Effects of shot peening on the bending fatigue limit of spring steel (SUP9A) containing an artificial semi-circular slit were investigated. Shot peening (SP) and stress shot peening (SSP) were conducted on the specimens containing an artificial semi-circular slit with a depth of a = 0.1, 0.2 and 0.3mm. Then, bending fatigue tests were conducted on the specimens. The fatigue limit was improved by SP and SSP. In the case of SP specimens and SSP specimens, the specimen having a semi-circular slit under a = 0.2mm fractured outside the slit, and they had considerably high fatigue limits almost equal to those of the shot-peened smooth specimens. Therefore, semi-circular slit whose depth of under a = 0.2mm could be made non-damaging by SP or SSP. It was found that the fatigue limit of specimens having a semi-circular slit which received SP or SSP determined by the threshold condition for non-propagation of fatigue cracks emanated outside the slit. The criterion that the semi-circular slit is non-damaging or not is decided by relationship between the stress intensity factor range of semi-circular crack and threshold stress intensity factor range.
mprovement in the threshold stress intensity factor by tensile overload in large crack was investigated. The crack retarded zone and the non-propagating crack size was validated based on the fracture surface observation. It was observed that crack propagation was retarded by the overload during the fatigue tests. The size of non-propagating cracks and the crack retarded zone corresponded well to the calculated value . Moreover, in order to evaluate the overload effects on the small crack like defect, bending fatigue tests were carried out and discussed the effect of crack size on the improvement of the ∆Kth.
PurposeSurface defects reduce fatigue strength and may greatly reduce component reliability, particularly in pressure vessel weld regions, springs, and other applications. The fatigue strength of components, and thus their reliability, can be substantially increased by tensile overloading prior to use. The purpose of this paper is to investigate the effect of tensile overload on small cracks by applying a tensile overload to steel plates containing semicircular slits that simulate small surface cracks and by determining the degree of increase in the fatigue strength. The effect of tensile overload on the apparent fatigue threshold stress intensity factor range (ΔKth) was also investigated.Design/methodology/approachA tensile overload stress of 1,000 or 1,200 MPa was applied once to all test pieces. Then, bending fatigue tests were conducted with a stress ratio R=0.1. The slit region was subjected to applied cyclic tensile stress by four‐point bending throughout the fatigue test. A test specimen to which no overload stress was applied was tested for comparison.FindingsThe improvement in ΔKth by tensile overload is observed in the specimen with a small crack like surface defect. However, in the specimen with a small crack like surface defect, the improvement in ΔKth by tensile overload is saturated as increasing tensile overload. The improvement rate of ΔKth by tensile overload and the upper limits of improvement in ΔKth were predicted. The predicted values of the improvement rate of K were well in agreement with the experimental results.Practical implicationsThe proposed method can be applied to pressure vessels and springs.Originality/valueThe overload effects on fatigue strength are studied for large cracks. However, the effect is not understood at all for small cracks. This study focused the over load effects for small cracks. This is the original point of the present study.
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