Dynamics analysis and damage constitute equation of triaxial coal mass under impact load

To study the dynamics characteristics of triaxial coal mass under impact load,the triaxial separated hopkin- son pressure bar (SHPB) experiment system was built,and the dynamic impact tests were carried out under the ran- dom combination of axial static load,confining pressure and impact load. The dynamics characteristics at these condi- tions were studied. The experimental results showed that there was no compaction stage in the dynamic stress-strain curves of triaxial coal under impact load. The pre-axial static load would cause the closure of protogenetic fractures, which indicated the feature of complete elastic body at the initial loading stage. When the stress reached 60% -85% of the peak strength,the stress-strain curves presented a “leap” phenomenon,which might be related to the role of char- coal in coal crystal micro-fracture. When the stress exceeded the dynamic compressed strength of coal samples,it de- creased and coal samples were destroyed. Under the impact load,both the dynamic strength and failure strain of triaxial coal increased linearly with the strain rate,which indicated the obvious strain rate effects. And the strain rate effects made the influence of axial static load,confining pressure and impact load to the dynamic strength and failure strain of coal was of comparability. Based on the strength theory of rock mechanics and the statistical damage theory,the dynam- ic damage constitutive model of triaxial coal under impact load was established,in which the factors of axial static load,confining pressure and impact load were taken into account. It was determined that the influences of these three factors to dynamics characteristics. The axial static load would damage coal and lead to the decrease in dynamic strength,while confining pressure and impact load were helpful to improve the dynamic strength. The characteristics re- flected by the theoretical model were consistent with the experimental results. Using the built theoretical model and ex- perimental stress-strain data,the theoretical stress-strain curves were fitted,which were identical with the experimental results,and the higher the strain rate,the higher the fitting degree.