Abstract
The effect of the hole edge shape formed by a punching process including thickening on the fatigue strength of the hole of ultra-high strength steel sheets was investigated. The bending and tensile fatigue tests of the thickened hole edge were performed. For the bending fatigue strength, the high height of hole edge was effective because of high bending stiffness. Although the high tensile stiffness was important for the high tensile fatigue strength, the shape of hole edge was alternative. To increase both the bending and tensile fatigue strengths, a compressive process in rollover of hole edge was added for the thickened sheet having a high bending fatigue strength. It was found that the thickened and compressed hole edge of ultra-high strength steel sheet was effective in improving both the bending and tensile fatigue strengths.
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