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. (C) 2017 Elsevier Ltd. All rights reserved.