The aim of the ACCURATe is to develop a prototype system based on Laser UT (LUT) technology to be used for non-destructive inspection (NDI) of hybrid composite structures, containing damping or very attenuative materials, as well as thick laminates.
This project is part of a wider context of work in the framework of the Airframe ITD of Clean Sky 2. In particular, the Work Package B-4.3 “More Affordable Composite Fuselage”.
Existing state of the art composites such as carbon fibre reinforced polymers and hybrid polymer-metal multilayer sandwich structures (laminates) offer much greater fatigue strength to weight ratio and elastic modulus to weigh ratio than metals. Thus they are the key pathway to reducing fuel costs and emissions. This is why state of the art aircraft contain up to 80% by weight of such composites in their load bearing structures. However there are two key barriers to overcome: (i) that they are more expensive that traditional aluminium alloy structures and (ii) the risks of the development of both internal defects and impact damage leading to structural failure are higher.
The project concept is to use a laser based NDT system for fast and contactless testing of large carbon fiber reinforced polymer (CFRP) aircraft structures. The approach is based on a non-contact laser generated and detected pulsed ultrasound technique with delivery of both the laser ultrasound excitation and detection pulses through flexible optical fibres. The backscattered light is also collected into a fiber.
The excitation and detection lasers are based on diode pumped Nd:YAG lasers which enable a low profile casing with low weight and very long lifetime with little maintenance and high scanning speed of around 500Hz. Both lasers are based on commercial laser designs by Innolas and are specially redesigned for the required specifications. For the demodulation of the ultrasonic waves, both a balanced two wave mixing interferometer (B-TWM) and a dual confocal Fabry-Perot Interferometer (D-CFPI) are tested.
The measurement head, which contains the two beam outputs and the light collection optics is scanned over the surface by a 6-axis lightweight robot arm, to provide an area coverage (scan window) exceeding 1.5m x 1m from a single location of the robot base. The robot arm will move along a rail track that runs the length of one side of the barrel demonstrator panel which will be positioned and fixed within the system cell. The robot arm will raster the laser head system over the part surface, and move in increments along the track to inspect the whole component. It is envisioned that the part will be scanned in less than 4 scan windows.
The LUT robot system and track will be housed within a cell which will be lightproof and have interlocking safety doors. The components will be positioned within the cell using a jig and support structures designed during this project.
There are three main objectives of this project:
- To design a LUT prototype system for the NDI of the fuselage barrel demonstrator component panel, supplied by the Topic Manager. This will involve the innovative design of a generating laser system, detection laser system, analysis software (with post processing applications), and the design of an inspection cell with a robotic arm to manipulate the laser systems.
- To develop the prototype LUT system to meet the NDI requirements specified by the Topic Manager. This will involve assembly and integration of the sub-systems, as well as optimisation of the system using test samples supplied by the Topic Manager.
- Install the prototype system at the Topic Manager’s facility, and perform a final validation of the system, with the inspection of the fuselage barrel demonstrator component panel.