Dynamic and Damage Characteristics of Mortar Composite under Impact Load

In the process of underground construction, it is common to spray concrete on the working surface, which will form a concrete-rock composite structure, to achieve the expected reinforcement effect. Compared with pure concrete or rock structures, composite structures have different mechanical properties, energy evolution and damage characteristics. Understanding the dynamic mechanics and damage characteristics of composite structures under impact loads is important to the safety of underground engineering construction. First, dynamic compression tests on different grades of mortar-sandstone composite specimens were carried out by using a Hopkinson pressure bar system to discuss the effect of impact velocities on the mechanical properties and failure modes of composite specimens. The results show that the dynamic peak stress, strain energy density and fragmentation block of the mortar-sandstone composite have an obvious strain rate effect. With an increase in impact velocity, the internal cracks of the sample do not have sufficient time to expand, and the energy absorbed by the sample leads to the continuous initiation of new cracks in the sample, resulting in an increasing damage degree of composite specimens. The increase in mortar strength can significantly increase the peak stress, improve the load-bearing capacity of the sample, and reduce the degree of sample damage. At the same impact velocity, the fragmentation degree of the mortar sample is more severe than that of sandstone, and the mortar sample is broken into powder debris, while the sandstone is broken into fragments. In addition, according to the failure properties of the composite sample, the mortar and sandstone samples are described by different damage models, and the dynamic damage constitutive model of the mortar-sandstone composite is established based on the overlay principle. The dynamic test results were fitted. The research shows that the fitting curves of the model are well fitted with the test curves and that the model accurately reflects the changes in the mechanical parameters of the composite specimens.