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Opis: A system of edge cracks was applied to polished (010) surfaces of K-rich gem-quality alkali feldspar by diffusion-mediated cation exchange between oriented feldspar plates and a Na-rich NaCl–KCl salt melt. The cation exchange produced a Na-rich layer at and beneath the specimen surface, and the associated strongly anisotropic lattice contraction lead to a tensile stress state at the specimen surface, which induced fracturing. Cation exchange along the newly formed crack flanks produced Na-enriched diffusion halos around the cracks, and the associated lattice contraction and tensile stress state caused continuous crack growth. The cracks nucleated with non-uniform spacing on the sample surface and quickly attained nearly uniform spacing below the surface by systematic turning along their early propagation paths. In places, conspicuous wavy cracks oscillating several times before attaining their final position between the neighboring cracks were produced. It is shown that the evolution of irregularly spaced towards regularly spaced cracks including the systematic turning and wavyness along the early propagation paths maximizes the rate of free energy dissipation in every evolutionary stage of the system. Maximization of the dissipation rate is suggested as a criterion for selection of the most probable evolution path for a system undergoing chemically induced diffusion mediated fracturing in an anisotropic homogeneous brittle material.
Ključne besede: chemically induced fracturing, alkali feldspar, crack spacing, wavy cracks, dissipation rate, thermodynamic extremal principle
Objavljeno v DKUM: 17.07.2023; Ogledov: 299; Prenosov: 31
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Opis: This paper presents a case study, examining the influence of a sharp bimaterial interface on the effective crack driving force in a fracture mechanics specimen. The inhomogeneity of the elastic modulus in linear elasticand non-hardening and hardening elastic-plastic bimaterials is considered. The interface is perpendicular to the crack plane. The material properties and the distance between the crack tip and the interface are systematically varied. The effect of the material inhomogeneity is captured in form of a quantity called "material inhomogeneity term",▫$C_inh$▫. This term can be evaluated either by a simple post-processing procedure, following a conventional finite element stress analysis, or by computing the J-integral along a contour around the interface, ▫$J_int$▫. The effective crack driving force,▫$J_tip$▫, can be determined as the sum of ▫$C_inh$▫ and the nominally applied far-field crack driving force, ▫$J_far$▫. The results show that ▫$C_inh$▫ can be accurately determined by both methods even in cases where ▫$J_tip$▫-values are inaccurate. When a crack approaches a stiff/compliant interface,▫$C_inh$▫ is positive and ▫$J_tip$▫ becomes larger than ▫$J-far$▫. A compliant/stiff transition leads to a negative ▫$C_inh$▫, and ▫J_tip$▫ becomes smaller than ▫$J_far$▫. The material inhomogeneity term, ▫$C_inh$▫, can have the same order of magnitude as ▫$J_far$▫. Based on the numerical results, the dependencies of ▫$C_inh$▫ on the material parameters and the geometry are derived. Simple expressions are obtained to estimate ▫$C_inh$▫.
Ključne besede: mechanics of structures, fracture toughness, inhomogeneous materials, J-integral, crack driving force, interface, material force
Objavljeno v DKUM: 01.06.2012; Ogledov: 1720; Prenosov: 34
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