Application of asymmetric semi-circular bend test for determining mixed mode I + II fracture toughness of compacted soil material

Abstract

Soil is a common material for constructing the sub-grade layer in multi-layer pavement systems, buried pipelines and embankment dams. Failure and fracture of this material may result in undesired deformation and bad effect on the long-term performance of such structures. So far a number of testing methods and data are available for tensile type fracture (or mode I fracture) of soil materials. However, soil materials can experience mixed mode tensile-shear deformations in real situations due to external applied loads. Hence, a suitable testing method called asymmetric semi-circular bend (ASCB) specimen was proposed and utilized in this paper for characterizing the mixed mode tensile-shear cracking behavior of compacted soil materials. Full mode mixities ranging from pure mode I to pure mode II were tested on a clay soil material using the ASCB specimen. Depending on the mode mixity, the soil fracture toughness value was varied between 0.025 and 0.035 MPa.m0.5. The lowest fracture toughness was obtained at mixed mode I/II with nearly the same contribution of tensile and shear deformations. In this loading situation the effective fracture toughness (Keff) was approximately 30% less than KIc and KIIc (i.e. pure modes I and II fracture toughness values, respectively). The fracture toughness values under pure mode I and pure mode II were almost equal. The maximum tangential stress criterion was also able to predict well the experimental mixed mode I/II soil fracture toughness results.