Fatigue Damage and Small Crack Growth During Biaxial Loading
by
Chang-Tsan Hua
Department of Mechanical and Industrial Engineering
Abstract
The progressive nature of fatigue damage under biaxial loading has been investigated. Experiments were performed on thin-wall tubular specimens of 1045 steel in tension, torsion, and combined tension-torsion loading. Two equivalent strain amplitudes, one in the high cycle fatigue (HCF) region and one in the low cycle fatigue (LCF) region were employed in this study for all loading patterns. The test program included constant amplitude loading and variable amplitude loading. Two types of variable loading patterns, step loading and block loading, were selected.
Different types of crack systems were formed in the HCF and LCF regions. In the HCF region, cracks nucleated on a few planes. Usually one of them grew into a dominant crack and led to failure. This type of cracking is called Typed-S (side) crack system. In the LCF region, equally developed microcracks are observed over the entire gage section. The failure is due to a linking process in which the microcracks join up during the last few cycles of the fatigue life. This type of cracking is called Type-R (ring) crack system.
Two approaches, the damage curve approach and the fracture mechanics approach, were selected to analyze the test results. The two crack systems can be analyzed by the damage curve approach. The results showed that the damage curve for pure tension can be used to evaluate damage behavior under combined tension-torsion within the tested strain ratios. Fatigue lives under variable amplitude loading can be predicted from constant amplitude damage curves if a single type of crack system dominates the fatigue process. During block loading, two competitive crack systems can be developed in a single specimen. This results in conservative prediction of the fatigue life.
The fracture mechanics approach is only applicable for Type-S crack. The mixed mode small crack growth rate can be correlated with the strain based intensity factor and total energy release rate. The agreement with the long crack data justified the approach.
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