliminated using linear transformations. The minimum significant detectable volume ranged from 11 cc to 19 cc for the ipsilateral frontotemporal, contralateral frontotemporal and posterior fossa mass lesions. These volumes are lower than indications for surgical intervention.

11:00 AM JJ5.8 
MEASURING DAMAGE EVOLUTION IN CERAMIC MATRIX COMPOSITES USING THERMOELASTIC STRESS ANALYSIS. Tom Mackin, Mark Roberts, University of IIllinois at Urbana-Champaign, Mechanical Engineering Department, Urbana, IL.

A new generation of relatively 'ductile' CMCs is being developed that expands the general utility of structural ceramic composites. These new materials rely on inelastic mechanisms such as interface failure, matrix cracking, fiber failure and fiber pullout to redistribute stress away from locations of stress concentration (Evans et al, 1995). In most materials, an interphase is utilized to attain the requisite interfacial properties. That is, the interphase provides a weak link that enables debonding between the matrix and fiber, thereby decoupling the locations of matrix and fiber failures. A novel alumina fiber reinforced alumina matrix material was developed at GE with variants developed at other locations (Lange et al.,1995)(Levi et al.,1997) that achieves the needed debonding through the use of porosity rather than fiber coatings. This material offers a new paradigm: relatively ductile ceramics can be fabricated without a second-phase weak interface if the matrix can be processed with ample and controlled porosity. To evaluate the utility of this new all-oxide material, experiments were conducted to determine the damage mechanism and the extent of stress redistribution in standard test specimens. Thermoelastic stress analysis TSA) was used to classify the damage mechanism and measure the extent of stress redistribution (Mackin et al.,1995). Tests were performed on 0/90 and +/- 45 weave double notched specimens, and the evolution of the stress concentration factor with increasing specimen damage was measured. The results indicate that the subject alumina/ alumina composite exhibits a measurable degree of ductility by developing a shear damage zone.

11:15 AM JJ5.9 
INVESTIGATING DISTRIBUTED MICRO-CRACKS IN PLATE SHAPED MATERIALS WITH ULTRASONIC REFLECTION COEFFICIENT. Wei Yang and Surendra P. Shah, NSF Center for Advanced Cement-Based Materials, Northwestern University, Evanston, IL.

The proposed research aims at quantitative evaluation on the density of distributed microscopic cracks in concrete materials that are sometimes extremely important since micro-crack accumulation is usually the most dominant stage during the life time of a structure. The effects on mechanical properties of materials caused by distributed micro-cracks can be correlated to certain amount of reduction on elastic moduli and increase on acoustic attenuation. From discretizing the concrete plate into a number of layers of different effective moduli and attenuation factors, acoustic reflection coefficient corresponding to certain profile of distributed cracks through the thickness of concrete plate is computed and plotted. The convergence of the discretized approach was verified from a numerical example of a plate with micro-cracks distributed linearly through the depth. Immersion ultrasound tests were performed on plate-shaped concrete specimens cast with defined artificial cracks of certain levels and variational crack densities through the thicknesses of specimens. Refection coefficients were measured under different incident angles and it shows that, experimentally, the refection coefficients are very sensitive to the crack density and the distributed crack gradient. The computed reflection coefficient variation with respect to incident angle curves are compared with experimental measurement to get reliable judgment on the level and profile of micro-scaled defects in the material.

11:30 AM JJ5.10 
HOW TO SEE THROUGH MURKY CONDITIONS: USE OF AN OPTICAL PARAMETRIC AMPLIFIER. David E. Bliss, Stewart M. Cameron, Sandia National Laboratories, Albuquerque, NM.

Anyone who has tried to drive an automobile on a foggy night can appreciate the practical difficulties associated with optical imaging through a highly scattering, turbid medium. This is just one common example of how visualization of phenomena occurring in turbid or ''fog-like'' media is fundamental to solving many problems of current technical interest including tissue imaging, atmospheric and oceanographic lidar, and nondestructive metrology of buried interfaces. The authors have developed a broadly wavelength tunable, 470-2400 mm, ultrashort amplifying optical gate for reflective and transillumination spectral imaging based on optical parametric amplification in a nonlinear crystal. The time-gated image amplification process exhibits low noise and high sensitivity, with gains greater than 104 achievable for low light levels. We report results in which this system was used to reconstruct, spectrally upcovert and enhance near-infrared two-dimensional images with feature sizes of 65 m in background optical attenuations exceeding 1012. Phase images of test objects exhibiting both absorptive contrast and diffuse scatter were acquired using a self-referencing Shack-Hartmann wavefront sensor in combination with short-pulse quasi-ballistic gating. The sensor employed a lenslet array based on binary optics technology and was sensitive to optical path distortions approaching l/100.

