Fatigue crack growth curve estimation method, estimation program, and estimation device

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 setting an initial value of the crack length to “0”, and setting an initial value of a crack mode index to “1”, which represents a shear mode crack and an opening mode crack by “1” or “0” and
• then calculating, when a cyclic load is applied on a stress concentrated area, an inherent displacement which forms a tensile residual deformation layer at a maximum load from a tip position of a tensile plastic zone which is calculated from vertical equivalent distributed stress applied on an arbitrary x axis at the maximum load, vertical equivalent distributed stress in the x axis caused by a static load, equivalent distributed stress corresponding to residual stress applied on the x axis, and a yield stress and a plastic constraint factor under the cyclic load;
• a second step of calculating the inherent displacement at a minimum load from the vertical equivalent distributed stress applied on the x axis in the case of applying the minimum load and a unit external load of the cyclic load, the vertical equivalent distributed stress in the x axis caused by the static load, the equivalent distributed stress corresponding to the residual stress applied on the x axis, and the inherent displacement at the maximum load, and then calculating a tip position of a compressive plastic zone at the minimum load from the inherent displacement at the maximum load and n inherent displacement at the minimum load, and calculating the inherent displacement in a compressive yield zone from the inherent displacement at the minimum load and the yield stress and the plastic constraint factor under the cyclic load;
• 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 in order 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 from the variation of the inherent displacement 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 from the variation of the inherent displacement, 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 setting the crack mode index to “0”, and
• calculating the inherent displacement taken in a crack incremental region and the yield stress and the plastic constraint factor under the cyclic load;
• and
• a fifth step of calculating a yield stress under the next cyclic load at the next maximum load from the vertical equivalent distributed stress applied on the arbitrary x axis at the next maximum load, the vertical equivalent distributed stress in the x axis caused by the static load, the equivalent distributed stress corresponding to the residual stress applied on the x axis, and the plastic constraint factor, and returning the process to the first step.

A computer readable medium having stored thereon an estimation program of a fatigue crack growth curve for making a computer, which sets an initial crack length to “0” and sets an initial value of a crack mode index to “1” which represents a shear mode crack and an opening mode crack by “1” or “0”, perform a method comprising:

• a first step of calculating, when a cyclic load is applied on a stress concentrated area, an inherent displacement which forms a tensile residual deformation layer at a maximum load from a tip position of a tensile plastic zone which is calculated from vertical equivalent distributed stress applied on an arbitrary x axis at the maximum load, vertical equivalent distributed stress in the x axis caused by a static load, equivalent distributed stress corresponding to residual stress applied on axis, and a yield stress and a plastic constraint factor under the cyclic load, and storing the tip position of the tensile plastic zone and the inherent displacement in a memory;
• a second step of calculating a inherent displacement at a minimum load from the vertical equivalent distributed stress applied on the x axis in the case of applying the minimum load and a unit external load of the cyclic load, the vertical equivalent distributed stress in the x axis caused by the static load, the equivalent distributed stress corresponding to the residual stress applied on the x axis, and the inherent displacement at the maximum load read from the memory, and then calculating a tip position of a compressive plastic zone at the minimum load from the inherent displacement at the maximum load read from the memory, and the thickness distribution of the tensile residual deformation layer at the minimum load, and calculating the inherent displacement in a compressive yield zone from the inherent displacement at the minimum load and the yield stress and the plastic constraint factor under the cyclic load, and storing the inherent displacement and the tip position of the compressive plastic zone in the memory;
• 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 which are read from the memory, and calculating an increment of the crack from the fatigue damage accumulated region in order to add the increment of the crack to the crack length and store a result in the memory;
• 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 from the variation of the inherent displacement read from the memory, in order to calculate cumulative plastic strain from the increment of plastic strain and store it in the memory, 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 from the variation of the inherent displacement read from the memory, and calculating the cumulative plastic strain from the increment of plastic strain to store it in the memory, 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 setting the crack mode index to “0” to store it in the memory, and
• calculating the inherent displacement taken in a crack incremental region and the yield stress and the plastic constraint factor under the cyclic load to store it in the memory;
• and
• a fifth step of calculating a yield stress under the next cyclic load at the next maximum load from the vertical equivalent distributed stress applied on the arbitrary x axis at the next maximum load, the vertical equivalent distributed stress in the x axis caused by the static load, the equivalent distributed stress corresponding to the residual stress applied on the x axis, the plastic constraint factor, and the crack mode index read from the memory, and storing it in the memory, and then returning the process to the first step.

An estimation device of a fatigue crack growth curve comprising:

• memory means for storing an initial value of a crack length set to “0” and an initial value of a crack mode index to “1”, which represents a shear mode crack and an opening mode crack by “1” or “0”;
• first operation means for calculating, when a cyclic load is applied on a stress concentrated area, an inherent displacement which forms a tensile residual deformation layer at a maximum load from a tip position of a tensile plastic zone which is calculated from vertical equivalent distributed stress applied on an arbitrary x axis at the maximum load, vertical equivalent distributed stress in the x axis caused by a static load, equivalent distributed stress corresponding to residual stress applied on the x axis, and a yield stress and a plastic constraint factor under the cyclic load, and storing the tip position of the tensile plastic zone and the inherent displacement in the memory means;
• second operation means for calculating a inherent displacement at a minimum load from the vertical equivalent distributed stress applied on the x axis in the case of applying the minimum load and a unit external load of the cyclic load, the vertical equivalent distributed stress in the x axis caused by the static load, the equivalent distributed stress corresponding to the residual stress applied on the x axis, and the inherent displacement at the maximum load read from the memory means, and then calculating a tip position of a compressive plastic zone at the minimum load from the inherent displacement at the maximum load read from the memory means and the inherent displacement at the minimum load, and calculating the inherent displacement in a compressive yield zone from the inherent displacement at the minimum load and the yield stress and the plastic constraint factor under the cyclic load, and storing the inherent displacement and the tip position of the compressive plastic zone in the memory means;
• third operation means for 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 which are read from the memory means, and calculating an increment of the crack from the fatigue damage accumulated region in order to add the increment of the crack to the crack length and store a result in the memory means;
• fourth operation means for 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 from the variation of the inherent displacement read from the memory means in order to calculate cumulative plastic strain from the increment of plastic strain and store it in the memory means, 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 from the variation of the inherent displacement read from the memory means, and calculating the cumulative plastic strain from the increment of plastic strain to store it in the memory means, 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 setting the crack mode index to “0” to store it in the memory means, and
• calculating the inherent displacement taken in a crack incremental region and the yield stress and the plastic constraint factor under the cyclic load to store it in the memory means;
• and
• fifth operation means for calculating a yield stress under the next cyclic load at the next maximum load from the vertical equivalent distributed stress applied on the arbitrary x axis at the next maximum load, the vertical equivalent distributed stress in the x axis caused by the static load, the equivalent distributed stress corresponding to the residual stress applied on the x axis, the plastic constraint factor, and the crack mode index read from the memory means, and storing it in the memory means, and then returning the process to the first operation means.