Abstract: In the research of facture mechanics, although straight line crack is the most simple and useful crack model,One of the reason is that straight line crack can expend and changeinto bent line crack in the complex stresses condition.it is difficulty to calculate the SIF of bent crack. so it is also difficulty to research co mplex fatique extention law.
In this paper, the bent cracks were studied by the calculation of the SIF of a bent crack which contained three bent point specimen, the computations that verified by experimenting was applied to the research of complex type fatique extension law.