We Claim:
[Claim 1] A fracture determination method for determining a fracture of a metal structure, the method comprising:
a deformation analyzing step (S21-S23,S31-S33,S36,S37) of performing deformation analysis from start of deformation to end of deformation of the metal structure; and
a fracture determination step (S25,S26,S35,S39) of extracting a fracture determination target portion from a deformation state of the metal structure obtained in the deformation analyzing step, and when the extracted fracture determination target portion has returned from a plastic state to an elastic state,
given that a stress when the portion returned to the elastic state is (x, y) = (σ2, σ1) (maximum principal stress: σ1, minimum principal stress: σ2) on a (x, y) coordinate plane,
performing fracture determination of the fracture determination target portion using a re-yield stress determined by an intersection between a straight line satisfying a relation y = (σ1/σ2)x and an yield curve obtained from the plastic state of the fracture determination target portion, wherein
in the fracture determination step, there are obtained:
a coordinate point (A) of an initial plastic stress determined by an intersection between the straight line satisfying the relation y = (σ1/σ2)x and a yield curve of an initial state of the fracture determination target portion; and
a coordinate point (B) of a fracture limit stress determined by an intersection between the straight line satisfying the relation y = (σ1/σ2)x and a fracture limit stress

line of the fracture determination target portion, and
wherein a fracture risk of the fracture determination target portion is calculated using a distance from the coordinate point (A) of the initial plastic stress to the coordinate point (B) of the fracture limit stress and a distance from the coordinate point (A) of the initial plastic stress to a coordinate point (R) of the re-yield stress.
[Claim 2] The fracture determination method for determining a fracture of a metal structure according to claim 1,
wherein a fracture risk of the fracture determination target portion is calculated using a distance from an origin
(O) to the coordinate point (B) of the fracture limit stress and a distance from the origin (O) to a coordinate point (P) of a stress when the portion returned to the elastic state, and
one of the calculated fracture risks of the fracture determination target portion is selected and used.
[Claim 3] A fracture determination apparatus (10) determining a fracture of a metal structure, the apparatus comprising:
a deformation analyzing unit (4) performing deformation analysis from start of deformation to end of deformation of the metal structure; and
a fracture determination unit (9) extracting a fracture determination target portion from a deformation state of the metal structure obtained in the deformation analyzing unit (4) , and when the extracted fracture determination target portion has returned from a plastic state to an elastic state,
given that a stress when the portion returned to the elastic state is (x, y) = (σ2, σ1) (maximum principal stress:

σ1, minimum principal stress: σ2) on a (x, y) coordinate plane,
performing fracture determination of the fracture determination target portion using a re-yield stress determined by an intersection between a straight line satisfying a relation y = (σ1/σ2)x and an yield curve obtained from the plastic state of the fracture determination target portion, wherein
the fracture determination unit (9) is adapted to obtain: a coordinate point (A) of an initial plastic stress determined by an intersection between the straight line satisfying the relation y = (σ1/σ2)x and a yield curve of an initial state of the fracture determination target portion; and
a coordinate point (B) of a fracture limit stress determined by an intersection between the straight line satisfying the relation y = (σ1/σ2)x and a fracture limit stress line of the fracture determination target portion, and calculate
a fracture risk of the fracture determination target portion using a distance from the coordinate point (A) of the initial plastic stress to the coordinate point (B) of the fracture limit stress and a distance from the coordinate point (A) of the initial plastic stress to a coordinate point (R) of the re-yield stress.