Isotropic Brittle Materials Research Articles

Failure Criterion of Concrete Subjected to Multi-Axial Compression

Based on the postulate of stability of materials failure criteria of macroscopically homogeneous isotropic brittle materials under combined stresses are represented by convex surfaces in the principal stress space.The failure surfaces of both high- and low-strength concretes were compared in the compression rarge as the maximum first stress invariant up to 5.2 times of the uniaxial compressive strength.The surfaces of the both concretes are almost identical when they are represented in the non-dimensional principal stress space, though their absolute values are quite different.The surfaces are convex and have the space diagonal as a threefold rotation axis.Right sections of the surfaces are slightly bulged from equilateral triangles and expand almost isotropically with a decreasing ratio as the hydrostatic pressure increases.

Photoelastic investigation of brittle crack growth within a field of uniaxial compression

Griffith's theory of brittle fracture postulates that natural solid materials contain flaws that provide local sources of tensile-stress concentration, even when applied load is compressive. Fracture is predicted to ensue when the maximum tensile-stress concentration equals bond strength. This theory has proven to be fundamental in tensile fracture of isotropic brittle materials, and it appears to be fundamental in brittle shear fracture of rocks. Griffith assumed that the longitudinal cross-section of an elongate flaw could be represented mathematically by an excentric ellipse for analysis by the theory of elasticity. Photoelastic experiments support this assumption, provided the ellipse has the same length as the flaw and the same radius of curvature at points of maximum tension as those at the flaw boundary. The theory is extended mathematically to predict initial direction of propagation of such flaws or Griffith cracks. Unlike pure tensile fracture, an isolated critical Griffith crack ceases crack growth rapidly under applied compression by attaining stable configuration. Hence, interaction between several cracks apparently is required to form the shear-fracture surface. Photoelastic experiments indicate that coalescence of neighbouring cracks may not occur for crack separation greater than a few crack lengths, unless applied stress is appreciably higher than that needed to initiate growth, and that certain en echelon arrays of Griffith cracks are more favourable for coalescence than others. Brace (1964) reports that straight segments of grain boundaries, in crystalline rocks tested, behave like Griffith cracks; those in en echelon array are apparently activated preferentially to form the shear-fracture surface.