Cohesionless crack at peak load in a quasi-brittle material

A three point bending fracture test was performed on a typical quasi-brittle material (Berea sandstone). The loading was continued into the post-peak region where crack growth was visible along the center line of the beam. Subsequent inspection of a portion of the specimen showed that part of the fracture offered no resistance to loading – a cohesionless crack existed. Digital image correlation was used to study the nature of the displacement discontinuity associated with the cohesionless (traction free) crack. The pattern identified in the post-peak region was used as a guide to study the displacement discontinuity at peak load. Two possibilities are offered: (1) The critical opening was developed at peak load. (2) A cohesionless crack, a few millimeters in length, existed at peak load, an observation that is not consistent with linear fracture mechanics. Some justification for cohesionless crack extension at peak load is discussed.