Predictive modeling and experimental analysis of the drilling process of Carbon Fiber Reinforced Polymer composite laminates

Abstract

Carbon fiber reinforced plastic (CFRP) is promising composite material which is a combination of carbon fiber and polymer matrix. CFRP has been actively utilized in the aerospace, automobile, and sport goods due to their superior mechanical properties. High strength-to-weight ratio is one of the biggest advantages of CFRP composite laminates which contribute to development of lighter structural component with abundant strength and stiffness. Although CFRP materials are fabricated in the near-net shape, they still demand the post processing such as drilling, trimming and surface finishing processes to be used as a final product. Among them, the drilling is the most frequently used process for assembly and joining of the components. However, anisotropic and non-homogeneous characteristics of CFRP composite make the drilling process much more difficult than general metallic materials. Above adversities include the reduced life of drill bit with excessive tool wear by materials and leads to defect occurrence in the composite workpiece. Therefore, investigation of optimal process parameters is essential to overcome the adverse effect which can be appeared during drilling process and to make high quality CFRP hole. This study aims at predictive modeling of drilling process of CFRP composite to understand mechanism of force and defect generation. Predictive modeling can be divided into two sections, analytical and numerical studies for a deeper understanding of the process. First, experimental studies were conducted to investigate to figure out the process parameters affecting the drilling process of CFRP composites. Machining process involved with multiple factors, such as machine tool dynamics, tool geometry, material properties, and cutting conditions. Especially, fiber volume fraction, and thermo-mechanical properties of CFRP composite was also considered in the study. Cutting force, and delamination were observed with dynamometer, optical microscope and computational tomography image. It was observed that both thrust force and delamination were intensely related to the feed, and diameter of drill bit. Fiber volume fraction also contribute to the magnitude of cutting force and force changes in time domain. Support plate showed prevention effect of delamination without affecting the cutting force and tool wear. Second, an analytical investigation of the drilling forces for unidirectional CFRP composites according to different machining parameters. Analytical modeling focused on the prediction of thrust force, which was a force generated in the drilling feed direction. Process parameters were selected considering the geometry of drill bit, cutting conditions, and material properties for the accurate calculation. In the analytical modeling in light of drill bit, the chisel edge region is classified as an extrusion operation and the lip is considered as an orthogonal small-element cutting region. The chipping, pressing, and bouncing regions of the lip are incorporated into this thrust force model. In addition, the analytical model includes the thermophysical properties of CFRP by incorporating softening of the material due to heat generation during the cutting processes. Based on the developed model, the thrust force curve for all drilling stages is analyzed in time-domain considering both cutting conditions and material properties. Comparison between predictions and experiments was performed on three CFRP samples (USN 150Y, USN 150B, and USN150E) with different fiber volume fractions and sixteen cutting conditions. It was observed that the predicted forces can capture the trend of the experimental data with the error of the minimum and maximum percentage error for USN 150Y was 12 to 29%, 19 to 36% for USN 150B, and was 16 to 33% for USN 150E. Time-domain analysis and fluctuation evaluation were also used to better understand the mechanism of thrust force during CFRP drilling. Finally, investigation of the delamination of CFRP composites laminates during the drilling process has been conducted. When drilling the CFRP laminates, delamination is one of the severe defects that degrade the quality of CFRP products. However, delamination is the damage propagation inside the material which demand numerical analysis to observe stress-strain behavior. Therefore, finite element model of CFRP drilling was developed using commercial FE software to simulate the damages generated in the cohesive zone between the interface of laminates. Drilling simulation and tests were conducted to analyze effect of feed, spindle speed, and back-up plate on delamination as well as the thrust force. Before the delamination analysis, calculated thrust force results were validated by the experimental results to assure the accuracy of simulation. Delamination of the FE model were assessed by the evaluating damage criterion value of cohesive zone between the composite laminate plies. Quantified delamination factors of simulation were compared to the experimental results which acquired and processed by computational tomography image and image processing techniques and showed good agreement.