The rise in composites use on aircraft has not been without its challenges, as Dr Hao Cui from the Centre for Aeronautics at Cranfield, explains
Advanced lightweight materials and structures have largely contributed to the improved flight performance and fuel efficiency of aerospace products over the past few decades. Carbon fibre reinforced polymer (CFRP) composites have been extensively used in the new generation of commercial airliners including Boeing 787 and Airbus A350, thanks to their higher strength and stiffness per unit weight when compared to aluminium that was the primary aerospace material in the past. However, the nature of how CFRP was designed and manufactured has brought in many challenges in practice, and the legacy of metallic design experience is just not applicable due to the difference in its mechanical properties.
The deformation and failure processes of CFRP materials are largely dependent on the way they are loaded. The first type of the failure is dominated by the polymer matrix, where intra-laminar cracks aligned with fibres may split laminates transversely, or inter-fibre cracks, also called delamination, may separate the laminates. This damage can be caused by static loads that exceed the allowable design value, while in many scenarios this is caused by impact from some foreign object. For example, delamination cracks can be initiated on the upper skin of wings by impact from falling tools during maintenance work, which is very challenging to visually detect from the surface. Such inter-laminar cracks propagate under cyclic loading conditions, reducing the critical buckling load that potentially leads to catastrophic failure.
The second type of failure is fracture in the fibres, including fibre rupture when loaded in tension and fibre kinking under compressive stress, which usually results in considerable loss of structural loading capacity. Although carbon fibres technically feature high tension/compression strength, their measured strength on practical structures can be much lower, and the material is very sensitive to defects that inevitably occur during the manufacturing process.
The aforementioned failure modes of CFRP need to be properly studied for its industrial application. A building block approach is commonly used in the aerospace industry for the design and analysis of composite structures. In this approach, a lot of small-sized experiments must be conducted to characterise the material property; this data and knowledge will then be used for the testing and analysis of structures with increased size and complexity. Analysing tools were also developed and validated step by step, until the analysis of primary full structures. Such procedures have largely reduced the risk in composite structure development, and gradually builds up experience and confidence for applying CFRP composites in aerospace.
A series of experimental methods has been proposed for characterising the critical stress for different failure modes. Some of them have been nicely established, such as the uniaxial compression test for measuring the transverse compression strength and the double cantilever beam test for measuring the mode I delamination fracture toughness. Some of them have proved challenging, for example the transverse tension test has not been properly recognised because of massive scatter from traditional dog-bone specimens. Fibre failure strength has always been difficult to measure, since premature failure is highly likely to happen in the matrix first. Improving the material characterisation skills has been a shared interest of industry and the academic community. One recent piece of progress is the standardisation of the testing method for mode II delamination fracture toughness.
The mechanical response of CFRP composites can change depending on the rate of load applied. The failure strength may increase with the increase in loading rate, while the fracture toughness may have negative strain rate dependence. Comprehensive
study of CFRP material at different loading rates then becomes necessary for a safe design of structures that are potentially threatened with impact load. Similarly, fatigue damage resistance needs to be investigated for structures under cyclic load.
Experiments are expensive and time consuming, while numerical methods are deemed to be a cost-effective alternative. Several commercial finite element analysis packages have been widely used in aerospace industry, in which many of these mainstream failure theories for various damage modes have been successfully integrated. Some software also enables user-defined features, allowing highly sophisticated damage models for certain applications. The failure of CFRP composites is so highly complicated that it remains challenging for any existing damage model to successfully predict the failure behaviour of composites in all cases. Numerical simulation cannot completely replace experiments at the moment, however, it has proved useful for guiding the design of structures and analysing their mechanical performance. Throughout the building block approach, cross-validation between experiments and numerical simulations contributed to both improved reliability of experimental methods and predictive capability of numerical models.
Besides the study on composites materials themselves, the failure at joint interfaces is also crucial, as complex shapes are largely made by joining multiple parts together. The adhesive bonding has found increasing applications in composite structures; some novel welding methods are also being developed for thermoplastic composite structures. A combined experiment and simulation approach is also commonly used for the design and analysis of these joint interfaces.
Besides these experiments and simulations for analysis and design against failure, the improvement in damage resistance of composite materials has also contributed to the safety and efficiency of composite structures. The polymer matrix has
been widely toughened for increased fracture toughness, while the thermoplastic matrix also gains increasing interest for its higher ductility over a traditional thermoset matrix. Through-the-thickness reinforcement methods have been developed for increasing the resistance against delamination cracking, for example, stitching and Z-pinning were broadly studied for potential application on several aircraft.
Looking towards the future
Despite enormous efforts on investigating the failure of composites and the successful deployment of primary composite structures, our understanding and skills with using composites are still far from mature, especially when compared with metallic materials for which we have thousands of years of experience. Novel experimental methods need to be developed for accurate charactersation of composites in many failure modes; numerical tools may also be improved to reproduce the real physical fracture process at all scales.
Composite materials are produced simultaneously with the structures. Consequently, their mechanical properties may vary depending on each manufacturing process and even the shape of the structures. Those variations should be accounted for in future material datasheets, and may be handled by probabilistic methods.
Aircraft composite structures under repeated load tend to be designed with a large reserve factor, due to many issues, such as lack of fatigue damage knowledge and limited non-destructive testing performance. The structural fatigue damage is predominantly evaluated with testing, while numerical/theoretical analysis of such damage is also far from mature. The fatigue damage of composites will be characterised with experiments, and improved predictive capability will certainly enhance the structural efficiency.
New material systems keep emerging that feature great potential for aerospace applications. Functional materials that are able to detect damage and self-healing materials that can close cracks may result in a change in the design principles of composite structures.
The risk of damage in aerospace composite structures has been successfully managed through experiments and analysis at different scales that cross-validated each other, and advances in material systems and manufacturing methods also raised the composite damage resistance. There remains lots of room for improvement in all spectrums of academic research and industrial applications, as we keep aiming for safer, lighter and cheaper solutions in the next generation of composite structures.
The author is an active researcher on composite materials and structures.working in the Centre for Aeronautics, Cranfield University. He can be reached on firstname.lastname@example.org