Experimental investigation on dynamic mechanical properties and deformation failure mechanism of anchored coal samples

The dynamic mechanical properties of anchored bodies，which determine the stability of anchor bearing structures，is the scientific basis for further revealing the deformation and failure mechanism of anchor bearing structures under impact dynamic loads. In this study，the dynamic mechanical properties and deformation failure processes of three types(unanchored，end-anchored，and fully-anchored) of specimens subjected to dynamic impact loads under varying pre-tightening forces were investigated with the aid of the split Hopkinson pressure bar(SHPB) technique. Based on the test results，the dynamic stress-strain curves，dynamic strain field evolution characteristics，and anchor rod axial force characteristics of anchored coal specimens were yielded. Furthermore，the interactions among anchor rods，anchoring agents，and pre-tightening forces during impact failure of anchored coal specimens were analyzed，and the dynamic deformation and failure mechanism of anchored coal specimens was disclosed. The following beneficial results were obtained：(1) Under dynamic impact loads，the stress-strain curves of anchored coal specimens exhibit significant elastic-plastic behaviors，and their dynamic strength and average peak strain values both grow with the rise of pre-tightening force. Among the three types of specimens，the dynamic strength values of end-anchored specimens are 3.7%–7.9% lower than those of fully-anchored specimens. (2) Affected by anchor rods and pre-tightening forces，crack development and propagation in the specimens slows down，and the position of crack initiation shifts towards both sides. An increase in the pre-tightening force and anchorage length brings about a decrease in the fragmentation degree of specimens，and these two factors exert a stronger influence on fully-anchored specimens than end-anchored specimens. (3) The deformation compatibility process between specimens and anchor rods is divided into two stages，i.e.，“advance response” and “delayed response”. For end-anchored specimens，the strain response of anchor rods precedes that of the specimens. In contrast，for fully-anchored specimens，it lags behind that of the specimens. (4) Under dynamic impact loads，a larger anchoring length and pre-tightening force correspond to a higher support stiffness and sensitivity of specimens and a smaller peak axial force of anchor rods. The pre-tightening force should be controlled below a value that neither diminishes the strength of surrounding rock nor exceeds the yield load of the rod body，as an excessively high pre-tightening force is inconducive to the overall impact resistance of the anchored coal body. These findings are expected to offer guidance and reference for grasping the anchoring mechanism of roadway support under dynamic impact loads and the degradation mechanism of anchored bearing structures in surrounding rock.