Failure and fracture analysis of brittle and ductile composites using the computer simulation method

Unidirectional and cross-ply glass fiber-reinforced epoxy (GFRE) composites with approximately 50% reinforcement were investigated to predict their unnotched strengths. Relations were also developed to predict the notched strengths and the critical stress intensity factors for off-axis composites. The agreement between the predicted and the experimental results was very good. It was found that there was a critical region beyond which the singular, continuum solution was not valid. For brittle GFRE composites, they were reasonably constant. The typical magnitude of the critical region was 10^-2 in. for these composites.

Considering the importance of the changing order of the singularity of the stresses at the crack tip, a gradually graded finite element method was developed to study the failure of ductile composites. It was shown that the simple method took a reasonable amount of computer time to solve elasto-plastic problems using conventional elements. The Tsai-Wu failure theory of brittle composites was used to predict the initial yielding of ductile composites. An anisotropic plasticity analysis was done using isotropic-hardening and associated flow rules. The material was assumed to be work-hardening.

A fracture specimen was monotonically loaded until failure was i reached in at least one of its elements. The corresponding fracture parameters were assumed to be the critical fracture parameters for crack initiation.

A relationship was developed between the J-integral and the crack-opening displacement (COD) using small-scale deformation in a strain-hardening material. Using this relation and the failure loads predicted from the computer simulation, it was concluded that the critical J, being dependent on fiber orientations (unidirectional) and crack lengths (cross-plied), was not a material parameter for B-A1 composites. Conversely, the value of cod at failure was reasonably constant for all the fiber orientations (unidirectional) and crack lengths (cross-plied) investigated; thus, it could be considered to be a material parameter.