Rock strengths under axial tension and flexure

Summary

• 1. The tensile strenghts of rocks differ from their flexural strengths; the reasons for the discrepancy include the statistical factor (which depends on the fraction by volume of the rock which undergoes the greatest tensile stress), the occurrence of plastic deformations, and the fact that the elasticity moduli of the rock are different for tension and compression.

• 2. Since the elasticity modulus of a rock is greater under compression than under tension, the maximum tensile stress under flexure is lower than the value given by the formular σ=M/W. By using formula (15) we can allow for the ratio of the moduli, E_c/E_t, when calculating σ_max.

• 3. From measurements of the deformations of the most strongly stretched and most strongly compressed filaments, we can, using formula (16), directly determine the ratio E_c/E_t.

• 4. The most convenient and reliable method of determining E_c/E_t is to measure the deformations of the edge filaments using wire gauges. It is inadvisable to determine E_c/E_t from direct experiments on axial tension and compression. In such experiments it is difficult to eliminate other factors such as the rate of loading, the temperature. the humidity etc., which can distort the true value of E_c/E_t.