Dirlik Stress PSD Fatigue

This script calculates the Dirlik fatigue damage from a stress PSD using S/N Coefficients from MIL-HDBK-5J, MMPDS or NASGRO

This is an absolute damage fatigue script which gives a cumulative damage index R.

There are various "not-to-exceed" thresholds for R.

R=1 per Palmgen-Miner Theory

R=0.7 per Steinberg

R=0.25 per Sarafin

R=0.1 per Wirsching, Paez, Oritz

The lower values account for potential scatter in the S/N curve.