13:30   Session 14: Fatigue crack growth and life prediction methods  IV Chair: Tomi Viitanen 13:30 20 mins Fatigue life prediction of metallic materials using the Tanaka-Mura-Wu model Siqi Li, Rong Liu, Xijia Wu, Zhong Zhang Abstract: In this research, the recently developed Tanaka-Mura-Wu (TMW) model is applied to common engineering materials including nickel-based superalloys Haynes 282 and Inconel 617, aluminum alloys 7075-T6 and 2024-T3, alloy steels SAE 4340 and SAE 1020, and titanium alloy Ti-6Al-4V, as well as a high entropy alloy (HEA) CoCrFeMnNi over the full fatigue range comprised of low cycle fatigue (LCF) and high cycle fatigue (HCF). The TMW model is derived from the mechanism of dislocation dipole pileup, which is presented with a plastic strain-based expression and a stress-based expression for fatigue crack nucleation. The plastic strain-based equation can provide class-A predictions for the LCF life with the plastic strain above 0.001. The stress-based equation is more suitable for HCF life prediction where macroscopic plasticity is discernible, but the lattice resistance needs to be calibrated to one S N point close to the fatigue endurance limit. The TMW model is shown to be applicable to a wide range of common engineering alloys for fatigue life prediction within a scatter factor of 2 as compared with the experimental data and/or Coffin-Manson-Basquin relations. By virtue of the physics of failure, it offers a greater applicability for crystalline materials. A relationship of fatigue life versus the total strain is established with the use of the Ramberg Osgood equation. The TMW model describes the full range fatigue life in terms of material’s elastic modulus, Poisson’s ratio, surface energy and the Burgers vector. Thus, it establishes a physics-based baseline for characterizing the effects of other contributing factors such as microstructure and surface roughness, which contributes to the uncertainty in the fatigue scatter. 13:50 20 mins Effect of the build direction on defect distribution and crack initiation of LPBF TI6AL4V Yu-E M Abstract: Additive manufactured (AM) Ti6Al4V titanium alloy has a potential market in aerospace and biomedical industries due to its excellent corrosion resistance and high specific strength. Compared with the conventionally manufactured parts, a relatively better static mechanical performance of AM metal materials has been achieved by the optimized processing parameters. However, the defects in AM metal alloy cannot be eliminated thoroughly, and then this can lead to the degradation of fatigue properties and the anisotropic mechanical response. A comprehensive study on the defect characterization and its effect on crack initiation behaviour were performed in this paper. Laser powder bed fused (LPBF) Ti6Al4V titanium alloy in 0°, 45°, and 90° build directions were designed and manufactured. The defects were measured based on X-ray computed tomography and reconstructed using the Dragonfly software. The defect distributions along the build direction, the scanning direction, and the radial direction were compared. Defect size, sphericity, and defect orientation were characterized. Finally, the real defects were extracted and imported into a finite element model, effects of the defect orientation and the porosity on crack path were analysed. It was shown that porosity of the as-built samples decreased with the increasement of the build direction, and stress relieving process can cause the coalescence of small defect and higher porosity. The lack of fusion defects is prone to growing along the build direction. The defect size distribution can be fitted by the lognormal function, while the sphericity distribution of defects was fitted by the two-phase exponential growth function. The crack initiation site was controlled by the defect orientation and the effective bearing aera. 14:10 20 mins Development of methods for microtexture characterization and dwell fatigue life prediction of dual phase titanium alloys Masayuki Tsukada, Koichi Inagaki, Itsuki Kawata, Shigeru Yasuda, Toko Hamaguchi, Yoshihiro Otani, Yuta Kitamura, Shinya MIyazaki, Naoya Yamada Abstract: Near alpha and alpha plus beta titanium alloy can exhibit large reductions of dwell fatigue life. These reductions result from the formation of commonly oriented microscopic alpha-phase regions called microtexture (MTR). In this study, electron backscatter diffractions (EBSD), spatially resolved acoustic spectroscopy (SRAS) and ultrasonic testing have been used for characterization of MTR. The results of in-situ dwell fatigue test by digital image correlation (DIC) and the related quantitative fractography have been utilized to establish the role of MTR for dwell fatigue fracture. To develop a physical model to predict dwell fatigue life reduction depending on MTR, crystal plasticity analysis also has been conducted. This recently acquired information aims to be used to create a tool to estimate reduction of dwell fatigue life by non-destructive ultrasonic evaluation of titanium forgings, which will enable classification of materials from a MTR perspective and will support improvement of material quality in actual production. Results for Ti-64 will be presented here as it is a widely used alloy. 14:30 20 mins Consideration of life prediction model for ceramic matrix composite(CMC) with cooling hole Hayao Sato, Daichi Haruyama, Hiroshi Nakamura, Tatsuhito Honda, Masahiro Hojo Abstract: Ceramic matrix composites (CMCs) have higher heat resistance and specific elasticity than Ni-based alloys, thus it is desired to be applied to aircraft engines such as turbine parts. In high temperature parts, small holes should be pierced to inject cold fluid. However, there are few research on the effect of small hole against the CMC fatigue life. To investigate them, fatigue tests were conducted using flat plates pierced with single hole and multiple holes. Multiple holes were allocated vertically against load direction. Multiple holes have more advanced hole shapes which is called diffuser hole. In the test results, different fracture mode and crack propagation were observed between single holed and multiple holed type. Multiple holed specimens had shorter lives than single hole. In addition, the fatigue lives of diffuser hole were shorter than circular hole. For evaluating life shorting of multiple holed specimens, life prediction models were reconsidered, which includes stress prediction and life prediction. The strength parameters were calculated by averaging stress field, which were predicted by finite element analysis (FEA), in area of CMC unit cell. After evaluating lives of hole specimens, multiple holed lives were predicted by using a smooth test S-N curve. Furthermore, to improve the stress prediction, the strength parameters were recalculated after resizing diffuser hole shape to fit the actual specimen. After that, all specimen lives were predicted around smooth specimen’s S-N curve regardless of the number of holes. Finally, we have established the great life prediction model for the holed CMCs. 14:50 20 mins Microstructure-based computational fatigue life prediction of polycrystalline alloys Xijia WU, Siqi Li, Zhong Zhang, Rong Liu Abstract: In this research microstructure-based fatigue modeling is conducted to predict the low cycle fatigue (LCF) crack nucleation life of a nickel-based alloy, Haynes 282. A three-dimensional representative volume element (RVE) consisting of polycrystalline aggregate is constructed for the material using Voronoi tessellation with the grain size distribution from the real material and grain orientations randomly assigned for isotropy. Hill’s yield criteria and linear strain hardening are employed to describe the anisotropic plasticity of each grain, using the finite element method (FEM), such that the overall deformation response matches the cyclic hysteresis behaviour of Haynes 282 alloy at the macroscopic scale,. The fatigue crack nucleation life of Haynes 282 alloy is predicted using the Tanaka Mura Wu (TMW) model based on the material surface energy, shear modulus, Burgers vector, and the plastic strain range at the microstructural level. The fatigue crack growth life under LCF conditions is described using the Tomkins equation. It is demonstrated that this approach can computationally predict the fatigue life of Haynes 282 alloy and estimate the scattering of the fatigue life by the simulations with different sets of the grain distribution functions. The model predictions are in good agreement with the coupon tests, in a statistical sense. Furthermore, the effect of grain orientation on the fatigue crack nucleation is discussed.

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