Numerical investigation of the influence of the tightening force on the geometric stress concentration factor at the root of the bolt thread

In this article, a numerical study of the influence of the bolt tightening force on the geometric stress concentration factor at the root of the bolt thread was carried out. A three-dimensional simulation of the load and stress of the M12 metric bolt was made using the finite element method (FEM). The geometry of the bolt, nut and metric thread was modeled according to the ISO 4014, ISO 4032 and ISO 724 standards. The tightening force of the bolted joint with M12 thread was varied from 2.5 to 25 kN. A nonlinear characteristic of the geometric stress concentration factor in relation to the tightening force was obtained. Numerical research has shown that the geometric stress concentration factor at the root of the thread increases as the tightening force increases. For the specified test values of the tightening force from 2.5 to 25 kN, the geometric stress concentration factor at the root of the bolt thread increased non-linearly between 3.03 and 4.46. This result theoretically proves that the influence of the tightening force on the geometric stress concentration factor in the bolted joint is not negligible and that for high tightening forces due to this effect, an unexpected fracture in the root of the bolt thread may occur in the case of dynamic cyclic loading.