In this paper hydrogen effect on fatigue strength of structural materials related to the internal hydrogen embrittlement and hydrogen environment embrittlement is reviewed. The emphasis is focused upon the hydrogen assisted corrosion fatigue behavior of structural materials in aggressive gas and sour crude oil environments. The proposed mechanism of hydrogen effect on fatigue crack propagation behavior of structural materials is briefly summarized. Finally recommended researches in hydrogen effect on fatigue strength of structural materials are touched on briefly.

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Fatigue tests were conducted for hydrogen charged and work-hardened SUS 316 L austenitic stainless steel, using smoothed and notched specimens of 1 mm thickness at frequency of 150-200 Hz under stress ratio of R=O. The work-hardened steel was prepared with cold rolling at 20 or 40% reduction of cross-sectional area, and the hydrogen charging was conducted for 500 hours, leading to hydrogen content of 20-50 mass. ppm. In the case of smoothed specimens, fatigue strength of the 40% rolled version was decreased due to the hydrogen charging, while the 0 and 20% rolled versions showed negligible difference between hydrogen charged and uncharged specimens. The hydrogen charged specimens of the 40% rolled version revealed internal fractures which occurred below a fatigue limit of the uncharged specimens and eliminated the fatigue limit. In the case of notched specimens, fatigue limits were equal between hydrogen charged and uncharged specimens even in the 40% rolled version. However, fatigue lives were a little decreased due to the hydrogen charging in case of the 40% rolled version at frequency of 150-200 Hz.

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In order to investigate the hydrogen effect on the fatigue strength at 107 cycles of type 316L stainless steel, the rotating bending fatigue tests in atmospheric air and the plane bending fatigue tests in 1 MPa dry hydrogen gas and in air were carried out. The observed fatigue behavior showed that the fatigue strength at 10⁷ cycles in both environments is determined by the non-propagation of fatigue crack of the order of the grain size. And, the strength at 10⁷ cycles in hydrogen is slightly higher than that in air. In the region of long fatigue life, the fatigue life in hydrogen is longer than that in air, which is mainly caused by the longer crack initiation life in hydrogen. The crack propagation life in hydrogen is shorter than that in air, but has only a small ratio to the fatigue life in this region.

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The effect of absorbed hydrogen on the torsional fatigue in high cycle region was studied under continuous cathodic polarization condition using three kinds of austenitic stainless steels SUS304, SUS316 and SUS316L. The tensile strengths of these materials were varied into different levels by work hardening, hot drawing and solution heat treatment. The absorbed hydrogen gave a detrimental effect in the case of hardened materials and the fatigue strength was substantially lowered irrespective of chemical composition. In a solution heat-treated material, however, no significant effect was observed. The fatigue behavior of an actual coil spring made of work hardened SUS316 was studied as an application of work hardened material to a component of a hydrogen utilization machine. The fatigue strength was much more significantly lowered than in the case of plain specimen under cathodic polarization. Small meshwork groove existing on the surface of actual spring acted as a defect. This resulted in a significant reduction of fatigue life under cathodically polarized condition. These results suggested that high tensile strength and small defect were detrimental factors for the torsional fatigue of work hardened austenitic stainless steel used in hydrogen environment.

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Fatigue tests on SCM435 steel quenched at 860°C and tempered at 580°C were carried out under the frequencies of 0.2, 2 and 20 Hz with specimens containing a small artificial hole. An additional test in which the frequency was alternatively switched between 0.02 Hz and 2 Hz was also carried out. Hydrogen charging to specimens was carried out by immersion method. The fatigue life of hydrogen-charged specimens decreased in comparison with that of uncharged specimens. The acceleration of fatigue crack growth rate da/dN was observed at dα/dN<1.0×10^-7m/cycle in test frequency f=20 Hz, dα/dN<4.0×10^-7 m/cycle in f=2Hz and dα/dN<1.5×10^-6 m/cycle in f=0.2 Hz. The acceleration of dα/dN saturated at ΔK<17 MPa√m in f≤2Hz. It was found that there is the upper bound of the effects of hydrogen and test frequency on fatigue crack growth rate in hydrogen environment. The test with switching two frequencies between 0.02 Hz and 2 Hz showed that hydrogen concentration in the vicinity of the crack tip remarkably affects fatigue crack growth behavior and crack morphology. The most remarkable change of the slip bands around the crack tip was less and discrete in the hydrogen-charged specimens compared to uncharged specimens.

