Characterisation and Modelling of Brittle Fracture in Two-dimensional Soil Cutting

Abstract Brittle fracture by the propagation of tensile cracks accounts for soil failure during many soil cutting and tilth forming operations. However, this mechanism is little understood and cannot be analysed using the classical Mohr–Coulomb failure theory in which soil is characterised by its cohesive strength c and angle of internal shearing resistance φ. Based on previous experimental observations of its deformation characteristics, a theoretical analysis was developed for the mechanism of brittle fracture using linear elastic fracture mechanics methods. The soil was characterised by a fracture strength property namely the critical stress intensity factor KIc. Using five model plane blades of rake angles 25, 40, 55, 70 and 90°, respectively, soil cutting experiments were carried out in the laboratory on sandy loam and clay loam soils prepared to different soil strength and soil–blade interface conditions. During these experiments, clod sizes and cutting force components were measured. The results showed that in brittle fracture, the cutting force is cyclic in nature and is essentially concentrated in the region of the blade tip. For experiments where brittle fracture occurred, clod size estimates based on critical crack lengths predicted using the present brittle fracture theory were in closer agreement with measured experimental rupture distances than rupture distances predicted using existing shear failure theory. The theoretical analysis, which comprises inter alia two regimes of crack behaviour termed the short and long crack models, covers the complete process of crack initiation and subsequent clod formation. The analysis further shows that there is a transition from short to long crack behaviour and the cutting force attains a maximum during this transition. However, the inability to deduce short crack behaviour under mixed mode loading limits further extraction of this maximum cutting force using the present theoretical analysis.