The fatigue life along a towed tether can be estimated in a time domain analysis. The formula used to calculate the fatigue life is

(10–19)

where is the stress range computed from a rainflow count of time history stresses, is the number of cycles per day for this stress range from a probability distribution by a rainflow count, is the stress concentration factor, and and are the S-N curve slope and the S-N curve intercept coefficient respectively.

The extreme values of stress are based on the assumption that stress has a Rayleigh distribution. The peak stress is given by

(10–20)

where is the mean stress, is the root mean square of the stress, and is the number of cycles per hour (based on the mean number of positive and negative peaks per hour in the stress time history).