PSI - Issue 5

Shayan Eslami et al. / Procedia Structural Integrity 5 (2017) 1433–1438 Shayan Eslami et al./ Structural Integrity Procedia 00 (2017) 000 – 000

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2. Materials and experimental procedures

Two thin sheets of dissimilar polymers were welded in the lap-joint configuration using position control FSW process. 2 mm thick High Molecular Weight Polyethylene (PE-HMW) and 1.2 mm thick Polypropylene (PP) were joined, and its fatigue life was evaluated. The parent materials were cut to 120×120 mm 2 plates and fixed on the 3 axis CNC table with a sophisticated clamping device. The welding tool used in this study consists of a stationary shoulder made of Teflon with highly conductive sleeve around the rotating probe. The frictional heat between the rotating probe and the sleeve generates adequate heat to plastically deform polymers under the axial force without using external heating source. The schematic representation of the tool used to weld those polymers is illustrated in Figure 2. The optimized welding parameters were determined in a previous study, which optimized the welding parameters by Taguchi statistical approach [5].

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Figure 2-FSW tool used to weld polymers: Front view (a); section view (b) and bottom view (c) [5]. Figure 1-Designed tool geometry: front view (a), section view (b) and bottom view (c).

High frequency load-controlled fatigue tests were performed in the tension-tension mode at the ambient temperature. The fatigue life for the welded specimens was obtained for three different load levels: 90%, 80% and 70% of the Ultimate Tensile Strength (UTS) of the highest performing parent material (PP). The stress ratio was R=0.1 (R=F min / F max ) with a frequency of 25 Hz using electromechanical Instron ElectroPuls™ testing machine at ambient temperature. Generally, polymers failure mode (fatigue failure or thermal failure) under cyclic stress conditions determined by cyclic frequency [10]. In this study, four specimens were tested for each load level according to ASTM D-7791 standard for uniaxial fatigue properties [11] of plastics. With the intention of comparing the fatigue life of the produced joints, Polypropylene dog-bone specimens were prepared and tested under the same cyclic stress conditions. The parent materials (Figure 3c) and the welded specimens (Figure 3a) were machined into the predefined dimensions as illustrated in Figure 3. The remote stress-strain curves for PP, PE-HMW base materials and the optimized welded specimen under lap-shear stress is demonstrated in Figure 3b. The tensile strength properties were determined at a loading rate of 2 mm/min. The tests were performed according to ASTM D-638 tensile properties standard [12] at ambient temperature.