ABSTRACT
ABSTRACT: The analysis and measurement of the damage variable of materials is important for research on damage mechanics. For this study, pure iron test samples were fabricated by powder metallurgy. The crack formation and propagation mechanism was investigated during a dynamic changing process under tensile load by environmental scanning electron microscopy. The results indicate that, at the initial stage of loading, micro-cracks easily appear at different positions around the prefabricated notch, connecting with surrounding micro-cracks and propagating along the grain boundaries, eventually forming a dominant crack. The dominant crack is formed at the notch and connects with neighboring micro-cracks while propagating along the grain boundaries in the middle period of loading. When the crack propagation is disrupted, new cracks form in front of the older cracks and connect with each other, again propagating along the grain boundaries. Finally the samples lose stability and fracture rapidly. The fracture surfaces do not show visible necking phenomena along the thickness direction. Most dimples are oval in shape, indicating that they are formed by normal stress. The damage evolution law of the microstructure of the pure iron samples is discussed and the evolution equation is established for future application to other materials fabricated by powder metallurgy.
