Fatigue in Materials: Fundamentals, Multiscale Modeling and Prevention : Creep, Fatigue, and Environmental Interactions
Sponsored by: TMS Structural Materials Division, TMS: Advanced Characterization, Testing, and Simulation Committee, TMS: Computational Materials Science and Engineering Committee, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Ashley Spear, University of Utah; Jean-Briac le Graverend, Texas A&M University; Antonios Kontsos, Drexel University; Tongguang Zhai, University of Kentucky

Wednesday 8:30 AM
March 1, 2017
Room: 23C
Location: San Diego Convention Ctr

Session Chair: Jean-Briac le Graverend, Texas A&M University

8:30 AM  
Fatigue Deformation Mode in a Polycrystalline Nickel Base Superalloy at Intermediate Temperature: Oxidation Assisted Process: J.C. Stinville1; M.P. Echlin1; P.G. Callahan1; W.C. Lenthe1; J. Miao2; T.M. Pollock1; 1University of California Santa Barbara; 2University of Michigan
    At component service temperatures, the effect of the environment on fatigue life of polycrystalline nickel-base turbine disk superalloy may be significant. Oxidation assisted processes that induce fatigue initiation for specific conditions are closely correlated with strain localization and microstructural configuration. A major challenge is to understand the strong dependence of the environmental effect that operates during fatigue in relation to the microstructure of the alloy. Therefore, experimental data on fatigue initiation induced by oxidation assisted processes spatially correlated with the microstructure has been gathered using HR-SEM digital image correlation and TEM analysis. Crack initiation by oxidation-assisted mechanisms is detailed for a René 88DT polycrystalline nickel-base superalloy during fatigue at intermediate temperature. This process is observed to control the fatigue lifetime at high stresses. Moreover, the specific microstructural configurations and plastic deformation modes that lead to crack initiation by oxidation assisted process during fatigue at intermediate temperature have been identified.

8:50 AM  
Fatigue Crack Initiation and Fatigue Crack Growth Behavior of AA7050-T7451 with Different Corrosion Morphologies: Noelle Easter Co1; James Burns1; 1University of Virginia
    Aluminum-based aerospace structures are often connected via stainless steel fasteners, resulting in a galvanic couple when an electrolyte is trapped in between the aluminum substructure and the stainless steel fastener. This research investigates the initiation and growth behavior of fatigue cracks forming at the corrosion damage representative of the galvanic corrosion in AA7050-T7451. Electrochemical techniques are employed to induce the corrosion damage on the surface of AA7050-T7451 and corrosion morphologies are characterized prior to fatigue loading. Results show that macro-scale corrosion metrics such as pit depths, volume, area, density do not correlate to the location of the fatigue initiation. This finding points out to possible strong effect of micro-geometry or microstructure on the location of fatigue crack formation. Grain orientation and constituent particle location are considered to understand local plasticity at the location of fatigue initiation. Results of this study offer guidance to structural integrity management and corrosion inhibitor design.

9:10 AM  
The Influence of Operating Slip Systems on the Dwell Sensitivity of Titanium Alloys: Samuel Hemery1; Patrick Villechaise2; 1ENSMA; 2CNRS
    The introduction of a dwell stage in low cycle fatigue tests at room temperature results in a reduced life of α/β titanium alloys. The sensitivity to dwell may vary depending on the alloy considered. Ageing of α/β Ti alloys may also lead to further dwell-fatigue life reduction due to Ti3Al precipitation. In the present work, metallurgical parameters such as composition and precipitation have been used to investigate the potential influence of the operating slip systems during the early deformation processes on dwell sensitivity. In order to characterize the slip activity at the local scale, in-situ SEM tensile testing coupled with EBSD investigations were applied to different polycrystalline specimens.

9:30 AM  
Creep-fatigue Damage Mechanism in Cyclically-Softened Mod.9Cr-1Mo Ferritic-Martensitic Steel: Meimei Li1; Weiying Chen1; Ken Natesan1; 1Argonne National Lab
    Mod.9Cr-1Mo ferritic-martensitic steel is an important structural material in advanced sodium-cooled fast reactors. Creep-fatigue is a major concern for its applications in high-temperature nuclear reactors. Creep-fatigue interaction is a complex dynamic process involving combined effects of external temperature, loading and environmental variables and internal metallurgical variables, and is the most damaging and least understood mechanism in high temperature structural designs. Mod.9Cr-1Mo experiences softening under cyclic loading at elevated temperatures, in contrast to austenitic stainless steels that cyclically hardened and whose creep-fatigue behavior is well understood. Our recent study of creep-fatigue interaction in Mod.9Cr-1Mo at 550-600C revealed that the conventional creep-fatigue theory cannot explain the unique creep-fatigue response of cyclically-softened Mod.9Cr-1Mo steel. Instead, its creep-fatigue process can be well described by a thermally-activated deformation process, similar to the creep deformation. A new creep-fatigue life prediction model was developed for cyclically-softened Mod.9Cr-1Mo steel, which was linked with microstructural evolution under creep-fatigue loading.

