Patent ID: 7480573

Claim:
A method for estimating a fatigue crack growth curve from a sound area using equivalent distributed stress which reproduces a relation between a crack length and a stress intensity factor in an actual structure by external force and internal force due to residual stress into a straight crack in an infinitely wide plate, the method comprising: a first step of calculating a tip position of a tensile plastic zone and an inherent displacement which forms a tensile residual deformation layer at a maximum load during cyclic loading on a stress concentrated area; a second step of calculating the inherent displacement at a minimum load of the cyclic load, and calculating a tip position of a compressive plastic zone from the inherent displacement; a third step of calculating a fatigue damage accumulated region formed ahead of the crack from the tip position of the tensile plastic zone and the tip position of the compressive plastic zone, and calculating an increment of the crack from the fatigue damage accumulated region to add the increment of the crack to the crack length; a fourth step of setting, when the crack length from the sound area is smaller than a first grain size at a notch root, an increment of plastic strain to “0”, the plastic strain being formed by a pair of the maximum load and the minimum load in applying the cyclic load inside the grain only in the compressive plastic zone in a load cycle in which the tensile plastic zone at the maximum load is larger than the compressive plastic zone at the minimum load, or only in the tensile plastic zone in a load cycle in which the tensile plastic zone at the maximum load is smaller than the compressive plastic zone at the minimum load, then calculating an increment of plastic strain outside of the grain to calculate cumulative plastic strain from the increment of plastic strain, and, when the crack length from the sound area is equal to or larger than the first grain size at the notch root, calculating the increment of plastic strain and calculating the cumulative plastic strain from the increment of plastic strain, and, when the cumulative plastic strain inside the grain reaches a ductility limit specific to a material composing the stress concentrated area, judging the crack to have changed into an opening mode crack, and calculating the inherent displacement taken in a crack incremental region; and a fifth step of calculating a yield stress under the next cyclic load at the next maximum load, and returning the process to the first step.