TWI Industrial Member Report Summary 2/1976
By T R Gurney
In cumulative damage calculations there is some difficulty in taking account of n stresses below the fatigue limit, because N in the cycle ratio - then becomes N/n infinite, so that - becomes zero. A method is presented of overcoming this N difficulty. This consists of calculating the length to which the crack must grow before each stress becomes effective, and then calculating the number of cycles which are ineffective - i.e. which are applied while the crack is too small for them to be effective. By considering each stress in turn it is shown that the cumulative damage ratio can be calculated taking account of these ineffective cycles. A set of cut-off stresses is proposed which should enable design offices to continue to use the simple linear summation approach.