A Study of Fracture Processes in Concrete Using Laser Holography
Final rept. Aug 1988-Sep 1989
NORTHWESTERN UNIV EVANSTON IL DEPT OF CIVIL ENGINEERING
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Conventional linear elastic fracture mechanics cannot explain fracture in concrete and mortar. Unlike traditional LEFM materials which show a constant fracture toughness, concrete and mortar toughen show more resistance to fracture as cracks propagate until an instability results and the structure fails. This increase in toughness is thought to occur because of many possible complex mechanisms which take place during fracture in concrete and mortar. These mechanisms include formation of a zone of microcracking around the crack tip crack bridging, where aggregate particles span the crack and act as ligaments to hold the crack together and crack deflection, where the crack changes direction. All of these mechanisms require additional energy input into the system and may explain the apparent toughening of the material. It is generally assumed that the closing pressure is a function of the crack opening displacement w. In this study, crack profiles as well as the full field microscopic displacements on the fracture process zone were accurately measured by laser holographic interferometry. The first two part of this report discuss mode I fracture of concrete which was studied using center notched plate specimens. In the last part of this report, a study of mixed mode crack propagation under compression is described.
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