The axial fatigue tests were conducted using Al-Mg-Si casting Al alloys with some impurity chemical compositions to investigate the effect of impurity chemical composition on the fatigue behavior. The simulated impurity chemical compositions were Fe, Mn and Ni. The effect of weight% of Fe was also investigated by changing Fe content. By adding impurity chemical compositions, intermetallic compounds (IMCs) were formed in the matrix, while the size and distribution of IMCs were similar to those of eutectic Si. When the weight% of Fe exceeded 1.5%, long IMCs with high aspect ratios were formed. The effects of impurity chemical compositions on the static mechanical properties, such as 0.2% proof stress, tensile strength and hardness, were small. However, samples with 1.5% and 2.0% Fe exhibited higher proof stress and hardness. The fatigue strengths of the samples with impurity chemical compositions were higher than those of the standard samples. When the weight% of Fe exceeded 1.0%, some samples showed lower fatigue strengths than the standard samples. In those cases, large IMCs were found at the crack initiation sites, which reduced crack initiation resistance and shortened fatigue lives.

This paper describes an experimental investigation of the deformation characteristics of SN490B, a construction steel with good weldability and an excellent yield ratio, under multiaxial non-proportional cyclic loading. Steel beams in buildings are plastically deformed under severe loading during heavy earthquakes, resulting in multiaxial and non–proportional loading owing to the complexity of the external loading and the discontinuities in the geometry of the structure. The stress response of the SN490B hollow cylinder specimens was investigated by conducting circular– and diamond–shaped strain waveform control tests using an axial–torsional fatigue testing machine. Using Mises–type equivalent stresses and strains, it was possible to obtain stress–strain relationships in a unified manner, even for multiaxial non–proportional loading. For equivalent strain amplitudes of 0.3% and 0.5%, most of the total strain range for both the circular and diamond shapes was plastic strain. The occurrence of plastic strain was found to be larger than that in the uniaxial tension–compression test. For both the circular and diamond shapes, the stress amplitude in the strain–controlled tests increased significantly from the initial cycles to approximately 10 cycles, indicating cyclic hardening. The subsequent increase in the stress amplitude slowed, but hardening continued until the specimen failed. An expression for the change in the stress amplitude under multiaxial non–proportional loading of SN490B was proposed.

Low-alloy steels were subjected to peening with fine titanium-nitride (TiN) particles under ambient conditions to form surface modified layer having TiN and improve their fatigue properties. Steels treated with fine particle peening using TiN particles (TiN-FPP) were assessed using electron probe microanalysis and X-ray diffraction to characterize the surface microstructure. Heterogeneous TiN particles transferred layer was formed on the steel surface. Rotating bending fatigue tests were performed for TiN-FPP treated steels in ambient air. It was found that TiN-FPP increased the fatigue limit due to high degree of compressive residual stress and surface hardness. In addition, the combined effects of residual stress, surface hardness and surface morphology on the fatigue limit of the TiN-FPP treated steel were quantitatively investigated based on a modified Goodman diagram considering residual stress relaxation during fatigue tests. The proposed method can achieve the fatigue limit estimation of the TiN-FPP treated steel.

Solution treatments at high temperatures in air were performed on maraging steel to facilitate the reaction between the elements contained in the steel and the atmosphere. The aim was to improve the wear resistance and fatigue limit of the steel by forming a high-hardness layer on its surface. The microscopic properties of maraging steel specimens were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, electron backscatter diffraction, nano-indentation test, and micro-Vickers hardness test. The wear resistance and fatigue properties were examined by ball-on-disc wear and four-point bending fatigue tests, respectively. Solution treatments at high temperatures created oxide layers with pores on the surface of maraging steel. Although this process decreased the hardness inside the steel, aging after the solution treatment improved the hardness reduction. The wear resistance of maraging steel was improved by solution at a high temperature, owing to the formation of an oxygen compound layer. The fatigue limit of maraging steel treated with solution at a high temperature and aging was higher than that of the sample treated with conventional solution and aging. However, the brittle compound layer promoted fatigue crack initiation, which decreased the fatigue limit of the maraging steel.

It was shown that the fatigue life of an elbow pipe made of carbon steel was shorter than that obtained under uniaxial loading conditions, while an elbow pipe made of stainless steel did not exhibit a reduction in fatigue life. To investigate the influence of stress triaxiality on the fatigue life reduction in the elbow pipe, fatigue life under equi-biaxial loading conditions was examined using disc bending fatigue testing equipment. A 15C carbon steel and Type 316 stainless steel were used for the fatigue tests. It was found that the fatigue life obtained from the disc bending fatigue test was longer than that obtained from the uniaxial fatigue tests for the same equivalent strain range, which was used as the driving force for fatigue damage in the design of nuclear power plant components. Therefore, it was concluded that the reduction in fatigue life of the elbow pipe was not caused by stress triaxiality. Although a reduction in fatigue life was observed only for the carbon elbow steel, the degree of change in the fatigue life due to stress triaxiality was comparable for both the carbon steel and stainless steel. This implies that stress triaxiality plays a minor role in the reduction of fatigue life in the elbow pipe.

