The J-integral, an extension of the J-integral by Rice, is path-independent irrespective of the constitutive relation of material and is interpreted as the energy release rate associated with extension of a crack. In the present study, the J-integral for a preloaded tensile specimen with a center crack was analyzed on the basis of the flow theory of plasticity by the finite element method. The conclusions obtained are as follows:
(1) Under monotonically increasing load, the value of J is in good agreement with that of J.
(2) If the load is reduced from its current value, J decreases almost linearly and takes a negative value owing to the compressive residual stress produced by yielding at the crack tip.
(3) In the case where the load in reloading stage is brought over its original value, the value of J agrees approximately with that of J calculated from the load versus load-point displacement curve with the simple formula by Rice, Paris and Merkle.
(4) If the specimens unloaded from various load levels are reloaded until the crack tip opening displacement (CTOD) reaches a constant value, J also becomes a constant value.