Effect of edge trimming on surface quality and tensile strength of CFRP composites

Carbon fiber reinforced polymer (CFRP) composites offer exceptional specific strength and stiffness properties that make it critically important material for aerospace applications. Even though composite parts are produced near net shape, they have to undergo machining processes to attain required surface finish and dimensional accuracy. Conventional machining processes such as edge trimming, milling and drilling produce machining damage which may lead to undesirable changes in mechanical strength properties. In this work, edge trimming of CFRP coupons was conducted to determine the effect of machining quality on tensile strength. The edge trimming operation was carried out on CNC machining centre by varying cutting speed and feed rate by three levels and keeping depth of cut constant (1.25 mm). The levels of cutting speed and feed rate were (75, 150, 300 m/min) and (2.54, 5.08, 10.16 m/min), respectively. Machining quality was evaluated by surface roughness parameters and delamination damage. The delamination damage of machined laminate was described by depth and frequency of occurrence using optical microscope. Surface roughness was described by Ra and Rz parameters using stylus roughness tester. The tensile strength was measured on MTS (Material Testing System) for every machined coupon. It was found that the most common delamination type was Type I followed by Type I/II. The roughness parameters and delamination frequency charts showed that surface roughness and delamination damage increased with the increase of feed rate and the decrease of cutting speed. The best machining quality was obtained when cutting at the highest cutting speed and lowest feed rate (i.e. the smallest chip thickness). Statistical analysis of the results showed that feed rate has the most significant influence on Ra and Rz. Delamination depth was also most significantly influenced by feed rate. Tensile strength was also found to decrease with the increase in feed rate. The variation of tensile strength with cutting speed was different; it decreased with the increase of cutting speed up to 150 m/min and with further increase of cutting speed the tensile strength increased. The correlation between tensile strength and machining parameters was not very strong. This is due to the great scatter in the experimental data that is caused by material inhomogeneity. Nevertheless, regression analyses have shown that tensile strength decreases with the increase of feed rate, surface roughness and delamination depth.