An upper bound on strain rate for wedge type fracture in nickel during creep
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1. INTRODUCTION
THE distinction between "wedge" type and "r" type of intergranular fracture during creep has been well recognized in the literature. Grant I and coworkers studied wedge type fracture in considerable detail and have proposed a physical mechanism in which incompatability of sliding from adjacent grains at a triple junction node leads to the opening up of a wedge crack} There have also been attempts to model wedge cracking on the basis of a sliding mechanism.3,4 Typically, wedge cracks appear at high strain-rates and intermediate temperatures. As the strain rate is lowered, or the temperature is raised, there is a transition to "r" type of cavitation failure. Sliding is unquestionably important in wedge cracking, (as seen in a classical picture from Ref. 5 which is reproduced in Fig. 1); therefore, in a uniaxial test, the wedge cracks usually form adjacent to boundaries that are aligned for maximum shear, since it is these boundaries that are likely to suffer the maximum amount of sliding. In "r" cavitation, cavities form exclusively at boundaries that are aligned normal to the tensile axis; in this instance there is little metallographic or theoretical6 evidence that sliding is a vital part of the cavity nucleation and growth mechanism. The main distinguishing feature of a wedge crack, therefore, is the large sliding offset which provides the "crack opening displacement" for the propogation of the wedge crack, as illustrated schematically in Fig. 2. The actual mechanism of crack propagation is probably similar to the phenomenon of creep crack growth where it is believed that the crack is propelled forward by the nucleation and growth of cavities* ahead of the crack tip. 7,8 * Most commonly, cavities form at second phase particles in the grain boundaries although cavitation in "clean" boundaries, at structural discontinuities such as ledges and dislocation-grain boundary interactions, cannot be ruled out.
Although it is true that wedge cracking usually prevails at high strain-rates, we thought that there must come a strain rate, above which wedge cracking would cease because the sliding component of the total strain would decrease beyond a certain critical strain rate. This would occur because sliding is a time and temperature dependent phenomenon, whereas deformation of ductile crystals is always possible at all temperatures through a variety of dislocation mechanisms. Therefore, at intermediate strain-rates, when boundary sliding can keep up with the applied strain-rate, there will be considerable sliding; whereas at the very high strainrates, the boundaries will remain essentially frozen. The transition from sliding to no-sliding behavior has been considered by Crossman and Ashby." How broad the transition is depends to a large extent on the difference in the strain-rate sensitivity of pure sliding and pure crystal deformation. The greater this difference, the sharper the transition. In this paper we show that the upper bound strainrate, i.e., the strain rate above which wedge cracking is not poss
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