An experimental investigation of crack-path directional stability

Using the criterion that a crack will extend perpendicular to the maximum circumferential stress,σ _θ, we show that the directional stability of crack growth is governed by the location of microcrack initiation ahead of the crack tip. At distances greater than a geometrical radiusr _o, the maximum value ofσ _θ deviates from the position of symmetry. Thus, if we assume that the physical processes involved in fracture lead to crack initiation at a distancer _c ahead of the crack tip, the criterion for directional stability isr _o>r _c. Experimental and theoretical values ofr _o verify that, asr _o becomes small, the crack's directional stability deteriorates. Observing that a lengthwise compressive stress increasesr _o, a center-cracked specimen was developed which allows the application of controlled lengthwise compression independently of the opening-mode load. A detailed photoelastic analysis of the specimen has provided the value ofr _o as a function of the crack length. The value ofr _o is then compared with the expected microcrack initiation distances in ductile fracture. By applying sufficient lengthwise compression, we are able to make the crack grow straight and obtain numerous data points from this specimen which would otherwise be directionally unstable. The results indicate that, as the total lengthwise tensile stress at the crack tip increases, the fracture toughness also increases. Using this information we can then adjustK _Ic for zero lengthwise loading and obtain a geometry independent fracture toughness.