Influence of the Specimen Geometry on R-Curve: Numerical Investigations

Abstract

During fracture test of quasibrittle materials such as concrete, rocks, or wood, a fracture process zone (FPZ) is observed ahead of the crack tip. Various toughening mechanism take place in this FPZ such as microcracking, crack branching or crack bridging. If Linear Elastic Fracture Mechanics (LEFM) cannot be directly applied to such quasibrittle fractures, an adaptation of LEFM, which consists in considering an equivalent linear elastic problem, provides useful approximations of fracture properties [1],[2],[3]. Within the framework of ‘equivalent LEFM’, the increase of the specimen compliance due to the FPZ development is attributed to the propagation of an elastically equivalent crack. On this basis, the fracture behaviour of quasibrittle materials is characterised by a more or less pronounced rising resistance curve, briefly called R-curve [4]. For instance, R-curve obtained from a Norway spruce (Picea Abies L.) Double Cantilever Beam (DCB) and two kinds of Tapered DCB (TDCB) specimens extracted from [3] are plotted in Fig. 1. Such rising R-curves are common among materials that exhibit toughening mechanism. In fracture perpendicular to grain of wood, it has been shown that the main toughening mechanism is crack bridging [5],[6]. After a characteristic equivalent crack length a c, the resistance to crack growth becomes independent of the equivalent crack length defining a plateau value of the resistance to crack growth also called critical resistance and noted here as G RC.