Local instability in fracture of granite and observation of crack growth by means of ink penetration test

Local instability of an initially smooth bar made of a quasi-brittle material which is subjected to a uniaxial tensile test under the condition of displacement control is discussed, where the local instability means unstable increase in the width of the fracture zone in which damage localizes. The condition for the local instability is given in the tension-softening curve which is the relationship between the stress and the width of the fracture zone. Next, the results of ink penetration test were presented in which the test was carried out by using compact tension specimens made of granite to observe main crack growth. The boundary loading level was found, below which the region colored by ink near the initial notch tip spread not all points through the specimen thickness but above which the ink penetrated through the thickness. It is shown that the boundary loading level corresponds to a point on the tension-softening curve at which the stress is not zero. This result is discussed in connection with the local instability.