Effects of machined circular holes on the mechanical properties of unidirectional carbon fiber-reinforced plastics

The effects of machined circular holes on the mechanical properties and failure characteristics of a unidirectional CFRP were investigated. Our approach was to change the location and the number of holes: (i) a two-set hole was machined with different modes: (A) parallel, (B) 45°and (C) a direction perpendicular to the loading direction; and (ii) multiple holes from n = 0 to 5 were made in the sample parallel to the loading direction. The higher tensile and higher fatigue strengths were obtained for the CFRP sample produced by mode A, compared to modes B and C. This was attributed to the different extend of the maximum stress and stress distribution, caused by the geometrical effects on the sample. The ultimate tensile strength (σ_UTS) of the sample was well predicted by the geometrical criterion. If the number of holes was increased from n = 0 to n = 2, the tensile strength decreased dramatically. However, the tensile strength did not strongly decrease further for samples with multiple holes from n = 2 to 5. The tensile strength is correlated with the maximum stress adjacent to the hole(s). Those stress values were verified by a 2D digital image correlation and a finite element analysis. Material failure of the CFRP during tensile loading was revealed by nondestructive testing using a piezoelectric ceramic, and debonding of the fibers occurred even at a low applied stress of approximately 35% σ_UTS.