SHAPE FORMING
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
Delamination failure of T-joints, and Z- pins for arresting delamination cracks
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.
SUMMARY
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. ●
LEAP fan
The author is an active researcher on composite materials and structures, working in the Centre for Aeronautics, Cranfield University. He can be reached on
hao.cui@
cranfield.ac.uk, or +44(0) 1234 754494
28
www.engineerlive.com
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