Detection of crack onset in double cleavage drilled specimens of plaster under compression by digital image correlation

Abstract : Geomaterials such as rocks and concrete are brittle or quasi-brittle materials. Tensile tests carried out to observe the initial phases of crack nucleation are dif ficult to achieve because of the unstable nature of the tests. Instead, compression tests on drilled specimens offer a greater stability. When subjected to a compressive loading, two opposite cracks take place and grow from the cavity, parallel to the load. This crack nucleation is experimentally studied in rectangular drilled specimens of plaster with a centred cylindrical hole which size is assumed to be small with respect to the dimensions of the specimen. The results are compared to a theoretical prediction of the crack onset derived from the coupled criterion of Leguillon. Due to the difficulty of determining the crack initiation directly by the naked eye, 2D Digital Image Correlation is used. The nucleation event is determined by analysing the history of deformations at some points where the crack is expected to start. The predictions are proving to be in good agreement with the experimental results.