An analytical tool wear model for drilling of continuous fibre reinforced composites
Carbon fibre reinforced polymers (CFRP) are increasingly being used in aerospace structures due to high specific stiffness and good corrosion resistance. Drilling holes to facilitate the mechanical joining of CFRP to other, metallic components is a key operation during aircraft manufacture and high rates of tool wear are experienced in commercial practice. Unmonitored tool wear can lead to increased cutting forces, heighted cutting temperatures, and subsequently, delamination, fibre pull out or thermal degradation of the polymer matrix. Understanding the tool wear mechanism is crucial as it not only affects the hole quality, and therefore structural integrity of the component, but also production planning and cost. Existing research has focused on experimentally observing the relationships between tool wear and drilling parameters (cutting speeds and feed rates), tool design (geometry and tool coatings) composite properties (polymer matrix and reinforcing fibre) and environmental conditions (drilling temperature). However, knowledge gaps are apparent when it comes to understanding the physics relating these variations to the magnitude and rate of tool wear. The optimum method of addressing this research gap is through the use of computational models or simulations, however existing works do not produce informative results regarding the progression of tool wear throughout the course of the tool’s life. Thus, the aim of this thesis is to define a method of modelling the severity and progression of tool wear when drilling fibre reinforced composites. This is based on the hypothesis that by building an analytical tool wear model based on known machining, geometric, structural and environmental parameters, the magnitude and shape of tool wear can be calculated, giving informative results in a time efficient manner. Consequently, informed through a dense literature review, this research has identified an appropriate, cyclic analytical modelling methodology for the drilling of continuous fibre reinforced composites. The problem is discretised down to a two dimensional simplification, and through the use of unique helical path contact equations the total number, chronological order, and angle of each idealised fibre-tool wear contact is identified. Hertz’s elastic equations for a cylindrical body indenting an elastic half space are used to model the penetration depths of each abrasive fibre-tool contact. The resultant overestimation of the idealised contacts compared to existing tool wear studies, enforced the need for a corrective non-linear efficiency factor to be applied to each individual wear calculation. A select set of test cases are proposed, investigating the model’s sensitivity to changes in specific input parameters, themselves directly related to changes in the idealised number of abrasive wear contacts, the tool material and the cutting temperature. Subsequently, the model’s outputs are validated against a system of commercially representative drilling experiments, enabling the accuracy of the physics and mechanical principles built into the model to be critiqued. Firstly, the model found the idealised number of fibre-tool abrasive contacts to have a significant impact on the magnitude and rate of cutting edge rounding wear. The validating experiments did not agree with this finding, reporting negligible change in wear magnitude or rate irrespective of changes in the number of fibre-tool abrasive contacts. Secondly, the validation experiments found that exchanging the uncoated tungsten carbide tool for a polycrystalline diamond coated variant, the rate and magnitude of tool wear significantly decreased. By changing the associated elastic modulus parameters within the analytical model, this same trend was identified. Thirdly, the validation experiments found that irrespective of drilling speeds or feed rates, the tool tip temperature exceeded that of the glass transition temperature of the resin matrix constituent of the CFRP laminate. By modifying the associated material stiffness parameters within the analytical model, the estimated rate and magnitude of tool wear decreased. This brought the model and experiment finding closer together. Lastly, the combined studies have identified the shape of the edge rounding wear to be significant. The proposed model describes worn cutting edges using a single radius, disconnected from the original flank face of the tool. As such, further investigations are necessary to determine the relationships between the shape of edge wear and the drilling conditions. In order to achieve this, there is an underlying need to generate a method of definitively measuring tool wear, internationally recognised for its ability to describe the magnitude and shape of the wear at the cutting edge.
Drilling, CFRP, tool wear, analytical methods, abrasion
Networked Digital Library of Theses and Dissertations