Brittle Fracture Initiation Characteristics under Bi-axial Loading

Abstract

In this paper, brittle fracture initiation characteristics of plate with an inclined crack which is subjected to bi-axial tensile load is investigated by using cruciform specimens made of polymethylmethacrylate (PMMA) which is perfectly brittle material, and mild steel which is accompanied with small scale yielding. The brittle fracture tests of the cruciform specimen under proportional loading ratio of 1/ 2, 1/1 and 0/1, and the non-cruciform or straight edge specimen under uni-axial loading for comparison are conducted.The test results on direction of fracture initiation and load at fracture are analyzed by applying the following concepts : (1) maximum tangential stress, σθ, max (2) minimum strain energy density factor, Smin (3) maximum decreasing rate of total potential energy, Gθ, max.The test results can be explained fairly in terms of σθ, max in which a certain ambiguity remains. The experimental values except the case of direction of fracture initiation for load ratio 1/2 agree with the calculated ones by Smin concept. However, the physical meaning of Smin is not understandable. Gθ, max concept is based on the Griffith-Orowan energy condition. Practically, Gθ, max is evaluated by finite element method instead of analytical method. Therefore, the experimental results can be explained most clearly by applying Gθ, max concept.Test results lead to the following conclusions :(1) Gθ, max concept is the most suitable one in discussing the brittle fracture behaviors under mixed mode condition. The mixed mode means the combination of tensile and in-plane shear ones.(2) In case of load ratio 1/2, as the inclined angle of the notch to the direction of the main load, β, decreases, the load at fracture increases at maximum up to 2. 4 times as that for β=90°.(3) In case of load ratio 1/1, that is, equi-axial load, the load at fracture does not vary with β, and the direction of fracture initiation coincides with the line of notch.(4) In case of load ratio 0/1, that is, uni-axial load, the load at fracture increases as β decreases. However, the direction of fracture initiation is almost normal to the load irrespective of β.