EFFECT OF LOADING RATE ON TRANSGRANULAR-INTERGRANULAR FRACTURE IN CHARCOAL GRAY GRANITE

An inverse relation was found between rate of loading and transgranular-intergranular fracture in Charcoal gray granite discs loaded to failure at four different loading rates (Lr's) by the point load test. Transgranular-intergranular fracture is described by means of distance increments within mineral grains and along grain boundaries that are measured along a fracture trace in a thin-sectioned disc. Measurements were made with a mechanical stage and petrographic microscope and were used to compute a transgranular-to-intergranular ratio (T/I) for each disc. A weighted T/I (R) was obtained as a product of T/I and the percentage (F) of each mineral along a fracture trace. Computer facilities using regression analysis fitted a logarithmic equation to the data with a very high correlation coefficient. The equation, Rc = 9·93 - 0·57 In Lr (Lr in lb/min), was shown to be statistically valid within the limits of the analytical data. Although the equation is strictly empirical, it probably expresses the dependence of strain energy dissipation on the degree to which favorably oriented defects dilate under different loading rates. © 1969.