The performance and strength of a composite aero-structure is established incrementally through a programme of analysis and a series of experimental tests conducted using specimens of varying size and complexity. The process utilises a so-called 'building block' or 'testing pyramid' approach with tests at each of the following levels: (i) Coupon, (ii) Structural detail, (iii) Component, and (iv) Sub-structure or full structure. The 'building block' approach provides a comprehensive and systematic methodology to demonstrate airworthiness and structural integrity, and as such represents the backbone of the certification processes for composite aero-structures. The vast majority of certification tests are conducted at the coupon level, whereas far fewer certification tests are conducted at the subsequent higher pyramid levels. The complexity, cost and time of each test escalates up through the testing pyramid. The underlying assumption is that the material properties derived from tests at the lower levels can be used to define the requirements and design allowables at higher length scales and component complexity. At the mid-pyramid level, the as-manufactured strength of parts is currently assessed by empirical 'manufacturing knockdown factors', and the uncertainties in this assessment, together with uncertain in-service damage, propagate up the pyramid to the full component and structure levels. At best, this leads to conservative, over-constrained design. At worst, there is risk that potentially unsafe scenarios can develop where combinations of weakening events cascade into premature failure. Thus, the very time consuming and expensive testing at the coupon level, produces conservative strain limits with questionable relevance to the strength of large parts or at the full structure level. Also, innovative material and technology developments, which facilitate lightweighting, safer and more damage tolerant composite design, are only relevant at the sub-structure and component levels, and therefore cannot be incorporated into applications because of the current validation practices. Accordingly there is increasing evidence that the building block approach has severe limitations, particularly the high cost of certification, time to market, and the general inability to characterise and predict limit states that may lead to failure at structural scales. There is increasing awareness that, in its current form, the 'building block' approach prevents the innovative use of composites, and consequently that the potential benefits of using advanced composites in terms of lightweighting and efficiency cannot be fully realised under current certification and regulatory procedures.
The vision and ambition of the PG are:
AMBITION: To enable lighter, more cost and fuel efficient composite aero-structures through developing the scientific foundations for a new approach for integrated high-fidelity structural testing and multi-scale modelling and 3D product quantification based on Bayesian learning and statistical Design of Experiments (DoE), incorporating understanding of design features at structural lengths scales.
VISION: To enable more structurally efficient and lightweight airframes that are essential for meeting future fuel and cost efficiency challenges and to maintain and enhance the UK's international position in the aerospace industry.
The PG provides a route for lessening regulatory constraints, moving towards a more cost/performance optimised philosophy, by reducing the multiple coupon level tests at the bottom of the test pyramid. Instead structural behaviour will be accounted for in a new culture of virtual design and certification focusing on the higher levels of the testing pyramid. This will promote a change towards virtual testing, enabling reduction of empiricism, significant mass savings, expansion of the design and performance envelopes, and reduction of design costs and associated development time.