各種荷重速度下における岩石の力学的挙動について

抄録

The mechanical behaviors of rocks under various loading-rates which ranged from 1×10^-2kg/cm²/sec to 12×10⁵kg/cm²/sec, have been investigated in this paper. The higher rates of loading have been obtained by the use of a drop-hammer testing machine, while the lower rates of loading have been obtained by the use of a hydraulic testing machine.
In the case of the high-speed loading test, which is undertaken for the purpose of comparing it with the low-speed loading test, the shock waves should be eliminated perfectly, so as not to expose the rock specimens to impact loading. Or the rock specimens would only be definitely exposed to an elastic pressure.
From the above consideration, a new drop-hammer testing machine has been designed. It consists of an anvil, a drop-hammer and a cylindrical dynamometer. The anvil weighs about 800kg and is mounted on four dampers. The drop-hammer weighs about 40kg, having a damper in its body. On the other hand, the dynamometer is equipped with eight resistance-type wire strain gauges.
From the test, the following results were obtained, using sandstone, marble, or cementmortar.
(1) The compressive strength of rocks increases with the loading-rate, sandstone at the rate of about 1.6 in respect of the ratio of the high-speed compressive strength to that of low-speed, marble at the rate of about 1.9, and cementmortar at the rate of about 1.5. In these cases, the average loading-rates of the high-speed test are about 3.0∼12×105kg/cm²/sec, and the average loading-rates of the low-speed test are about 9.5∼10×10^-2kg/cm²/sec. Also, the values of the secant modulus of elasticity of rock increase with the loading-rate. Poisson's number of each rock decreases with the increase of the compressive load, but Poisson's number on the same compressive stress has a tendency to increase as the loading-rate.
(2) The compressive strength of rocks increases with the strain-rate; for instance, the relation between the compressive strength and the strain-rate of marble is expressed by
σ_d=730+26logv_d/v₀+2.6(logv_d/v₀)^2.12
where σ₀: strength in the low-speed loading test (kg/cm²)
σ_d strength in the test of voluntary loading speed (kg/cm²)
υ₀: strain-rate in the low-speed loading test
υ_d: strain-rate in the test of voluntary loading speed
(3) The strength and the secant modulus of elasticity of rocks in the tension test increase with the loading-rate; for instance, the ratio of the high-speed tensile strength to the low-speed one is about 2.1, and the ratio of the modulus of elasticity in the high-speed test to that in the low-speed test is about 2.5 for marble. In these tests, the average loading-rates of the high-speed test are about 2.7×10⁴kg/cm²/sec, and the average loading-rates of the low-speed test are about 9.9×10^-3 kg/cm²/sec.