Chair: R. Bruce Thompson 
Wednesday Afternoon, December 3, 1997 
Fairfax B (S)

8:30 AM *JJ6.1 
NONDESTRUCTIVE CHARACTERIZATION OF MECHANICAL PROPERTIES ON MICRO-AND NANOSCALES. S. I. Rokhlin, The Ohio State University, Nondestructive Evaluation Program, Columbus, OH.

Recent developments in the aerospace and electronic industries demand new materials with complex microstructure on different scales. This necessitates the determination of mechanical properties on these scales, including thin surface and thin interfacial layers. Therefore nondestructive methods for characterization on these scales become necessary. This talk reviews various recent developments in the determination of material properties on these scales using ultrasonic methods, and evaluates the potential of atomic force microscopy for these applications. Several examples of assessment of the environmental degradation of thin-surface and interphasial layer properties using ultrasonic techniques are given. Their capabilities, degree of localization, and limitations are described. New opportunities of nanoscale properties determination have been opened with the discovery of atomic force microscopy. Quantitative models of the cantilever tip/sample interaction in atomic force microscopy are given for tapping and force-modulation modes. These modes are analyzed for measurement of elastic properties of materials on the nanoscale. The relation of the atomic force microscopy to the well-known tapping NDE method is discussed.

2:00 PM JJ6.2 

Pulsed, or transient eddy-current methods are an effective tool for quantitative characterization of hidden corrosion and cracking in multi-layer aircraft structures. Eddy currents are the method of choice for this task, since they penetrate multiple layers of metal, whether or not the layers are mechanically bonded. The pulsed eddy current technique provides an important advance over typical single-frequency methods because it rapidly provides data over a wide range of frequency. Much more information is contained in a pulsed eddy current signal than would be available from a conventional eddy current instrument. We have combined a pulsed eddy-current instrument with a portable two-axis scanner to produce an instrument capable of rapidly scanning aircraft lap splices in situ and producing pseudo-color images representing maps of hidden corrosion or cracking. The ability to produce images of the scanned areas enhances the interpretation of data, allowing the operator to use his or her natural skills at pattern recognition. A unique feature of time-domain presentation of eddy current data is the ability to selectively filter clutter from the image by time-gating the pulsed signal. Time-gating permits the user to select the inspection depth, thereby eliminating interference from upper layers, air gaps, lift-off variation and fasteners. Because the pulsed eddy current system has been modeled theoretically, it is possible to understand the data quantitatively, producing quantitative maps of corrosion damage. Some of the same advantages of the pulsed eddy current technique apply to the characterization of hidden fatigue cracks, although the theory in this case is less advanced. But accurate measurements of crack lengths under one or two layers of skin or reinforcement has been demonstrated.

2:15 PM JJ6.3 
DEVELOPMENT AND TESTING OF A HIGH TEMPERATURE SUPERCONDUCTING EDDY CURRENT PROBE FOR NONDESTRUCTIVE EVALUATION. James R. Claycomb, Nilesh Tralshawala, Hsiao-Mei Cho, and John H. Miller, Jr., University of Houston, Dept of Physics and Texas Center for Superconductivity, Houston, TX.

We report on the design and performance evaluation of a high temperature superconducting eddy current probe for SQUID (Superconducting Quantum Interference Device) based electromagnetic nondestructive evaluation. An eddy current probe, the design of which is based on our numerical modeling studies, has been fabricated and tested in our lab. The probe utilizes high-Tc superconducting and mu-metal cylindrical shields in order to reduce exposure to electromagnetic interference and to focus magnetic flux due to the eddy current drive coil into the object under test. Measurements are made by varying the size, depth and orientation of simulated flaws in conducting samples. The magnetic response due to cracks and corrosion is also measured in actual FAA test specimens. The measured SQUID data is then compared to analytical defect and corrosion models.

