Factors influencing the tensile strength of carbon fiber reinforced plastic laminates for laser machining method and the underlined mechanisms

This paper presents an investigation into the influence of laser drilling on the tensile strength of carbon fiber reinforced plastic (CFRP) laminates of 1 and 2.5 mm thickness. The CFRP laminates were drilled using the microsecond laser, nanosecond laser, and picosecond laser, and the heat-affected zone (HAZ) was characterized and measured by optical microscopy. Through setting laser parameters, specimens with different HAZs were prepared, and then tensile strength tests were conducted. The results indicate that the tensile strength linearly depends on the width of HAZ. The tensile strengths of 1 mm specimens decrease by approximately 122 MPa/mm HAZ, and 2.5 mm specimens decrease by approximately 33.9 MPa/mm HAZ. Comparing the results of different thicknesses of CFRP, the effect of HAZ on tensile strength would be weakened with the increasing of laminate's thickness. Actually, besides HAZ, cutting-induced geometric defects also seriously influence tensile strength, because of the stress concentration around these defects during the stretching process. Additionally, the mechanism why the HAZ influences tensile strength was revealed visually through FEM simulation: the resin matrix damage within HAZ causes the tensile loads not be transferred effectively between the fibers and then weakens the strength. It was the first time the underlined mechanism is revealed for HAZ-inducing strength reduction through FEM simulation. The laser machining method is more suitable for processing the thicker CFRP composites because the negative effect of HAZ is weakened for the thicker material.