This paper identifies two ways to extract the energy (or power) flowing intoa crack tip during propagation based on the power balance of areas enclosed bya stationary contour and a comoving contour. It is very interesting to find acontradiction that two corresponding energy release rates (ERRs), asurface-forming ERR and a local ERR, are different when stress singularityexists at a crack tip. Besides a rigorous mathematical interpretation, wededuce that the stress singularity leads to an accompanying kinetic energy atthe crack tip. The local ERR represents the driving force to overcome thesurface energy and the accompanying kinetic energy, while the surface-formingERR represents the driving force to overcome the surface energy only. Theiradvantages and disadvantages are discussed. We recommend using thesurface-forming ERR based fracture criterion for a crack propagation inelastic-plastic materials, since it has a wide applicability and conciseformulae which are easy to compute among all energy based criteria.
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