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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weight and comply with CO2 emission regulations consists in replacing current steel grades by thinner gauge ones having higher mechanical properties (see example of a subframe Fig. 1). Steel solutions led by ArcelorMittal for chassis and suspension parts have shown that weight reduction of components is usually reachable whereas satisfying stiffness and monotonic behaviour by using AHSS in place of standard products and by design optimization in the same time. On chassis components, the assembly of different parts is ensured by Gaz Metal Arc Welding (GMAW) that remains the mainstream joining solution for economical reasons. Several meters of welds are needed for the component assembly. On the base material, as fatigue properties increase with the increase of mechanical properties, the substitution of grades by thinner steel is easily compensated to withstand the durability requirement. For GMAW, their fatigue strength could be an issue as it is well-known that higher strength grades do not bring higher fatigue resistance.

Fig. 1: Application of S-in motion® on a front subframe of C-segment vehicles, achieving a weight reduction of 15 percent compared to the baseline with the use of hot rolled complex phases CP800SF and CP1000SF instead of high strength low alloyed and ferrite-bainite grades [1].

Former and more recent internal R&D studies have demonstrated that the fatigue resistance of lap welded joints (Fig. 2) was independent to mechanical properties of grades (Fig. 3), with a fatigue strength expressed in nominal stress between 60 and 100 MPa. For the thinnest sheets (2 mm) and due to local stress distribution between tension and flexion, finite element analysis based on real weld cross-section geometries explained that the stress concentration factor is lower for thin sheets than for thicker sheets.

Fig. 2: geometry of the fatigue lap welded joint specimen.

Fig. 3: Fatigue properties (R = 0.1, 2 million cycles) of lap welded joints specimens for different steel grades (from mild steel to AHSS) and thicknesses (from 2 to 4 mm).