Effect of Matrix Properties on Quasi‐Static and Fatigue Properties of Thermosetting Polyurethane and Its GFRP Composites

ABSTRACT In this study, the influence of resin cross‐link density on thermomechanical properties and fatigue behavior of polyurethane resins and glass fiber‐reinforced polyurethane (PUR‐GFRP) composites was investigated. This material is a candidate for renewable energy applications like rotor blades in wind turbines. The thermomechanical behavior and morphologies of three different PUR resin systems were analyzed using dynamic mechanical analysis (DMA) and atomic force microscopy (AFM). Results show that the system with low cross‐link density exhibits earlier crack initiation but higher strains before failure, indicating initially a more brittle behavior but greater polymer chain reorientation capacity. This phenomenon was correlated to the different phase morphology and the hybrid nature of the investigated PUR resins. Residual strength analysis showed minimum influence on tensile strength and a slight modulus decrease after extensive creep tests. This highlights the importance of fiber–matrix interaction on the composite fatigue failure. The static mechanical properties of the composites and SN fatigue tests allowed assessing the fatigue performance and lifetime analysis under specific loading conditions. Highly cross‐linked systems show a significantly longer lifetime compared to the medium cross‐linked system. In conclusion, the study highlights the importance of understanding resin cross‐link density's impact on fatigue behavior in GFRP composites for better material and part design.