Abstract: In this study, we investigated the reliability assessment of exhaust manifold used in
thermomechanical condition. Overlay model proposed by Besseling was modified to consider the strain range dependence on elastic limit. By combining geometrical relation in hysteresis loop and temperature dependence of elastic limit with isothermal overlay model, temperature dependent cyclic plasticity model was proposed. Continuous damage model based on isothermal fatigue data was generalized for non-isothermal condition. Finite element analysis and life prediction of exhaust manifold were performed under severe operating conditions.
Abstract: The purpose of this paper is to investigate the effect of solid solution strengthening on the high temperature creep strength of STS310J1TB. To make clear the effect of solid solution strengthening, the stress abrupt change test was carried out to observe the instantaneous plastic strain and the stress relaxation test was carried out to investigate the ratio of solid solution strengthening to applied stress. The instantaneous plastic strain was observed in the stress abrupt change test of STS310S, while it was absent in STS310J1TB. This clearly indicates that the dislocations glide viscously in STS310J1TB, while they move in a free flight manner in STS310S. As a result of TEM observation, the dislocation structure of STS310J1TB is more uniform than that of STS310S. From the analysis of the stress relaxation curve, the effective stress to total applied stress was found to be about 16 %.
Abstract: The fatigue properties of austenitic stainless steels become inevitably important when using in structural materials. The authors have performed fatigue tests to investigate the effect of nitrogen content on fatigue properties of typical austenitic stainless steels (SUS304) and two kinds of nitrogen-contained SUS304 (SUS304N and YUS170). The main results obtained in this study are as
follows; (1) The knee point in S-N curves exists for SUS304 and SUS304N, but does not exist for YUS170. (2) The fatigue limit of SUS304N is higher than that of SUS304. On the other hand, the fatigue strength by 107 cycles of YUS170 is lower than those of SUS304 and SUS304N. (3) For SUS304, the transformation ratio increases with increase in stress amplitude. On the other hand, the transformation ratio of SUS304N is very small and that of YUS170 remains essentially zero.
Abstract: The aged degradation of material is observed when heat-resisting steel is exposed for long periods of time at high temperatures. In the present study, the degraded 1Cr-0.5Mo steel that is used for long periods of time at high temperature(about 515°C) and artificially reheat-treated materials are prepared. These materials were used to study the effect of aged degradation on fretting fatigue behavior. Through this experiment, it is found that the fretting fatigue strength of reheat-treated 1Cr-0.5Mo steel is approximately 46 percent lower than that of the plain fatigue strength of the
same material. Furthermore, the fretting fatigue strength of degraded 1Cr-0.5Mo steel was less than 53 percent of the same material™s plain fatigue strength. The maximum value of fatigue strength difference is observed as 57 percent between the fretting fatigue of degraded material and plain fatigue of reheat-treated material. These results can be used as basic data in a structural integrity evaluation of heat-resisting steel considering aged degradation effects.
Abstract: Mechanical and fatigue properties of the SiCp/Al with three different SiC particle volume fractions: 5%, 7.5% and 10% as well as Al6061 alloy were examined with tensile test, hardness measurement and fatigue crack propagation test. Tensile strengths and hardness of the SiCp/Al increased together with SiC particle volume fraction. It was checked that tensile strengths of the SiCp/Al had a certain relation with the corresponding hardness. On the other hand, fatigue crack propagation behavior (FCPB) of the SiCp/Al was faster than that of the AA6061. Besides, difference of the FCPB among SiC particle volume fractions was not pronounced. However, The FCPB slightly increased together with SiC particle volume fraction. It was noted that fatigue properties of the SiCp/Al was more affected by local volume fraction in front of crack tip than bulk volume fraction. In addition, the presence of SiC particles delayed fatigue crack initiation effectively because the percentage of fatigue crack initiation life to the whole fatigue life was a maximum of 38%, larger than that of ordinary structural materials.
Abstract: In the present study, fatigue tests have been performed to study the effect of pre-strain on fatigue properties of NHH (New Head-Hardened) rail. The objectives of this study were: (1) to observe the microscopic behavior of specimens during pre-strain process, (2) to research the influence of pre-strain on fatigue strength of NHH rail and (3) to investigate initiation and propagation behavior of the fatigue crack. The results showed that plastic pre-strain decreased the fatigue strength of
NHH rail; fatigue limits had no obvious variation among the different pre-strain ratios. Fatigue cracks initiated in the microscopic cracking or slip lines that were originated in the pre-strain process and propagated from these sites in the later fatigue test.
Abstract: Peening can be used to produce a layer of compressive residual stress at the surface of components which are subject to fatigue or stress corrosion, thereby retarding crack initiation and/or impeding the development of new cracks and hence improving their fatigue life. We have developed a new peening method, Cavitation Shotless Peening (CSP), which makes use of cavitation impacts induced by the collapse of the cavitation bubbles to produce compressive residual stress and work hardening on the material surface. CSP is a surface enhancement technique which differs with shot peening in that shots are not used. CSP uses a submerged high-speed water jet with cavitation, herein referred to as a cavitating jet, whose intensity and occurring region can be controlled by parameters such as upstream pressure and nozzle size. Cavitation number , which is defined by the ratio of upstream pressure to downstream pressure, is the main parameter of the cavitating jet. In this paper, the pit distribution on the specimen was observed with cavitating numbers = 0.0057 and = 0.0142. The improvement of fatigue strength and introduction of residual stress were investigated for both conditions using carburized alloy steel (JIS SCM415). It was evident from a comparison between non-peened and cavitation shotless peened specimens that the cavitation number has influence on the fatigue strength of metallic materials. Comparison of shot peened and CSP specimens has also been discussed.
Abstract: In this paper hydrogen effect on fatigue strength of structural steels is mainly reviewed on the basis of the author's experimental results. The emphasis is focused upon the hydrogen related corrosion fatigue crack propagation behavior of the ship structural steels and their welded joints in sour crude oil environment. It can be concluded that the environment enhancement of the fatigue crack propagation rate in sour crude oil is dependent of hydrogen evolved by reaction of H2S and H2O in the sour crude oil with structural steels.
Abstract: About 90% of steel structure™s failure was induced from the notched parts due to the stress concentration. Therefore it was very important to research how to improve the fatigue property of notched parts when the stainless steel was used as structural material. Rotating bending fatigue test was preformed to investigate the effect of different notch-machining process (Conventional lathe-machining, Combination of lathe-machining and roller-working) on fatigue strength of the
typical austenitic stainless steel SUS304. Fatigue limit of roller-worked specimen was improved to 285% compared to that of lathe-machined specimen. It was found that, work hardening effect induced in roller-working process played an important role to improve the fatigue strength. In addition, the residual compressive stress and the fibriform structure in the specimen surface-layer induced by roller-working were also effective to improve the fatigue strength. Non-propagating fatigue crack was observed in the notch surface of the roller-worked SUS304 for the first time.