Internal Morphology of Cracking of Two 3-D Pre-Existing Cross-Embedded Flaws under Uniaxial Compression

This paper studies the internal morphology of cracking of two 3-D pre-existing cross-embedded flaws in poly(methyl methacrylate) specimens under uniaxial compression. In this study, observable cracking behaviors include the crack initiation, growth, wrapping and coalescence, and the ultimate fracture of the specimen. The experimental results show six modes of crack initiation and two types of crack wrapping. The initiation of silky cracks in 3-D cracking behaviors is reported first, as well as the closed-mode wrapping of petal cracks. Substantial wrapping of petal cracks, which plays an important role in crack coalescence, appears to be the major difference between the 2-D and 3-D cracking behaviors of pre-existing flaws. Moreover, three modes of crack coalescence, which include the coalescence of petal cracks, antiwing cracks, and oblique secondary cracks, are found. Crack coalescence is predominantly created by the linkage of petal cracks, which is further classified into six categories. Note that only limited vertical growth of petal cracks is observed, and the ultimate failure of specimens is induced by the continued growth of the propagated wing crack. It was found that the ultimate fracture face of the specimens consists of continuous wing cracks and fragmented silky cracks. This paper provides the internal morphology basis for the 3-D fracture mechanism of specimens containing two 3-D pre-existing cross-embedded flaws.