Fracture Mechanics of PGX Graphite

Abstract

Fracture mechanics tests were performed on grade PGX graphite. A compact tension (CT) specimen configuration that yields consistent values of the opening mode critical stress-intensity factor KIc was designed. For the calculation of the fracture toughness and crack growth rate the concept of the effective crack length is used. It corresponds to the crack length of a machined notched specimen with the same compliance. Fracture toughness testing was performed in two environments, air and helium, all at room temperature. The critical stress-intensity factor, KIc, is calculated based on the maximum load and the effective crack length. With grain KIc was found to be 0.91 and 1.04 MPa · m1/2 with the applied stress field perpendicular to and parallel to the grain direction, respectively. The in-helium values are about 10 percent higher than the in-air values. The fatigue crack growth test was performed in air only. A “break-in” period was observed for the machined notch to develop into a naturally occurring crack path. Half of the fatigue life was spent in this period. The average KI at the end of this period is about the same as obtained in the fracture toughness test. This is followed by a stable crack growth period. In the log-log plot of the crack growth rate versus the stress-intensity factor a very steep slope is found. The onset of the rapid crack propagation starts at approximately 90 percent of the fatigue life. The corresponding KI values are 0.97 and 1.12 MPa · m1/2 for stresses applied perpendicular to and parallel to the grain direction respectively. They are about 8 percent higher than the KIc values.