PREDICTION OF DELAY CYCLES DUE TO INSTANT OF SINGLE OVERLOAD CYCLES

Crack closure experiments were performed on 6061-T6 aluminium alloy using a crack opening displacement gauge for various overload ratios (1.44, 1.67, 1.87 and 2.06). The single overload cycle is given at the beginning, after exhausting 25% of the constant amplitude loading (CAL) life and 50% of the CAL life of the specimen. On the basis of these experiments one expression is developed to predict delay cycles due to application of overload cycle. The overloads produce a beneficial effect when they are given early. Later overloads, although giving an increase in life, produce a response of lesser magnitude. The experimental results are plotted on a log-log graph of N(D)/N(CAL) versus overload ratio. The experimental data are fitted on a straight line. One power law is developed.