Stress analysis is carried out for a double cantilever beam (DCB) specimen of unidirectional carbon-fiber-reinforced plastic (CFRP) by means of finite element method (FEM). Three models are used in the analysis; (1) composite model I in which the specimen is composed of elastic fibers and elastic matrix, (2) composite model II in which the specimen is composed of elastic fibers and elastic-perfectly plastic matrix, and (3) homogeneous model in which the specimen is homogeneously orthotropic and its elastic constants macroscopically coincides with those of the composite model I. The distribution of normal stress, σy, ahead of the delamination crack tip in the composite model I can be divided into three zones. In the first zone (i.e., the nearest zone from the crack tip), the stress distribution is represented by a stress intensity factor, Kcom, evaluated by the energy release rate and elastic constants of the matrix. In the second zone, the stress distribution is undulated. The stress distribution in the third zone is represented by a stress intensity factor, Khomo, in the homogeneous model. The analysis of the composite model II shows that the stress distribution in the third zone is hardly affected by plastic deformation at the crack tip. Since the length of this zone in crack propagation direction is much larger than those of the other two zones, Khomo could characterize the crack propagation. However, when the composite is composed of very brittle matrix, Kcom could be applied in stead of Khomo.
