Cumulative damage under reversed transverse shear superimposed upon static tension

Abstract

Cylindrical specimens of 7075-T6 aluminum alloy were subjected to reversed shear stress and static mean stress to study the effect of mean static tensile stress on cumulative fatigue damage under reversed shear stress. The reversed shear stress amplitude varied in three steps. Test specimens were of double shear model, with minimum diameters of 0.3 + 0.002 inches. These specimens were tested, under different combinations of reversed shear and static tension. The specimens were tested at three different shear stress levels of 8, 12 and 18 Ksi on each shear area in the constant-amplitude reversed shear stress tests. Two tensile stress levels were chosen in combination with the above three shear stress levels in the combined stress tests. These tensile stresses were 10% and 15% of the yield strength at 0.2% offset. The number of cycles to failure obtained from the constant-amplitude tests were used in analysis of variable-amplitude test data. In the variable-amplitude reversed shear stress tests each of the specimens was subjected to 8 Ksi for a number of cycles n-j, 12 Ksi for a number of cycles n2 and 18 Ksi for a number of cycles n^ until failure. The number of cycles n-j, n2 and n3 were randomly chosen. In the variable-amplitude combined fatigue tests each specimen was subjected to the same three shear stress levels plus a constant static tensile stress. The order of application of the three shear stress levels was changed in the variable-amplitude tests to study the effect of sequences of loading on cumulative fatigue damage.