Relation between crack density and acoustic nonlinearity in thermally damaged sandstone

This paper investigates thermal damage (microcracking) in sandstone using ultrasonic methods. Previous investigations have demonstrated that microcracks and lightly compressed interfaces in solids can generate strong higher harmonic signals in a propagating time-harmonic wave. Distributed cracks in rock will generate such nonlinearity and thus measurements of the acoustic nonlinearity can be a useful experimental means to quantify microcracking damage. In fact, experimental results have shown that the acoustic nonlinearity increases with the progress of microcracking in different rocks and concrete. This research attempts to experimentally determine the relation between the crack density and the acoustic nonlinearity in thermally damaged sandstone. The dynamic Young's modulus and longitudinal wave velocity are measured in sandstone specimens that are exposed to high temperatures, from which the crack density is calculated using a micromechanics model. A recently proposed standing wave based ultrasonic technique is employed to measure the acoustic nonlinearity parameters in these specimens. It is shown that the relation between the crack density and the acoustic nonlinearity are approximately linear. This result indicates that the acoustic nonlinearity measurement can be useful for evaluating thermal damage in rock materials.