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Cumulative Damage In Fatigue: A Step Towards Its Understanding

R. H. W. Brook, J. S. C. Parry

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A method is described which uses changes of apparent dynamic modulus and damping during fatigue cycling, for estimating accurately the fatigue lives of stainless steel (Rex 535) specimens. This technique for estimating well in advance of failure the fatigue lives of individual specimens avoids the usual difficulties caused by scatter of fatigue results, and has enabled a more precise quantitative investigation to be made of cumulative fatigue damage (i.e. damage due to multi-level loading) than would have been possible using conventional experimental methods. By estimating the remaining life of a specimen at one stress amplitude before measuring the equivalent remaining life at a second stress amplitude by cycling to failure, it was possible to determine lines of equal damage on a plot of stress amplitude versus remaining fatigue life. These lines of equal damage were used to predict the fatigue lives of specimens subjected to programmes of multi-level loading, and the accuracy of these estimates, when compared with the subsequent experimental results, is much better than has been achieved hitherto. Depending on the stressing programme chosen, Miner's linear damage rule is shown to be very good, rather pessimistic, or very dangerous.