PSI - Issue 19

M. Duchet et al. / Procedia Structural Integrity 19 (2019) 585–594 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Oscillation of the torch

Fig. 4: principle for transversal oscillation of the welding torch.

• Shot-peening Shot-peening is a well-known mechanical process where round steel shot (cut steel wire for instance) impacts components to be reinforced. Thus, the upper skin is plastically deformed resulting in the introduction of compressive residual stresses on surface where fatigue cracks initiate. Depending on the diameter of the shot particle, the recovery rate and the intensity of the process, the residual stresses profile can be modified with certain limitations by changing the maximal compressive stress or the affected depth. These compressive residual stresses reduce the local tensile stress, especially at the weld toe. These three processes are then used on lap joint specimens and subjected to fatigue tests. 2.2. Fatigue results on lap joint specimens reinforced by TIG remelting or oscillating welding Here are reported only the results on these two reinforcement solutions. Nevertheless, the choice of the appropriated reinforcement solution is not only driven by the increase in fatigue resistance, but also by the cost of such an operation, especially in the automotive industry where it is one of the key factors. Fatigue tests on lap joint specimens are carried-out on servo-hydraulic machines with a 50 kN capacity at a frequency 30 Hz. The applied loading ratio is equal to 0.1. The specimen is composed by two coupons of FB590 (microstructure having a ferritic matrix reinforced by homogeneously distributed islands of bainite and a tensile strength greater than 600 MPa). The thickness of the material is 2.5 mm. A reference fatigue curve in the as-welded conditions is first obtained on the lap joints. A fatigue strength of 66 MPa is obtained at 2 million cycles. After TIG remelting, the fatigue strength reaches 99 MPa (+50%) as shown in the Fig. 5. The Fig. 6 shows the smoothing of the geometry of the weld: a reduction of the toe radius and an increase of the toe angle as well as a wider weld from 7.3 to 8.7 mm. For transversal oscillating welding, the fatigue strength at 2 million cycles is increased by 68% at 111 MPa. In comparison with TIG remelting (Fig. 6), the enlargement of the weld width is strongly greater, resulting in an additional increase of fatigue properties by a more important reduction of the local stresses (Fig. 8).