Stresses occurring during transient events should be evaluated on the basis of cumulative fatigue. Cumulative fatigue analysis is used to estimate how many cycles of a certain stress level may be endured before shaft failure would occur. EDI uses Miner's rule to determine the cumulative fatigue damage incurred by each shaft during one startup as a result of stresses in excess of the shear endurance limit. The number of allowable starts can then be determined.
Many different fatigue analysis techniques are available for calculating cumulative fatigue damage. The major differences usually appear in the S-N curves, fatigue reduction factors, and methods for counting stress cycles. EDI uses the rainflow method, which is a widely accepted technique to evaluate stress cycles. Experimental fatigue tests show that identical specimens have widely different fatigue lives (factors of 10 are not unusual), so calculations can be used only as a tool to predict average life.
The procedure used by EDI for calculating cumulative fatigue damage during a transient event is as follows:
1. Obtain stress versus time histories using a time-domain torsional modeling technique.
2. Process the stress-time histories using the rainflow counting method to obtain equivalent stress ranges when numerous stress cycles in excess of the endurance limit occur.
3. Compare the calculated stresses to the S-N curve and calculate the fatigue damage per event using Miner's rule.
