Fatigue Life Prediction of Welded Joint by Microstructure-based Simulation

This paper proposes a numerical framework to predict fatigue life on welded joints by integrating several computational techniques. The framework consists of five steps: i) materials properties estimation; ii) welding simulation using thermo-mechanical finite element method; iii) macroscopic stress field analysis under cyclic loading; iv) mesoscopic stress field analysis using crystal plasticity finite element method (CPFEM); v) analysis of fatigue crack growth. The total number of cycles to failure is eventually obtained by the sum of initiation life calculated by CPFEM and propagation life calculated by X-FEM. A fatigue life of butt joint is evaluated by the proposed method. The results demonstrated the possibility of evaluating the fatigue life and its scattering by the proposed framework

構造用鋼の疲労性能予測のための微細構造を考慮したモデリング

学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 榎 学, 東京大学教授 小関 敏彦, 東京大学教授 佐藤 英一, 東京大学准教授 井上 純哉, 東京大学教授 酒井 信介University of Tokyo(東京大学

Using global existing fiber networks for environmental sensing

We review recent advances in distributed fiber optic sensing (DFOS) and their applications. The scattering mechanisms in glass, which are exploited for reflectometry-based DFOS, are Rayleigh, Brillouin, and Raman scatterings. These are sensitive to either strain and/or temperature, allowing optical fiber cables to monitor their ambient environment in addition to their conventional role as a medium for telecommunications. Recently, DFOS leveraged technologies developed for telecommunications, such as coherent detection, digital signal processing, coding, and spatial/frequency diversity, to achieve improved performance in terms of measurand resolution, reach, spatial resolution, and bandwidth. We review the theory and architecture of commonly used DFOS methods. We provide recent experimental and field trial results where DFOS was used in wide-ranging applications, such as geohazard monitoring, seismic monitoring, traffic monitoring, and infrastructure health monitoring. Events of interest often have unique signatures either in the spatial, temporal, frequency, or wavenumber domains. Based on the temperature and strain raw data obtained from DFOS, downstream postprocessing allows the detection, classification, and localization of events. Combining DFOS with machine learning methods, it is possible to realize complete sensor systems that are compact, low cost, and can operate in harsh environments and difficult-to-access locations, facilitating increased public safety and smarter cities

The effect of the 18R-LPSO phase on the fatigue behavior of extruded Mg/LPSO two-phase alloy through a comparative experimental-numerical study

The fatigue behavior of four extruded Mg-Y-Zn alloys containing different volume fractions of long-period stacking ordered (LPSO) grains was investigated through a comparative study combining experiments and crystal plasticity finite element simulations. Strain controlled low-cycle fatigue experiments were conducted at different strain amplitudes and revealed a limited cyclic hardening in Mg89Zn4Y7 alloy or softening in Mg99.2Zn0.2Y0.6 and Mg97Zn1Y2 alloys. A decrease in the fatigue life against the plastic strain with the increase in LPSO phase volume fraction was observed and was related the limited ductility of extruded LPSO grains. Stress-strain hysteresis curves were used to calibrate and validate a crystal plasticity model taking into account twinning and detwinning. The interaction of the different phases on the distribution of local micro-mechanical fields at the grain scale was then analyzed on synthetic microstructures under strain-controlled conditions. Deformation twinning activity was predicted in coarse unrecrystallized grains and tended to disappear with the increase in the LPSO phase volume fraction. Cleavage-like facets observed in LPSO grains were related to high tensile stress, especially at the Mg/LPSO interface, due to the limited number of deformation mechanisms in LPSO crystal to accommodate out-of-basal plane strain. The increase of the fatigue limit with the increase in LPSO phase volume fraction was finally associated with the decreasing presence of coarse unrecrystallized α-Mg grains due to a higher dynamic recrystallization activity during the extrusion process

Prediction of Fatigue Crack Initiation of 7075 Aluminum Alloy by Crystal Plasticity Simulation

The 7075 aluminum alloy is a promising material for the aerospace industry due to its combination of light weight and high strength. This study proposed a method for predicting fatigue crack initiation of the 7075 aluminum alloy by crystal plasticity finite element analysis considering microstructures. In order to accurately predict the total fatigue life, it is necessary to calculate the number of cycles for fatigue crack initiation, small crack growth, and long crack growth. The long crack growth life can be estimated by the Paris law, but fatigue crack initiation and small crack growth are sensitive to the microstructures and have been difficult to predict. In this work, the microstructure of 7075 aluminum alloy was reconstructed based on experimental observations in the literature and crystal plasticity simulations were performed to calculate the elasto-plastic deformation behavior in the reconstructed polycrystalline model under cyclic deformation. The calculated local plastic strain was introduced into the crack initiation criterion (Tanaka and Mura, 1981) to predict fatigue crack initiation life. The predicted crack initiation life and crack morphology were in good agreement with the experimental results, indicating that the proposed method is effective in predicting fatigue crack initiation in aluminum alloys. From the obtained results, future issues regarding the prediction of fatigue crack initiation were discussed

Mechanical properties and failure mechanisms of Mg-Zn-Y alloys with different extrusion ratio and LPSO volume fraction

In long period stacking ordered (LPSO) phase containing Mg-Zn-Y alloys, high elastic modulus and deformation kinks of LPSO phase considerably enhance the tensile yield strength, with slight detriment of or benefit to the ductility depending on its volume fraction. In present work, uniaxial tensile tests and fracture toughness tests are carried out using Mg99.2Zn0.2Y0.6, Mg97Zn1Y2, Mg89Zn4Y7 and Mg85Zn6Y9 (at%) materials with different extrusion ratios. Extrusion processing enhances both strength and ductility due to the recrystallization of Mg grains. Variable plastic deformation mechanisms are activated depending on volume fraction of Mg and LPSO phase as well as their relative size during bending. {101¯2} tensile twins in Mg grains and deformation kinks in LPSO phase are observed, which dissipate large amount of deformation energy favoring for toughness. However, inherently brittle LPSO phase is detrimental to toughness. Microstructure-motivated empirical models for yield strength and fracture toughness prediction based on rule of mixtures are calibrated by experimental data. Energy release rates of individual mechanisms are estimated, which quantitatively indicate strong Mg/LPSO interaction