Loosening on Thin Plate Bolted Joint Caused by Plastic Deformation of Thin Plate Subjected to Cyclic Offset Load

In previous studies, the loosening of bolted joints has primarily focused on self-loosening with rotation under axial, transverse, or torsional vibrations applied to the joint. Loosening without rotation has also been investigated, with emphasis on embedment at the bearing surfaces of clamped parts and bolt creep deformation. In our previous study, we incidentally observed slight loosening caused by slippage between clamped plates when high-tensile-strength thin steel plates, fastened with a bolt and nut, were subjected to offset loads. In the present study, we investigate loosening behavior when plastic deformation occurs in thin plates, using rolled steel plates with relatively low yield stress. The materials used in this study were cold-reduced carbon steel plates with a thickness of 3.2 mm and hot-rolled mild steel plates with a thickness of 4.5 mm, both having nearly the same ultimate tensile strength. Experiments and finite element (FE) analyses were conducted by applying repeated offset loads to thin-plate bolted joints fastened with steel bolts and nuts. The results showed that a decrease in clamp force of nearly 10% occurred when the plates were relatively thin and significant plastic deformation had developed. In contrast, a decrease in clamp force of several percent was observed due to slippage between the clamped plates when the plates were relatively thick and plastic deformation was not significant, similar to the behavior of high-tensile-strength steel plates. This difference was found to depend on the bending moment induced by the offset load acting on the bolted joint.