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cumulative fatigue
3

cumulative fatigue

cumulative fatigue

(OP)
Hi,

I am in the lengthy process of carrying-out the fatigue analysis of a transportable pressure vessel. I am using PD 5500:2003 as my guide. Have no problems so far in calculating the fatigue life associatiated with individual stress ranges.  However, I dont understand the need for check (C-6), ie that sum of n/N 's are <= 1. I guess I am struggling to understand the need to combine the effects of unrelated fatigue lives to provide a cumulative value.  As it stands, my figures comply, but would like to understand the need for the check. Hope someone can illuminate me !    

RE: cumulative fatigue

I am not familiar with PD5500, but the check of cumulative life is a basic requirement for any fatigue analysis.

In the older simpler fatigue desing codes, one might have over a 50 year period 100 cold startups with each consuming 1/2000 life fraction, 2500 hot restarts with each consuming 1/10,000 life fraction. The cumulative fatigue life fraction consumed from predictable transients would then be
100/2000 + 2500/10,000 = 0.29   .  If the value had turned out to be greater than 1.0, then the component would not survive the nominal 50 yr life.

Newer codes use a more sophisticated means of predicting fatigue life consumption- using the "rainflow cycle counting algorithm" to combine successive strain -time histograms - this newer procedure allows one to discard or ignore many of the smaller alternating strain ranges.

RE: cumulative fatigue

2
This is called Miner's rule or Palmgren-Miner linear damage hypothesis.  The idea, as Dave pointed out is that each cycle uses up a certain fraction of the materials fatigue life.  

Note: even with rainflow counting Miner's rule is still used to accumulate fatigue damage due to different types of cycles.

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