Bimetallic joints and threaded fasteners under assembly and service loads.

摘要

An analytical method is proposed for determining the stress distributions in steering knuckle/tapered stud assemblies. It is shown that the analytical and finite element results are in good agreement for several load and frictional conditions, and the hoop and radial stress solutions presented are good engineering solutions to the knuckle/tapered stud problem where the draw distance is provided. These tapered bimetallic joints are then investigated with and without consideration of assembly stresses under assembly loads. Two-dimensional plane stress and three-dimensional finite element models are developed to investigate the interaction between the steel stud and aluminum disk, with and without an interference fit, under the cyclic loads applied to the stud. The computational results show that the assembly stresses can significantly affect the stress and strain histories and the distributions of the stresses and strains near the bi-material interface under the applied load. Next, experiments are conducted to determine the relationship between the assembly parameters of draw load and draw distance for coated and uncoated steel studs. These experimental results are compared to the corresponding computational results to determine the effective coefficient of friction needed in computational simulations of the assembly process and subsequent service loads. The results for the coefficients of friction from the combined experimental and computational analysis are then used to analyze the thread fatigue failure of bimetallic tapered joint assemblies under cyclic service loads with consideration of assembly loads. Finally, the stress and strain distributions at the first thread of a joint under monotonic and cyclic uniaxial loads are examined by finite element analysis. The distribution of load to the threads is determined and compared with an analytical approximation. Then, the stress and strain histories of material elements are used in conjunction with multiaxial fatigue theories and the critical plane approach to determine the location, life, and direction of crack initiation for both high cycle and low cycle fatigue loading conditions. It is shown that the crack initiation occurs at approximately 20° from the center of the thread root at an inclination of approximately 45° to the free surface, and fatigue life decreases with increasing mean stress.