Effect of Crack-Parallel Stresses on Fracture of Concrete and Other Quasibrittle Materials

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This research will address prediction and control of fracture in concrete, with implications in other heterogenous materials such as rocks, fiber composites, ceramics, sea ice, rigid foams, shale, various bio-, biomimetic and printed architectured materials. Concrete fracture is a serious safety concern generally, and particularly during earthquakes. Fracture is always accompanied by cracking which, in the long term, leads to environmental degradation as well. Realistic fracture predictions based on standardized laboratory tests and computer simulations is therefore critical to strong and durable civil infrastructure. Recent research reveals that crack tips in quasibrittle materials are surrounded by a wide zone of visually undetectable microcracking damage, which controls crack growth yet is very sensitive to stresses that are parallel to the cracks. The effects of these crack-parallel stresses are currently unknown. This research will devise a new type of laboratory test, which can measure the changes of energy required for fracture growth at various crack-parallel stress levels. Testing will be conducted to better understand the effect of crack-parallel stresses on normal and high-strength concretes as well as fiber-reinforced concretes, and the results will be used to formulate a new mathematical model for quasibrittle fracture. The crack-parallel stress effects have gone unnoticed because they do not appear in the currently standardized fracture tests and are not thermodynamic variables in existing linear elastic fracture mechanics and cohesive crack models. The key idea of this research is a modification of the notched three-point bend test with four crucial features: 1) plastic support pads at notch mouth introduce constant notch-parallel compression; 2) the end supports installed with gaps engage only when the pads are yielding; 3) the test setup switches from one statically determinate configuration to another, allowing unambiguous interpretation; and 4) the size effect method, most effective for fracture energy testing, is made possible. A finite element crack band model with a tensorial damage softening law will be developed and calibrated by optimal fitting of the test results. Finally, a multiscale model that incorporates mesoscale mechanisms of frictional slip, microcrack opening, interlock and splitting causing the crack-parallel stress effects will be devised. The results are expected to transform fracture mechanics of quasibrittle materials.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.