Fatigue failure is one of major problems of the structural components under cyclic loading; however the fatigue test is a time-consuming and high-cost process. Therefore, it is better to establish the relationship between the fatigue property and static mechanical properties of materials. The present studies mainly focus on the relationship between fatigue strength and tensile strength of 316L austenitic stainless steel prepared with four different technologies. The rolling results in severe plastic deformation; dislocation multiplication occurs inside the grains, and the elongated grains are parallel to the rolling direction. With the tensile strength increasing, the optimum fatigue strength reaches at an appropriate tensile strength. The fatigue strength is controlled by the tensile strength and the ability of work hardening, and based on this analysis a new method is established to estimate the fatigue strength. The competitive mechanism between fatigue crack initiation and propagation is also proposed to account for the different shapes of S-N curves.
For many applications, the understanding of very long life fatigue in materials becomes extremely important. In this study, the fatigue behavior of bearing steel GCr15 (conforming to AISI 52100) at very high number of cycles has been examined. Experiments on hourglass specimens were conducted in air at room temperature, for fully reversed loading condition (R=-1), using a piezoelectric
fatigue testing machine operating at a frequency of 20kHz. The results indicate that the S-N data does not reach a horizontal asymptote (signifying the fatigue limit) at 107 cycles, as conventionally believed, and that the material can fracture up to 109 cycles. Therefore, to quote a fatigue limit at 107 cycles may not hold good for the material studied. The influence of defects (such as inclusions) on the crack initiation and fracture was analyzed by scanning electron microscopy.
Ultrafine-grained (UFG) pure Cu processed by equal channel angular pressing (ECAP) was subjected to cyclic deformation and subsequent ageing treatment at room temperature (RT) in order to investigate the stability of defects and grain size. Cyclic deformation for 1000 cycles at RT leads to a large decrease of internal stress. X-ray diffraction (XRD) shows that the stability of defects and grain size at RT in as-cyclic deformed sample is lower than that in as-processed sample and that a reduction of internal stress takes place prior to grain growth. TEM observations show that the microstructural evolution during ageing is characterized by normal grain growth accompanied with recovery within grain interior.
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