2:30 PM JJ6.4 
SHORT-TIME FOURIER TRANSFORM OF CONTINUOUS WAVE DOPPLER SIGNALS. Shi-Chang Wooh, Coach Wei, and Arthur Clay, Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, Cambridge, MA.

This research was motivated by the need of developing a high-speed monitoring technique for detecting defects in railroad tracks. A concept based on the Doppler effect was introduced in order to exploit the relative motion between an ultrasonic sensor and a moving target. It was shown from the feasibility tests that low frequency air-coupled transducers can be effectively used for detecting surface defects on a rapidly moving object, thereby eliminating the requirement of contact between the sensor and the material surface. In a laboratory test conducted using a rotating aluminum disk with a peripheral notch, the Doppler effect was observed. The frequency of the continuous-wave (CW) ultrasonic signals reflected from the notch was Doppler-shifted, resulting in wave packets in the as-obtained waveform. Due to the narrow bandwidth of the signal, the effect of the shifted frequency clearly appeared in the frequency spectrum. A method was developed to process the CW signal using the Short-Time Fourier Transform (STFT). By monitoring the peak magnitude of the STFT spectra within a specified window, it was possible to reconstruct a signal representing Doppler-shift in the time domain. The locations of surface defects were pronounced and marked as sharp peaks corresponding to the spatial position of the flaw.

3:15 PM JJ6.5 

Object - Damage and failure behavior of metallic structures under severe service loading are important factors in connection with design or building regulations, because metal fatigue is the principal cause of failure in structural and mechanical systems. These fracture phenomena, which until recently were poorly understood, are those which often occur unexpectedly at relatively low stress levels. In recent years, some progress has been made in design methods. Nevertheless, these methods must be used in conjunction with reliable experimental procedures. From an engineering point of view, fatigue criteria and fracture control are needed, together with practical and economic considerations. 
Method - Recently, in response to these problems, diverse analysis methodologies have been developed which isolate the factors affecting crack initiation and growth, and enable the prediction of their cumulative effects on the technical performance of the structural components. The aim of the present paper is to illustrate the use of quantitative infrared thermography as a nondestructive and noncontact technique to detect the manifestation of the physical process of fatigue, and to evaluate rapidly the fatigue limit of metallic materials. 
Results - The proposed method has been applied successfully to metallic materials: different types of steels, cast iron, et.), on diverse stress paths (reversed tension, rotating bending, cyclic torsion, etc.). The fatigue limit of connecting rods has been readily determined using this technique. The experimental tool is used here to detect the onset of intrinsic dissipation or damage of material due to the thermomechanical coupling when the specimen is excited by a fluctuating load. Information about the location and significance of defects, can be obtained through inspection and nondestructive evaluation. 
Concluding remarks - Thanks to the thermomechanical coupling, infrared thermography offers a nondestructive, noncontact technique, to evidence metal degradation or damage. Intrinsic dissipation is a scalar parameter, readily evaluated by infrared thermography. It provides a simple measure of the limit of a progressive damaging process under repeated loading beyond which the metal is damaged. This endurance limit is traditionally determined by a very time-consuming procedure.

3:30 PM JJ6.6 
DAMAGE IDENTIFICATION OF CONTINUA FROM VIBRATION MODAL DATA. Huapeng Chen, Nenad Bicanic, Dept of Civil Engineering, Glasgow University, Glasgow, UNITED KINGDOM.

A novel procedure for damage identification of continua is proposed, where both the location and the extent of structural damage in continuum structures can be correctly determined using only a limited amount of measurements and incomplete modal data. On the basis of the characteristic equations for the original and the damaged structure, a set of equations which represent the exact relationship between structural parameters and modal parameters is established. A computational technique based on direct iteration and directly using the incomplete modal data is developed to determine structural damage from the derived equations. Structural damage is assumed to be associated with a modification of the original element stiffness matrices, equivalent to a scalar reduction of the material modulus, associated with a Gauss point level. Finally, numerical examples for plane stress problems and plate bending problems are utilised to demonstrate the effectiveness of the proposed approach.

