David Armstrong, University of Oxford
David Bahr, Purdue University
Megan Cordill, Erich Schmid Institute of Materials Science
Corinne Packard, Colorado School of Mines
Symposium Support Hysitron, Inc.
T3: Time-Dependent Behavior and Testing I
Monday PM, November 30, 2015
Hynes, Level 1, Room 102
2:30 AM - T3.01
Ultra-Small-Scale High Cycle Fatigue Testing Using Micro-Cantilevers
Jicheng Gong 1 Angus J. Wilkinson 1
1Univ of Oxford Oxford United KingdomShow Abstract
A new method has been developed for testing high cycle and very high cycle fatigue properties of materials at the micro-scale based on micro-cantilevers. Focused ion beam was employed to cut micro-cantilevers with triangular cross-section into the surface of a selected grain in a bulk polycrystalline commercial pure Titanium. The bulk specimen was pre-examined by EBSD, so the crystal orientation of all micro-cantilevers were known. The bulk sample containing the micro-cantilevers was then attached to a high power ultrasonic generator, which can generate mechanical vibration at the frequency 20KHz. The bulk of the specimen moves with the ultrasonic generator, but the micro-cantilever lags somewhat behind. The resulting deflections generate cyclic stress in the micro-cantilevers. The high vibration frequency means it can easily test into the high cycle and very high cycle regime. The design challenge is to generate enough stress to cause fatigue in these ultra-small specimens because the stress amplitude achieved in vibration is inverse to the cantilever size. Previous finite element modelling and experiments had shown that the classic micro-cantilever with uniform cross-section can only generate stress of a few MPa, even with the acceleration as high as 107m/s2. Instead, we designed a new ‘hammer-shaped' micro-cantilever with a widened region at the free end to significantly increase the inertia. This design now generates sufficient stress and enables fatigue testing even in Titanium, which is a challenging material due to the high strength to weight ratio (both high stress and low density require higher acceleration).
SN curves in the testing range from 105 to 108 cycles have been obtained for 2 mu;m wide micro-cantilever single crystal Ti samples using a step test protocol. The stress to failure decreases systematically as the number of cycles to failure increases. However, there is strong dependence on the crystal orientation with the fatigue strength at 107 cycles for test pieces cut along the direction being approximately twice that of those cut in the direction. The fatigue strength of micro-fatigue test is significantly lower than the static strength measured on micro-cantilevers of identical size using a nanoindenter. Due to the small specimen size, it is suggested that the results reflect the behavior of fatigue nucleation rather than propagation.
2:45 AM - *T3.02
In Situ TEM Stress Relaxation and Cracking Investigation of Nanocrystalline Ultrathin Films
Ehsan Hosseinian 1 Saurabh Gupta 1 Marc Legros 2 Olivier N. Pierron 1
1Georgia Inst of Technology Atlanta United States2CEMES-CNRS Toulouse FranceShow Abstract
This talk presents an experimental investigation of the stress relaxation and cracking mechanisms in ultrathin nanocrystalline metals (Au and Al). Tensile tests that include stress relaxation segments were performed on ultrathin (<100 nm) specimens with micron-sized dogbone shapes, using a MEMS-based nanomechanical testing setup. The setup allows quantification of stress and strain (and their evolution with time) as well as in situ TEM observations of the governing mechanisms. The results (based on in situ TEM observations and activation volume calculation as well as evolution of strain rate with applied stress) show that the transient relaxation deformation in 100-nm-thick Au (average grain size: 75 nm) is highly localized and dominated by intergranular and intragranular dislocation activities. These dislocation activities slow down within a timescale of 1 hour, leading gradually to a diffusion-dominated deformation (steady-state creep). In contrast, the 30-nm-thick Au films (with a much smaller average grain size) do not show these sustained dislocation activities; instead their deformation behavior appears to be dominated by grain boundary activities. In addition to the stress relaxation properties, this talk will discuss the cracking behavior in ultrathin nanocrystalline gold and aluminum films.
