The Effective Surface Energy of Brittle Materials

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, gamma(I). The third method, the work of fracture test, is concerned with the mean effective surface energy over the whole fracture process, gamma(F). The two estimates of gamma(I) give consistent values, and there is no systematic variation of gamma(I) with notch depth. Values of gamma(F) decrease with increasing notch depth as the fracture process becomes more controlled. For alumina gamma(I) similar to gamma(F). For PMMA and glass gamma(I) > gamma(F) because of a multiplicity of crack sources during fracture initiation. For graphite gamma(I) < gamma(F) because of subsidiary cracking as fracture proceeds.