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.
