Know the drill

SGS Carbide Tool's composites machining specialist, Martyn Wiltshire says that while composites are attractive for aerospace applications because of their exceptional strength and weight properties, these benefits can come at a price.

The determination of the aerospace industry to enhance the performance of aircraft is constantly pushing forward the development of improved high performance structural materials. Composite materials can provide a much better strength-to-weight ratio compared to metals. The lower weight results in reduced fuel consumption and emissions and, because composite structures need fewer riveted joints, it enhances aerodynamic efficiencies and lowers manufacturing costs. By way of comparison, the ultimate strength of aerospace grade aluminium alloys is typically 450MPa, a carbon fibre composite would be five times that value. Additionally, composites are only 60% the density of aluminium, so the potential for weight reduction in an aircraft is very apparent. In addition to strength and weight, composites are thought to be virtually immune from ‘fatigue'. Because of the non-homogeneous structure of composites cracks do not spread, allowing structural engineers to perform design and analysis assuming much higher resistance to stress, and increased long-term durability. A material whirl The composite materials used in the aerospace sector comprise CFRP and GFRP, and consist of carbon and glass fibres, both of which are stiff and strong, but brittle, in a polymer matrix, which is tough but neither particularly stiff nor strong. By combining materials with complementary properties, a composite material with most or all of the benefits of strength, stiffness, toughness and low density is obtained with few or none of the weaknesses of the individual component materials. It's with the advent of the latest generation of airliners, that these materials have been deployed extensively in the primary load carrying structure. The Airbus A380 uses composite materials in its wings, which helps enable a 17% lower fuel use per passenger than comparable aircraft. Composite materials comprise more than 20% of the A380's airframe, where it is used extensively in wings, fuselage sections such as the undercarriage and rear end of fuselage, tail surfaces, and doors. The A380 also makes extensive use of glass laminate aluminium reinforced epoxy (GLARE), which features in the front fairing, upper fuselage shells, crown and side panels, and the upper sections of the forward and aft upper fuselage. GLARE laminates are made up of four or more 0.38mm thick sheets of aluminium alloy and glass fibre resin bond film. GLARE offers weight savings of between 15% and 30% over aluminium alloy along with exceptional fatigue resistance. The top and bottom skin panels of the A380 and the front, centre and rear spars contain CFRP, which is also used for the rear pressure bulkhead, the upper deck floor beams, and for the ailerons, spoilers and outer flaps. The belly fairing consists of about 100 composite honeycomb panels. Following the Airbus lead a number of current large aircraft development programmes are looking to use composites more extensively within the wings and fuselage. The Boeing 787 Dreamliner, for example, will be as much as 50% composites. This revolutionary aircraft uses a novel process of ‘winding' composite layers, like the winding of a cotton reel, in the fabrication of large, joint-less, fuselage sections. Meanwhile the Airbus A400M, the next generation of military airlifter expected to make its first flight later this year, similarly has wings made from carbon fibre composites. This aircraft is designed to withstand the severe loads associated with operations from informal landing strips like deserts and fields, and it benefits from the superior fatigue resistance of carbon composites. Another advantage of composite materials is that, generally speaking, they can be formed into more complex shapes than their metallic counterparts. This reduces the number of parts making up a given component, thereby reducing the need for fasteners and joints. The advantages of which are twofold: fasteners and joints may be the weak points of a component and fewer fasteners and joints can shorten assembly times. The hole truth However, hole making, predominantly for joining, represents around 90% of carbon fibre machining requirements in the aerospace industry. Structural aircraft components such as wing boxes, spars, stringers and skins are made from various composite materials, while other complex structures, such as fuselage central wing boxes, are made from several types of composite material often stacked with alloys. All require hundreds of holes drilled in them. So, the drills selected must be able to withstand the harsh demands of the latest CFRP materials and aluminium stacks. However, attributes such as long tool life and precision need to be matched with impressive performance. The geometry should be designed specially to counter common problems such as splintering or fraying that compromise composite hole quality. Thrust force is a critical factor in the onset of delamination and splintering. Geometries should be designed to reduce thrust and to achieve correct cutting of carbon fibres in order to achieve rigorous hole quality demands. In the aerospace sector, typical hole requirements include surface finish of better than Ra 4.8µm, delamination of less than 1mm over the diameter, and no composite splintering. Production engineers should be looking to their industry partners to help increase hole making performance in composite and stacked material applications. The performance data is dependent purely upon application, and SGS has gathered a significant amount of cutting tool data in various production and development scenarios. The cutting tool edge preparation and point geometry play a significant factor in the tools performance and life, and here experience and the ability to modify parameters count. Machining strategies must also be considered with techniques such as orbital drilling being developed and exploited. Mixed material stacks create the most demanding applications, with sandwiches of very disparate materials such as carbon composite, titanium and aluminium all requiring different cutting conditions. This is being compounded by many manufacturers now looking for one-shot solutions, so as well as the hole being created the external surface is finished with a countersink or a spot face. No doubt the considerable benefits offered by composites have yet to be fully exploited and as knowledge and understanding grow, composite materials will play an increasingly significant role. This role will expand not only as a result of improved material performance, but also as our ingenuity finds more and diverse areas where composite materials can be beneficially and advantageously applied. www.sgstool.com