Fatigue, fracture and size effect in particulate and fabric composites: constitutive modeling, theory and experiments.

摘要

Quasibrittle materials, such as particulate and fabric composites, are heterogeneous materials with brittle constituents, widely used in various engineering structures. The monotonic and fatigue failure of these materials is typically characterized by a large fracture process zone that is not negligible to the structure dimensions. Further, quasibrittle fatigue damage often occurs in combination with tri-axial softening damage whose localization is governed by a finite material characteristic length. This engenders a strong and unique size effect in the strength and fatigue behavior. Objective mathematical description of these phenomena necessitates accounting for the fracture energy of the material, or alternatively, introduction of the characteristic length scale in the constitutive models. In the first part of the thesis, a general and realistic model is proposed, that can capture these effects for fatigue in concrete, the most widely used particulate composite. A new fatigue damage measure and a law for hysteresis and fatigue degradation is introduced, to realize accurate predictions of the quasibrittle fatigue behavior. The size effect in fatigue crack growth is analyzed numerically and experimentally, where a discrepancy in the fatigue transitional size and the physical cyclic fracture process zone size is revealed. A new size-adjusted Paris law involving two length scales is proposed as a possible solution. In the next part, a microplane damage model for woven composites is proposed with application to car-crashworthiness. The model incorporates a hierarchical multi-scale scheme to predict the orthotropic elastic constants for various woven composites. The quasi-brittle character is captured by considering the fracture energy of the material, and by coupling the model with the crack band theory. The model is shown to realistically predict not only the elastic behavior but also the structural size effect, and the axial crushing of composite crush cans. Following these developments, some aspects of the stochastic and probabilistic nature of quasibrittle fracture are analyzed. First, an application of the microplane model M7 is presented where the fracturing and size effect in bi-axial flexural failures of concrete disks is analyzed using 3D stochastic FEA. Presented next is a formulation for the probabilistic distributions of the residual strength of quasibrittle structures under a sustained load, based on nano-scale crack growth arguments.