A novel reinforcement method for carbon fiber reinforced plastic (CFRP) bolted joints is proposed considering the cylindrical stress fields around bolt holes. The C and R layers are circularly and radially steered reinforcing layers in the directions of the maximum tensile and maximum compressive stresses, respectively. The effects of the C and R layers on the bearing strength of CFRP bolted joints were investigated by conducting double lap shear tests. The tests showed that the C layer was effective at preventing the shear-out failure and increasing the bearing strength of joints during the ultimate stage of failure. By contrast, the R layer was effective at increasing the bearing strength of joints during the initial and ultimate stages of failure. The reinforcement mechanisms were also studied according to the local stress fields around the bolt joints.

Thermoplastics are viscoelastic-viscoplastic materials whose mechanical behavior depends on temperature and strain rate. In this study, the viscoelastic-viscoplastic characteristics of polybutylene terephthalate are evaluated on the basis of the mechanical model consisting of Schapery nonlinear viscoelasticity and Perzyna viscoplasticity. Also, the viscoelastic-viscoplastic behavior of the polybutylene terephthalate is predicted by finite element simulation. The evaluation reveals that the initial yield stress depends on temperature and strain rate; however, viscosity depends on only strain rate. Furthermore, the rate sensitivity is independent of temperature and strain rate. The finite element simulation using identified viscoelastic-viscoplastic characteristics can reproduce the stress–strain relationship in the uniaxial tensile tests under various testing conditions. It is expected that the time-temperature superposition principle can be applied to not only the maximum stress but also the initial yield stress.

The effect of stress concentration on compressive failure behavior of carbon-fiber reinforced plastic e laminates with open holes was experimentally and numerically investigated. The aspect ratio of the elliptic hole was changed to determine the dependence of the failure behavior on stress concentration. The finite element analysis was implemented by using the ABAQUS 2017 software and a dynamic explicit solving method. Continuum shell elements incorporating Hashin’s failure criteria and cohesive interaction, which is a combined function of the general contact definition and cohesive traction-separation law, were used to model lamina and interlaminar damage growths, respectively. The damage initiations were defined by critical stresses, while the damage evolutions were defined by energy assumptions of softening behavior for the continuum shell elements and cohesive interactions. The numerical results of the stresses at initial damage occurrences and ultimate failures agreed well with the experimental results for the specimens of various aspect ratios. Moreover, the numerical and experimental results of the damage behavior of the 0º ply were in good agreement. Furthermore, we discussed internal damages and delamination behaviors, which are difficult to observe through experiments, based on the numerical results.