13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- (CSS-44100). Compressive load was controlled by displacement mode at a speed of 0.06mm/min. During the whole loading stage, a CCD camera (1004×1003 pixels, 8bits) under the illumination of two light sources was installed perpendicularly to the samples so as to capture the images that were simultaneously stored on a computer disc for subsequent processing. The length-pixel ratio of the images is 61.7μm/pixel and recording frequency was 1frame/s. It is worth mentioning that only the images in the area as indicated by the white rectangle in Fig.1(a) will be used to obtain the displacement fields. The experimental setup is illustrated in Fig.1(b). (a) (b) Fig.1 (a) sandstone sample and (b) experimental setup In the present work, the image was processed by our in-house DIC software. A relatively small calculation subset size (21×21 pixels) and step length (10 pixels) were chosen to increase the spatial resolution of the measurement. The displacement measurement accuracy in our test was at least 1μm based on our calibration. 3. Results and discussions The load-displacement (at the contact interface) recorded by the electric universal machine is present in Fig. 2. It can be seen that except at the initial stage, the load is almost proportional to the displacement even till failure (with macroscopic crack). The sample fails at a load level of about 68kN via axial splitting. Fig.2 Load-displacement curve during the uniaxial compression of a sandstone sample
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