Extended stress intensity factor model from short to long cracks

In this work, an extension of the stress-intensity factor model to long semi-elliptic shaped cracks is proposed considering the generalizing of Murakami limited to short cracks. The latter allows the calculation of the stress intensity factor by using the new surfaces of the crack created by propagation with an acceptable efficiency. The extension we propose to take into account the effect of two factors which are directly influencing the propagation behaviour of this cracks type, the premature closure, and the load ratio. The validation of the new model was carried out using the fatigue results reported in literature on thick plates by three-point bending with three load ratios. The calculation of the propagation velocity of the crack front and the number of cycles of the transition from one front to another is possible by adapting the Paris law to the new model of Delta K, that is representative of the entire crack front and not only a few points generally chosen on the front and is considered as a characteristic. This methodology can provide engineers with a simplified method to evaluate the harmfulness of this type of cracks, especially in the case of pressure vessels. It can also open new perspectives on the use of a crack surface in fatigue studies.