IMPACT DAMAGE IN A COMPOSITE-MATERIAL

Damage development in plates of a glass fiber-reinforced polypropylene is recorded, using short pulse photography, during instrumented falling weight tests using the excess energy approach. It is seen that the damage is progressive throughout the test but its initiation cannot be detected either by freeze-frame photography or visual observation. Specimens are therefore subjected to low-energy impact followed by microscopic observation of the tensile face. It is found that the initial damage mechanism is cracking of the matrix at the fiber-matrix interface, the crack propagating along the fiber. The finite element method is used to estimate the stress distributions at damage initiation in 4-ply and 8-ply samples cut from unidirectional and cross-ply plaques. The computed results indicate that initial damage occurs when the transverse tensile stress reaches a critical value.