Study on the Safety Evaluation for the Micro-cracks in Structures

J-integral is the most important parameter to evaluate the safety of structures with cracks. When the structure's crack length is on the order of the materials' inner characteristic length parameter, e.g., micro-cracks in rocks or metal structures are on the order of the structures inner grains' size, J-integral based on the conventional fracture mechanics is not effective because size effects will dominate in such small scale. An updated safety assessment parameter J-integral based on the strain gradient theory is studied. Results showed that this parameter is still irrelative to the integral paths and conservative. For the infinite structure with semi-infinite cracks, the results are the same as the classical fracture mechanics predicts. However, for the finite size structure with micro-cracks, size effects become significant and the J-integral is strongly influenced by the structures inner characteristic length parameter. The stronger is the size effect, the smaller is the J-integral.