Modeling and experimental investigations of drilling carbon fiber reinforced composites
Fattahi Massoom, Zahra Sadat
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The application of composites in different industries especially aerospace industry has significantly increased due to their phenomenal mechanical characteristics. Drilling woven composite parts is one of the most common operations in assembly, and therefore selecting the ideal process parameters and tool geometry is a vital key to control the drilling-caused damages (i.e. delamination). In this study, a novel mathematical model is proposed to predict the critical thrust force at which delamination starts. The model takes into account the thermo-mechanical loads with considering a mixed-mode of fracture that takes place in the delamination area for unidirectional composites. The proposed model has been benchmarked with five previous models including different composite materials, drill bits and feed rates. It was found that the proposed model offered the highest average accuracy among all studied cases. This model explains how the delamination area starts with a circular cross-sectional area in the entrance layer and grows into an ellipsoid, in the same direction of the fibers, in the exit layer. In addition, the visualization of this phenomena is executed through Finite Element Analysis. Furthermore, the second phase of this work offers an integrated approach to fully understand the machining process of woven composites. This integrated approach includes three stages; (i) experimental investigation of drilling thrust force, (ii) investigating the importance of drill bit geometry, and (iii) Effects of feed rate scheduling on minimizing delamination. It was perceived that only scheduling the feed rate is not sufficient for minimizing the delamination. However, updating the chisel edge ratio based on the drill bit geometry results in predicting a more accurate thrust force which can sufficiently minimize the delamination. Finally, the results obtained in this work provide a valuable guideline to better utilize common machining tools and resources while maintaining the quality of drilled holes.