Mechanical properties and microstructure evolution of resin-based composites during thermo-oxidative aging and anisotropic crack propagation kinetics

Prolonged exposure of resin-based composites to elevated temperatures induces thermo-oxidative aging, a process that significantly degrades their mechanical properties and limits service life. This study systematically investigates the thermo-oxidative aging behavior of a carbon fiber-reinforced bismaleimide composite intended for aero-engine bypass casings. Thermogravimetric analysis (TGA) was employed to determine the thermal decomposition kinetics of the neat resin. Activation energies of 240.17 and 239.51 kJ·mol –1 were calculated using the Kissinger and Ozawa methods, respectively, indicating excellent intrinsic thermal stability. The correlation between thermo-oxidative aging and the evolution of mechanical properties was further analyzed. The aging behavior arises from the competing effects of post-curing and resin decomposition. In the early stages, strength decreased while modulus increased; in the later stages, both strength and modulus declined. Resin-dominated properties, such as interlaminar shear strength and transverse tensile performance, were most severely affected. Using the weight loss rate as an indicator of aging kinetics, the influence of specimen geometry on weight loss was examined. Specimen thickness was found to have the most pronounced effect. The propagation of microcracks during aging was quantitatively characterized using scanning electron microscopy (SEM). The results show that aging begins with surface resin degradation, followed by microcrack formation. Crack propagation is highly anisotropic, with the highest rate occurring along the fiber direction. The relationship between crack depth and aging time was well-described by a linear model ( y 3 = 3.97 x , R 2 = 0.989).