Level excursion analysis of probabilistic quasibrittle fracture

It has widely been acknowledged that no structures can be designed to be risk free, and therefore reliability analysis plays an essential role in the design of engineering structures. The recent focus has been placed on structures made of brittle heterogenous (a.k.a. quasibrittle) materials, such as ceramics, composites, concrete, rock, cold asphalt mixture, and many more at the microscale. This paper presents a level excursion model for the analysis of probabilistic failure of quasibrittle structures, in which the failure statistics is calculated as a first passage probability. The model captures both the spatial randomness of local material resistance and the random stress field induced by microstructures (e.g. randomly distributed flaws). The model represents a generalization of the classical weakest-link model at the continuum limit and it recovers the classical Weibull distribution as an asymptotic distribution function. The paper discusses two applications of the model. The model is first applied to the strength distribution of polycrystalline silicon (poly-Si) MEMS specimens. It is shown that the model agrees well with the experimentally measured strength distributions of poly-Si MEMS specimens of different sizes. The model predicts a complete size effect curve of the mean structural strength transitioning from a vanishing size effect at the small-size limit to the classical Weibull size effect at the large-size limit. The second application is concerned with the left tail of strength distribution of quasibrittle structures. By taking into account both random stress and strength fields, the model is used to investigate the origin of the power-law tail distribution of structural strength.