Fatigue in Materials: Fundamentals, Multiscale Modeling and Prevention : Fatigue Behaviors of Engineering Alloys
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 2:00 PM
March 1, 2017
Room: 23C
Location: San Diego Convention Ctr

Session Chair: Tongguang Zhai, University of Kentucky

2:00 PM  Invited
Non-local Stored Energy and J-integral Methods for Microstructure-sensitive Crack Growth: Fionn Dunne1; David Wilson1; 1Imperial College
    The stage of crack growth during which a strong microstructural sensitivity exists may comprise a quite substantial fraction of fatigue life. The path of the crack growth is often tortuous being influenced by local grain-level anisotropic elasticity and slip, as well as by morphological, crystallographic and grain boundary features (and many others); mixed trans- and inter-granular growth modes are often observed. The rate of growth is also significantly influenced by these features. Hence this study addresses techniques for the quantification of crack growth at the microstructurally-sensitive level. A J-integral approach has been established within a crystal plasticity framework in order to address quantitatively the role of microstructural features. A new non-local stored energy technique has also been established and the two approaches are compared in the context of single and oligocrystal crack growth. The diminution of microstructural sensitivity of crack growth is also addressed.

2:20 PM  Invited
A Physically Based Law for S-N Fatigue Behavior of Metals: K. S. Ravi Chandran1; 1University of Utah
    A physically based law for the S-N fatigue behavior of structural materials is presented. This has been formulated on the basis of a universal, macroscopic mechanism of fatigue crack growth. A constitutive equation has been derived which is shown to describe quite accurately the S-N high cycle fatigue data of various materials. It is preferable over the empirical laws of fatigue such as that of Basquin or Coffin-Manson equations. Metallurgical fatigue strengthening effects due to pre-strain, alloying and grain refinement have also been shown to be accurately predictable. The equation is then expanded to include the mean-stress effects, creating a master S-N fatigue equation, which facilitated the prediction of stress-life behavior and fatigue limit, for any mean stress, solely from the S-N behavior of fully reversed fatigue. A diverse set of experimental data has been used to verify the validity of this approach.

2:40 PM  
Fatigue Mediated Lattice Rotation in Al Alloys at Room Temperature: Ramasis Goswami1; Syed Qadri1; Chandra Pande1; 1Naval Research Laboratory
    We observe significant lattice rotation in the plastic zone mediated by fatigue crack growth at room temperature in Al 1100 and Al-7075 (T6) by employing X-ray diffraction (XRD) and transmission electron microscopy (TEM). A series of XRD at different locations showing the relative variations of 111, 200 and 220 Al peaks were obtained close to the crack. The intensity of 111 peak was observed to gradually increase as compared to that of the 200 peak as we approach the crack, suggesting lattice rotation as a result of fatigue crack growth at room temperature. We ascribe such rotation to glide of large number of dislocations along {111} planes across grain boundaries, which results in increase in the misorientation during crack growth. This establishes a new the energy dissipation mechanism with implications for predicting material life under cyclic load.

3:00 PM  Invited
Effects of Induced Surface Defects on Crack Initiation and Fatigue Strength for HCF and VHCF of a Structural Steel: Youshi Hong1; Qingqing Jiang1; Chengqi Sun1; 1LNM, Institute of Mechanics, Chinese Academy of Sciences
    Surface defects of structural steels have remarkable influence on crack initiation and fatigue strength for high-cycle-fatigue (HCF) and very-high-cycle fatigue (VHCF). In this paper, we performed fatigue tests with rotary bending method on specimens with different types of induced surface defects for a structural steel, such that to investigate the effects of surface defects on crack initiation and fatigue strength of HCF and VHCF. The S-N data show the degradation of fatigue strength due to the existence of surface defects, which is described by available models. The observations via SEM and TEM on the region of crack initiation demonstrate the detailed microstructure morphologies and selected area X-ray diffraction patterns, which reveal the micro-mechanism of crack initiation in HCF and VHCF regimes with the effects of induced surface defects. The results are compared with the case without surface defects.

3:20 PM  
Strain Mapping and Mining to Quantify the Extent of Cyclic Damage and Transverse Necking in Thin Metallic Sheets: James Collins1; Wade Lanning1; Yoon Joo Na1; Syed Javaid1; Christopher Muhlstein1; 1Georgia Institute of Technology
    Fatigue crack tips in thin, ductile metal films are often considered to be ideally plane stress, constraint-free because of their thin sheet form. While plastic collapse analyses can be used to characterize crack driving forces under these conditions, they do not provide insights into the crack growth mechanisms or the scale of the degradation ahead of the crack tip. In this presentation we will explore how monotonic and cyclic plastic zones evolve during fatigue of metallic thin sheets, with an emphasis on the link to through-thickness (transverse) necking. We will also illustrate how strain field mining can be used to characterize the extent and distribution of large scale plasticity regions between crack tips and back edges of specimens.

