Hello Fellow Stressers!!

Expansion stress range in the fatigue analysis will use the Fatigue Curve in ASME (which is given in Stress amplitude) to be multiplied by 2. Essentially, the alternating stress being 1/2 of expansion stress range.

But how can we take into account the mean stress & loading ratio because normally, the loading ratio will not be zero at some point in the cycle life. Say the tie-in temperature is 70 deg. F and design temperature of 170 deg. F and minimum ambient temperature of 27 deg. F. So approximately the loading ratio (ratio of 2nd load to 1st load) will be (27-70/170-70) equals -0.43 and there will be some shutdowns at varying ambient temperature, which will give different loading ratio also at some point in the cycle life.

Would it be prudent to use the ASME Fatigue Curve (Stress Amplitude) to compare directly to stress range in lieu of a more detailed fatigue evaluation (considering mean stress effects & loading ratio) or this is already accounted in the fatigue curve itself.

Your expert advise is highly appreciated.

Warm Regards,