Probabilistic Assessment of Fatigue Delamination Growth in Fibre Reinforced Composite Laminates

This First Grant research project aims to expand the current understanding of the uncertain behaviour of polymeric fibre reinforced composites subjected to fatigue loading. The primary goals are characterising and predicting the effect of scatter in delamination propagation data, which is intrinsic to compositesComposite materials are employed in primary load bearing elements of aerospace, automotive and naval structures. The excellent specific stiffness and strength of composites allow reducing the structural weight with respect to alloy based design solutions, thus maximising vehicle payloads and reducing fuel burn. Composite structural elements exhibit peculiar failure modes, having excellent in-plane strength but being extremely weak through-the-thickness, i.e. prone to delaminations, i.e. fracture between contiguous plies. The latter are held responsible for about 60% of structural failures affecting composite elements in aerospace structures. Fatigue is particularly important in rotating components subject to high vibration and cyclic loading, e.g. aero engines; moreover delamination growth from barely visible damages represents a significant hazard for composite fuselage and wing panels. Therefore the prediction of the onset and growth of delaminations due to fatigue is crucial for assessing the operating performance and reliability of composite structures. Modelling the delamination propagation under fatigue represents the corner-stone of a damage tolerance approach to the design of composite structures. Delamination growth exhibits a significant statistical dispersion, so damage tolerance design requires estimating the probability of having interlaminar cracks reaching critical detectable lengths between inspections, following which the defective structural elements can be eventually repaired or substituted. The majority of primary composite structural elements are still designed according to safe-life criteria, i.e. limiting the allowable strains so that initial defects do not grow. Thus designers employ large safety factors, and there is potential for substantial weight savings as a result of the work proposed here, which essentially aims to better understand the variability in fatigue behaviour.

In a medium term scenario the research project outlined here will provide benefits primarily to the UK aerospace industry. This is due to two main reasons: firstly the material considered for the tests is a carbon fibre reinforced plastic (CFRP), which is a high performance composite. The use of CFRPs is particularly important in the aerospace field, since for that kind of application the weight reduction is a primary design goal and the benefits of a smaller structural mass usually offset the penalty in manufacturing cost. For aerospace structures maximum reliability needs to be achieved for certification purposes and guaranteed throughout the whole operative life. The damage tolerance design of composites is not considered fully mature. Therefore the certification of aerospace composite structures is still based on a safe-life , i.e. no damage growth, philosophy; on the contrary more conventional alloy based solutions can be designed benefiting the weight savings which damage tolerance allows achieving. Consequently it is still not possible to exploit the full potential of composites in terms of structural weight reduction and more traditional alloy based solution are still competitive due to the higher manufacturing cost of CFRPs. Thus at present there exists a strong market drive towards the damage tolerance certification of composites in primary aerospace structural elements and large R&D investments have been made by major companies in order to fulfil this demand; this research will provide a strong contribution towards achieving the market demands as well as expanding the state-of-the-art understanding of composite material behaviour. Fibre reinforced composites also have a wide range of applications beyond the aerospace market, some at the stage of conceptual studies, some already successfully implemented. Thus, still in the medium term scenario, the research proposed here will also have a positive impact on the automotive and marine fields, since the usage of composites is picking up for both those sectors and the general consensus is that this trend will continue in the future. Application of composite damage tolerance design to the civil engineering field can also be envisaged, particularly regarding lightweight, easy to transport, construction components, such as portable bridge decks. Another engineering area where the results of this research will be extremely useful is that of renewable energy; fibre reinforced composites are widely employed in wind turbines and damage tolerance design methods will help considerably in improving the efficiency of those devices. In order to ensure the industrial relevance of the proposed research, this project will be carried out in partnership with the following UK based companies 1) Rolls-Royce Plc; 2) Airbus UK; 3) Hexcel Composites Ltd. In order to provide an efficient communication mechanism with the industrial partners during the project, a technical customer will be identified within each of the supporting companies. Technical costumers will act as members of a research steering panel; they will attend progress meetings and provide their feedback. The progress meetings will be organized every five months throughout the project duration, with a kick-off presentation to be held at the start of the research activity. Progress reports will be issued before each presentation, in order to provide a consistent base for discussion. Further communication will take place informally between meetings via visits, phone calls and email. The engagement of technical costumers will ensure a smooth transfer of the research results to the industrial supporters, so that the project outcomes will be successfully exploited by leading UK industries in a short term scenario.