抄録
Although natural cracks such as fatigue cracks are favourable as a pre-crack for fracture toughness testing of rocks, it is very difficult to measure the exact length of natural cracks because of the opaque nature of rock and the unevenness of the crack-front.
In this paper, by measuring the compliance of the crack opening displacement under bending, the effective length of fatigue cracks was estimated for two kinds of rocks; Ogino tuff and Tohoku marble, and the result was compared to the actual crack length on the side of the specimen. Furthermore, in order to know the fracture toughness of the rock containing natural cracks by using an artificial notch, the fracture toughness of rock specimens with a chevron-notch was measured and compared with those with another types of pre-cracks including fatigue crack and straight-through notch.
The main results obtained are as follows:
(1) The relationship between the observed length and the effective length of the fatigue crack depended on the texture of the rock. The effective crack length of the marble was mostly smaller than the observed one while the effective crack length of the tuff is larger than the observed one. The stable values of the fracture toughness were obtained from the effective crack length.
(2) The fracture toughness obtained from the chevron-notch was nearly equal to that from the fatigue crack using the effective crack length.
(3) The fracture toughness obtained from the natural cracks of chevron-notch and fatigue crack was considerably larger than that from the artificial straight-through notch because of the unevenness of the crack-front.
