Strength Of Quasi-brittle Materials Research Articles

Laboratory investigation of scale effects in bursting failure of quasi-brittle materials

Understanding the scale effect in the behavior of solid materials is important when small-scale models are used to predict the behavior of real structures. Tremendous efforts have been devoted to investigating the scale effect in the strength of quasi-brittle materials. However, limited studies have focused on the whether there is a scale effect in bursting failure, which is a typical failure pattern of quasi-brittle structures. Here, we conduct a series of unconfined compression tests on coal samples using a specialized designed loading machine to produce bursting failure in the laboratory. We use cubic coal samples with a large range of sizes from 50 to 300 mm to investigate the scale effect in bursting failure of quasi-brittle materials. We use acoustic emission technology to monitor microcreaking within the samples and a high-speed camera to capture the bursting failure process. We demonstrate that there is a strong scale effect in bursting failure of quasi-brittle materials in terms of mechanical properties including compressive strength and deformability, strain energy density, bursting severity, and precursor time. We found that ejection of localized pieces from the skin of the structure is a precursor of catastrophic bursting failure of quasi-brittle materials. We conclude that the relationship between the precursor time (T) and structure size (M) of bursting failure is logarithmic and can be described as logT = a + bM where a and b are constants. This relationship provides a guideline when models obtained from laboratory tests are used to predict strength and behavior at the intermediate (mines and rock slopes) and large scales (earthquakes).

A generalized weakest-link model for size effect on strength of quasi-brittle materials

The work contributes to the statistical approach to unitedly characterize the random variation and size effect of brittle fracture strength of materials. Current practices rely on the conventional Weibull statistics and more recently on the weakest-link statistical model conforming to the uniform spatial distribution of microcracks. Based on the understanding that the weakest-link postulate is built on the mutual independence of microcracks and is not necessarily bounded to the uniform spatial distribution of microcracks, this work develops a generic weakest-link statistical formulation pertaining to a power-law spatial distribution of mutually non-interactive microcracks to synchronize the random variation and size effect of brittle fracture strength. The formulation encompasses both Weibull statistics and the uniform spatial distribution law-based weakest-link model as its subordinate members. A resultant master curve behavior between a compound parameter and the nominal fracture strength is identified for size scaling of strength and is validated by the strength data of a wide spectrum of quasi-brittle materials including wood, concrete, coal, gamma titanium aluminum alloy, nuclear-grade graphite and aluminum foam on geometrically self-similar specimens under same loading conditions.

Analysis of size effect on strength of quasi-brittle materials using integral-type nonlocal models

The influence of the averaging operator of nonlocal continuum damage models near specimen boundaries on the size effect on strength of quasibrittle materials is investigated. Two phenomenological approaches, namely standard rescaling and distance-based models, are considered. The numerical results are compared to data from three-point bending tests of notched and unnotched beams recently reported in the literature. It is shown that both approaches can reproduce the experiments well for one type of geometry with one set of input parameters. However, only the distance-based model provides a good agreement for both unnotched and notched beams with the same set of parameters.

Asymptotic Analysis of Boundary-Induced Size Dependence of Fracture Properties

The recently-developed boundary effect concept and associated asymptotic model are used to explain the size effect phenomena in fracture of quasi-brittle materials. It is demonstrated that the size dependence of the fracture toughness and strength of quasi-brittle materials is indeed due to the influences of specimen boundaries on the failure mode and therefore, on the strength of the specimen. To verify the boundary effect concept, fracture tests on a high strength concrete reported by Karihaloo et al are analysed and predicted using the asymptotic model. The results show that the predictions of the asymptotic boundary effect model agree very well with those experimental results.

Failure analysis of quasi-brittle materials involving multiple mechanisms on fractured surfaces

We present a variational formulation for the quasi-static boundary value problem of a structure with quasi-brittle materials, involving (i) unknown states of contact, (ii) deformation-dependent frictional forces, (iii) crack opening and closing with cohesive traction, and (iv) configuration change due to the initiation and the evolution of cracks, and propose a new finite cover method (FCM) capable of reflecting those multiple mechanisms in the failure analysis. The cover-division strategy is also introduced to judge the generation of cracks, and to locate and orient them within the framework of the FCM. A relevant numerical algorithm is designed to be consistent with the mathematical representation of the multiple mechanisms. Several numerical examples are presented to validate the proposed method and to demonstrate its promise and potential for evaluating the ultimate strength of quasi-brittle materials. Copyright © 2006 John Wiley & Sons, Ltd.