9:50 AM  
Damage Evolution in Thin Tin Sheets During Creep Fatigue Loading: Syed Javaid1; Wade Lanning1; James Collins1; Christopher Muhlstein1; 1Georgia Institute of Technology
    Creep-Fatigue interactions are observed in certain materials at high temperature and intermediate hold times / strain rates. In this study, we present and experimental study of the damage evolution due to creep-fatigue loading in thin Sn sheets. This ductile, non-cubic model system is a platform to study how twinning, dislocation motion, and diffusion contribute to damage accumulation processes in non-cubic, ductile metals. The crack propagation rates and process zones were characterized at various fatigue loading and temperature conditions through strain mapping with Digital Image Correlation (DIC).

10:10 AM Break

10:30 AM  Invited
Micromechanics of Biaxial Cold Dwell Fatigue Mechanisms in Ti-7Al Elucidated Using Far-field High-energy Diffraction Microscopy: Aaron Stebner1; Garrison Hommer1; Adam Pilchak2; 1Colorado School of Mines; 2Air Force Research Laboratory
    Alpha-titanium alloys have limited plasticity mechanisms due to hexagonal close-packed (HCP) crystal structure, and also twinning deformation, observed in many HCP alloys, is suppressed by adequate aluminum content. Critically resolved shear stress anisotropy between remaining slip systems gives rise to soft grains preferentially oriented for slip, and hard grains that are not. Dwell fatigue is known to adversely affect life compared to regular cyclic fatigue for uniaxial loading in Ti-7Al. It is generally postulated that this dwell debit is due to load shedding between soft and hard grains that occurs because of low hardening and propensity to creep at low temperatures. However, biaxial dwell fatigue life and mechanisms still lack understanding as evidenced by a persisting inability to accurately model fatigue lives of aircraft turbine compressor blades. We present new micromechanical mechanistic insights gained by studying tension-tension dwell fatigue in-situ using far-field high-energy diffraction microscopy.

10:50 AM  
On the Effects of Multiaxial Stress on Facet Nucleation in Cold Dwell Fatigue: Mitch Cuddihy1; Adam Stapleton2; Steve Williams2; David Rugg2; Fionn Dunne1; 1Imperial College London; 2Rolls-Royce plc
    Cold dwell fatigue (CDF) is a critical failure mode observed in α-titanium alloys used in the aerospace industry. This condition arises due to stress holds in fatigue cycles at ambient temperature. CDF is characterized by the presence of basal plane cracks, known as facets. The investigations presented in this talk will address an ongoing effort to combine industrial scale experimentation with fundamental, dislocation level crystal plasticity. The range of multiaxial stress states present in the operating conditions of in-service compressor discs using an elastic analysis (including temperature and residual processing effects) is investigated. This is used to inform the crystal plasticity level analyses which demonstrates the significance of load shedding, through capture of the key rate-sensitivity creep effects, understood to be key in CDF. This approach has facilitated the development of a predictive capability for CDF facet nucleation sites and is validated by extensive disc spin tests.

11:10 AM  
A Continuum Damage Model for Creep-Fatigue Interactions: Jean-Briac le Graverend1; 1Texas A&M University
    Damage by void nucleation and growth limits the lifetime of components subjected to mechanical loading at high temperatures. X-ray tomography analyses recently showed that creep damage in single-crystal Ni-based superalloys may be seen as isotropic since pores are not nucleating at specific locations. Based on the Continuum Damage Mechanics (CDM) theory and therefore on an effective stress concept, a new continuum damage model for single crystal superalloys has been developed and implemented in a microstructure-sensitive constitutive model written in a crystal plasticity framework. It is especially designed for anisothermal loading and is intrinsically taking into account the non-linear accumulation of damage. It is shown that the resulting material model is capable of predicting the lifetime in the high-temperature regime for creep/fatigue loading.

11:30 AM  Invited
Creep, Fatigue and Environmental Interactions and Their Effect on Crack Growth in Superalloys: Jack Telesman1; Tim Gabb1; Louis Ghosn1; 1NASA GRC
    To gain an understanding of the complex creep/fatigue/environmental interactions in superalloys, a study of dwell and cyclic fatigue crack growth behavior was conducted on a powder metallurgy disk alloy given various heat treatments to vary creep resistance. Testing was conducted at elevated temperatures both in air and in vacuum. It was shown that environmental embrittlement was the dominant mechanism of crack advancement with creep crack growth having only a secondary effect. However crack tip stress relaxation did have a major, yet indirect, effect on the dwell crack growth by controlling the magnitude of the remaining crack tip tensile stresses which governed crack growth. A simple, empirically based correction to the linear elastic stress intensity factor was developed by considering the crack tip stress relaxation behavior.