PSI - Issue 48

V.M.G. Gomes et al. / Procedia Structural Integrity 48 (2023) 142–148 Gomes et al/ Structural Integrity Procedia 00 (2023) 000–000

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variants, such as stress shot peening. Stress shot peening consists in applying a bending pre-stress on the leaves with posterior application of shot peening. Applying a bending pre-stress permits that a higher quantity of the kinematic energy is converted into plastic deformation, and hence a higher magnitude of compressive residual stress is achieved (Kaiser, 1987). After applying the mechanical surface treatment, the defect and geometry requirements are verified. After that, spring leaves are assembled. In the assembly process of the leaves of the spring, a pre-initial bending deformation is generated due to their different curvatures. Depending on the leaf in the assembly, it may be subject to tensile or compression loads (Wang et al ., 2018). Once the leaf springs are assembled, they are immersed in protective paint against corrosion (UIC, 2017). Taking into account the importance of residual stresses and average stress in the fatigue strength of materials, in this investigation, two experimental campaigns are carried out to quantify the magnitude of the initial stress state in leaf springs due to the stress shot peening and assembly process. Estimation of its effect on the fatigue limit of chromium-vanadium steel of UIC parabolic leaf springs is also made.

2. Experimental Details 2.1. Stress Relief Method

The quantification of the initial stress state of UIC parabolic leaf springs due to the assembly process is made by the stress relief method with the aid of electric extensometry. The stress relief method consists of relieving the stress of the leaves by incrementally removing the spring buckle. Two blocks with four screws are used to allow the spring leaf to be relieved incrementally while the CATMAN SPIDER 8 acquisition system gathers the surface strain readings from installed electric strain gauges. First, it is necessary to make a cut in the spring buckle on both sides. In order to keep the spring buckle in the initial position, a hydraulic press is used to apply a pressure of 70 bar (A) to the upper surface. Once the cuts are made, the screws are placed back and the threads tightened firmly (B). Removal of the top block is done incrementally until all spring leaves are completely relieved (C).

Fig. 3. A) Preparation of the specimen test, B) Specimen before stress relief testing, C) Specimen after stress relief testing.

2.2. X-Ray Diffraction Method

With respect to the residual stress analysis, the experiment was made according to ASTM E860 (ASTM E860-07, 2020). Figure 4 illustrates the portable diffractometer with two PSD detectors used to perform the x-ray diffraction analysis based on the iso-inclining β-method with ψ fixed (26 acquisitions). The analysis was run with the is Cr-Kα X-ray tube for a lattice plane {hkl} = {211}, and an angle 2θ of around 156º. The tube voltage and current intensity were respectively 20 kV and 20 mA. The distribution function to find the position of the XRD peak was the