TWI Industrial Member Report Summary 889/2007
By Y H Zhang
Background
Fatigue design of welded joints is based on fatigue data obtained under constant amplitude (CA) loading, used in conjunction with Miner's rule to estimate the damage introduced by cycles of various magnitudes in the service stress history. Although Miner's rule was often found to be either correct or conservative, there is extensive evidence suggesting that it can be unsafe (ie Σ(n/N) <1.0 at failure). This was often found when a loading spectrum with high mean stresses and short block length was applied. There is a lack of fundamental understanding of the mechanism behind the differences.
There are also doubts about the method of treating stresses below the CA fatigue limit (CAFL). The most widely used assumption is that their damaging effect can be represented by the CA S-N curve extrapolated beyond the CAFL, widely assumed to correspond to N=107 cycles, at a shallower slope, typically m = 5 instead of 3. However, there is evidence suggesting that in fact they are more damaging than this, ie a slope change should be introduced at an endurance beyond 107 cycles, or even not at all.
Objectives
- Investigate the effect of loading spectra with different mean stresses on the validity of Miner's rule.
- Investigate the effect of stresses below the CAFL on the fatigue performance of two types of weld joint.
- Understand the underlining mechanisms for any deficiency in Miner's rule by fracture mechanics fatigue crack growth analysis and residual stress measurements