Fatigue Crack Growth Process

As mentioned above, the agent which causes fatigue is crack growth and propagation through a material.

The process of fatigue can be considered to consist of five steps;

1. Dormant. The structure is devoid of cracking.
2. Initiation / Nucleation. This consists of new cracks forming.
3. Short or microcrack growth. Microcrack growth is the term given to the growth phase during which the crack is microstructurally small.
4. (Large) crack propagation. This is the phase of crack growth which has been extensively modelled in the literature.
5. Failure. This is when the structure finally fails. The crack grows very quickly in this phase and so it is relatively short.

Much work has previously been done on modelling crack growth behaviour, the great majority of this having been done for long cracks. The general cause of failure is a single crack growing exponentially. However, a substantial portion of the total time to failure can be spent in the short crack phase of development, and this needs to be taken into account in any estimation of reliability.

It is with this in mind that a study of microcrack propagation was undertaken. A Bayesian framework provides many practical advantages for this investigation. No previous Bayesian modelling of this problem has appeared in the literature.