Certification for Design - Reshaping the Testing Pyramid

Lead Research Organisation: University of Bristol
Department Name: Aerospace Engineering


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.

Planned Impact

The PG is essential to enable continued growth of UK aerospace industry and take economic benefits from the opportunities inherent in the move towards more sustainable aviation, as it fills a knowledge gap, where there is no equivalent capability in the UK or internationally. The ATI Technology Strategy and Roadmaps show a clear need for continuing improvement in aircraft efficiency, which will require step changes in performance to enable e.g. the move to hybrid-electric powertrains and ultimately all-electric aircraft. These transformative technologies will impact on every aspect of the aerospace industry, but will specifically set very challenging targets in terms of the mass of aero-structures and new aero-structural forms as the industry transitions to blended wing/body and other advanced concepts.
Driven by regulatory requirements, and the need for reduced CO2 emissions, sustainability and energy efficiency, similar opportunities exist in other sectors, and, whilst this PG is focussed on the developing needs of the aero-structures industry, it will also enable development and growth in other sectors through the delivery of improved structures development and certification methodologies. In particular, the scientific outputs of the PG are transferable across sectors that are also in need of innovative, cost effective and sustainable composite solutions, and which are also subject to regulatory constraints that inhibit the uptake of composites, including the automotive, renewables, rail, construction, marine and oil/gas sectors. Moreover, the radically new integrated approach to virtual and physical test and product validation, has a potentially strong crossover into emerging technologies such as Additive Manufacturing that currently lacks comprehensive routes to validation and certification.
The near-term impact of the PG is by providing the scientific basis and fundamental methodologies for unlocking the barriers for innovation and efficiency gains set by current certification procedures. The output of the PG will provide the UK with the capability to be the decisive 'first mover' and gain advantage in the global market place. The PG will enhance the UK position in the technical revolution that embraces new materials and processes, by addressing an urgent need in aerostructures design.
The long-term impact will be the development of new methods for verification (certification) that will provide the world-leading advantage required for winning UK design and manufacturing workshare. The research impacts directly on certification authorities, industry, as well as with the High Value Manufacturing Catapults including the NCC.
The impact and speed of uptake of the research outputs will be maximised by close engagement with an Industrial Steering Group, which will include representatives of industrial partners Airbus, GKN Aerospace, Rolls Royce, BAE systems and CFMS, and stakeholders NCC and the European Aviation Safety Agency EASA.
The impact into the industrial base will be further facilitated by secondments of Postdocs, researcher mobility and internships of the PhD students. Dissemination of research outputs will be through face-to-face meetings, industry seminars/workshops, scientific articles, articles/ communications directed towards industry, a PG sharepoint/website, conference presentations and learned society activities.
The PG will provide PhD/EngD graduates with unique and world-leading competence in testing, modelling and qualification/certification of advanced composite aerostructures, and will continue to have an impact well beyond the term of the PG, both as highly skilled engineers in industry and as a new generation of researchers and academics.


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