APPLICABILITY OF LINEAR ELASTIC FRACTURE MECHANICS FOR THE TESTING OF PLASTICS

Abstract

Linear Elastic Fracture Mechanics (LEFM) is based on the Williams-Irwin equations containing a singularity at the crack tip. To avoid the existence of infinite stresses a small plastic zone around the crack tip is assumed, the shape of which should be calculated by inserting the formula for the working stress into the Williams-Irwin equations. The choice of the type of working stress is free, usually the von Mises theory is applied producing the well-known liver-shaped plastic zone. The width of it along the crack plane is considered as an extension of the crack length. The state of stress at the crack tip in the case of plane strain is nearly hydrostatic, especially for values of the Poissons ratio approximating \nu = 0.5. As the von Mises criterion neglects the influence of the hydrostatic component of the stress state upon the elastic-plastic transition, this does not seem to be suitable if applied for investigating the fracture-mechanic properties of plastics. Two other hypotheses - which take into account the influence of the hydrostatic part of the stress state - were investigated for the determination of the size and shape of the plastic zone ahead the crack tip. The application of these for some types of plastics considered as `rigid´ revealed , that only thermosetting plastics are really suitable for strict LEFM investigation for producing the stress intensity factor as a material property.