Engineering structures that consist of brittle heterogeneous, or quasibrittle, materials such as concrete, fiber composites, ceramics, rocks, etc., exhibit a size-dependent failure behavior due to the fact that the size of fracture process zone (FPZ) is not negligible compared to structure size. This dissertation deals with both the probabilistic and energetic size effects caused by finiteness of FPZ of these materials.;The first part of this dissertation focuses on the size effects on the strength and lifetime statistics of quasibrittle structures failing at the initiation of a macrocrack from one representative volume element (RVE). This class of structures can be statistically modeled as a chain of RVEs, and the cumulative distribution functions (cdf's) of structural strength and lifetime are related to the strength and lifetime cdf's of one RVE. The strength distribution of one RVE is derived from atomistic fracture mechanics and multiscale statistical analysis. The lifetime distribution of one RVE under constant load is related to the strength distribution through the kinetics of subcritical crack growth. The power law for creep crack growth, previously considered empirical, is physically justified by atomistic fracture mechanics with certain plausible hypotheses about multiscale bridging. This theory indicates marked size effects on the types of cdf's of strength and lifetime, as well as on the mean structural strength and lifetime. The framework of this theory is also applied to the lifetime statistics of high--k gate dielectrics, based on a mathematical analogy between the failure of quasibrittle structures and the breakdown of gate dielectrics.;The second part of this dissertation, which investigates the size effect from a different viewpoint, deals with the energetic (nonstatistical) scaling of strength of metal-composite hybrid joints, which are a critical component of modern designs for large ships and aircraft. The strength of the joint is determined by the energy criterion for the macrocrack initiation at the bimaterial corner, from which the large-size asymptote of the size effect law is obtained. A general approximate size effect law, spanning all sizes, is further derived via asymptotic matching. The proposed size effect law is verified by size effect tests on geometrically similar specimens.
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