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CHALLENGING PROBLEMS IN FAILURE ANALYSIS OF DUAL-PHASE MATERIALS: CYCLIC MICRO-PLASTICITY AND SMALL FATIGUE CRACK TIP BEHAVIOR

Jinghong Fan(Department of Engineering Mechanics;Chongqing University Chongqing;Jinghong.

   In this paper, both analytical and numerical methodologies are developed to investigate cyclic micro-plasticity and small fatigue crack tip behavior in microstructures associated with heterogeneity and anisotropy. The improved self-consistent scheme proposed by Fan (1999) for evaluating across-scale transition of strain and stress is exemplified through micro/macro combined analysis of layered microstructure of pearlitic steel. The thin hard cementite layers are found to sustain very high stress due to load transferring from soft ferrite layers through intensive cyclic slip. It is found that the micromechanics-determined maximum applied stress, at which the calculated maximum local micro-plastic strain is zero, approaches closely from the lower bound fatigue limit to the experimentally-determined proof fatigue strength of a cast aluminum alloy. This result is encouraging due to its potential significant value in fatigue science and practice. Computational micromechanics analysis is also applied to investigate cyclic crack tip behavior for microstructurally small cracks in the Al-Si alloy. A relatively large cyclic plastic strain range is found in the crack-tip element when a micro-crack approaches the second phase. This result is inconsistent with the predictions of the existing models of microstructural fracture mechanics (MFM). It is found that in addition to the second phase s slip blockage mechanism, the geometrically necessary dislocation takes a role. This and other results in this work offer additional insight into the anomalous behavior of small fatigue crack.……