Research on macro-mechanics mechanism of bolt fracturing and its control with high pre-tensioned force

Aiming at the issue that highly pre-tightened bolts are prone to fracture, and adopting methods of field investigation and theoretical analysis, the morphology of fracture surfaces and bolt bodies of fractured bolts as well as the loaded types and forced states of highly pre-tightened bolts underground were analyzed.The relation between the pre-tensioned force and pre-tensioned torque of bolts was investigated, the stress model of bolts under the compound effect of lateral torsion load, axial tensile load and transverse shear load was built, the effect of pre-tensioned torque on the shear stress on bolt cross-sections and on the principal stress in the surrounding was revealed.The mechanics mechanism of fracturing of highly pre-tightened bolts under the compound effect of multiple types of loads was analyzed as combined with the third strength theory, and the mechanics mechanism and countermeasures to control the fracturing of highly pre-tightened bolts were proposed. The research results show as follows: the bolts working underground could not only bear large single-type load, but also bear the compound effect of multiple types of loads. The pre-tensioned force on bolts is proportional to the pre-tensioned torque, and is inversely proportional to the friction coefficient between the nut and tray the maximum principal stress in the surrounding of the cross-section of bolts that are under the compound effect of tension, torsion and bending is positively correlated with the pre-tensioned torque. Bearing the compound effect of torsion, tension and bending is the main reason for the fracturing of highly pre-tightened bolts, where the high pre-tensioned torque has a significant influence on the principal stress state in the surrounding of bolt cross-section and makes the stress state satisfy the strength condition. To take corresponding measures in the regards of reducing the extension of bolts, avoiding the influence of rock dislocation and decreasing the pre-tensioned torque is helpful in controlling the fracturing of highly pre-tensioned bolts.