Plane stress dynamic fracture resistance of automotive TPO under airbag deployment conditions
Paredes Mogica, Eduardo Arturo
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For the past three decades, polymers such as polypropylene (PP) and polyvinyl chloride (PVC) have replaced metals as the main material for automotive instrument panel construction. However, due to their high sensitivity to temperature, under-deployment of airbag systems might still happen at very low temperatures with these materials. Thermoplastic polyolefins (TPO), blends that consist of olefinic resins such as PP mixed with elastomers, have gained popularity in instrument panel construction due to their capability of deploying at very low temperatures without transitioning from ductile to brittle behavior. Nonetheless, the relationship between their fracture behavior and the different conditions common of an airbag deployment have not been thoroughly discussed yet. The purpose of this study is to investigate the fracture behavior of an automotive TPO material under plane stress conditions, which emulate the deformation and fracture process of the instrument panel tear line, at different temperatures and strain rates. To do so, Reversed Charpy tests were performed on TPO specimens with a ligament, or residual wall thickness, of 1 mm at different temperatures (-35, 0, 35 and 70 °C) and impact speeds (0.5 and 1 m/s). The estimated strain rates during the tests were between 219 and 686 s−1, which are among the strain rates reported during airbag deployment conditions (100 and 500 s−1). Experimental results showed that the yield stress, essential work of fracture we, and plastic work dissipation wp were higher at low temperatures, with a distinguishable transition (drop) around the glass transition temperature of the PP matrix. Also, the maximum value of we and wp occurred at about the same temperature as the peak in the tan δ curve, which can be related to the energy losses during fracture. As for the strain rates, we and wp increased with strain rates at low temperatures (-35 and 0 °C). It is suggested that the deformation process changes from isothermal to adiabatic when increasing the testing speed, resulting in lower average peak loads and higher displacements. A relaxation zone near the crack tip of specimens struck at 1 m/s at -35 °C, observed by SEM, evidences the possibility to thermal blunting due to adiabatic heating. However, this needs further investigation. Additionally, quasi-static tensile, tear and Single-Edge Notch Bending (SENB) tests were performed. Tensile and tear tests showed that the tensile modulus, yield stress, and tear strength decrease with temperature. SENB tests suggested that energy dissipation toughened the material near the crack tip but did not have an effect as significant with further crack growth. In general, results from quasi-static tests agreed with what has been reported in the literature for other TPOs. These results were compared to those obtained with Reversed Charpy and the importance of characterizing polymers at the temperatures and strain rates according to their application was confirmed.