IMaging and Probabilistic Assessment of Composite damage Threats (IMPACT)

Low velocity impact to Carbon Fibre Reinforced Plastic (CFRP) aerospace structures is common and can create damage that is almost undetectable from the surface yet may reduce compressive strength by up to 60%. Compression After Impact (CAI) strength of aerospace components is currently assessed through expensive and cumbersome experimental studies. The resulting design strategy - conservative thickening of vulnerable components to reduce in-service strains - is likely having a negative effect on airframe weight and fuel efficiency. This strategy is both a consequence of significant uncertainty in the factors that contribute to impact damage and compressive strength reduction, and of a lack of modelling capability for CAI strength that accounts for such uncertainty.
A recent project funded by Airbus UK, GKN Aerospace and ESPRC (EP/H025898/1) has led to the development of an analytical Damage Tolerance Model (DTM) that can capture the strain at which impact damage in a CFRP panel will grow under compressive loading. The DTM has computational efficiency that is sufficient to allow uncertainty in factors such as material properties and damage severity to be captured using large scale parallel computations i.e. Monte Carlo Simulations (MCS). However, the DTM relies on individual experiments to provide the size and structure of impact damage and this is currently limiting its efficiency and applicability in early stage design.
IMPACT will address the issue of damage structure by developing an empirically based predictive model. X-Ray Computed Tomography (XRCT) and ultrasonic inspection of impacted CFRP laminates, in partnership with generalised laminate design, will underpin the generation of empirically-based, but predictive, scaling laws that describe the structure of impact damage. The resulting model will be combined with the DTM and, exploiting MCS and new aircraft licensing body regulations on probabilistic methods , used to capture the effect of uncertainty in factors affecting the strength of damaged CFRP panels e.g. material properties varying with batch of CFRP. The resulting probability distribution for post-impact compressive panel strength will be linked with probability distributions for the detectability of impact and severity of both damage and compressive loading. The final overall distribution will indicate whether a specific design strain can be reached with an acceptable probability of failure.

Industrial: The principle of IMPACT is to reduce conservatism with regard to residual strength of composite aerospace components. Such a reduction should enable a lighter more fuel efficient next generation of aircraft in the next 5-20 years. Aircraft with increased fuel efficiency: are cheaper to operate; produce less emissions and potentially less noise; and are more a more competitive product. Increased aircraft sales and hence manufacturing activity will have a positive effect on the Aerospace sector and its supply chain and on the UK composites sector. UK aerospace is one of the most important national manufacturing sectors. With a 13% share of the global market it is second placed internationally only to US aerospace. Composites are a critical part of future UK manufacturing. Indeed, as noted by EPSRC, composites have been identified as a national priority area by the Department for Business Innovation and Skills (in the UK Composites Strategy (2009)) and also by the Technology Strategy Board (TSB). They are used extensively in aerospace structures and the composites supply chain has annual production revenues of £1.6bn. As impact damage and the lack of knowledge of its formation and growth are one of the major barriers to aircraft fuel efficiency, enabling a 10% weight reduction will be extremely relevant to aerospace companies. In particular, Airbus and GKN Aerospace will benefit directly from new knowledge about impact damage formation and reduced uncertainty in early design stage models for assessing residual strength. This will help them to provide products and technical solutions compatible with ACARE 2050 emissions targets. The incorporation of uncertainty will provide greater confidence in results allowing for reduced conservatism and test program size. Aerospace companies will benefit from an understanding of the role of uncertainty in stress method development. Rhead has ongoing collaborations with Airbus and GKN Aerospace through the EPSRC ABBSTRACT 2 project (EP/H025898/1), the University of Bath is an Airbus partner university and the CRU is supported by a strategic partnership with GKN Aerospace. The important two-way transfer of knowledge with UK aerospace and composite companies will be maintained via quarterly meetings of the project partners. Visits to the NCC, Airbus and GKN Aerospace will ensure a practical understanding of industry issues is maintained at all times. Where appropriate, results will be communicated to groups such as the British Airline Pilots Association via the CRU. Note that research from IMPACT should also be transferable to helicopter and military aircraft applications with similar impact.

Public: IMPACT will enable the delivery of future aircraft that are lighter and more fuel efficient than previous generations. The general public will see health benefits from reduced emissions and financial benefits as reduce flight costs are converted to reduce ticket prices. Lighter aircraft may also result in less powerful engines being required and subsequently a reduction in noise near airports. Open access publication and public data storage will ensure all information relating to the project is directly available to the public and to any aerospace or composite related UK company.

Policy: The UK is committed to reduction of emissions. Results from this project that enable such reductions will help deliver such commitments.

Staff: The Post-Doctoral Research Assistant (PDRA) will develop transferable skills in Non-Destructive Evaluation (NDE) and data interpretation/processing that will be applicable in all disciplines across the engineering sector.

Data access: Costs are included for storage of experimental data in publically accessible servers enabling open access and dissemination of results. The existence of such a database will be highlighted through conference dissemination, publicity through Rhead's online presence and through the use of a project twitter account.