3:45 PM JJ6.7 

A functional ultrasound camera system that employs an ultrasound sensitive multi-element array has been demonstrated and is currently in product development. It is a novel non-destructive testing tool capable of imaging internal defects (e.g. voids, delamination, corrosion) and provides real time, depth sensitive C-scan information. A large area ultrasound beam insonifies the target to be studied. An acoustic lens is used to collect the resulting transmitted or reflected energy and focuses the information onto the integrated circuit array. The ultrasound sensitive pixel elements (128 x 128) are coupled to a multiplexer which is designed to read out the individual voltages sequentially, producing a TV like image of high resolution. This contrasts to ultrasonic C-scan systems which require a mechanical point to point scan. The installation of the system could be similar to traditional C-Scan systems by immersing targets in water or by employing water squirter methods. However, since the camera system requires no mechanical scanning, the system can also be implemented as a stationery handheld probe with a couplant fluid if necessary (similar to medical ultrasound imaging devices) and would result in an immediate image. It would not require targets to be submerged in a fluid at all. The camera operates at either thirty frames a second (U S. standard scan rate) or at fifty frames a second corresponding to frame rates employed in Europe. These high frame rates allows rapid movement of the probe in order to cover larger areas in real time. If more signal gain is required, multiple frames can be integrated to allow frame integration for improved signal to noise.

4:00 PM JJ6.8 
EVALUATION OF PRESTRESSING WITH A SHAPE MEMORY ALLOY, Arup K. Maji, Air Force Phillips Laboratory (PL/VTV), Albuquerque, NM.

The paper addresses the possibility of using shape-memory effect in Nitinol (Nickel-Titanium) alloy for prestressing cement based and polymer matrix composites. Straight annealed NiTi in the low temperature martensite phase can undergo large plastic deformations. This deformation and strain energy is recoverable when the material transforms to the higher temperature (about 55C) austentite phase. This phenomena can be exploited to induce curvature, hence prestressing, by embedding NiTi wires and ropes in a structural member. Depending on the type of wire used its actuation effect can be reversed as necessary, creating the possibility of a smart prestressed concrete bridge of the future. Materials aspect studied in this project include the bond between NiTi wires and the host composite material, through mechanical testing and microscopy. An evaluation of the actuation phenomena is conducted through computer generated laser interferometry. The laser techniques used are electronic speckle interferometry and shearography, depending on whether deflections or curvatures are of interest. These non invasive techniques allow deformations along the entire beam specimens to be quantified and compared with theoretical models.

4:15 PM *JJ6.9 
NONDESTRUCTIVE INSPECTION OF THIN, LOW-Z SAMPLES USING COMPTON SCATTERING TOMOGRAPHY. Brian L. Evans, Jeffrey B. Martin, Larry W. Burggraf, Air Force Institute of Technology, Dept. of Engineering Physics, Wright-Patterson AFB, OH.

The viability of a Compton scattering tomography system for nondestructive inspection of thin, low Z samples is examined. This technique differs from conventional x-ray backscatter NDI because it does not rely on narrow collimation of source and detectors to examine small volumes in the sample. Instead, photons of a single energy are backscattered from the sample and their scattered energy spectra are measured at multiple detector locations. Compton scattered spectra are then used to computationally reconstruct an image of the object. This approach allows multiple volume elements to be interrogated simultaneously. Thin samples less than 1 cm thick and made of low Z materials are best imaged with gamma rays at or below 100 keV energy. At this energy, Compton line broadening becomes an important resolution limitation. An analytical model has been developed to simulate the signal collected in a demonstration system consisting of an array of 10 mm-thick planar high-purity germanium detectors with energy resolutions of less than 500 eV at 100 keV. A technique for deconvolving the effects of Compton line broadening and detector energy resolution from a signal with additive noise is also presented. A filtered backprojection image reconstruction algorithm with similarities to that used in conventional transmission computed tomography (CT) is developed, as is an iterative image reconstruction algorithm. Simulated results illustrate the capabilities of this demonstration system.

