Criteria for Crack Nucleation in Polycrystalline Ice

A theoretical analysis of crack nucleation in isotropic polycrystalline ice due to the elastic anisotropy of the constituent crystals has recently been presented by Shyam Sunder and Wu [1]. Subsequently, Shyam Sunder and Nanthikesan [2] have analyzed crack nucleation in polycrystalline ice that is isotropic but porous. The singularity of the stress concentrations near a grain boundary facet junction provides the mechanism for inducing microcrack precursors, if similar nuclei do not already exist. The total stress field is obtained by linearly superposing the microstructural stress field created by the elastic anisotropy mechanism on the applied stress field. Assuming plane stress conditions, the analysis of the nucleation stress is based on a solution to the problem of an extending precursor in a combined stress field including the effects of Coulombic frictional resistance. In the earlier papers, the local material resistance is characterized in terms of a critical value for the maximum principal tensile stress, MPTS, (Erdogan and Sih [3]). This paper compares the nucleation stresses for uniaxial and biaxial loading conditions obtained previously with those obtained from the use of a critical strain energy density, SED, factor (Sih [4]) to characterize the local material resistance. The results, synthesized into biaxial nucleation surfaces, are compared with the limiting tensile strain, LTS, criterion of Shyam Sunder and Ting [5]. The critical precursor orientation and the incipient growth direction for the two models are also compared.