Fatigue in Materials: Fundamentals, Multiscale Characterizations and Computational Modeling: Multi-mechanical Interactions during Extreme Environment Fatigue Loading
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
Tuesday 2:00 PM
February 25, 2020
Room: 11A
Location: San Diego Convention Ctr
Session Chair: Brian Wisner, Ohio University
2:00 PM Invited
The Effect of Molecular Transport on the Environmentally Assisted Fatigue Crack Growth Behavior of Aerospace Al-alloys in High Altitude Environments: James Burns1; Adam Thompson1; Luke Brown1; 1University of Virginia
High altitude environments are typified by low temperatures and low water vapor pressures, which reduce the crack growth rates (da/dN) observed in aerospace aluminum alloys by orders of magnitude. Incorporation of such behavior into linear elastic fracture mechanics (LEFM) modeling can result in a decreased inspection burden, increased airframe availability, and increased prediction accuracy. Such modeling approaches require a testing methodology that will rigorously generate environment specific crack growth kinetics that will ensure similitude between laboratory generated data and the behavior of engineering components. Despite a constant bulk environment and ΔK similitude can be compromised by variations in the molecular transport path. This study will provide clear evidence of this transport mediated behavior, present computational flow models that capture the microchannel flow at the relevant high Knudsen numbers, and will explore testing protocol recommendations to enable generation of data that can be rigorous used in engineering scale predictions.
2:20 PM
A Multi-scale Characterization of the Effects of High Altitude Environments on the Damage Structure Evolution during Fatigue Loading of AA7075-T651: Adam Thompson1; Zachary Harris1; James Burns1; 1University of Virginia
Aerospace aluminum alloys often operate at high altitude (typified by low temperatures and water vapor pressures [PH2O]); such environments retard the fatigue crack growth behavior. Incorporating these benefits into structural life management requires an understanding of the governing damage physics. The low temperature behavior retardation may be due to changes in dislocation structure evolution and/or on the hydrogen environment embrittlement (HEE) process. The HEE process would be influenced by temperature via a reduction in bulk PH2O, crack tip reaction kinetics producing/absorbing atomic H, H diffusion in the process zone, and/or changing the nature of the H-dislocation interactions. A novel multi-scale characterization (e.g. EBSD, HR-EBSD, GND calculation, FIB-TEM) of damage structure in the crack wake of different samples at a constant ΔK and constant PH2O but different temperature, is performed to provide insights into the governing mechanisms.
2:40 PM
Macrozones and Dwell Fatigue Failure on a Near-α Titanium Alloy: Beatriz Fernandez-Silva1; Bradley Wynne1; Martin Jackson1; Matthew Thomas2; Katharine Fox3; 1University of Sheffield; 2TIMET UK Limited; 3Rolls Royce plc
The susceptibility to cold dwell fatigue of near-α titanium alloys has been a matter of investigation in the aerospace industry for more than 40 years. Alloys such as Ti834 have a reduction in fatigue life when the material is exposed to high stress during the loading cycle. The failure source is observed as subsurface cracks through primary alpha grains leading to quasi-cleavage facets, which are nearly normal to the loading direction and unfavourably orientated for slip to occur. The presence of strong textured regions called macrozones has been linked with dwell fatigue failure where the main texture component comes from these grains. However, in bimodal microstructures the transformed beta may contribute to the final macrozone size and density. In this work, the nature of the underlying macrozones, the contribution of the morphologies of the bimodal microstructure and their relation to the faceted regions on LCF specimen fracture surfaces is examined.
3:00 PM
Impact of Temperature and Microstructure on Dwell Fatigue in Near-alpha Titanium Alloys: Michelle Harr1; Samantha Daly2; Adam Pilchak3; 1University of Michigan; 2University of California, Santa Barbara; 3Air Force Research Laboratory
Near-alpha titanium alloys, such as Ti-6242, experience a reduction in fatigue lifetime when the peak stress is held for each loading cycle. This type of sustained peak loading, also known as dwell fatigue, mimics the long periods of high mean stress experienced by titanium fan and compressor components during takeoff and cruise. The significant lifetime reduction is attributed to the phenomenon of load shedding, where the relaxation of a soft grain requires a neighboring hard grain to carry a higher load, thereby creating a favorable location for early crack nucleation. Both local microstructure and temperature are known to impact load shedding, but the underlying mechanisms are still under active investigation. This work utilizes Digital Image Correlation (DIC) and Electron Backscatter Diffraction (EBSD) at multiple length scales to experimentally investigate and statistically quantify the effect of temperature and local microstructure on the distribution of stain and damage accumulation during dwell fatigue.
3:20 PM
Oxygen, α2, Macrozones and Dwell Fatigue Initiation in α-Ti: Felicity Worsnop1; Paraskevas Kontis2; Baptiste Gault2; Jan Ilavsky3; Joel Bernier4; David Rugg5; David Dye1; 1Imperial College London; 2Max-Planck-Institut fur Eisenforschung; 3Argonne National Laboratory; 4Lawrence Livermore National Laboratory; 5Rolls-Royce plc
Predictive capability for fatigue in critical rotating parts in aero engines is essential for safe and efficient operation. Fatigue crack initiation is thought to occur by pri<a> slip bands proceeding across similarly oriented hcp α grains in a macrozone to cause high localised stress upon impingement on a grain poorly oriented for slip. Ordered Ti3Al α2 precipitates and interstitial oxygen promote slip heterogeneity by suppressing cross-slip, promoting localisation into slip bands. Comprehensive characterisation of α2 formation and growth across a model alloy series using TEM, APT and SAXS, incorporating different interstitial O contents, was followed by exploration of dislocation behaviours with TEM. Mesomechanical stress evolution has been explored using ff-HEDM to observe spatial and temporal slip heterogeneity during in situ deformation.
