Understanding Delamination Suppression at High Deformation Rates in Through-Thickness Reinforced Laminated Composites

This proposal focuses on the impact performance of state-of-the-art composites in the form of fibre-reinforced plastics (FRPs) with through-thickness reinforcement introduced via Z-pinning.

The application of composites in primary lightweight structures has been steadily growing during the last 20 years, increasing the requirement for new and advanced composites technologies. Recent examples include large civil aircraft, such as the Boeing 787 and the Airbus A350, high performance cars, such as the McLaren 650S, and civil infrastructure, such as the Mount Pleasant bridge on the M6 motorway.

FRPs are made of thin layers (plies) of plastic material with embedded high stiffness and strength fibres. The plies are bonded together in a stack by applying heat and pressure in a process known as "curing". The resulting assembly is the FRP laminate. The main reasons for the increasing usage of FRPs in several engineering fields are the superior in-plane specific stiffness and strength with respect to traditional alloys and the long-term environmental durability due to the absence of corrosion. Another key advantage of FRPs is that they can be tailored to specific design loads via optimising the orientation of the reinforcing fibres across the laminate stack.

FRPs are, however, prone to delamination, i.e. the progressive dis-bond of the plies through the thickness of the laminate. This is due to the fact that standard FRP laminates have no reinforcement in the through-thickness direction, so the out-of-plane mechanical properties are significantly lower than the in-plane ones. According to the US Air Force, delamination can be held responsible for 60% of structural failures in FRP components in service. Impacts are the main cause of delamination in FRP laminates with energies usually in the order of 20J, sufficient to produce multiple delaminations in FRP plates. A representative scenario for such energy level is that of a 2cm diameter stone impacting a laminate at a speed of 110 km/h. In aerospace impact scenarios can be much more severe. For example, the certification of turbofan engines requires the fan blades to be able to withstand an impact with a bird whose mass is in the order of a few kilograms at speed in excess of 300 km/h, with impact energies of thousands of Joules.

Introducing through-thickness reinforcement in FRPs is a viable strategy for improving the through-thickness mechanical properties and inhibiting delamination. Z-pinning is a through-thickness reinforcement technique whereby short FRP rods are inserted in the laminate before curing. Z-pinning has been proven to be particularly effective in inhibiting delamination under quasi-static, fatigue loading and low velocity/low energy impact loading. Nonetheless, little is known regarding the performance of Z-pinned laminates withstanding high energy/high speed impacts, whose effects are governed by complex transient phenomena taking place within the bulk FRP laminates and multiple ply interfaces. Overall, these phenomena are commonly denoted as "high strain rate" effects.

There is some evidence that Z-pinning is beneficial also for high-speed impacts, but this is not conclusive. The current lack of knowledge may be circumvented with overdesign and expensive large-scale structural testing, but this is not a sustainable solution in a medium to long-term scenario.

This project aims to fill the knowledge gap outlined above, by combining novel experimental characterisation at high deformation rates with new modelling techniques that can be used for the design and certification of impact damage tolerant composite structures. The development of suitable modelling techniques is particularly important for industrial exploitation, since it will reduce the amount of testing required for certification of composite structures, with a significant reduction of costs and shorter lead times to mark

Major stakeholders of the UK aerospace industry (Rolls-Royce plc and BAE Systems) and composites supply-chain (Hexcel Composites UK) are supporting this project, together with the National Composite Centre (NCC). High rate impact represents a major challenge composite aerospace components, for example the certification of next-generation Rolls-Royce aero-engines, which will be required to operate with substantially lower fuel consumption, noise and emissions than state-of-the-art propulsive units. In the design of lightweight composite airframes, especially in the booming UAV market in which BAE Systems is a key player, mitigation of impact damage represents a major design driver. These structures use state-of-the-art composite material systems such as those supplied by Hexcel. Finally, the NCC represents the UK hub for applied research on composites, as part of the High Value Manufacturing Catapult, focussing primarily on the industrial exploitation of research generated by academia.

A number of mechanisms will be in place in order to ensure the rapid dissemination of the research results among the industrial partners and the exploitation of the analysis tools developed during the project. The industrial partners will sit in a project review board, which will meet on a quarterly basis. The aim of these meetings will be to report the project activity to the industrial supporters and gather their feedback regarding the overall progress of the research. All investigators have extensive background experience of running such progress meetings with industry, in the framework of other projects funded by EPSRC, TSB, EU and Industry. As the project progresses, the numerical tools developed for the analysis of Z-pinned composites subjected to impacts will be made available to the supporters, so that they can be tested on relevant design cases of increasing level of complexity. This will ensure that the analysis strategy will be relevant to industrial requirements, ensuring a fast deployment at the end of the project.

Dissemination opportunities will arise at the events organised by the composite centres: ACCIS (annual conference, faculty receptions), and NCC (seminars for industry, academic sandpit meetings). At the end of the project an open one-day showcase event will be held at the NCC for academic and industrial participants and those interested in the outcomes of the project. A one-day training course for industrial practitioners will be developed and delivered at the NCC.

In a wider context, a targeted academic and industrial audience will be reached through international conferences, such as the International and European Conferences on Composite Materials (ICCM and ECCM), SAMPE Conferences and ICMAC15.

Appropriate consortium and non-disclosure agreements will be put in place, supported by the universities' technology transfer offices, to manage IP and sensitive information from the industrial partners. The investigators will leverage additional funding available for working with industry at the higher TRLs such as Knowledge Transfer Partnerships (KTPs) and the universities' EPSRC Impact Acceleration Accounts. An additional route for exploitation is to feed into the industry PV funded projects at the Universities, which are focussed on more applied problems, e.g. the Rolls-Royce Composites UTC at Bristol, for which Prof. Stephen Hallett is the Technical Director, and the Rolls-Royce Solid Mechanics UTC at Oxford, through Prof. Nik Petrinic.