Chair: Brian Maclean 
Thursday Morning, December 4, 1997 
Fairfax B (S)

8:30 AM *JJ7.1 
A DIGITAL MEMS-BASED STRAIN GAGE FOR STRUCTURAL HEALTH MONITORING. Brian Maclean, Marc Whitaker, Marc Olivier, and Stephen Jacobsen, Sarcos Research Corp., Salt Lake City, UT; Mike Mladejovsky, University of Utah, Center for Engineering Design, Salt Lake City, UT.

One approach for structural health monitoring of aging aircraft is to take discrete airframe strain measurements and record the flight loads history. A complementary method consists of measuring changes in dynamic response due to fatigue crack growth. The challenge in implementing such methods is the need for inexpensive networks of distributed strain sensors which possess high resolution with no drift over time. The Uni Axial Strain Transducer (UAST) has been developed as a digital, absolute encoding device to address these very issues. The UAST is a micro-electro-mechanical system (MEMS) which exploits the capacitive coupling between an array of 64 electrostatic field emitters and 64 field effect transistor gates. The slightly different array element spacings form a vernier scale and digital signal processing of the detector outputs is used to calculate the absolute translational displacement of the emitter array relative to the CMOS detector chip. The UAST provides a dynamic range of +/-5000 micro-strain and displacements of 25 Angstroms have been resolved. The sensor sampling rate is dynamically configurable for 125, 250, 500, or 1000 Hz, providing 15, 14, 13, or 12 bits of resolution (equal to 0.3, 0.6, 1.2, or 2.4 micro-strain), respectively. The sensor network can communicate with up to 128 UASTs on a common 5-wire digital bus, eliminating the need for shielding and considerably reducing the number of wires which have to be routed through the airframe. A network technology demonstration is being conducted on a 1/2 scale F-18 vertical tail where dynamic loads are applied to evaluate network performance related to monitoring of fatigue crack growth or rivet-line failures. Application of the UAST in a helicopter rotor health usage and monitoring system, and the design of a bi-axial strain transducer under development, are also discussed.

9:00 AM JJ7.2 
MICROFABRICATED ULTRASONIC TRANSDUCERS FOR AIR-COUPLED NDE. B.T. Khuri-Yakub, I. Ladabaum, F. L. Degertekin, X. C. Jin, E.L. Ginzton Laboratory, Stanford University, Stanford, CA.

Microfabricated ultrasonic transducers (MUTs) are demonstrated to have a dynamic range of at least 110 dB. Air-coupled transmission of a 2.3 MHz tone burst through aluminum is reported. The received signal has an SNR of 30 dB. The transducers, which are made using techniques pioneered by the microelectronics industry, are also used to generate the first air-coupled scans of defects in aluminum plates. An aluminum plate is mechanically scanned between two 1cm X 1cm air transducers and the phase of the received tone burst is used to generate a map of defects. The transducers used for the experiment consist of a multitude of 100 micron diameter nitride membranes suspended above a silicon support structure. A theoretical model describing the electroacoustic behavior of the device is briefly presented, and the implications of the novel enabling technology are discussed.

9:15 AM JJ7.3 
A NEW SCANNING THERMAL MICROPROBE. Yongxia Zhang, Yanwei Zhang, Juliana Blaser, T.S. Sriram, Enver Ahsan, New Jersey Institute of Technology, Dept of Physics, University Heights Newark, NJ; and R. B. Marcus, Murray Hill Devices, Inc., Murray Hill, NJ.

A thermal microprobe has been designed and built for high resolution temperature sensing. The thermal sensor is a thin-film thermocouple junction at the tip of an Atomic Force Microprobe (AFM) silicon probe needle. Only wafer-stage processing steps are used for the fabrication. For high resolution temperature sensing it is essential that the junction be confined to a short distance at the AFM tip. This confinement is achieved by a controlled photoresist coating process. Prototypes have been made with a Au/Pd junction confined to within 0.3 microns of the tip, with the two metals electrically separated elsewhere by a thin insulating oxide layer. The device is designed for insertion in an AFM instrument so that topographical and thermal images can be made with the same tip. Large contact pads permit mechanical and ohmic contacting with spring clamps. Processing begins with double-polished, n-type, 4 inch-diameter, 300 um thick silicon wafers. Atomically sharp probe tips are formed by a combination of RIE, wet chemical etching, and oxidation sharpening. The metal layers are sputtering deposited and the cantilevers are released by a combination of KOH and dry etching. The thermal mass is kept low in order to cause minimal disturbance of the component under measurement and maximum temperature sensitivity. A resistively-heated calibration device was made for temperature calibration of the thermal microprobe over the temperature range 25-110 C. Over this range the thermal output of the microprobe is 4.5-5.6 microvolts per degree C and is linear.