Fatigue crack propagation tests were conducted under various nonproportional loading conditions on stainless steel SUS304 and titanium alloy Ti-6Al-4V. The crack opening displacement was also measured using digital image correlation to determine the effective resultant stress intensity factor range ΔK_eff. In addition, the influence of the material on the contribution of the shear stress τ_{θa, max}, which arises at the plane where the maximum resultant stress amplitude σ_{θa, max} occurs, to the crack closure was investigated. For three typical industrial materials, the effective resultant stress range Δσ_{θ, eff} could be uniformly arranged regardless of nonproportional loading conditions by using σ_{θa, max} + k'τ_{θa, max} with k'=1/2. Moreover, expressing the stress components Δσ_{θ, eff}, σ_{θa, max} and τ_{θa, max} in terms of strain components Δε_{θ, eff}, ε_{θa, max} and γ_{θa, max} established a unique relationship regardless of the material. Finally, the crack growth lifetime could be predicted with an error of approximately ±30% by using the proposed empirical formula.

The grain size of austenitic stainless steel JIS-SUS304 stainless steel was refined by rolling process and heat treatment to achieve higher strength. Rotating bending fatigue tests and tensile strength tests were conducted at room temperature. As a result，it was confirmed that both static strength and fatigue properties were improved by grain size refinement．In particular，for heat-treated GS1.6 or larger，the fatigue limit was significantly higher than the 0.2% proof stress，suggesting that plastic deformation occurred in the surface layer．The fatigue limits of all these specimens were significantly higher than those estimated from their hardness and tensile strength. This trend was especially pronounced in the heat-treated materials, which did not follow the conventional empirical rules．X-ray diffraction and hardness testing of these heat-treated specimens revealed that cyclic stress induces work hardening and stress-induced martensitic transformation in the surface layer．On the other hand, such progression was not observed in the GS0.9 specimen， although residual plastic strain from rolling may remain．Therefore，when estimating the fatigue limit of fine-grained stainless steels based on static strength，it is necessary to take the influence of microstructure into account.

In this study, a method for predicting low-cycle fatigue life using machine learning and a numerical model was investigated. The relationship between the remaining fatigue life and the distribution of fatigue cracks was calculated under various crack initiation parameters using the numerical model that simulates small crack initiation, growth and coalescence under low cycle condition. A prediction model for the remaining fatigue life was constructed using machine learning. Simulation results were used as training data for the machine learning model. The fatigue life from actual low-cycle fatigue tests was also predicted using the machine learning model. The remaining fatigue life predicted by the machine learning model was closer to the actual values than that predicted by fracture mechanics based on a single crack. In addition, the effect of parameters used for generating the training data on the prediction accuracy was investigated. The results showed that the number of training samples had a significant impact on fatigue life prediction accuracy. In particular, the number of simulation conditions had the largest effect. All parameters used in this study (number of cycles, number of cracks, surface crack length, and the sum of surface crack lengths) contributed to reducing the prediction error of remaining fatigue life.

Fatigue tests with three different stress ratios were carried out on shot-peened SUS316L steel, and in situ X-ray stress measurements were performed to analyze the behavior of the residual stress change and related factors. The compressive residual stress decreased during tensile loading and increased during compressive loading. The reduced compressive residual stress was caused by the tensile yielding of the core region, whereas the increase in compressive residual stress was caused by the compressive yielding of the core region. This behavior was observed when the yield strength of the surface layer was significantly increased by shot peening, and the compressive yielding of that layer and the resulting relaxation of compressive residual stress were prevented. In this case, the behavior of the residual stress change during cyclic loading was contingent on the stress ratio. It is because the compressive residual stress, which decreased during tensile loading, increased under subsequent compressive loading, and the amount of increase is based on the magnitude of the applied compressive loading.

This study investigates the factors influencing the fatigue strength of welded joints made of ultra-high-strength steel sheets (980 MPa grade) and high-strength steel sheets (440 MPa grade) for automotive chassis components. Although the application of high-strength materials does not necessarily improve the fatigue strength of welded joints, the underlying mechanisms remain unclear. This research focuses on the effects of stress concentration and microstructural characteristics near the weld toe. Experimental results reveal that, although the hardness near the fusion line in the 980 MPa grade joint is comparable to that of the base material, its fatigue strength is lower. In contrast, the 440 MPa grade joint exhibits a fatigue strength consistent with its hardness. This study proposes a correction factor based on microstructural parameters, which is incorporated into a fatigue crack initiation model proposed by Toyosada et al., resulting in improved agreement between calculated and experimental fatigue life results. Overall, the findings underscore the critical roles of microstructural characteristics and stress concentration in determining the fatigue performance of welded joints.