An experimental study on the fracture of a unidirectional carbon fiber-reinforced composite under quasistatic torsion

Carbon fiber-reinforced composites exhibit excellent mechanical properties for a variety of applications. Using the spirally notched specimens, we demonstrate how unidirectional carbon fiber-reinforced polymeric composite specimens with different fiber orientations fail under quasistatic torsion. By collecting data at 200,000 points per second, type I specimens exhibited limited fluctuations at the fiber-matrix interfaces before the catastrophic failure, whereas type II specimens exhibited stepwise failure after the peak loads with torque avalanches described by Gaussian models. Distinctive crack-tip stress contours and energy release rates were obtained through the finite element modeling using anisotropic elastic properties of the unidirectional composite. The fractographic characterization showed that the primary failure of type I specimens occurred along fiber-matrix interfaces, whereas circular cracks switched between polymeric matrices and fiber-matrix interfaces when type II specimens failed progressively. Our findings could help further insights on the mechanical behavior of different fiber-reinforced composites.