Fatigue Properties of Ultra-Fine Grain Austenitic Stainless Steel and the Effect of Hydrogen

The fatigue properties of ultra-fine grain austenitic steel (UFG16-10), which has a 1 μm average grain size, were studied as part of the project aimed at the development of high-strength low-cost stainless steels for hydrogen service. The fatigue properties of the UFG16-10 were compared with that of a coarse grain material with the same chemical composition (CG16-10) and two kinds of commercial steels, JIS SUS316 and JIS SUH660. The fatigue strength of the UFG16-10 was 2.8 times higher than that of the CG16-10. The effect of hydrogen on the fatigue limit of the UFG16-10 was not significant. However, the fatigue life of the UFG16-10 was reduced by hydrogen in the short life regime. In the fatigue crack growth test, the UFG16-10 showed a good crack growth resistance that was equivalent to that of the SUH660 and significantly higher than that of the SUS316. However, the crack growth rate was significantly accelerated by hydrogen. The cause of the hydrogen-assisted fatigue crack growth of the UFG16-10 was transformation of the microstructure at the crack tip from austenite to strain-induced martensite. This was also the cause of the reduced fatigue life of the hydrogen-charged UFG16-10.

Effect of Rolling Process on Fatigue Property of 2A66 Aluminum-Lithium Alloy Sheet

The effect of rolling process on the mechanical property and microstructure of 2A66 Al-Li alloy sheet was investigated. Three different rolling processes for 2A66 AL-Li Alloy Sheet were picked for fatigue crack growth test in T3. The results showed that enhancing the amount of deformation in the range of moderate or reducing the rolling step of rolling process will increase the fatigue crack growth rate.

Hydrogen Diffusion Analysis in the Fatigue Crack Growth Test Under High Pressure Hydrogen

Prevention of hydrogen embrittlement is one of the most important issues for the materials used in hydrogen environment. Various mechanisms for hydrogen embrittlement were proposed, such as the decohesion mechanism and hydrogen enhanced localized plasticity. However, the process of hydrogen diffusion and accumulation into the fracture point is necessary for understanding of hydrogen embrittlement. In this paper, hydrogen embrittlement behavior during the fatigue crack extension test under high pressure hydrogen was first conducted. Then, hydrogen diffusion and accumulation behavior in the crack tip region was analyzed by the numerical simulation based on the Fick’s diffusion theory. Alpha multiplication method which multiplies stress gradient induced terms was found to be valid to realize correct particle diffusion behaviors driven by the stress gradient term. The significance of the concept of alpha multiplication method for the numerical analysis was discussed by comparing to the experimental hydrogen embrittlement behavior.

Quantification and Prediction of Residual Stresses in Creep Crack Growth Specimens

An important issue to be considered in the life assessment of power plant components is the effects of prior creep damage on subsequent fatigue crack growth and fracture behavior. To examine these effects, creep damage has been introduced into 316H stainless steel material by interrupting creep crack growth (CCG) tests on compact tension, C(T), specimens at 550 °C. During the CCG tests, the specimen is loaded in tension, crept and unloaded after a small amount of crack extension. This process introduces compressive residual stress fields at the crack tip, which may subsequently affect the fatigue crack growth test results. In this work, neutron diffraction (ND) measurements have been conducted on interrupted CCG test specimens, which contain creep damage local to the crack tip, and the results are compared to predictions obtained from finite element (FE) simulations. Reasonable agreement has been found between the FE predictions and ND measurements.

Experimental and Analytical Studies on the Effect of Excessive Loading on Fatigue Crack Propagation in Piping Materials

The seismic design review guide in Japan was revised in September 2006 to address the occurrence of a large earthquake beyond the design basis. In addition, Japanese nuclear power plants (NPPs) experienced multiple large earthquakes, such as Niigata-ken Chuetsu-Oki Earthquake in 2007 and the Great East Japan Earthquake in 2011. Therefore, it is very important to assess the structural integrity of reactor piping under such a large earthquake when a crack exists in the piping. In this work, crack growth behavior after excessive loading during the large-scale earthquake were experimentally and analytically evaluated for carbon steel and austenitic stainless steel. Some cyclic loading patterns with increasing and decreasing load amplitudes and maximum loads were applied to fatigue crack growth test specimens. From the results, the retardation of crack growth rate was clearly observed after excessive loading. In addition, the applicability to the retardation effect of the modified Wheeler model was confirmed. It is also concluded that the retardation effect has little influence on the failure probability due to seismic loading using probabilistic fracture mechanics (PFM) analyses with the modified Wheeler model.