The effective surface energy of brittle materials

Abstract

The effective surface energy of four brittle materials, alumina, poly(methylmethacrylate), glass, and graphite, is calculated from load/deflection curves of notched bars deformed in three-point bending. Two of the methods, which are commonly used in fracture mechanics studies,viz the modified Griffith treatment and the compliance analysis method, are concerned with the effective surface energy at the initiation of fracture,γ _I . The third method, the work of fracture test, is concerned with the mean effective surface energy over the whole fracture process,γ _F . The two estimates ofγ _I give consistent values, and there is no systematic variation ofγ _I with notch depth. Values ofγ _F decrease with increasing notch depth as the fracture process becomes more controlled. For aluminaγ _I ∼γ _F . For PMMA and glassγ _I > γγ _F because of a multiplicity of crack sources during fracture initiation. For graphiteγ _I <γ _F because of subsidiary cracking as fracture proceeds.