Numerical investigation of the failure mechanism of concrete specimens under tri-axial dynamic loads

Abstract

The dynamic properties of concrete-like materials under true tri-axial loads were seldom studied because of the lack of proper testing facilities for conducting multi-axial impact tests. Although a three-dimensional Split Hopkinson pressure bar (3D-SHPB) device has developed in Tianjin University which could provide synchronized tri-axial dynamic loads with the same amplitude, conducting such tests is expensive and time consuming. As a supplement to physical tests, high fidelity numerical modelling is becoming more and more common to study the material and structural performances under different loading conditions. In this study, a mesoscale concrete model with the consideration of ITZ (interfacial transition zone) is developed and verified by the testing data of the 3D-SHPB tests. The dynamic properties of concrete material under different true tri-axial loading paths which the 3D-SHPB testing system could not provide are numerically investigated using the verified model. The results show that the strain rate effect on concrete compressive strength becomes less significant under tri-axial loading than that under uni-axial loading owing to the tri-axial loads acting on the specimen which limits the distribution and propagation of cracks. The change of the failure modes with strain rate under tri-axial loading is also less prominent compared to that under uni-axial loading. Based on the numerical results, an empirical relation is proposed to modify the DIF obtained from uni-axial impact loads to that under tri-axial dynamic loads for concrete material. The proposed empirical formulas can be used to more accurately model the dynamic strength increment of concrete material under tri-axial compressive loads.