A versatile device for measuring the mode I fracture toughness with application to estimate the impact of water weakening in chalks

Abstract

A newly built versatile device for mode I fracture toughness measurement is presented. To test this apparatus, measurements have been conducted on two crystalline rocks, the Aue Granite and the Äspö Diorite, and two chalks from the Paris basin, Obourg and Ciply chalks. The fracture toughness KIC can be measured with two different testing procedures, the Semi-Circular Bend (SCB) and the Straight Edge Cracked Round Bar Bend (SECRBB) methods, both known for the easiness of the notched sample preparation. For the SCB tests, ultrasonic sensors were mounted at the sample surface to monitor changes in P-wave velocity and record acoustic emission activity. Our results are in reasonably good agreement with published data on the same rocks. The SECRBB test provides values of the fracture toughness 37% higher compared to the SCB test for the Obourg chalk. This discrepancy may be explained by a sample size effect. The fracture toughness of water saturated chalks is strongly reduced compared to that of the dry chalks by almost 50%. This shows that fracture toughness is a valuable parameter to assess the importance of water weakening in porous rocks.