Fatigue in Materials: Fundamentals, Multiscale Characterizations and Computational Modeling: Crack Initiation Mechanisms and Crack Growth Behavior
Sponsored by: TMS Structural Materials Division, TMS: Computational Materials Science and Engineering Committee, TMS: Integrated Computational Materials Engineering Committee, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Garrett Pataky, Clemson University; Ashley Spear, University of Utah; Jean-Briac le Graverend, Texas A&M University; Antonios Kontsos, Drexel University; Brian Wisner, Ohio University

Monday 2:30 PM
February 24, 2020
Room: 11A
Location: San Diego Convention Ctr

Session Chair: Garrett Pataky, Clemson University


2:30 PM  Invited
Dislocation Patterns and Crack Initiation in Fatigued Nickel Single Crystal Microcrystals: Micro-Scale In-situ Scanning Electron Experiments: Steven Lavenstein1; Jaafar El-Awady1; 1Johns Hopkins University
    Here we present a unique approach combining high-frequency fully reversible cyclic uniaxial tension/compression loading in situ scanning electron microscopy experiments on microcrystals fabricated into bulk single crystal nickel. The cyclic loading is imposed using the high frequency actuator dynamics of a nanoindenters. The changes in the dislocation microstructure, and crack initiation and propagation in the microcrystals are monitored by observing changes in the microcrystal dynamic stiffness and SEM imaging. Surface extrusions and TEM characterization confirm the development of persistent slip bands (PSBs) in the microcrystals. However, dislocation vain structures were only observed sporadically, which is markedly different than in bulk crystals. The in situ surface observations, indicate that the PSBs nucleate locally within the crystal interior and then propagate until they spanned the entire slip region. Finally, we quantify crack initiation and propagation in these microcrystals.

2:50 PM  
Microstructure-interacting Short Crack Growth in Blocky Alpha Zircaloy-4: Weifeng Wan1; Fionn Dunne1; 1Imperial College London
    Microstructurally short fatigue crack growth in blocky alpha Zircaloy-4 is experimentally investigated in cyclic three-point bend testing. The short crack propagation is sensitive to the local microstructure with respect to grain crystallographic orientation and grain boundaries. Polcrystals with predominant c-axis texture aligned out-of-plane and normal to loading give alternating crack paths along prismatic planes. Samples with c-axis texture aligned in plane and normal to loading typically show straight paths along prismatic planes, sometimes tortuous paths, but always crystallographic. Prismatic <a>-direction crack growth rate is low (0.077 μm/cycle) compared to that for prismatic <c>-direction growth (0.458 μm/cycle) for given loading. Hence the crystallographic plane within which cracks grow is important for growth rate. For the tortuous cracks, with the predominant c-axis texture in plane and normal to loading, crack growth occurs along basal, prismatic and pyramidal planes, deflecting from one slip plane to another during transgranular propagation.

3:10 PM  
Mission Loading Effects on Small Crack Growth in an Alpha+Beta Titanium Alloy: Reji John1; Patrick Golden1; Sushant Jha2; W. Porter2; 1Air Force Research Laboratory; 2University of Dayton Research Institute
    Mechanism-based prediction of the minimum fatigue life requires accurate characterization of the early damage initiation from life-limiting microstructural features and small (microstructure-scale) crack growth. This approach has been used by the Air Force Research Laboratory to predict the limiting fatigue life in aerospace alloys typically under constant amplitude loading conditions. The probabilistic framework requires characterization of the variability of small crack growth emanating from microstructural features as small as 0.010 mm. In addition, component-scale demonstration of this technology requires validation under realistic loading conditions. Hence, this effort will characterize the small crack growth behavior in Ti-6Al-4V under mission loading conditions. Specimens with machined micro-notches (~ 0.010-0.015 mm depth) were used to monitor microstructure-scale crack growth during the variable–amplitude tests simulating mission loading. The presentation will discuss the effect of stress ratio and mission loading on small crack growth and characterization of its variability for probabilistic fatigue life prediction.

3:30 PM  
Characterization of Fatigue Short Crack Growth in Rare-earth Magnesium Alloy WE43 using High Energy X-ray Diffraction Microscopy: Duncan Greeley1; Jacob Adams1; Peter Kenesei2; Ashley Spear3; John Allison1; 1University of Michigan; 2Argonne National Laboratory; 3University of Utah
    Rare-earth magnesium alloys show promise for a variety of transportation structural applications due to their high specific strength and low density. To accelerate development of new magnesium alloys and processes, the Predictive Integrated Structural Materials Science (PRISMS) Center is developing simulation tools for predicting fatigue deformation behavior. To accurately model fatigue it is necessary to develop a greater understanding of the mechanistic phenomena involved in short crack growth. To this end, the three-dimensional crystallographic crack path during high-cycle fatigue of magnesium alloy WE43 was investigated using High Energy X-Ray Diffraction Microscopy (HEDM) at The Advanced Photon Source. Coupled far-field diffraction, near-field diffraction, and X-ray computed tomography provide detailed insights into the effect of grain boundaries and local crystallographic orientation on short crack growth and crack path tortuosity. Grain boundary misorientation and grain orientation characteristics are found to correlate to crystallographic and geometric short crack path selection.

