Improvement in Fatigue Limit by Shot Peening for High-Strength Steel Containing Crack-Like Surface Defect: Influence of Surface Crack Aspect Ratio

Abstract

The effect of shot peening (SP) on the bending fatigue limit of high-strength steel (SUP9A) containing a semi-elliptical surface slit was investigated. SP was conducted on specimens containing a semi-elliptical surface slit with an aspect ratio (a/c) of 0.4, where a was the crack depth (a = 0.1, 0.2, and 0.3 mm) and c was half the surface length of the crack. Bending fatigue tests were carried out under a stress ratio R equal to 0. The results showed that the fatigue limit of the shot-peened specimens with slits having a depth of less than 0.2 mm was almost the same as that of the shot-peened specimens without slits. Meanwhile, some of the specimens fractured at the surface in areas other than the slit. Thus, the maximum depth of the slit that could be rendered harmless by SP was 0.2 mm. The maximum depths of cracks with various aspect ratios that could be rendered harmless by SP were predicted, assuming that the cracks were arrested when the apparent stress intensity factor at the slit tip was less than the threshold stress intensity factor of the material. The estimated values were in good agreement with the experimental values. A harmless crack assessment diagram was proposed based on this estimation method.

Analysis on peculiar fatigue fracture behaviour of shot peened metal using new threshold stress intensity factor range equation

“…Figure 4 shows that most of the specimens had stage II nonpropagating cracks. 21 The results [21][22][23][24][25][26] of previous experimental studies have highlighted four problems with the peculiar fatigue fracture behaviour of SP precrack specimens. Regarding Figures 3 and 4, the following four problems should be investigated using Equations 10-13.…”

“…Recently, several studies have been published on the fatigue fracture behaviour of peened cracked materials. [21][22][23][24][25][26] Typical examples of the experimental results are shown in Figures 2-4. 21 The material is HV470 spring steel with a width (2w) and thickness (t) of 10 mm and 3 mm.…”

“…Relationship between the fatigue limit and crack depth for Non-SP and SP specimens of SUP9A (aspect ratio a/c = 1.0 and 0.4) 21 16 specimens whose precracks were rendered harmless showed the fatigue limit. The highest and lowest fatigue limits were 960 and 880 MPa, respectively.…”

“…The precrack that was rendered harmless was defined as the fatigue limit of the SP precrack specimen that was not less than 95% of the fatigue limit of the SP smooth specimen. [21][22][23][24][25][26] The threshold stress intensity factor range (ΔK th(l) ) of R = 0 for a large crack was 6.09 MPa ffiffiffiffi m p…”

“…. 21 The SP specimen showing the fatigue limit was heated at 280 C for 1 h after the test and fractured under high cyclic stress. Figure 4 shows that most of the specimens had stage II nonpropagating cracks.…”