ABSTRACT
Abstract The tensile/flexural response of quasi-brittle materials, such as concrete, rocks, and most ceramics is characterized by moderate strain hardening before, and significant tension softening after, the attainment of their ultimate tensile/flexural strength. These non-linear phenomena which are a result of the growth of existing flaws (microcracks and microvoids), and of the nucleation of fresh flaws, enhance the fracture toughness of the quasi-brittle materials. Based on a micromechanical model this paper estimates the contributions of microcracks and microvoids to the toughening of these materials and shows that the dominant contribution is made by the microvoids. The theoretical results are shown to compare favourably with the available test results for concretes. Keywords: Quasi-brittle Materials, Concrete, Rocks, Fracture Toughness, Tension Softening, Microvoids, Microcracks, Toughening
1 Introduction
The last decade or so has improved our understanding of the role of flaws in the tensile response not only of metallic materials (e.g. Tvergaard, 1988) but also of materials, such as concrete, rocks, and most ceramics that have traditionally been regarded as brittle. It is now generally known that concrete (Cornelissen et al, 1986), rocks (Labuz et al, 1985), and most ceramics exhibit moderate strain hard ening prior to the attainment of their ultimate tensile/flexural capacity (region AB in Fig. 1), reminiscent of the response of high strength metallic materials. Beyond the peak capacity, these materials are characterized by an increase in deformation with decreasing tensile carrying capacity, i.e. by tension softening (region BCD in Fig. 1). They may therefore be called quasi-brittle materials. This paper will attempt to quantify the contributions of the pre-peak nonlinearity and the postpeak tension softening to the fracture toughness of quasi-brittle materials. It will identify the roles of microcracks and microvoids in this toughening process and show that the dominant contribution to toughening is made by the microvoids.