3:40 PM Break

4:00 PM  
A Microstructure-Sensitive Fatigue Crack Growth Study Based on Experimental Measurements and Computational Modeling in Al-Si Cast Alloys: Tiantian Zhang1; Anthony Spangenberger1; Diana Lados1; 1Worcester Polytechnic Institute
    Fatigue crack growth (FCG) plays an important role in material/structural design and component lifing. Robust FCG models necessitate microstructure-sensitive experimental studies of the damage near the crack tip. In this regard, plasticity ahead of crack tip in hypoeutectic Al-Si cast alloys was captured using in-situ and ex-situ DIC, and its interactions with the microstructure were studied. The strain field was found to correlate with both location relative to the crack path and alloy’s characteristic microstructural features. The DIC allowed establishing relationships between crack tip plasticity, microstructure, and FCG rate, while measuring crack tip opening displacement at various growth rates. Compliance and a non-destructive evaluation method based on eddy-current detection were used to monitor FCG at all growth stages. Microstructural characterization was performed using optical microscopy and electron backscatter diffraction (EBSD). The experimental determinations were successfully used to validate a computational model developed to predict FCG in cast Al alloys.

4:20 PM  
Identifying Failure Locations in Nickel Based Superalloy R88DT under Cyclic Loadings, via Crystal Plasticity Simulations: Monica Soare1; Shenyan Huang1; Shakhrukh Ismonov1; Andrew Detor1; 1GE Global Research
    The current study focuses on developing visco-plastic models for polycrystalline material R88DT, capturing the critical deformation mechanisms at various temperature levels. The polycrystalline nickel – based alloys (as R88DT) are materials of choice for gas turbine disks due to their excellent high temperature properties as strength, creep resistance, corrosion and oxidation resistance as well as long fatigue life. The plasticity models were calibrated on tension and compression tests performed on representative single crystals with controlled crystallographic orientations and validated by comparison with experimental data performed for polycrystalline structures. The models were subsequently used in complex crystal plasticity simulations to identify the special microstructural features with high probability of failure under cyclic loading (as for example annealing twins with high Schmid factors in primary/secondary slip systems), and to dissociate between the crack nucleation locations at low (75F) and high (1200F) temperatures.

4:40 PM  
Grain Size Effects on Fatigue Crack Growth in Nanocrystalline NiTi: William LePage1; Aslan Ahadi2; Q.P. Sun3; John Shaw1; Samantha Daly4; 1University of Michigan; 2National Institute for Materials Science; 3The Hong Kong University of Science and Technology; 4University of California, Santa Barbara
    This talk discusses the effects of grain size on the fatigue crack growth of superelastic NiTi shape memory alloy with average grain size ranging from 10 to 1500 nm, which was investigated through the use of multiscale experimental techniques. Crack initiation and growth rates were characterized with optical digital image correlation (DIC), and then fatigue properties were elucidated on the microscale with in-situ, distortion-corrected SEM-DIC. High-magnification SEM micrographs were captured during crack growth in compact tension samples patterned for SEM-DIC by self-assembled nanoparticle deposition for sub-micron-scale displacement resolution. Crack opening and closure displacements in the crack wake were computed to quantify the effect of martensitic phase transformation as an energy-dissipating mechanism during crack propagation. Displacement fields from SEM-DIC also enabled assessing models of phase transformation strain fields at the crack tip. The enhanced resolution of SEM-DIC overcomes challenges that others have encountered for crack-tip and crack-closure measurements in nanocrystalline alloys.

5:00 PM  
Slip Transmission between Primary Alpha Grains during the Low Cycle Fatigue of Ti 6242Si: Sudha Joseph1; Ioannis Bantounas1; Trevor Lindley1; Hamidreza Abdolvand2; Angus Wilkinson2; David Dye1; 1Imperial College London; 2University of Oxford
    The dislocation structures formed in Ti6242Si during room temperature low cycle fatigue are examined. Fracture initiated by facet formation, propagated by striation formation and followed by ductile final failure. TEM analysis showed that the material deformed mainly by planar slip localized within primary alpha grains. Slip transfer between similarly oriented grains could be observed. However, strain transfer was also observed between soft and hard-oriented primary alpha grains. Prism a-type slip was observed in the soft grain. Stress concentration due to dislocation pile up then activated c-type slip in the adjacent hard grain, with these observed to be isolated dislocations rather than organized into bands. Minimal interaction was observed between the primary alpha grains and regions of retained beta with secondary alpha. High resolution EBSD has been carried out on the TEM foils to provide additional insight into the rotations arising from slip and the stress states of the deforming grains.

5:20 PM  
Fatigue Assessment of a Railway Wheel Steel in the VHCF-regime: Dietmar Eifler1; Michael Koster2; 1University of Kaiserslautern; 2European Patent Office
    The fatigue behavior of the railway wheel steel SAE 1050 in the Very High Cycle Fatigue regime was investigated with the in-house-developed ultrasonic test system “UltraFAST-WKK-Kaiserslautern” (UFK). Based on preliminary load increase tests, constant amplitude tests were performed. The UFK allows to measure process parameters like generator power, dissipated energy and specimen temperature as well as the oscillation amplitude at the free end of the specimens with a Laser Doppler Vibrometer. Additionally the system’s resonance frequency and the attenuation behavior of the specimens allow to determine fatigue induced changes in the materials microstructure with a very high sensitivity. The measured data were correlated with scanning and transmission electron microscopic observations. Fatigue tests were interrupted if fatigue induced microstructural changes were indicated by a significant increase of temperature or electrical resistance. With this method an increasing dislocation density as well as persistent slip bands could be verified before final failure occurs.