In this research, mixed mode crack growth behaviour of Dehbid marble is assessed by means of the cracked beam specimen under asymmetric four-point bend (AFPB) loading. The specimen is a suitable configuration for testing rocks and geo-materials. The stress intensity factors are also calculated for the AFPB specimens of different crack lengths. It is shown that the obtained mixed mode fracture data can be predicted well by means of the available fracture criteria. The well known maximum tangential stress criterion gives better.
Cracks, flaws, natural fractures and inherent discontinuities are frequently found in rock materials and structures. In many practical applications, such as tunnelling, mining, excavation process, blasting, drilling the gas and oil wells and stability analysis of rock slopes, the evaluation of crack growth is an important task for rock mechanics engineers. While, most of the previous rock fracture studies have concentrated only on crack opening mode (tensile fracturing), the cracked rock structures in practice, are usually subjected to complex loading and hence the fracture of rock masses may occur under a combination of tension-shear (mixed mode I/II) loading. For example, cracks and natural fractures surrounding the mines or tunnel walls are usually located in arbitrary directions with respect to the far field applied loads. Consequently, these cracks may grow catastrophically under the activation of both crack opening and crack sliding modes of deformation. Therefore, it is important to investigate mode I, mode II and mixed mode fracture in rock structures and geo-materials. Based on the principals of fracture mechanics, fracture toughness which defines the resistance and the load bearing capacity of cracked materials is the most important parameter for characterizing the fracture process. There are some theoretical and experimental methods for evaluating the fracture behaviour under mixed mode I/II loading conditions. Fracture toughness of cracked materials is generally investigated by means of suitable laboratory test specimens. These specimens often are in the form of disc, plate and beam shapes having either edge or internal cracks and loaded by various types of fixtures [1–10]. Some of the mixed mode fracture configurations are shown in Fig. 1. The edge cracked rectangular beam subjected to asymmetric four-point bend (AFPB) loading is one of the favourite test configurations frequently used by researchers investigating mixed mode fracture in rock materials [e.g. 2, 3]. The simple geometry and loading set up, the convenient pre-cracking of specimen and the ability of introducing full mode mixities (from pure mode I to pure mode II) are among the advantages of AFPB specimen. Additionally, the specimen is loaded by a compressive type of load which is more suitable for testing brittle and quasi brittle materials like rocks. In this paper, a combined numerical and experimental study using the AFPB specimen is presented for mixed mode loading of an Iranian marble (Dehbid marble).