Automation of Lay-Up in the Repair of Advanced Composite Aircraft Structures

Abstract

The use of lightweight composite materials in aircraft structure has become increasingly widespread over the past thirty years as the need for reduced fuel consumption and improved performance grew stronger. This has also raised concerns in the event of damage as damage patterns in composite structures can be unpredictable and difficult to detect when located under the surface. The high cost of fibre-reinforced composite materials has made replacement a less attractive option. Therefore, the need for an efficient and cost-effective repair method for composite structures has become significant. Nowadays, most composite repair operations are undertaken manually by repair engineers and technicians. However, manual composite repair is time-consuming and requires extensive training.

The challenge is to automate the process to reduce repair time and increase efficiency while maintaining strict aerospace quality requirements. The accuracy and repeatability offered by an automated process has the potential to meet such requirements. Previous research efforts have mainly focused on automated scarfing and automated inspection methods. Further research is required for the automation of composite repair patch manufacturing and application. This research project, supported by Boeing Research and Technology Australia, aims to complement global research efforts on automated composite repair. Two research aims were identified. Firstly, determine optimised repair patch shapes including joint parameters which are suitable for automated patch manufacturing. Secondly, investigate manufacturability and feasibility of composite repair patch manufacturing using the AFP method.

Extensive finite element modelling was performed to determine optimised repair shapes suitable for AFP. Two optimised repair shapes were identified: the octagon and the square-ellipse. The octagonal shape reduced the creation of adhesive rich areas at the parent-patch interface with composite prepreg tows in fibre directions used in this study (i.e. 0, ±45, 90). Finite element analysis was then performed for the optimised repair shapes. Stress results showed that the optimised shapes provided strength and stiffness in highly loaded areas while significantly reducing overall repair size compared to the traditional circular patch.

An experimental AFP apparatus was developed in-house and subsequently used for AFP composite repair patch manufacturing. Three-point bending tests were performed to characterise flexural strength for each repair shape. Experimental results validated the feasibility of using AFP for repair patch fabrication, and further strengthen the case for optimised AFP repair configurations which showed promising flexural strength results, particularly when compared to the traditional circular repair patch shape currently used in the industry. Several research opportunities have emerged from this research project which can be addressed in subsequent projects. They include the development of a mobile repair unit; the implementation of AFP repair patch fabrication for thermoset composite structures; the investigation of the elastic plastic behaviour of the adhesive in the optimised repair shapes under hot/wet environmental conditions; and opportunities for improvements to the in-house AFP equipment.