For commonly used aluminum alloy materials, the crack propagation life and crack nucleation (initiation) life account for about 30% ~ 70% of the whole fatigue life respectively. However, for the structure with casting defects, due to the influence of stress concentration, under the action of fatigue load, casting cracks rapidly nucleate and expand at the sensitive parts of casting defects. When fatigue cracks originate at the casting defects and the size of casting defects is large, casting defects quickly form a certain length and a certain number of cracks. And the nucleation life of casting cracks only accounts for a small part of the total fatigue life, which can be ignored. The fatigue life is mainly the life of casting crack propagation.
After the casting crack nucleates, the casting crack propagation quickly enters the long crack propagation stage. The driving force of the casting crack propagation can be described by the casting crack propagation rate (DA / DN). For the long crack propagation, the linear elastic fracture mechanics method is usually used to establish a relationship with the stress intensity factor K. there is a famous Paris erdogon formula:
Where: A is the casting crack length; N is the number of cycles; C and m are material constants.