Effect of prestress on low-cycle fatigue

An investigation was conducted on HY-80 steel to show how low-cycle fatigue life is influenced by imposition of various stresses prior to fatigue testing. One cycle of static prestress was imposed on each specimen at loads either above or below the yield strength of the material and fatigue testing was done at various levels of stresses. Tests were conducted on notched and tee-fillet welded beam specimens. The magnitude and type of residual stresses at the toe of tee-fillet welds were determined by a hole drilling technique. Experimental data show that residual stresses induced by prestressing can—according to their sign—influence fatigue life. It was observed in the notched beam tests that when stress cycling was performed at stress levels below the yield strength of the material, tensile prestress increased fatigue life and that the higher the prestress, the greater the increase in life. At stress levels above the yield strength, tensile prestress has little or no effect on fatigue life. Compressive prestresses were found to be detrimental to fatigue life regardless of the stress levels at which the fatigue tests were run. The effects of prestress on the tee-weldments were not delinated as clearly. Fatigue life was increased by tensile prestress but no adverse effects were observed for compressive prestress.