The use of readiness levels to describe system maturity has become an increasingly recognizable element of the development of technologies for future products in the aerospace and defence sectors. Technology readiness levels (TRLs), as originally defined by NASA, are now widely used by global organizations to articulate the relative maturity of key generic technologies. The implementation of radically new manufacturing capability can also benefit from such an approach. However, the direct use of TRLs in a manufacturing environment is problematic on the basis that it is hard to interpret them precisely, and because the effectiveness of the technology itself is only one of a number of success factors associated with a successful implementation. For several years now the aerospace industry has recognized the need for a manufacturing specific readiness interpretation to enable the delivery of new capabilities in a defined sequence of events. This paper describes an approach used within a single aerospace organization, Rolls-Royce plc. The framework has been identified as a good practice by the UK Aerospace Design and Manufacturing National Technical Committee.
The manufacture of an adhesively bonded structure which involves the joining of a carbon-fibre-reinforced plastic (CFRP) composite panel and an aluminium sheet is described. The thermal characteristics of CFRP composites and aluminium structures are very dissimilar. Hence careful design of the joint is required in order to achieve a bond which has the potential of durability when exposed to a high-humidity environment, such as is met in aircraft structures. The design of such a set of joint structures and their ageing in a high-humidity environment are described. The durability of the joints was assessed using high-frequency dielectric measurements as a non-destructive method and, in a parallel investigation, destructive shear strength and fracture toughness measurements were undertaken. Changes in the mechanical properties of the joints are correlated with the effects of moisture ingress into the joint structures. The joints obtained were surprisingly durable and this study implies that the design of aluminium-CFRP composite joints are a practical proposition.
The ageing behaviour of aluminium epoxy-aluminium, adhesively bonded structures is investigated using high-frequency dielectric analysis, mechanical shear and cleavage experiments. The joints are aged at 70°C and 100 per cent humidity and exhibit significant loss of strength over a prolonged period of exposure. The frequency domain dielectric data indicate the rate at which water is absorbed in the joint structure and whether or not significant corrosion of the oxide layer at the interface between the adhesive and the substrate has occurred. Time domain data indicate whether the joint cross-section has undergone significant change with exposure to the ageing environment. A good correlation appears between the extent of change in the joint structure as characterized by the dielectric data and the loss of mechanical strength. There is evidence of toughening due to plasticization of the adhesive from the fracture toughness data.
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