The reinforcement mechanisms of C (circular) and R (radial) layers, which are novel reinforcement techniques for CFRP (carbon fiber reinforced plastic) bolted joints, were investigated based on finite element results. In the case of the specimens having large edge distance (e/d=3), the longitudinal compressive stress, σ_LC, at θ≒45º on the 45º layer and at θ≒−45º on the −45º layer was reduced by the R layer, resulting in the improvement of the initial failure strength of the BR (bearing) mode. On the other hand, in the case of the specimen having small edge distance (e/d=1), the in-plane shear stress τ_LT, in the ranges −90º≦θ≦−45º and 45º≦θ≦90º on the 0º layer was reduced by the C layer, resulting in the transition of the failure mode from SO (shear-out) to the BR mode. Additionally, the longitudinal compressive stress, σ_LC, at θ≒45º on the 45º layer and at θ≒−45º on the −45º layer was reduced by the R layer, resulting in the improvement of the initial failure strength when the C and R layers were combined.

The fragmentation tests estimate the fiber-matrix interfacial shear strength based on the number of fiber breaks in the single-fiber composites induced by stretching. In our previous work, a method to estimate fiber strength and interfacial shear strength by analyzing the results of the fragmentation tests was developed. In the present study, this method is applied to estimate the fiber strength and interfacial shear strength of carbon fiber and glass fiber reinforced polypropylene composites. A viscoelastic model has also been proposed, explaining the phenomenon whereby fiber continues to break at a fixed length of the composites after they are stretched.

This study develops a micro-scale simulation scheme to predict in-situ damage and strength properties of CFRP laminates with various ply thicknesses. The in-situ properties depend not only on the microstructure (fiber configuration, volume fraction, etc.) but also on the constraint effect from neighboring plies (i.e., ply thickness, stacking sequence, etc.). In order to capture both of the initiation and the propagation of the transverse cracks, the microscopic random fiber configuration and the constraint effect from neighboring plies should be carefully taken into account. To this end, this study considers the representative volume element which consists of the “inhomogeneous” ply in which the fiber and the matrix were individually modelled by the solid elements and the “homogenized” plies which were homogeneously modelled by the shell elements. In the inhomogeneous ply, the matrix damage and the debonding between fiber and matrix were modelled to reproduce transverse crack propagation. The validity of the proposed tool was evaluated by comparing the predicted cracking behavior with the results of unidirectional tensile tests on cross-ply laminates having different 90º ply thicknesses. Finally, the effects of ply thickness on the in-situ damage and strength properties of cross-ply laminates were examined by the proposed scheme.

In this study, the mechanical behavior of short-fiber-reinforced thermoplastics is predicted by finite element analysis using various unit cells. Each unit cell contains a single elastic fiber and an elastoplastic or viscoelastic-viscoplastic matrix resin. Additionally, fiber break and interfacial debonding between thermoplastics and fibers are reproduced using the extended finite element method and the cohesive zone model, respectively. The analyses using the unit cells evaluate the stresses for various fiber lengths and fiber orientation angles, and then the macroscopic stress is predicted by the laminate analogy approach. The predicted results are compared with the tensile test results at various temperatures. In the plastic region, the predicted results reveal that the viscoelastic-viscoplastic analysis with fiber break and interfacial debonding is needed to predict the actual stress-strain curve. The tensile test results are within the results predicted by the analyses with assumed interfacial strength and with infinite interfacial strength. Therefore, the mechanical behavior of short-fiber-reinforced thermoplastics could be predicted by the viscoelastic-viscoplastic analysis with fiber break and interfacial debonding using appropriate interfacial strength.