3:40 PM Break
4:00 PM Invited
In-situ X-ray Microtomography to Elucidate Corrosion-fatigue Mechanisms in Aluminum Alloys: Arun Sundar1; Jason Williams1; Harsh Goyal1; Sridhar Niverty1; Sudhanshu Singh1; Tyler Stannard1; Xianghui Xiao2; Nikhilesh Chawla1; 1Arizona State University; 2Brookhaven National Laboratory
Aluminum alloys are frequently exposed to harsh environments in service. X-ray synchrotron micro and nanotomography provides a wonderful means of characterization damage in materials non-destructively. In this talk, I will describe experiments and simulations that address the critical link between microstructure and corrosion-fatigue behavior of aluminum alloys, by using three-dimensional (3D) x-ray synchrotron tomography. To investigate the effects of corrosion and fatigue on peak-aged 7075 aluminum alloy, corroded samples were tested via in situ x-Ray tomography to analyze the fatigue crack initiation and growth characteristics. Hydrogen bubbles were observed between the sample and the fluid upon crack initiation, indicating chemical changes in the sample during in situ corrosion fatigue. The effect of oxide layers forming during corrosion and 2nd phase inclusions, on fatigue initiation and propagation, will be discussed. New opportunities for x-ray microtomography, including lab-scale diffraction contrast tomography and the next generation of x-ray synchrotron tomography will be highlighted.
4:20 PM
Some Hardening and Softening Antagonist Processes Induced by Hydrogen on Cyclic Behavior of Nickel Singe Crystal: Guillaume Hachet1; Arnaud Metsue1; Abdelali Oudriss1; Feaugas Xavier1; 1University Of La Rochelle - Lasie
In order to understand the nature of the hydrogen-induced hardening and softening process in f.c.c. metals, a substantial effort was made to determine the effect of hydrogen on the cyclic stress-strain behavior of nickel single crystals. A multi-scale approach allows to evaluate the effects of hydrogen on the mechanical response from the macroscopic scale down to the atomic scale. At the macroscopic scale, cyclic tests are conducted on nickel single crystal with several hydrogen concentrations to evaluate its consequences on the hardening curve, back and effective stresses. Then, microstructural characterization of the dislocations distributions developed is conducted for several plastic deformations on nickel with and without hydrogen. Therefore, the impact of hydrogen on the elastic properties and some dislocations configurations are investigated using atomic scale calculations and specific experiments. We illustrate the hardening and softening antagonist processes associated with hydrogen and formed vacancy by hydrogen incorporation and plastic strain.
4:40 PM
Mechanical and Actuation Fatigue in Ni-Rich NiTiHf High Temperature Shape Memory Alloys: Behrouz Haghgouyan1; Benjamin Young1; Ibrahim Karaman1; Dimitris Lagoudas1; 1Texas A&M University
Shape Memory Alloys (SMAs) are a unique class of metals that undergo shape change as a function of temperature. SMAs, like NiTi, can be used as solid-state actuators that can be up to 1/10 the size and weight of conventional ones. The inherently low transformation temperatures of NiTi SMAs has led to development of high-temperature SMAs (HTSMAs). There is very limited knowledge on the fatigue and fracture behavior of HTSMAs, which is of great importance for their successful integration into applications. In this work, crack growth behavior in a Ni-rich NiTiHf HTSMA is investigated under two thermo-mechanical loading paths: isothermal, i.e. mechanical loading under constant temperature; and actuation, i.e. thermal cycling under constant load. The results indicate relatively brittle fracture behavior compared to that of NiTi SMAs. This can be explained by limited presence of dissipation mechanisms such as phase transformation, shown by digital image correlation, near the crack tip.
5:00 PM
Creep-fatigue Behavior of an Advanced Austenitic Stainless Steel (Alloy 709): Zeinab Alsmadi1; K.L. Murty1; 1North Carolina State University
The design and operation of high temperature systems that exhibit cyclic thermal stresses due to temperature gradients during start-ups and shut-downs introduce time-dependent effects like creep during on-load periods at elevated temperatures. Next-generation nuclear reactors such as Sodium-Cooled Fast Reactor (SFR) are subjected to creep-fatigue damage in reactor components. Therefore, the structural materials for SFR should have adequate mechanical properties that can withstand typical operating conditions expected in the nuclear reactor core. Fe-25Ni-20Cr (wt.%) advanced austenitic stainless steel (Alloy 709) is a potential candidate as structural material for SFR. Creep-fatigue interaction of the Alloy 709 is investigated by conducting strain-controlled creep-fatigue tests with hold times of 0, 600, 1,800 and 3,600 seconds at strain amplitudes of 0.3%, 0.5% and 0.6% at 750 °C. For each hold time, the number of cycles to failure was found to decrease with increasing strain amplitude. This research is supported by DOE/NEUP project 15-0008582.