Examination of the fatigue properties of vibration-welded nylon compounds

This study examined the fatigue strength of non-welded and vibration welded nylon 6 and nylon 66 compounds. The effects of variables including weld pressure (0.8 MPa and 4.0 MPa) and glass fibre content (0% and 30%) on the fatigue behaviour were studied. In addition, temperature profiles were measured experimentally to ensure mechanical fatigue. The experimental results showed in general that non-welded specimens have longer fatigue lives and higher endurance limits than the welded specimens. This is believed to be caused by the notches inherent from the vibration welding process. For unreinforced and glass fibre reinforced nylon 6, the specimens welded at low pressure have better fatigue lives than the ones welded at high pressure, while for unreinforced and glass fibre reinforced nylon 66, the effect of weld pressure is not obvious. The longer fatigue life associated with low weld pressure is due to thicker weld. The material effect on fatigue life is complex and more research is needed. Specimens made of glass fibre reinforced nylons have better fatigue life and endurance limit than the unreinforced ones. This is consistent with the material's tensile strength and the "strength-life equal rank assumption". The temperature profiles obtained from selected specimens showed a slight increase at the beginning of the fatigue process and remained stable until prior to failure, the temperature increased again. However, no physical thermal damage was observed. The fatigue endurance (sigmae/sigma u) ratio for vibration welded unreinforced nylons is 0.27 and 0.33 for the reinforced nylons. This indicates that the fatigue performance of welded thermoplastics is relatively equal to or greater than that of metals. Vibration welding of these materials appears to be viable for structural applications requiring fatigue resistance.