A B S T R A C T The fatigue behaviour of single-and double-rivet aluminum alloy 5754-O self-piercing riveted (SPR) lap joints has been investigated experimentally and analytically. With the single rivet, the experimental program involves a set of 27 cyclic tension tests on joints with 1-, 2-or 3-mm-thick sheet coupons. In most cases (85%), fatigue cracks are found to initiate in the gross section on the faying surface of the upper sheet. With two rivets (installed in two rows), the experimental program consists of nine cyclic tension tests, three for each of the three combinations of riveting orientation possible, on SPR joints consisting of 2-mm-thick sheet specimens. The fatigue life of double-rivet joints is found to be strongly dependent on the orientation combination of the rivets. Monotonic tests with the double-rivet joints also reveal an influence of orientation combination. In addition to experiments, values of local stress and rivet-sheet microslip in the single-rivet joints have been evaluated through three-dimensional elastic finite-element analysis. The analyses are used to interpret experimental observations of fatigue crack initiation location, life and fretting damage severity.

The fatigue life and fretting characteristics of aluminum alloy 5754-O self-piercing riveted lap joints have been investigated experimentally and analytically. The experimental program involves a set of 27 cyclic tension tests on three different joints consisting of either 1 mm, 2 mm or 3 mm-thick sheet specimens. In most cases (85%), fatigue cracks are found to initiate on the faying surface of the upper sheet, adjacent to the hole, and at an angular location that lies on the sheet loading axis towards the loading end. Three-dimensional finite element analysis of the three joints has also been performed. Computed distributions of local stresses and rivet-sheet slips are interpreted in terms of experimental observations of fatigue life, crack initiation location and fretting damage observations. Significantly, the calculations provide a rationale for the surprising crack initiation location.

With an increase in the use of advanced high strength steels in vehicle architectures, materials joining issues have become increasingly important. Among the various joining methods, adhesive bonding is increasingly used in automobile manufacturing. Successful implementation of adhesive bonding to improve structural crashworthiness and reduce vehicle weight requires the knowledge of issues related not only to processing but also to joint performance. In this study, the impact strength of adhesive bonded high strength steel joints is evaluated with the split Hopkinson tension bar (SHTB) technique. The influences of loading speed and thickness of the steels on the shear strength of the joints were examined. Comparative quasi-static lap shear tests were also conducted on a tensile testing machine. Test results showed that strength and energy absorption of bonded steel joints increase with loading speed, and is greatly affected by the thickness of the steels. As the loading rates are increased to 1100 s 21 (i.e. 20 m s 21 ), bonded 0?75 mm thick DP600 steel shows a 152% increase in strength and an 83% increase in energy absorption when compared to its quasi-static values. Examination of the impact tested specimens showed the failure mode changes from coarse cohesive mode to fine cohesive mode with increasing loading speed. The results from this study will provide the information for a better understanding of impact failure mechanisms of adhesive bonded high strength steels.

Hybrid laser-MIG welding technology has several advantages over laser welding alone or MIG welding alone. These include the possibility of modifying weld bead shape including the elimination of undercut, the change of weld compositions, and the reduction of porosity formation in the weld. Although the hybrid laser-MIG welding method is becoming popular in industry, its development has been based on the trial-and-error procedure. In this paper, mathematical models and the associated numerical techniques were developed to calculate the heat and mass transfer and fluid flow during the laser-MIG welding process. The continuum formulation was used to handle solid phase, liquid phase, and mushy zone during the melting and solidification processes. The volume-of-fluid (VOF) method was employed to handle free surfaces, and the enthalpy method was used for latent heat. The absorption (Inverse Bremsstrahlung and Fresnel absorption) and the thermal radiation by the plasma in the keyhole, and multiple reflections at the keyhole wall were all considered in the models. The transient keyhole dynamics, interactions between droplets and weld pool, and the shape and composition of the solidified weld bead were all predicted for both the pulsed laser-MIG welding and three-dimensional moving laser-MIG welding. Computer animations showing the fluid flow, weld pool dynamics, and the interaction between droplets and weld pool will be shown in the presentation.

Experiments have been completed in order to characterize the fatigue behavior of weld-bonded aluminum 5754-0/bis-phenol-A epoxy adhesive joints subjected to 100% relative humidity at 38°C. It was found that the presence of water vapor at elevated temperature decreases the fatigue strength of weld-bonded joints by as much as 33% at 5 X 10' cycles. Optical microscopy, scanning electron microscopy, dynamic mechanical analysis, and tensile testing of fatigued specimens and exposed bulk adhesive revealed that fatigue strength degradation is mainly due to the plasticization and micro-cracking of adhesive by the water vapor.KEY WORDS weld-bonding aluminum; vehicle applications; moisture and elevated temperature; concurrent fatigue testing; fatigue strength; adhesive plasticization.