Ductile Fracture Toughness Determined on an Energy Basis Using Polycarbonate.

Abstract

The quasi-static crack propagation method to measure fracture toughness has been investigated. This method has long been known to be valid when measuring the toughness of brittle materials. It is recognized as being basically different from other methods, e.g., K(,Ic), G(,Ic), J Integral, etc., because these methods apply to the onset of crack propagation whereas the investigated method applies to both initiation and propagation of a crack running under quasi-static conditions. This thesis reports the extension of the applicability of the quasi-static propagation method to a ductile fracture where displacement is not fully recoverable. It was found that by connecting the "pop-in" points of several specimens on a load-displacement diagram, each specimen having a different initial crack length, plane-strain fracture toughness can be found with specimens whose thicknesses are smaller than 2.5 (K(,Ic)/(sigma)(,y))('2), as recommended by ASTM Designation #E399-74. An important finding was in separating the energy expended between the plane-strain and plane-stress portions of the fractured surface, and the contributions of those two toughnesses to the total toughness was found.