TENSILE RESPONSE OF QUASI-BRITTLE MATERIALS

Concrete, rocks, non-phase-transforming ceramics and sintered metals exhibit non-linear behaviour in tension/flexure. The response prior to the attainment of the material's ultimate strength is similar to that of any moderately strain hardening material. However the response past the ultimate strength is characterized by increased deformation with decreasing tensile/flexural stress transfer capability (i.e., by tension softening) as a result of localization of damage (strain localization). Such materials may therefore be regarded as quasi-brittle. The non-linear response is a result of the growth of existing flaws (microcracks and microvoids) and of the formation of fresh flaws. The present paper will attempt to give a general micromechanical description of the various stages in the tensile response of quasi-brittle materials. It will take a damage mechanics approach to describing the pre-peak strain hardening response, followed by a combination of damage and fracture mechanics approaches to describing the post-peak tension softening. The reasons for the enhanced fracture toughness of quasi-brittle materials will be forwarded and the contribution of the pre-peak nonlinearity will be estimated. The fracture mechanics approach will also reveal the cause of the size effect observed in quasi-brittle materials, whereby large samples from such materials appear to behave in a brittle manner, but small samples in a ductile manner.