Abstract
A procedure has been developed to measure the fracture resistances of nominally brittle materials (monolithics and composites) using chevron-notched, three-point bend specimens at room temperature and elevated temperatures. The chevron-notched geometry promotes stable crack growth during quasistatic fracture tests. For each test a continuous record of load versus crack mouth opening displacement (CMOD) is obtained. The fracture toughness, work-of-fracture and the classical crack growth resistance curves are then determined from the stable crack growth loading curves and previously established relations between the CMOD compliance, the effective crack length, and the load point displacement (LPD).
Various materials have been tested which include monolithic ceramics: alpha silicon carbide, reaction bonded silicon nitride, and magnesium aluminate spinel and composites: titanium diboride particle/silicon carbide matrix, silicon carbide whisker/aluminium oxide matrix, and carbon fibre/carbon matrix. The R-curves for the various materials show flat, rising linear, and rising non-linear behavior indicative of linear elastic fracture behavior, constant but increasing fracture mechanisms, and constantly developing fracture mechanisms, respectively.
