Fatigue crack propagation in structural steel

Abstract

For several years now, structural steel consumers in Kenya have been using locally cast and rolled steel products. Some of these consumers, (notably fabricators of passenger vehicle structural frameworks) have been experiencing recurrent fracture and fatigue problems.

Fracture mechanics principles are used to investigate the fatigue crack propagation characteristics of the steel used in the construction of vehicle body structural frameworks. For this investigation, a test rig was designed and constructed. The effects of test frequency, stress ratio and of welding (both through weld metal and through the heat affected zone) on fatigue crack growth have been studied. All fatigue crack growth tests were performed on centre cracked tension specimens (CCTS) in laboratory air. R-Curve determination ("also on CCTS) was used to determine the material's fr9cture toughness. Preliminary tests were conducted to determine the chemical, mechanical and low cycle fatigue properties.

It was found that in the mid-range of stress intensity, a higher stress ratio increased the rate of crack growth: the crack growth rate through the heat affected zones was higher while that through the weld metal was lower than the crack growth rate through the present material. Lowering the test frequency increased the crack growth rate.

At low stress intensity ranges (near threshold), the rate of crack growth was higher at a higher R-ratio. The crack growth rate through both the weld metal and heat affected zone was lower than through the parent metal.

The chemical and some mechanical properties of the material did not satisfy the requirements of established standards.