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 degrees 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 (sigma(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% sigma(UTS).