3:15 AM - T3.03
Fatigue-Induced Abnormal Grain Growth in Nanocrystalline Metals: An In Situ Synchrotron XRD Approach
Timothy Allen Furnish 1 Brad Boyce 1 Apurva Mehta 2
1Sandia National Laboratories Albuquerque United States2Stanford Synchrotron Radiation Lightsource Menlo Park United StatesShow Abstract
The mechanical behavior of nanocrystalline metals has been the subject of extensive research, especially in terms of their high strengths and wear-resistance. However, the fatigue performance of these metals has received much less attention, despite initial reports of exceptional fatigue strengths. Particularly lacking is a comprehensive understanding of the cyclically-induced deformation mechanisms, structural evolution, and fracture behaviors in nanocrystalline metals. Recent studies using various post-fracture microscopy techniques have identified extensive regions of abnormally large grains near and along macro-scale cracks - but, the exact origins of these features and their overall effects on the fatigue performance and fracture are unclear. This is due, in part, to the general difficulties in performing non-destructive in situ characterization of through-thickness microstructures during fatigue testing. In the current study, a newly developed method of detecting abnormal grain growth (AGG) using synchrotron x-ray diffraction was employed to “watch” the microstructural evolution of various nanocrystalline Ni alloys during high-cycle fatigue. This in situ fatigue approach allowed us to verify with statistical confidence that the abnormally large grains do, in fact, grow during the fatigue process (i.e. they are not pre-existing) and that they develop well before final fracture of the material. The effects of the fatigue stress conditions on the microstructural evolution and AGG were also evaluated using a range of fatigue testing conditions. Additionally, traditional cross-sectional microscopy was performed after the onset of AGG, but before final fracture, to further explore the deformation and micro-scale crack mechanisms that lead to the macro-scale fracture during fatigue.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
3:30 AM - T3.04
Uniaxial Compression of Cellular Materials at a 10-1 Strain Rate Simultaneously with Synchrotron X-Ray Computed Tomographic Imaging
Brian M Patterson 1 Nikolaus L Cordes 1 Kevin Henderson 1 Mathew Robinson 2 Xianghui Xiao 3 Angel R Ovejero 4 Tyler Stannard 4 Jason Williams 4 Nikhilesh Chawla 4
1Los Alamos National Laboratory Los Alamos United States2Atomic Weapons Establishment Aldermaston United Kingdom3Argonne National Laboratory Argonne United States4Arizona State University Tempe United StatesShow Abstract
Understanding the effects of a material&’s morphology upon the overall material performance requires a detailed understanding of its initial morphology and how it changes under external stimuli. Laboratory based X-ray computed tomography (CT) systems can image polymer foams as they are compressed, in an interrupted in situ modality. Due to the lower flux of a laboratory X-ray source, stress relaxation must be allowed to occur in the material before a tomogram can be collected. Otherwise, the residual sample motion leads to significant image blurring. This relaxation process requires ~20 minutes before the tomogram can be collected over 1+ hours. Important information regarding compressive performance and changes in morphology is lost during this stress relaxation period.
Synchrotron light sources, such as the Advanced Photon Source, provide a significant increase in photon flux, compared to lab-based X-ray tubes. Coupling this high flux with a high speed camera allows for X-ray radiographs to be acquired every millisecond. This generates tomographic data in ~1 second. By coupling a custom loading stage to the X-ray microscope, it is possible to study the dynamic in situ deformation of polymeric foams at a 10-1 s-1 strain rate.
In this study, synchrotron-based X-ray CT, at beam line 2-BM, was used to capture the morphology changes in polymeric foam materials during dynamic uniaxial compression. Twenty tomograms of each sample were acquired while simultaneously compressing the samples up to 60%, within a timeframe of 100 seconds. The materials studied included hydrogen blown silicone foam (LK3626), prilled urea silicone foam, syntactic Sylgard-184, and additively manufactured foams.
With this technique, we can measure and correlate the differences in mechanical performance of polymeric foams to their physical/morphological changes. For example, LK3626 is a soft-structured hydrogen blown silicone foam. It has a weak ligament structure which quickly buckles with almost no bending resistive force, even at low compressive strains. Also, it compresses with no Poisson effect. Syntactic Sylgard 184 is a much stiffer material, due to the reinforcement from the 50 mu;m glass microballoon pore former; therefore, it exhibits a stress/strain curve with definitive bending/buckling/compressing regions. Importantly, the true stress can be calculated from CT data by measuring the full 3D change in area for each tomographic step. Additionally, the tomographic data can be converted into a hypermesh for importing into Abaqus for explicit analysis. Direct comparison of the reconstructed slices to the modeled result adds a high level validation to the modeled uniaxial compressive performance of the material that cannot be acquired with other techniques.
