A Fracture Strength Parameter for Brittle Agricultural Soils

Brittle fracture accounts for soil mechanical behaviour in different cultivation and tilth forming processes. The determination of soil fracture properties is therefore essential for quantitative analyses of these processes, which will lead to improvements in the design of tines for fracturing brittle agricultural soils. A simple fracture test, normally used for metals, has been adapted for determining the fracture strength of brittle agricultural soils. The procedure consists of subjecting single edge notch bend (SENB) specimens, each having a pre-formed crack of known depth, to a simple flexural test during which the applied load and deflection were monitored to failure. Specially designed specimen moulds were used to prepare SENB specimens of a sandy loam, a clay loam and a cemented sand soil. Fracture tests on soil SENB specimens showed that for a given soil type, soil fracture strength and brittleness increase as soil density increases. Similarly, the soil critical stress intensity factor KIcs was found to increase with increasing soil dry density. When the fracture test results were compared with previously reported results of soil cutting experiments on the same soils, it was found that for weak soils with relatively low dry densities, the value of the ratio Ws/F of the specimen self-weight to the failure load during fracture testing could be used to predict the type of failure mechanism expected during two-dimensional soil cutting with simple plane blades.