Gypsum (a mix of Hydrocal B-11 and Diatomaceous Earth) is used by the MIT rock mechanics group as a model rock material. Unconfined compression tests on pre-cracked specimens with high speed camera observation showed macro-cracking processes of gypsum similar to other materials; however in contrast to the other materials, no microcrack process zone could be observed. On the other hand, environmental scanning electron microscope (ESEM) observation of gypsum showed limited micro-cracking preceding the crack initiation. This indicates that a microcrack process zone in gypsum might exist but it is not visible in high speed camera photography because of the scale. It was therefore decided to use acoustic emission (AE) to determine if such microcracks occur. To this end, prismatic specimens containing pre-existing flaws were tested under uniaxial compression, and the cracking process was monitored with both AE and high-speed camera imaging. AE results revealed that although ESEM images show the occurrence of some micro-cracking before macro-cracking, so far it could not be detected by AE.
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