CRACK EXTENSION IN FIBERGLASS REINFORCED PLASTICS

Abstract

Abstract : Experimental investigation on initially cracked Scotchply plates indicated that the fracture strength of this fiberglass reinforced plastics can be described by the laws of linear elastic fracture mechanics. The stress intensity factors, k sub 1c and k sub 2c, were constants for symmetric and skew- symmetric loadings. Furthermore, k sub 1 and k sub 2 were also found to be constants for various combinations of symmetric and skew-symmetric loadings. As indicated by photographs of the fractured specimens and the crack displacement time curves it was observed that although the cracks propagated along lines which were essentially straight, it is probable that the cracks made microscopic skips across the fiberglass reinforcement in the vicinity of this line in a discontinuous manner. This phenomena occurs more frequently when the specimens were subjected to shearing stress. At present, because the modes of crack extension under different type of loading are not fully understood, and also due to the mathematical difficulties involved in the calculation of maximum normal stress, maximum shear stress and maximum energy release rate, a general law of fracture applicable to all orthotropic materials cannot be formulated without further analytical and experimental investigations. An empirical equation was found which was an adequate representation for balsa wood as well as Scotch-ply and could represent a general fracture law for orthotropic materials.