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 localisation. 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.
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