This paper presents a composite particle model with sphere clumps for modelling non-spherical particles in discrete element method (DEM) simulations. The formulation of the sphere clumps uses random selections of particle radii and positions, thus it can theoretically simulate all possible non-spherical particles encountered in engineering projects. In this study, the generation of sphere clumps in space was discussed in detail, and different particle nonsphericities were compared. This numerical model has been employed to study the shear behaviour of granular assemblies under undrained triaxial compression conditions. It is evident that the use of composite particle model in the DEM can largely increase the shear strengths of granular assemblies. For granular samples consisting of various types of sphere clumps, different mechanical responses have been observed. In particular, the sphere clumps with high non-sphericity can lead to very high peak/residual shear strength, and high material internal friction angle. The use of sphere clumps mixture can modulate the shear behaviour, with its mechanical properties being close to those of real quartz sand grains.

The cracked chevron notched Brazilian disc (CCNBD) specimen, suggested by the International Society for Rock Mechanics for testing mode I fracture toughness of rocks, usually yields rather conservative toughness measurements, and the reasons have not been fully explored. In this study, the CCNBD method is compared with the cracked chevron notched semicircular bending (CCNSCB) method in the fracture process zone (FPZ) and its influence on the fracture toughness measurement. Theoretical analysis reveals that the FPZ is longer in the CCNBD specimen than in the CCNSCB specimen using a relatively large support span, the toughness measurement using the former is affected more seriously by the presence of FPZ, and thus the CCNBD method is usually, more or less, conservative compared with the CCNSCB method. These inferences are further validated by experimental results, which indicate that the CCNBD test indeed produces much lower fracture toughness values and even the results of 75‐mm radius CCNBD specimens are still lower than those of 25‐mm radius CCNSCB specimens. Consequently, due to smaller FPZ, the CCNSCB specimen with a relatively large span is more likely to produce comparably accurate or representative toughness value, and it may be more suitable than the CCNBD specimen for the engineering applications that require more representative or less conservative fracture toughness.

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