An experimental investigation of the characteristics of and conditions for brittle fracture in two-dimensional soil cutting

Although the occurrence of different soil failure mechanisms during soil cutting operations is well attested to in the literature, not all have been successfully characterised and modelled. Such quantitative and qualitative models are essential for improving the design, selection and use of soil cutting implements, in different field soils under different field soil conditions. This paper describes an experimental investigation of the failure characteristics of, and conditions for brittle fracture in two-dimensional soil cutting. Comprehensive tests were carried out on sandy loam, clay loam and cemented sand soils using a glass-sided box apparatus and five model plane blades of rake angles 25°, 40°, 55°, 70° and 90°, respectively. Cutting forces were measured and soil deformation patterns were studied using a bead tracer technique. The failure characteristics of brittle fracture are shown to be quite different from those of shear failure. Whilst shear failure is characterised by extensive shear distortion, compaction and the regular formation of distinct slip planes, brittle fracture is characterised by crack propagation and negligible deformation within separated soil clods. This indicates the possibility of using fracture mechanics methods to model the mechanism of brittle fracture. The results further show that transitions between these two modes of failure do occur and are governed by certain soil and implement factors, namely blade rake angle, soil strength and soil–blade interface condition. For example, for three different levels of soil strength of the sandy loam at 176.0 g kg−1 soil moisture content, the mean transition rake angle from brittle fracture to shear failure increased from 32.5° in S1 (c=11.20 kN m−2,φ=29°) to 47.5° in S2 (c=16.00 kN m−2,φ=35°) and to 62.5° in S3 (c=23.93 kN m−2,φ=49°). These factors should provide a basis for the reliable prediction of the failure type, and hence the quality of soil tilth expected in two-dimensional soil cutting operations.