Open-hole compression (OHC) strength is a standard design parameter for carbon fiber reinforced plastics (CFRP) laminated structures. However, the actual structures often use fasteners, resulting in a filled-hole compression (FHC) strength that typically exceeds the OHC strength. Accurate characterization of FHC strength can facilitate lightweight design and improve the testing standards. This study focused on the effect of the clearance ratio between the pin and the hole on the FHC strength. The failure process was investigated for a clearance ratio of 0.4％ using strain measurements and X-ray computed tomography/microscopy. The results showed that, upon pin-to-CFRP contact, the strain increased in the upper part of the hole and decreased at the side edges. As the load increased, the strain growth at the upper part saturated, and the final failure was initiated from the side edges. Tomographic observations confirmed that the damage in the contact region progressively propagated with the load. These findings suggest that the load-transfer path at the contact interface mitigates the stress at the hole edges, thereby increasing the FHC strength. However, the associated damage eventually reached a critical limit, ultimately governing the fracture strength at the hole edges.

Injection-molded short-fiber reinforced plastics (SFRPs) exhibit residual stress, which is caused by their internal fiber orientation distribution. However, measuring the internal fiber orientation distribution of SFRPs is challenging, thus resulting in no investigations into their residual stress. In this study, first, the fiber orientation distribution tensor in the thickness direction of an injection-molded SFRP flat plate was obtained using X-ray micro-computed tomography (micro-CT). Second, the anisotropic material properties arising from the fiber orientation tensor were calculated using Digimat. Third, residual stress analysis was performed by applying the material properties that correspond to the fiber orientation tensor, to a multi-layer meshed finite element model. Finally, to verify the results of the residual stress analysis, they were compared with the released strain obtained using the hole drilling method. Based on these results, released strain was thus qualitative agreement with that calculated via residual stress analysis.

In this study, the viscoplastic mechanical behavior of CFRP angle-ply laminates was analytically investigated. Based on experimental observations, a viscoplasticity model with overstress was used. This model additively decomposes the total stress into equilibrium stress and overstress. An orthotropic plasticity model (Hill model) was used for the equilibrium stress, and Lion’s model was employed for overstress. A lamina-level plane stress model was developed and used for the laminate analysis. A return-mapping algorithm with the backward Euler method was used to solve the elastoplastic problem of equilibrium stress. The Newton-Raphson technique was used to solve the resulting system of algebraic equations. The model qualitatively describes the strain rate effect, stress relaxation behavior, and hysteresis behavior during the loading-unloading process.