In the connection of threaded fasteners, the main possible factors that lead to the relaxation of the pre-tightening force are: contact surface subsidence, mutual influence of bolt groups, seal ring deformation, high temperature creep and torsion relaxation. 1. The contact surface sinks. When the threaded fastener is tightened for the first time, the internal and external threads

, the bearing surface of the bolt and the nut, and the raised areas in the microstructure of the contact surface of the connected part first come into contact, as shown in Figure 1. The material in these local protrusions will be overloaded, first exceeding its yield point, and then the material: creep deformation occurs, until there are more and enough contact surfaces to withstand the load.

The sinking of the contact surface is the main cause of the relaxation of the pre-tightening force; the main reason is also the most common situation encountered in the threaded connection. These relatively short-term stress relaxation effects are called 'sinking.' Studies have shown that the relaxation rate just after tightening is the largest, and it decreases exponentially, usually after an initial period of time, that is, it decreases to a very low creep rate, as shown in Figure 2.

Usually, the Multi-Function Home Repair Tool of the pre-tightening force due to subsidence accounts for about 5% to 10% of the initial pre-tightening force. 2. The mutual influence of bolt groups The mutual influence of bolt groups mostly occurs in the occasions where multiple bolts are connected in a plane such as bolt flange connection. When the first bolt is tightened, the bolt is elongated and the vicinity of the connected part is compressed. When the nearby bolts are tightened, the connected parts near the first bolt will be further compressed, causing the pre-tightening force of the first bolt to relax.

https://www.lizhan-hardware.net/product/home-tool/xm-set-of-sets/lz000xm000013.html