Improvement fatigue limit of steel containing a small crack‐like surface defect by overload effect

Abstract

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.

Improvement of fatigue limit by shot peening for high‐strength steel containing a crack‐like surface defect

“…As a result, the fatigue limit was increased 71 percent by tensile overloading. It was pointed out by Mizukami et al (2010) that compressive residual stresses were formed in the vicinity of the surface slit after tensile overloading. Thus, the fatigue limits were increased after tensile overloading.…”

Improvement of fatigue limit by overload for high-tensile strength steel containing a crack in the stress concentration zone

“…However, in the range of a ¼ 0.1-0.3 mm, s w of specimens with the 0.9 Mp process (deep-colored bar) has an almost constant value, s w ¼ 520-540 MPa. Mizukami et al (2010) explain that the effect of overload at the crack tip improves as the slit IJSI 4,3 size increases. As a result, the fatigue limit improves considerably.…”

“…The size of the yield zone v r is calculated using equation (4). When a crack propagates in this yield zone v r , a negative stress intensity factor is generated and prevents crack propagation (Mizukami et al, 2010;Houjou and Ando, 2012). The width of the non-propagating crack, indicated by B in Figure 14(b) and (c), is less than v r which is calculated by substituting K ov into equation (4).…”

“…Many researches on improvement of fatigue limit by cold-work had been reported (Norton, 1946;Rosenthal and Sines, 1951;Frost, 1958;Dugdale, 1959;Pattinson and Dugdale, 1962). However, there is almost no research on the improvement of fatigue limit by plastically deformation for steel containing a crack in the stress concentration zone (Mizukami et al, 2010). Figure 1 shows the distribution of residual stresses in the section of the beam after removal of the load of bending moment (Mp).…”