Static tensile and bolting shear mechanical properties of micro NPR bolt steel in underground engineering

Resource mining and underground space construction are gradually moving deeper，which results in some new challenges to the deep surrounding rock support system. It is imperative to develop some surrounding rock support materials with high strength，high toughness，super energy absorption，and high preload. Based on this，a new material of anchor steel with high strength，high ductility，and microscopic negative Poisson’s ratio effect has been independently developed，which is also known as the micro NPR material. This material can be processed into bolts，steel bars，anchor cables，steel belts and other types of engineering application materials. This paper focuses on the experimental research on the shear mechanical properties of micro NPR bolt. Firstly，the basic concepts and research history of PR material，macro NPR structure and micro NPR material are introduced. Then，the static tension and bolting shear tests of micro NPR bolt steel and ordinary PR bolt steel are carried out. Finally，the failure modes，tensile mechanical properties and shear resistance properties of micro NPR steel are compared and analyzed. The experimental results show that under static tensile condition，compared with ordinary bolt steel，the tensile curve of micro NPR steel has no yield platform，and has the quasi ideal elastic plastic characteristics of constant resistance and large deformation. It greatly improves the strength and deformation properties of bolt material. During tensile process，it is uniformly deformed and its fracture neck shrinkage is significantly lower than that of ordinary steel. The bolting shear experiment shows that the bolted jointed rock mass has significant ductile failure characteristics under shear load，which effectively improves the brittle failure phenomenon of the unbolted jointed rock mass. The shear strength of micro NPR bolt steel is much higher than that of ordinary bolt steel，and its lateral shear deformation ability is more than 2.5 times that of ordinary bolt steel. Experimental results show that the micro NPR anchor steel balances the contradiction between high strength and high ductility of metal materials，greatly improves the tensile and shear capacity of anchor steel，and essentially realizes the characteristics of large deformation and super energy absorption of steel.