ABSTRACT
Abstract For brittle or quasi-brittle materials such as ceramics, concrete and rocks, linear elastic fracture mechanics is generally inadequate for fracture toughness and failure characterization of laboratory-size specimens due to the formation of large process zones (as opposed to the small scale yielding in ductile metallic materials). Methods such as the nonlinear tension-softening testing has been successful in obtaining valid, size-independent fracture parameters needed for the design of full-size structures involving the use of these materials. The National Science Foundation has been supporting this and other basic research projects in the size-scale effects and failure mechanisms of brittle and quasi-brittle materials. For composite structures reasonable theory of size/scale effects on the failure mechanism is not available yet. Composite manufacturers have experienced decreased strength of fibers when the size of the structural fiber bundle increases. Such experiences have been kept as the company proprieties and not much information has been exchanged. In this paper, various failure theories are reviewed. An equation expressing the rate of decrease in tensile strength of glass fibers used for filament wound tubes is derived, providing estimate for the decreased strength of glass fibers as the size of the tube increases. Keywords: Size effects, brittle materials, fracture mechanics, composite
structures, failure mechanism, strength criteria. 1 Introduction Size effects influence the material properties of quasi-brittle materials (e.g. concrete and rocks). In case of rock masses, the larger the volume the greater is the probability of larger flaws [1,2,3]. In a NSF/CNRS workshop [4] the strain localization and size effect due to cracking and damage were investigated. More
recently, the mechanics of materials were studied at various scales ranging from atomic scale to microns to large macro or structural behavior [5]. A NASA/IMM (NSF) workshop on scaling effects in composite materials and structures [6] was held in Hampton, Virginia.
