A carbon fiber reinforced thermoplastic polymer composite was fabricated using plainwoven carbon fiber and a thermoplastic polyimide (CF/PI). Bending properties at elevated temperature and the bending creep behavior of the CF/PI material were studied and compared with those of a carbon fiber-reinforced epoxy composite (CF/EP). Additionally, the same tests were performed using 130ºC cured CF/EP. The CF/PI bending modulus remained almost constant under 240ºC, until a decrease was observed at 250ºC, which coincided with the glass transition temperature of the thermoplastic polyimide. Bending creep tests were performed at various elevated temperatures to obtain the creep compliance master curves. It was found that the Arrhenius-type time–temperature superposition principle was applicable to the CF/PI. Comparing this material with CF/EP, CF/PI has advantageous bending properties at elevated temperatures and a lower creep deformation.
Biaxial stress tests of carbon fiber reinforced plastic (CFRP) laminates were performed to investigate a failure criterion under biaxial loading. Specimens of unidirectional CFRP prepreg sheets were subjected to a tensile load in the longitudinal fiber direction and a compressive load in the transverse fiber direction. A dedicated jig was used to perform biaxial stress tests with a commonly-used single-axis testing machine. Measurements were obtained by controlling the displacement ratio, the ratio between compressive and tensile displacements. The maximum tensile and compressive stresses were then calculated using a constitutive equation. Longitudinal tensile strength markedly dropped with increasing compressive stress in the transverse direction. The failure criterion of the biaxial stress tests was expressed as an ellipse, of which the major and minor axes were the longitudinal tensile strength and transverse compressive strength, respectively. Moreover, scanning electron microscopy (SEM) observations suggest that fiber/matrix interfacial debonding occurred due to compressive stress, and such debonding could decrease the tensile strength in the longitudinal fiber direction.
Damage problem occurring in quasi-isotropic composite laminates subjected to low velocity impact of foreign object is analytically studied. The damaged portion was modelled as piled equal-thickness circular plates where delaminations do not open. This assumption is a crude but mechanically acceptable approximation of the impact damage problem. Closed-form expressions of displacement and energy release rate are obtained in terms of applied force for simply-supported and fixed boundary conditions, where the effect of transverse shear is considered. The solution helps us to clearly understand the fundamental mechanism of impact damage problems, despite the solution being valid only at the initial stage of damage growth owing to the strong geometrical nonlinearity of the delaminated portion.
Carbon fiber reinforced thermoplastics (CFRTP) have attracted intense attention because of their potential application in both weight reduction and high-volume manufacturing of automotive components. In this study, we focus on the flexural behavior of automobile panel parts by finite element method (FEM) to verify that the optimal structural design of CFRTP is significantly lighter than that of steel with the same stiffness and strength. There are three typical reinforcing structures for CFRTP: ribbed, gradual thickness distribution, and sandwich structures. We investigate the optimal structural design and verify its applicability by taking account of the analytical accuracy for each structure. Our previous research showed that the numerical simulation of ribbed structures does not conform to the experimental results because of low analytical accuracy. Instead, structures with a gradual distribution of thickness and sandwich structures have been used in this study to minimize the weight using a shape optimization technique. The results not only demonstrate nearly 70% reduction in weight as compared with a steel panel possessing the same stiffness, but also indicate that the strength criteria are satisfied.