An Experimental Investigation on Self-Loosening of Bolted Joints

Abstract

The self-loosening process of a bolted joint consists of two distinct stages. The early stage of self-loosening is due to the cyclic plastic deformation of the materials. The second stage of self-loosening is characterized by the backing off of the nut. The current study is concentrated on the experimental investigation of the second stage of self-loosening with the aim to explore the mechanisms responsible for the phenomenon. Over a hundred bolted joints were experimentally tested using a specially designed testing apparatus. M12×1.75 bolts and nuts were used. The experiments mimicked two plates jointed by a bolt and a nut and were subjected to cyclic transverse shear loading. During an experiment, the relative displacement between the two clamped plates, denoted by δ, was a control parameter. The clamping force, the relative rotation between the nut and the bolt, and the applied transverse load were monitored and recorded for each loading cycle. For a given preload, the relationship between, Δδ/2, the amplitude of the relative displacement between the two clamped plates, and, NL, the number of loading cycles to loosening followed a pattern similar to a fatigue curve. There existed an endurance limit below which self-loosening would not persist. A larger preload resulted in a larger endurance limit. However, a large preload increased the possibility for the bolt to fail in fatigue. It was found that the surface condition, which influenced the friction coefficient, had a significant influence on self-loosening resistance. The results also reveal that the use of a regular nut is superior to the use of a flange nut in terms of self-loosening resistance. An “endurance diagram” concept was developed that can be used directly for the design and evaluation of the bolted joints.