9:30 AM JJ7.4 
A NOVEL STRESS-STRAIN MICROPROBE FOR NON-DESTRUCTIVE EVALUATION OF MECHANICAL AND FRACTURE PROPERTIES OF MATERIALS. Fahmy M. Haggag, Advanced Technology Corporation, Oak Ridge, TN; M.D. Mathew and K. Linga Murty, North Carolina State University, Raleigh, NC.

A novel portable/in-situ Stress-Strain Microprobe (SSM) system was used to measure true-stress/true-plastic-strain behavior of several metallic materials, welds, and their heat-affected-zones (HAZs) in various metallurgical and damage conditions. SSM is based on the principle of automated ball indentation (ABI) which is rapid, is non-destructive, is adaptable for in-situ testing and could be made operator-insensitive. The microprobe provides a localized direct stress-strain curve determination based on strain-controlled multiple indentations (at a single penetration location) of a polished surface by a spherical indenter and the indentation depth is progressively increased to a maximum user-specified limit with intermediate partial unloadings. Recent advances include single cycle method, development of high and low temperature testing, and fracture property evaluation. The technique permits measurement of yield strength, stress-strain curve, strength coefficient, and strain-hardening-exponent (uniform ductility). From temperature variation of true stress vs strain curves, a new fracture parameter known as Indentation Energy to Fracture (IEF) was derived which depicted the transition from ductile-to-brittle fracture in correlation with Charpy and fracture toughness data. The capabilities of the SSM system have been further extended to carry out stress-relaxation tests, and results obtained on electronic solders (Sn5%Sb) will be presented. Effects of cold-work, radiation exposure and thermal annealing of post-irradiated nuclear pressure vessel steels will be described in terms of ABI derived parameters.

9:45 AM JJ7.5 
ULTRASONIC SPECTROSCOPY OF HETEROGENEOUS POLYMERIC MATERIALS. Gregory Schueneman, Bruce Novak, and Alan Lesser, Univ. of Massachusetts, Dept. of Polymer Science and Engineering, Materials Research Science and Engineering Center, Amherst, MA.

Ultrasonic Spectroscopy is a new technique that utilizes high frequency (10-100 MHz) transducers to transmit and receive broad band ultrasonic signals through materials. Taking the fast Fourier transform of the received time domain signals allows one to obtain a frequency spectrum of the studied specimen. Ultrasonic waves are absorbed and dispersed as they pass through materials. Absorption and dispersion are connected by the Kramers-Kronig relationship. This relationship was originally derived for electromagnetic radiation, but can be applied to ultrasonics as the requirements of causality and linearity are satisfied. Kramers-Kronig relations allow dispersion to be calculated from measured absorption and vise versa. In this paper, we present results from studies on rubber modified and porous epoxies. We have examined the excess absorption and dispersion to determine the rubber particle and void sizes and to probe the epoxy/rubber and epoxy/void interfaces.

10:00 AM JJ7.6 
A SMALL ANGLE LIGHT SCATTERING TECHNIQUE FOR DETECTION OF MICRON-SCALE DEFECTS IN POLYCRYSTALLINE MATERIALS. Hsing-Yi Maria Ko, Karen McNamara Rutledge, Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester, MA.

In the past, small angle light scattering (SALS) has been applied with great success to the study of polymer solutions. This technique allows the quantification of micron-scale particles in terms of concentration, particle size, and particle shape. In this work, we have extended the technique to the solid state. Of partcular interest are polycrystalline materials, such as those used in the semi-conductor industry, which may include structural defects on a relevant length scale, such as intergranular voids, as a result of processing conditions. Here, we discuss the development of the experimental technique and theoretical analysis as applied to commercial polycrystalline chemical vapor deposited diamond.