Search

The role of the process deformation history on the mechanical performance of self-pierce riveted (SPR) joints is examined in detail. The SPR process was simulated using a non-linear finite element model coupled with a damage criterion sensitive to the stress state. Results of the FE simulation compared closely to cross-sectional images of experimental SPR joints. The FE simulation results suggest that the strain hardening due to the piercing process is the main mechanism for joint strength under quasi-static condition. Aspects of fatigue damage were modeled including crack initiation, propagation, fretting and number of cycle to failure, and compared to experimental results. Results from the study indicate that residual stresses and plastic strain may alter the crack initiation location and crack propagation plane, leading to more accurate results when included in finite element analysis of SPR joints. Including these residual stresses and plastic strains also changed the predicted fretting locations leading to better agreement with experimental results. Lastly, linear elastic fracture mechanics were performed to estimate fatigue lives based on initial crack location and angle of crack plane. The estimation of the number of cycles to failure that included residual stresses and plastic strains resulted in a better correlation to experimental data when compared to the calculations that excluded residual stresses and plastic strains.

Mixed-mode fatigue crack growth behavior of an automotive adhesive bonding system (AA5754O- A951-BM4601) has been investigated using a compound compact mixed-mode specimen. The test was performed under load control with 0.1 R ratio and 3Hz frequency. A long-distance moving microscope was employed to record the real-time length of the fatigue crack in the adhesive layer during testing. The strain energy release rate components of the crack were analyzed by finite element method. The pure-mode fatigue crack growth test results show that a given value of G I results in a fatigue crack growth rate over an order of magnitude higher than for an equal value of G II. The mixed- mode tests results indicate that under a given loading angle, the fatigue crack growth behavior is also influenced by the test load due to the changing shear mode ratio when the crack grows. The fatigue crack growth data were then processed by a generalized Paris relation formula and employed in the direct fatigue life prediction of single lap shear joints with the same bonding system. The life prediction shows good results in the low-cycle region but slightly conservative results in the high- cycle region when compared to coupon fatigue tests.

A numerical model of fatigue crack propagation based on the Theory of Critical Distances has been developed in a previous paper (Zheng et al., 2013 [1]). The present study aims to investigate the capability of the crack propagation model to predict the effects of overloads and underloads. The plastic energy density at the critical distance ahead of crack tip is affected by the changes in the plasticity-induced crack closure levels and has been used as a measure of the fatigue damage. It is concluded that the predicted crack retardation and acceleration after the overload and underload, respectively, are in satisfactory agreement with the experimental observations.

A numerical model based on the theory of critical distances has been developed to simulate the fatigue crack propagation for cast aluminum alloys under mode I loading. The accumulated plastic energy density at the critical distance point ahead of the crack tip is used as a measure of the fatigue damage. The effect of different R -ratios on the crack propagation rate is correctly predicted using the proposed methodology. For the 319 cast aluminum alloys investigated in the present paper, the critical distance is of the same order of magnitude as the secondary dendrite arm spacing (SDAS).