Influence de la temperature de coupe sur les propriétés mécaniques en traction uni-axiale d'un composite carbone/époxy et étude de l'usinabilité d'un composite lin/époxy

Abstract

CFRP composites have seen a significant increase in their use over the last years thanks to their high specific properties. Despite the continuous improvement of molding methods to produce laminated parts, trimming operation is still necessary to achieve the functional dimensions required by the specifications. However, carbon fibers are very abrasive. This causes rapid tool wear and high cutting temperatures which damage the epoxy matrix. The first part of this study aims to highlight the influence of cutting temperature generated by the tool wear on the surface finish and tensile properties. To highlight the edge defects and describe the machining, 6 mm and 12 mm wide test specimens are formed, on a carbon/epoxy laminate, by a trimming operation in order to amplify the effect of defects on the measured properties. A new tool and a worn tool are used to generate different cutting temperatures. The measures show a cutting temperature of 300 °C reached for the new tool and 475 °C for the worn tool. The analysis of surface reveals that the specimens machined with the new tool have no thermal damage and the cut is clean. Specimens machined with the worn tool show a degraded surface and a carbonized matrix. We can’t see the plies after cutting, which is due to a degraded resin spread on the machined surface creating a surface with a lower roughness, giving the artificial impress of good cutting conditions. The tensile tests show no variation of the mechanical properties for 12 mm wide specimens regardless of the machining condition. A 10% loss in mechanical performance for 6 mm wide specimens is observed. These results suggest that the thermal defects caused by the tool wear affects the tensile properties, but only from a certain low width of the specimen from which the influence of machining defect start to affect the tensile properties (the defect size representing a larger proportion of the specimen cross-section for lower width specimens). The second part of this study focuses on flax fibers composites. These fibers have a great economic and environmental interest because they are natural products, biodegradable, and entirely recyclable unlike synthetic fibers. They also are less expensive than synthetic fibers such as glass or carbon fibers. Moreover, they are slightly abrasive which provides a greater tool lifetime and their specific properties (strength-to-weight ratio) are comparable to glass fibers. So, they provide economic and environmental benefits for companies. Unfortunately, the machining knowledge of this kind of material is low and researches in this way have barely started. The objective of this study is to describe the machinability of a flax/epoxy composite (NFRP) and observe the influence of cutting parameters and fiber orientation on the cutting forces and the surface finish. Milling tests are made on unidirectional composites laminates with two different tools. The results show that the surface finish and cutting forces depend on the feed rate and are independent of the cutting speed. Tool #2 (PCD tool) shows the best results compared to the tool #1 (CVD tool), a better surface finish, a lower delamination factor and lower cutting forces. This material is easily machinable and low abrasive because no tool wear can be seen after cutting. In a cutting forces minimization objective, roughness minization objective and productivity maximization objective, and intermediate feed rate and highest cutting speed seem to be the best parameters.

Date	12 Jun 2015
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Jean-François Chatelain (Supervisor) & Gilbert Lebrun (Co-supervisor)