3:45 AM - T3.05
Dynamic Fracture Tests of Brittle Microspheres and Development of a Pulverization Parameter
David F. Bahr 1 Wayne Chen 1 Mohamad B. Zbib 1 Niranjan D. Parab 1
1Purdue Univ West Lafayette United StatesShow Abstract
Impact loading of spherical particulates occurs during materials handling, processing, and service conditions. This can lead to fracture of the particles and commutation into smaller particles. The dynamic fracture behavior of micro spherical particles of 5 brittle spherical solids, ranging from soda lime glass, to polycrystalline silicon and yttrium-stabilized zirconia (YSZ) was characterized with high speed, in situ X-ray phase-contrast imaging to examine the failure mechanisms in situ for spheroidal particles with diameters (d) from 0.600 to 2 mm. A new pulverization parameter was developed and is shown to predict the failure mechanism of the materials under fixed grips loading (i.e. overdriven in load) relating the hardness, toughness, modulus, and the size of the given particulate. The high speed behavior will be compared to quasi-static loading of particulates to demonstrate the effects of strain rate on particle fracture.
T4: Structural Materialsmdash;Mechanical Behavior of Materials for Extreme Environments
Monday PM, November 30, 2015
Hynes, Level 1, Room 102
4:30 AM - T4.01
Onset of Plasticity in Zirconium due to Hydrides Formation
Wojciech Szewc 1 Sandrine Brochard 1 Emmanuel Clouet 2 Laurent Pizzagalli 1
1Institut Pprime Chasseneuil France2SRMP - CEA Saclay FranceShow Abstract
Zirconium is a material of great importance in nuclear applications, as it is commonly used for fuel cladding. One of the known factors potentially degrading the mechanical properties is hydrogen precipitation, leading to the formation of brittle zirconium hydride needles. Different hydride phases form, depending on the amount of hydrogen in these needles. The associated volume expansion of hydrides leads to an increasing applied strain on the zirconium matrix. Available observations suggest that such strains may induce the nucleation of dislocations in zirconium from the zirconium/hydrides interface, thus further altering the mechanical properties of the material. But these observations are rather limited, and little is known about the elastic limits, and the nucleation mechanisms.
To address this issue, we have performed molecular dynamics simulations of a strained Zr surface, using two different interatomic potentials (EAM and COMB). Different loading modes were tested, in order to mimic the influence of the hydride precipitate. We found that the onset of plasticity is controlled by heterogeneous nucleation of partial dislocations in the first-order pyramidal planes, immediately followed by partial dislocations in the basal plane. This surprising result can be understood from the analysis of generalized stacking fault energy surfaces . The apparent blocking of the prismatic-plane slip, which unquestionably dominates in bulk zirconium, follows from the loading conditions imposed by the precipitate. The elastic limit depends on the geometric features of the surface making contact to the precipitate. In particular, it is lowered by the presence of surface steps. In specific circumstances, it is possible to favor the basal slip over the pyramidal one.
 C. Varvenne et al., Acta Mater. 78 (2014) 65 - 77
4:45 AM - T4.02
New Experimental Methods for Determining the Fracture Toughness in Brittle Metals: Tungsten Case
Teresa Palacios 1 Jens Reiser 2 Jose Ygnacio Pastor 1
1Universidad Politeacute;cnica de Madrid Madrid Spain2Karlsruhe Institute of Technology Karlsruhe GermanyShow Abstract
The objective of this work is to show some new and original methods to determine the fracture toughness in brittle metals. In this case the analysis was performed for tungsten due to its thermo-physical properties, tungsten is a suitable material for plasma facing applications in the future fusion reactors. However, its structural use is compromised due to its inherent brittleness. Therefore, to improve this critical feature is relevant to accurately measure its fracture toughness as a real property independent of geometrical parameters.
The most common methods to produce cracks such as fatigue, indentation microfracture or electro-discharge machining cannot be used here because they are difficult to apply, do not produce reproducible cracks or induce extended thermal damage in the material. For these reasons, in a first part of this work four experimental methods, that gradually approach a crack-like notch, were compared to determine the effect of the notch root radius on the measured fracture toughness of a nanostructured bulk tungsten material. Three-point bending tests were performed on these four types of specimens with notches introduced with: a classical diamond disc, a diamond wire, a razor blade and an ultra-short pulsed laser. The results showed that the best alternative is to introduce a notch with a femtosecond pulsed laser. This method, which produces crack-like notches without thermal damage, possesses good reproducibility, high accuracy and reliable fracture toughness. It was previously used on ceramics but there is no evidence of its use on metals.
