R‐Curve Behavior of a Polycrystalline Graphite: Microcracking and Grain Bridging in the Wake Region

Abstract

The contributions of nonlinear fracture processes both in the microcracking frontal process zone and in the following wake region and of grain bridging to crack‐growth resistance parameters are discussed in terms of the R‐curve behavior of an isotropic polycrystalline graphite. The R‐curve behavior of the graphite is characterized by rapidly increasing values at the initial stage of crack extension (Δa≤1 to 2 mm) followed by a steady‐state plateaulike region and then a distinct decrease when the primary crack tip approaches the end surface of the test specimen. Scanning electron microscopy of fracture mechanics specimens revealed a dominant role of grain bridging in the following wake regions on the rising R‐curve behavior and confirmed the significant size effect of the large‐scale microcracking process zone on the falling R‐curve behavior. The stress‐derived fracture toughness (K R ) and the energy fracture toughness (R c ) are discussed in relation to the micro‐cracking residual strain.