On the assessment of strength and durability of weld lines in injection-moulded unreinforced and short glass fiber reinforced polypropylene

Identifying and predicting the performance of process-induced defects, such as weld lines in injection-moulded thermoplastic components, is critical for failure assessment. While previous studies have addressed various weld line phenomena, accurately predicting their instantaneous and long-term strength remains a challenge. This study investigates the thermo-mechanical performance of unreinforced, and 30% glass fiber-reinforced isotactic polypropylene (iPP) processed by injection moulding into specimens containing a central weld line due to a stagnating flow. Short-term experiments on smooth tensile bars at various strain rates (from 10 -6 to 10 -2 s -1 ) and temperatures (-20°C, 23°C and 80°C) identify plasticity driven failure kinetics with a transition towards brittle crack-growth driven failure at high temperatures and low strain rates. Long-term plasticity driven performance is assessed through creep-to-rupture and cyclic fatigue tests on tensile bars and identify the same transition from ductile-to-brittle failure, as well as a clear discrepancy in perceived plasticity-governed strength when comparing short- and long-term experiments. The key novelty of the work is found in the demonstration that stress rate-controlled tensile experiments rationalise this observed discrepancy in strength that is explained by strain localization. We therefore propose the stress rate-controlled tensile test to be more suitable to assess the strength of a weld line compared to the traditional strain-rate controlled tensile test. All observations are accurately captured using Ree-Eyring flow theory. The crack-growth governed failure of the weld line is explored by including fatigue crack growth experiments on CT specimens on both unreinforced and reinforced materials and general findings are presented. • stress-rate-driven tests capturing short and long-term strength of weld lines • localisation effects in strain-rate tests hindering assessment of real weld line strength • Eyring flow derived model from stress-rate data predicts creep and cyclic failure • slow-crack-growth is better evaluated in energy terms than stress intensity factor • crack-growth behaviour, unlike ductility, sensitive to geometry and processing