Loading history and structure of fracture of material

Abstract

The bearing capacity of the structured materials and features of their fracture depend on local reaction of elements of structure on loading history. The initial structure of material can be modified in the course of prefracture at the initial stage of loading such that at further deformation, including deformation with the changed ratio of loadings on axes, this induced structure determines the scenario of fracture of material and its strength. The effective strength of heterogeneous materials and scenarios of their fracture are influenced by the level of the intermediate main stress. At compression of a cracked body lack of significant reaction to stresses along the sliding plane is characteristic for initial microcracks (areas of sliding). In a porous body such effect is absent. Therefore at tests with the proportional mode of loading according to the Karman or Böker schemes the effective measured strength can differ.To estimate an influence of a type of initial structural elements – pores and microcracks - on a final phase of fracture at uniaxial compression within a plane problem, an approach based on their equivalence to action of some concentrated forces is used. If the main crack is formed by joining a system of microcracks – the sliding areas, the movement on which causes the wedging forces, then weakening of the action in the process of the main crack growth is small. In case of a porous structure an influence of pores, remote from the crack tip, on the effective stress intensity factor KI in the process of crack sizes increasing weakens much faster. Therefore in a porous body development of echelons of rather short cracks joining small number of pores (N ~ 2-3) is energetically preferable, while occurring of the main cracks is characteristic for a cracked body at a final stage of fracture at compression. For a body with microcracks in which after coalescence of cracks from two and more concentrators the stress intensity factors for the formed defect become larger than it was at the moment of microcracks coalescence, the choice of the fracture scenario – development of the main fault occurs at the initial stage of fracture.In the porous body, forming the cracks with an influence of pores, favorable conditions for initiation of fracture in the new source of fracture at the small size of cracks are only created near the crack tips. The induced structure of fracture arising at partial unloading also influences the fracture mechanism at the subsequent loading. Development of a system of the feathering of cracks in the vicinity of the main fault also belongs to such structures. The last can be used as a method for the main crack arresting.