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Brittle solid under compression. Part I: Gradient mechanisms of microcracking

1997; Elsevier BV; Volume: 34; Issue: 20 Linguagem: Inglês

10.1016/s0020-7683(96)00177-1

1879-2146

I. Blechman,

Microstructure and mechanical properties

The venerable problem of the origins of cracking and failure of a brittle heterogeneous solid (heterogen) under compression is analyzed here from a new point of view, with the cause of its microcracking and atrophy (degeneration) under load, found, first of all, in the differences in Poisson's ratio of its components. Four fundamental phenomena, found in experiments, are the basis of the new approach to the behavior and failure of a heterogen under compression. (1) The non-linear part of the ascending branch of SSc is due to formation and accumulation of stable microcracks. (2) The intrinsic elastic modulus remains constant up to the peak point. (3) Concrete and rock fail in splitting. (4) The pattern of SSc is the same for different types of concrete and rock under different types of loading. It is found that some kinds of gradient mechanisms can induce local transverse strains of tension and cause microcracking in a heterogen under compression. The first creates local strain gradients among the components due to difference in their Poisson extension, when the components with lower Poisson's ratio are tensioned in the lateral direction. There is also a mechanism which creates an internal thrust due to gradient in the elastic moduli of the component. It is shown that, in a brittle solid built from randomly oriented crystals, a population of laterally tensioned crystals, called "acrons", are created due to gradients in Poisson's ratio of a single crystal along its three axes. The models of gradient strain in the acrons are given, including the equation of critical strains. The problem of crystals acting as "pistons" due to a process of sliding is also discussed. The gradient models explain the appearance of microcracks and their stochasticity and why, instead of growing into macrocracks they are stable, in good accordance with a vast number of experiments. Gradient mechanisms, especially that of Poisson, are universal, descriptive and based on measurable parameters. They suffice to exhaust the bearing capacity of a heterogen under increasing compression without recourse to shear stresses. They affect every heterogen under compression: rock materials, concrete, ceramics and do not need initial microcracking to initiate and realize the process of atrophy (degradation) of the brittle solid.