Analytical, numerical and experimental study of self-loosening of bolted joints

Abstract

The large-scale demand for bolted joints across diverse engineering applications is indisputable because of their proven functional reliability coupled with simplified structural design. The issue of self-loosening presents a significant problem that potentially leads to catastrophic outcomes in the respective applications even though they offer a good rigidity to the joined connections. There are several factors responsible for self-loosening to occur. For instance, axial vibration can contribute to the problem that is persistent to only a certain number of cycles. Torsional vibration can also trigger the loosening of joints. However, the most significant cause of its occurrence is the transverse cyclic loading. Bolts experience rotation in the loosening direction due to a cyclic transverse force that produce slip between threads and/or under bolt heads leading to a relative rotation. Because of the cyclic transverse loading, a reduction of the clamping force is likely to occur followed by a relative rotation between the bolt and nut in the last stage of each half a cycle. In addition to these, there are other factors responsible, and a comprehensive understanding of the self-loosening mechanism remains an area in need of further investigation. Hence, this study aims to identify the principal mechanisms behind the self-loosening phenomenon in bolted joints and to explore how various factors contribute to its occurrence. To achieve this goal, the project involves an analytical investigation to comprehend the behavior of self-loosening, coupled with numerical analysis through Finite Element Modeling (FEM) analysis, and experimental validation of the results using a laboratory test rig. A numerical axisymmetric finite element (FE) modeling-based approach is developed to effectively evaluate accurately the stiffness of bolted joint components and particularly the stiffness of the clamped members and the bolt stiffness as current models are oversimplified. In this work, for the purpose of improving the analytical models, a wide range of bolts from M6 to M36 along with various joint grip lengths are covered in the analysis to develop a unique relationship of the stiffness of clamped members based on the bolt head or nut bearing contact. By performing validation with various load cases, the new method reveals that several wellknown numerical and analytical methods from the literature overestimate the stiffness of clamped members. Since friction within various contact interfaces of a bolted joint significantly contributes to the occurrence of self-loosening, the study investigated the contribution of the various torques involved in a bolt, namely pitch, bolt and nut bearing friction and thread friction torques, and their variations during the tightening process, at rest after tightening is achieved and then during the untightening process. An analytical approach is elaborated to evaluate the three different torques, relative angular rotation between bolt and nut, interrelationship among the coefficient of friction and the nut factor, and stiffness of joint. This shows good agreement with the data from an earlier experimental study (Eccles, 2014) on tightening and untightening loading cycles. A three-dimensional (3D) numerical FE model is developed of a M12×1.75 hex threaded bolted joint to simulate the tightening and untightening loading cycles, which validates the analytical model as well. The project also conducted experimental tests on a test rig already developed, which replicates the self-loosening phenomenon in a bolted joint under cyclic transverse loading. It included tests with M12×1.75 hex bolt and clamped members of different sizes and materials for different tightening conditions. As the stiffness and surface friction in a bolted joint are crucial factors influencing its loosening shown in earlier investigations, and both parameters depend on the types of material, a comparison is demonstrated between the tests conducted with steel and High-Density Polyethylene (HDPE) clamped members. A comprehensive understanding of the stage-II loosening (loosening by relative rotation between bolt and nut) is attained as every test end with a total preload drop, unveiling the effect of clamped member stiffness on the self-loosening phenomena patterns for comparative analysis.

Date	18 Dec 2024
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Hakim A. Bouzid (Supervisor)