The Effect of Stress Ratio on Fatigue Crack Growth in a Biaxial Stress Field

Abstract

The effect of stress ratio (mean stress) on fatigue crack growth in a biaxial stress field was investigated. The biaxiality was introduced by means of anticlastic bending of rhombic plates. A crack initiator in the form of an EDM slot was embedded in the top surface of plate specimen. The crack tip was subjected, as a result of specimen bending, to two principal stresses, one tensile and normal to the crack and the other parallel to it but in a compressive direction. Three mean stress ratios of R = Kmin/Kmax were applied, one in a zero to tension mode, the second in a completely reversed cycling where the stress ratio R = −1, and the third in an intermediate ratio of R = −0.5. All stress ratios were imposed in an existing biaxial stress field. Uniaxial concepts such as Walker’s approach were analyzed and applied to biaxial stress system. Experimental results have shown that fatigue crack growth is influenced by both variables: mean stress ratio and stress biaxiality ratio. Each variable was isolated by using the concept of effective stress intensity range ΔK. The crack growth and the fracture mode under each type of loading are discussed and analyzed.