| The quasi-brittle behavior of cement-based materials has been traced to the existence of a "fracture process zone" made up of microcracks, particle interlocks, and other phenomena which lead to an increase in toughness over other brittle solids. Since microstructural properties ultimately govern the bulk mechanical properties of these materials, any attempt to gain a better understanding of microstructural behavior will lead to a better understanding of overall behavior. It is toward this goal that quantitative acoustic emission techniques were applied to basic problems of microfracture in cement-based materials.; An acoustic emission is a sudden release of strain energy in a material under load. This strain energy propagates to the surface where it can be detected by sensitive ultrasonic transducers. Acoustic emissions in cement-based materials result from microcracks and other dynamic phenomena in the fracture process zone. The goals of this research program were to characterize microcracking in various cement-based materials, to track the evolution of damage in those materials, and to examine the relationships to overall mechanical behavior. Advanced analysis techniques were developed to process the large amounts of data involved with acoustic emission testing. In addition, the wave propagation characteristics of these materials were examined for the effects on measured acoustic emission signals.; Characterization of the microcracks showed a dependence on the degree of inhomogeniety in the material. Fine-grained materials showed different microfracture characteristics than coarse-grained materials. Microcracks were characterized according to their fracture mode. The fine-grained materials tests showed primarily mixed mode microfracture, whereas the coarse-grained materials showed primarily mode II (shear) microfracture. It is shown that there exists a relationship between the microcrack characteristics established through quantitative acoustic emission analysis and the fracture toughness of the material. Nonlinearities in the load-displacement relationships are also traced to microcracking phenomena. |
