Fatigue crack propagating behavior from inner surface crack on bent sheet of ultra-high strength steel

Abstract

Fatigue fracture of structures made of bent sheets often occurs from cracks on the sheet inner surface. This study investigated fatigue crack growth and non-propagating behavior in bent sheets of ultra-high strength steel (UHSS) using artificial cracks introduced by a picosecond pulse laser. A newly-designed fatigue test showed that the internal compressive residual stress introduced by bending arrests crack propagation in the depth direction. A crack propagation analysis using the software SCANP revealed that this internal compressive residual stress suppressed not only the stress intensity factor in the depth direction, ΔKa, but also that in the width direction, ΔKc, even though the surface residual stress was tensile. A stress relief heat treatment test demonstrated that the crack arrest behavior disappeared and the propagation life was shortened in the absence of this compressive residual stress. Non-propagating cracks were observed in the specimens after 1×107 test cycles. The relationship between the fatigue limit and the artificial crack depth (Kitagawa-Takahashi diagram) was generally fitted by the Line Method with a critical distance of 46.7 μm. This study provides insight into the fatigue behavior of cracks on bent sheets of UHSS, which are widely used in many industries such as automobile manufacturing.