Understanding the effect of residual stresses and deformation on the fatigue behavior of permanent fasteners

In this work, the complex relationship between deformation history and mechanical performance of self-pierce riveted (SPR) joints is elucidated. This study focuses on understanding how the material deformation that occurs during the riveting process impacts the quasi-static and fatigue behavior of the joint. Experimental results show that fatigue crack initiation in SPR joints can occur away from the rivet. Numerical simulations predicted the deformation of the riveted aluminum-alloy joint, revealing a strong correlation between residual contact pressure due to the riveting process and fretting induced fatigue crack initiation. Furthermore, the number of cycles to failure was calculated by applying linear elastic fracture mechanics approach, which correlated to the experimental fatigue results and further supported the hypothesis that the deformation induced by the riveting process altered the failure mode under high cycle fatigue. Since residual contact stresses were evaluated and correlated to fretting, a more in-depth analysis was performed. To aid in understanding the role of deformation in the riveting process, residual stresses originating from elastic strains within a magnesium-to-aluminum SPR joint were evaluated using neutron diffraction and X-Ray diffraction (XRD) measurements. Furthermore, micro hardness mapping was performed in order to quantify the plastic deformation on a cross-section of the SPR joint. The experimental characterization of the stress and strain state of the SPR joint were then compared to finite element simulations showing good agreement. After validation, the macro lap-shear mechanical response of the SPR joint was simulated and compared with experimental results. In particular, SPR simulations were carried out in order to assess the effect of including the residual stresses and plastic strains in the modeling approach. The simulation results reveal that the inclusion of plastic strains is the main driving force for strength and joint stiffness under quasi-static lap-shear loading, while the residual stresses have a negligible effect. Also, the role of residual stresses and plastic strains was investigated on cyclic loading. Results show that including the deformation history captured in the process simulation changes the prediction of the location of crack initiation, resulting in an estimated number of cycles to failure that differs by a factor of two in the high cycle regime. (Published By University of Alabama Libraries