EXPERIMENTAL INVESTIGATION OF MECHANICAL BEHAVIOR OF GRADE 8.8 AND 10.9 SMALL-SIZED (M14-M20) BOLTS WITH THE THREAD INTO THE BOLT HOLE

Abstract

This paper is a study on the mechanical behavior of grade 8.8 and 10.9 small-sized (M14-M20) bolts with the thread into the bolt hole. A total of eighty specimens with different thread depth, surface treatment, bolt diameter, and bolt strength grade were subjected to static loading tests. The effects of different parameters on the properties of specimens such as ultimate bearing capacity, initial slip load, ductility, and initial stiffness were systematically investigated. It was demonstrated that the typical failure mode is bolt shear failure. The increase in thread depth causes the ultimate bearing capacity of bolted connections to decrease, and other design parameters also have effects on the ultimate bearing capacity. The ultimate bearing capacity of bolted connections with shot-blasted surfaces is greater than that of bolted connections with wire-brushed surfaces. Moreover,bolted connections with a greater thread depth have a lower yield strength. The method of contact surface treatment, bolt diameter, and strength grade also affect the yield load of bolted connections. The initial stiffness does not change much with the increase of the thread depth. In addition, the increase in thread depth decreases the ductility coefficient and ultimate displacement, which has a negative effect on deformation performance, and the ductility coefficient of bolted connections also decreases with the increase in bolt grade. Finally, based on the test results, a design formula for predicting the ultimate bearing capacity was proposed, and the calculation results match the experimental results well. The difference in ultimate bearing capacity due to thread depth can be well described by the formula.