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In recent years, the fuel cell vehicle that uses hydrogen as clean energy has been developed among automobile companies. To store high compressed hydrogen gas in the fuel cell vehicle, a special tank is needed and Al-Mg-Si base alloys have been regarded as one of the candidate materials for the tank liner. However, little is know about the effect of atmospheric hydrogen on the mechanical properties of the Al-Mg-Si alloy. In this study, effect of aging microstructure on hydrogen embrittlement (HE) sensitivity of the Al-Mg-Si alloy was investigated by means of the tensile testing machine equipped with quadruple mass spectrometer in an ultrahigh vacuum chamber, which was originally developed by our group. It is proved that the elongation decreases when the slow strain rate testing (SSRT) is carried out in humid air. The amount of hydrogen evolution at the moment of fracture is clearly decreased particularly in the OA specimen. The hydrogen desorption amount from the OA specimen is smaller than that from the PA specimen after the predeformation by SSRT. As the aging proceeds, the composition of the grain boundary precipitates is changed. It is assumed the PA condition has higher capability to trap hydrogen than the OA condition in the Al-Mg-Si alloy.

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The influence of aging after hydrogen absorption on the fatigue properties of a β-type titanium alloy, Ti-20 V-4 Al-1 Sn, was examined. Hydrogen was absorbed to the alloy through acid-cleaning in order to make three model materials possessing different hydrogen quantities. Since hydrogen is a strong β-phase stabilizer, the precipitation of a fine α phase during aging was strongly suppressed and age-hardening was delayed with increasing absorbed hydrogen quantity. In the materials aged after hydrogen absorption, even if their hardness and tensile strength were leveled out by adjusting the aging time, fatigue strength was slightly decreased with the increase in hydrogen quantity. It was thought that such reduction in the fatigue strength resulted from the existence of weak parts where the density of the α phase precipitated with aging was lower.

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The objective of this study is to clarify the effect of hydrogen gas environment on fretting fatigue strength of the materials for hydrogen utilization machines and structures. The materials used in this experiment were a low alloy steel SCM435H and a precipitation hardened stainless steel SUH660. And the effectiveness of nitriding, which has been used to improve fretting fatigue strength, was examined in hydrogen gas environment. Hydrogen gas pressure was 0.12MPa in absolute pressure. Continuous measurement of fretting fatigue crack propagation during the test in hydrogen gas was done by using an electric potential drop technique in order to consider the mechanism of hydrogen function. Fretting fatigue lives of both materials in the short-life region were increased in hydrogen gas than in air. This was caused by that start of stable crack propagation was delayed in hydrogen gas environment. However, in SUH660, fretting fatigue strength in hydrogen gas was decreased than that in air in the long-life region, which is more than 10⁷ cycles. In the fretting fatigue test of SUH660 in hydrogen gas at 18% lower stress amplitude compared with the fretting fatigue strength in air at N=3 x 10⁷ cycles, a failure occurred at N=3 x 10⁷. In SCM435H, fretting fatigue strength was the same in both environments even in long-life region.

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Fatigue fracture mechanisms of the Al alloy liner/FW-CFRP composite pressure vessels were evaluated through experimental and simulation approaches. Burst tests and cyclic pressure tests were conducted for a specially designed composite vessel. Fracture surface morphologies were observed microscopically and the fatigue crack propagation mechanisms were investigated using fractography. Fatigue behaviors of Al liner were examined through simulated stress and strain amplitudes and its mixed mode fatigue crack propagation characteristics. The effects of CFRP layer thickness on the fatigue life of Al liner are presented in the context of optimum fatigue design.

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Storage tanks and pipelines of gaseous fuel are frequently installed in low temperature environment. So, when gaseous fuel contains hydrogen, not only cryogenic embrittlement but also hydrogen embrittlement may occur in the equipments. In this report, the effect of hydrogen on brittle-ductile transition of COR-TEN O steel was studied by carrying out fracture experiments of hydrogen charged and non-charged materials over the temperature range from -196°C to room temperature. The variations of fracture resistance expressed by CED with temperature for hydrogen charged and non-charged materials are compared. As the result, it is shown that the brittle-ductile transition temperature of hydrogen charged material is lower than that of non-charged one and, although the fracture resistance of charged material naturally becomes smaller in the upper shelf range, not the fracture resistance of non-charged material but that of charged one is a little bit larger in the brittle-ductile transition range.

