Introduction: Size effects in materials

Abstract In material science, size effects are described as the variation of material properties as the sample size changes. The size effects include various properties such as optical and Photocatalytic properties, thermal conductivity, Young’s Modulus, material strength, diffusion processes, electrical conductivity, and magnetic properties. In this book, the size dependency of the material strength is addressed as size effects. The size effects underlying mechanisms depend on the nature of the considered material. The governing mechanisms of size effects in brittle materials, quasibrittle materials, and crystalline metals are different from each other. In the case of brittle materials, the size effects have a statistical nature. It may be considered to be originated from the random nature of the material strength that can be generally captured using Weibull statistical theory. The basic assumptions of the theory are the structure failure occurs by failing any element of the structure, and the strength of the structure elements is a random function that can be described by the Weibull distribution. The Weibull theory can successfully capture the size effects in brittle materials. In the case of quasibrittle materials, however, the nature of the size effects is deterministic and not statistical and it is due to the stress redistribution around the cracks. In the case of crystalline metallic structures, the sources of size effects are more versatile. The size effects in crystalline metallic samples can be related to some internal length scales, such as grain size, the average distance of second phase particles or precipitates, and dislocations mean free path. Finally, the external geometrical properties of the specimen such as the thin film thickness, pillar diameter, or indentation depth may lead to size effects.