3:50 PM  
Characterization of Fatigue Crack Growth Behavior in CrCoFeNi High Entropy Alloy: Wm Williams1; Mitra Shabani1; Garrett Pataky1; Paul Jablonski2; 1Clemson University; 2National Energy Technology Laboratory
    Fatigue crack growth experiments were performed at room temperature on single edge notch samples of CoCrFeNi, a single phase high entropy alloy with an FCC structure. The experiments consisted of cyclic loading at load ratios of 0.05 and 0.2 and a naturally increasing stress intensity factor. Images taken during the experiment were used for DIC and to identify the crack growth rates. Displacement fields were calculated using digital image correlation and used to quantify the stress intensity factor ranges and mode II displacements. Post-mortem fractography revealed significant surface roughness and microstructural features indicating high levels of roughness-induced crack closure for low load ratios R. Fractography showed "fin-like" features and surface cracking and TEM analysis was performed for further investigation of the features. The material exhibited good fatigue crack growth resistance.

4:10 PM Break

4:30 PM  Invited
Watching High-cycle Fatigue in Nanocrystalline Pt and Pt-Au: Nathan Heckman1; Christopher Barr1; Khalid Hattar1; David Adams1; Timothy Furnish1; Brad Boyce1; 1Sandia National Laboratories
    High-cycle fatigue of nanocrystalline Pt and Pt-Au is investigated through automated, interrupted scanning electron microscopy experiments. Topographical evolution as well as crack formation and propagation are observed at a notch throughout the entire fatigue life of microns thick samples to understand the deformation process, and the role that mechanisms such as abnormal grain growth and compositional crack arrest play in each of these materials. Structures that lead to the eventual failure of the materials are observed as early as 1% of the total life, and the complete transformation of these structures is seen over the fatigue lifetime. Post-mortem transmission electron microscopy characterization reveals the specific microstructural transformations occurring during deformation. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

4:50 PM  
Volumetric Defect Quantification of Pure Aluminum During Fatigue Loading Below the Yield Stress: Joseph Indeck1; Jefferson Cuadra2; Kavan Hazeli1; 1University of Alabama, Huntsville; 2Lawrence Livermore National Laboratory
    This research focuses of the evolution of bulk defects as a function of fatigue life in pure aluminum loaded below the yield stress, and what effect those defects have on subsequent mechanical behavior. Experimental studies have shown that fatigue induced substructures, such as persistent slip bands, can form at stress amplitudes well below the yield stress. Formation of PSBs and resultant internal microscopic changes are not constrained to the free surface and can influence the mechanical behavior. X-ray computed tomography was used to quantify internal void changes at different stages of the fatigue life. Then the pre-fatigued samples were tested under tension at quasi-static and dynamic strain rates in order to investigate the influence of fatigue-induced defects on the subsequent mechanical properties. Presented results connect size and distribution of internal voids with respect to the subsequent mechanical properties at various stages of fatigue life.

5:10 PM  
Propositions for Functional Fatigue and Fatigue-crack Initiation in Shape Memory Alloys: Ahmedsameerkhan Mohammed1; Huseyin Sehitoglu1; 1University of Illinois Urbana-Champaign
    Shape Memory Alloys (SMAs) are unique materials that hold the potential to offer enhanced fatigue life in applications involving appreciable strain amplitudes. This is done by way of favoring reversible phase transformation over irreversible plastic slip accumulation. Yet, there are fatigue-induced slip irreversibilities associated with transformation, an emissary mechanism for which was proposed by the authors. The present study proposes to model the ramifications of slip emission on the diminishing recoverable strain and hysteresis under fatigue loading. An atomistic approach involving Molecular Statics (MS) and Density Functional Theory (DFT) was chosen. The Burgers’ vector magnitudes and fault energies associated with the emitted slip in relation to the twin-partial on the internal martensitic TB is shown to dictate this behavior. In addition, the implications of slip-emission, over multiple fatigue cycles, to dislocation pile-up and subsequent fatigue crack initiation at the austenite-martensite phase boundary is explored.

5:30 PM  
Moving Cracks Form White Etching Areas During Rolling Contact Fatigue in Bearings: Lutz Morsdorf1; David Mayweg1; Yujiao Li2; Annika Diederichs3; Dierk Raabe1; Michael Herbig1; 1Max-Planck-Institute fuer Eisenforschung; 2Ruhr University Bochum; 3Denmark Technical University
    White etching areas (WEAs) and the associated white etching cracks (WECs) are responsible for failure of widely spread engineering applications such as bearings and railways. We propose a mechanism where WECs move, i.e. continuously change their position in the material, leaving behind a severely plastically deformed area - the WEAs. Cracks have so far only been reported to propagate, i.e. to continuously extend, not to move normal to their plane through the material. Our results suggest that cracks initiate at inclusions below the surface. During compressive loading cycles fracture surface rubbing occurs. This leads to the formation of WEA by local severe plastic deformation. It also leads to partial cohesion and material transport between the abutting fracture surfaces. The WEC opens during unloading at a slightly shifted position with respect to its former one. Repetitive loading/unloading cycles lead to a macroscopic change of the crack position.