Additionally, the ultra-short pulsed laser was used to introduce notches in tungsten foils of 0.1 mm thickness to perform in situ tensile tests inside a scanning electron microscope and determine the fracture toughness. Two types of geometries were also produced in the foils: single-edge and double-edge notched specimens. The introduction of the notches is a challenging task because of the reduced size of the samples and their slenderness. Single-edge notched specimens, easier to produce, were not suitable to provide accurate fracture toughness since the slenderness and the non-symmetric geometry resulted in complex stresses and momentum apart from the uniaxial tensile being applied to the sample. Double-edge notched specimens, however, produced accurate fracture toughness results, although to introduce both notches aligned at both sides of the specimens involves higher complications during the notching process.
5:00 AM - T4.03
In Situ Micro-Cantilever Testing of the Deformation and Fracture of Neutron Irradiated Graphite
Dong Liu 1 Peter Flewitt 1
1University of Bristol Bristol United KingdomShow Abstract
Irradiation in a fast neutron environment is known to degrade the mechanical properties of materials, therefore, to ensure safe operation it is necessary to undertake measurements of these properties. To minimise doses to personnel during post irradiation testing it is appropriate to minimise the sample mass and select small test specimens. In addition, in materials with a complex microstructure, it is necessary to undertake these measurements at the required length-scale to provide input parameters for computer modelling. In this paper, we undertake measurements using micro-metre length scale cantilever beam specimens prepared by ion beam milling in a Helios Dualbeam workstation. These cantilever beams were loaded using a customised system developed at the University of Bristol. This allows load-displacement to be measured and the associated mechanical properties, such as the elastic modulus and flexural strength to be determined. With these techniques, it is possible to observe specimens throughout the duration of a test.
Pile Grade A (PGA) graphite was used as moderator and structural material within the core of UK gas-cooled Magnox reactors. It is an anisotropic, polygranular graphite comprising aligned filler particles in a matrix of graphitised binder with around 20 vol.% porosity caused by the manufacturing process. Irradiated and radiolytically oxidised PGA graphite samples were extracted from the fuel channels as part of an overall monitoring programme to support safe operation. Three cylindrical samples (7 mm height by 12 mm dia.) trepanned from bricks with increasing distance from the centre of the reactor were selected to provide a decreasing neutron dose. The microstructure, in particular the fine-scale porosity has been characterised using serial sectioning, high spatial resolution tomography. To extrapolate small-scale testing data to size of reactor core bricks, a microstructure-based multi-scale modelling approach has to be adopted. This requires the input parameters at a suitable length-scale to eliminate size effects introduced by the heterogeneity of the multi-scale microstructure. Measurement of load-displacement, elastic modulus and fracture strength have been made using 2 µm x 2 µm x 10 µm micro-cantilever test specimens in the dualbeam workstation to evaluate the changes in properties with neutron dose. This has revealed the effect of irradiation damage, independent of the macro-sized pores present in the PGA graphite. The data are discussed with respect to the possible mechanisms leading to changes in properties and their benefit as input parameters to multi-scale computer models.
5:15 AM - T4.05
Evolution of Microcrack Density under Tensile Loading in beta;-Eucryptite
Ryan C Cooper 1 Giovanni Bruno 2 Amit Shyam 1
1Oak Ridge National Laboratory Oak Ridge United States2BAM, Federal Institute of Materials Research and Testing Berlin GermanyShow Abstract
The microcracking phenomenon in ceramics initiates above a critical grain size due to anisotropy in the coefficient of lattice thermal expansion. This microcrack network can result in an increase in light scattering, electrical and thermal resistance, and a decrease in the bulk coefficient of thermal expansion-even to the point of negative thermal expansion. The tunable material properties of microcracked ceramics provide a unique design opportunity but in order to utilize these materials, there is a need to understand the effect of mechanical loading on microcrack populations. In the current study, we probe the effects of microcracks in ceramics by varying the microstructure in beta-eucryptite samples. We mounted specimens with varying grain sizes into a microtensile apparatus and loaded until failure. The resulting stress-strain curves exhibit nonlinear behavior and irrecoverable strain accumulation. Digital image correlation (DIC) was used to measure the two-dimensional strain fields under applied stress for noncontact microstrain measurements. We develop a constitutive model that predicts the evolution of microcrack density within a linear elastic continuum and then applied the model to fit the experimental tensile curves. The constitutive model accounts for the accumulation of irrecoverable strain due to the increasing crack density and the evolving effect of linear-elastic modulus due to changes in crack density.
T1: Thin-Film Mechanical Behavior