Various sample thicknesses and punch diameters were used in punch-shear tests for investigating the size effects in evaluating the shear strength of machinable ceramics. The ratio of the sample thickness to the punch diameter was defined as the relative thickness that strongly affects the measured shear strength in a certain range. It was found that there was a threshold value in the relative thicknesses (similar to0.1 for Ti3AlC2 and similar to0.13 for Ti3SiC2) above which the measured shear strength increased linearly with increasing thickness. The threshold provided a thickness-requirement for valid punch-shear tests, and stable shear strength was obtained under this requirement using 3 mm and 1.8 mm punch diameters, respectively. Experiments using various loading rates suggested that the shear strength measured by the punch-shear test was almost immune from the loading rates. The mechanism of the size effects was analysed through optical and SEM observations.
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