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Stress distributions in stepped-lap adhesive joints subjected to static tensile loadings are analyzed using a three-dimensional finite-element method (FEM). For establishing an optimum design method of the joints, the effects of adhesive Young's modulus, the adhesive thickness and a number of steps on the interface stress distributions are examined. As the results, it is found that the maximum value of the maximum principal stress σ₁ occurs at the butted edge of the adhesive interfaces. The maximum value of σ₁ decreases as the adhesive Young's modulus and the number of steps increase, and as the adhesive thickness decrease. In addition, the difference in the stress distributions between the 2-D and 3-D FEM calculations was shown. A method for estimating the joint strength is proposed using the interface stress distributions. For verification of the FEM calculations, experiments were carried out to measure the strains of the adherends and the joint strengths. Fairly good agreements are found between the numerical and the experimental results.

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This work is concerned with the thermoelectromechanical fracture behavior of two coplanar cracks in a piezoelectric material strip under a uniform heat flow far away from the crack region. The crack faces are supposed to be insulated thermally and electrically. By using the Fourier transform, the thermal and electromechanical problems are reduced to singular integral equations, respectively, which are solved numerically. Numerical calculations are carried out, and detailed results are presented to illustrate the influence of the crack location and length on the thermal stress intensity factors.

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The copper electroplating stress measurement method uses the grain growth in copper foil. The grain growth is a kind of thermal recrystallization that is affected by the magnitude of prestraining. Since a lot of grain nucleation occurs at the large prestraining, the large shearing stress should also lead to the same effect. On the basis of the above, we investigate the stress dependence of the grown grain size. We confirmed that the size of grown grains is expressed by the three-parameter Weibull distribution, and the relationship between scale parameter g and arithmetic mean of grain size d^* has a linear relationship until the grain growth rate r^* reaches 50%. It is also recognized that the mean grown grain size becomes smaller with increasing shearing stress amplitude. To measure the shearing stress amplitude, the relationship between mean grain size, shearing stress amplitude, and the number of cycles is experimentally formulated. Moreover, even if the number of cycles is unknown, measuring both r^* and d^* enables us to estimate the shearing stress amplitude.

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In this study, we investigated a method to evaluate fatigue damage of steels in the whole view field without contact using laser. In high cycle fatigue of mild steels, slipbands are firstly produced on material surface and the slipband density increases with progress of fatigue damage. In our previous work, we used a laser spot beam illuminating the fatigue damaged area of a specimen surface and diffusion pattern of the reflected light from the specimen surface changes due to surface change. We are able to evaluate fatigue damage based on diffusion pattern change due to surface property change caused by slipbands at an observing point. But it is important to find first the position where fatigue damage starts. In this study, we tried three types of optical setups to detect fatigue damaged area in the whole view field of the observing area with a CCD camera and evaluate fatigue damage. The diffusion pattern change accompanied by the loading cycles was observed under constant stress amplitude with the three optical setups. The results showed that there is a possibility of detecting fatigue damaged area and also evaluating fatigue damage by the optical setup using slit beam. Observation in the whole view field is possible scanning the slit beam on the specimen surface.

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Fatigue life prediction of solder joints is one of the most important areas of research in the development of reliable electronic packages. In the present study, the new method of estimating the crack propagation, called 'damage path simulation', was applied for solder bumps under the mechanical fatigue tests. The new method is based on finite element analysis without geometrical crack model, and on the characteristic damage model assumed for solder. Damage paths from the entire circumference in the vicinity of the interface were simulated, and they were finally combined at the center of bumps. The extension of the damage path showed good agreement with the behavior of crack propagation observed in the mechanical fatigue tests. In the case of solder bumps, the effects of the assessment of the previous damage on the extension rate of the damage path might be critical issue. However, the effects of the assessment of the previous damage were relatively small, in consideration of the large scattering in the results of mechanical fatigue tests. It was confirmed that the number of cycles to the fatal failure of solder bumps can be accurately estimated by the damage path simulation. It was concluded that the estimation of the crack propagation based on the proposed method is satisfactory for engineering purposes.

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