Mechanical performance of unidirectional glass fiber-reinforced thermoplastic composites under varying aging conditions

Continuous fiber-reinforced thermoplastic composites are gaining attention in the industry due to their high toughness and recyclability, making them a promising alternative to traditional thermosetting composites for primary structural applications. However, challenges such as hydrothermal aging at elevated temperatures must be addressed before these composites can widely be adopted. Elevated temperature conditions increasingly fall within the operating range in field conditions for thermoplastic composite pipes. Although designs aim to prevent direct water exposure of the load-bearing reinforcement phase in these pipes, unexpected or accidental water ingress must be considered for safety-critical pressure piping, as it can lead to the degradation of the reinforcement material over time and eventual pipe failure. This study focuses on evaluating the mechanical resistance of unidirectional glass fiber-reinforced thermoplastic composite tapes (UGFTC) under elevated temperature and different aging conditions. Two types of thermoplastic matrices, polypropylene and polyethylene, were examined. Samples were subjected to thermal aging at 95°C and water immersion in distilled and deionized water at the same temperature for up to 4 weeks. Mechanical resistance was assessed by measuring strength reduction, with a decline of over 25% being considered severe, across the three aging conditions during this period. Water uptake behavior was also analyzed by weighing the samples over the 4-week aging period and after its completion. To further understand the damage, optical microscopy was used to compare the condition of the UGFTC samples under each aging condition. This study provides novel insights into the damaging effects on the fiber-matrix interface and the polymer matrix itself when exposed to aging in different conditions, which are key to the mechanical performance of UGFTC materials. Results indicate that samples aged in deionized water experience greater strength reduction compared to those in distilled water. Additionally, samples with polyethylene matrices show more surface damage and reinforcement degradation than those with polypropylene matrices.

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