Pure Mode I Fracture Toughness Determination in Rocks Using a Pseudo-Compact Tension (pCT) Test Approach

Mode I fracture toughness (K_IC) quantifies the ability of a material to withstand crack initiation and propagation due to tensile loads. The International Society for Rock Mechanics (ISRM) has proposed four suggested methods for determining K_IC. However, these methods present some drawbacks such as insufficient post-peak control, complex sample preparation and considerable material requirements. Here we present an alternative approach, called the pseudo-compact tension (pCT) method, to measure K_IC in rocks using disc-shaped specimens loaded in pure tension. The pCT specimen has favourable features such as a simple geometry, small sample volume and minimal machining requirement. The tensile load is transmitted to the specimen through two high-strength, high-stiffness steel jaws that fit into a U-shaped groove cut in the specimen. An additional thin straight notch is introduced to act as a stress concentrator. The crack propagates from the notch tip along the ligament plane, splitting the specimen into two halves. The effects of specimen size and notch length on K_IC are determined by testing specimens 100, 50 and 38 mm in diameter with different notch length ratios (0.1 ≤ a/b ≤ 0.4). Tests were performed under ambient conditions and a slow loading rate (0.1 mm/min). Our results show that the pCT method is convenient for the assessment of K_IC of both fragile and ductile rocks. The method offers good control even beyond the maximum load, making it possible to study the post-peak behaviour of the material.