The use of composite materials in aerospace manufacture is accelerating fast, with the most modern aircraft in the world’s fleet now made of more than 50% composite materials. These new-generation aeroplanes are lighter, more fuel-efficient, and so more profitable, as well as significantly reducing CO2 emissions compared to traditional aluminium planes. However, composite materials are much more expensive to produce, partly because they are not yet as well-understood as metals, so the industry spends millions every year slowly inspecting each part for flaws before it is deemed safe enough to take its place in an aircraft. And inspecting composite components is not easy.
However, for all of the advantages, there are still challenges associated with the use of composites in aircraft manufacture. Performing differently to the metals that are traditionally used, carbon fibre parts are anisotropic, with laminar construction and can fail without showing signs of visible deterioration. Furthermore, manufacturing defects, such as damaged or misaligned fibres, voids and delamination, can lead to reduced material performance.
It is therefore important to use non-destructive evaluation (NDE) techniques to ensure components meet the required mechanical properties. These NDE requirements currently account for 10% of manufacturing time and 5% of manufacturing costs, meaning that there is plenty of scope to improve and accelerate these processes. In addition current techniques have drawbacks, with ultrasonic inspection requiring the use of a couplant that adds moisture to the manufacturing environment, while X-ray can be dangerous and expensive for shop floor use.
While ultrasonic inspection is the most commonly used technique, with top speeds of around 1m2/hour, there have been attempts to introduce automation to the process. However, this has proven difficult due to the sensitivity of the process. As a result, TWI has been working alongside a consortium of project partners on the CFLUX project to develop an inspection system that has the potential to work at 30m2/hour, which is 30 times faster than manual ultrasonic NDE and eight times faster than an ultrasonic gantry system.
The CFLUX consortium have developed innovative sensor technology that can give sensitivities 1000 time greater than before, retrieving high-quality, high-resolution signals that were previously unachievable. Not only that, but this technology is tiny, making it easy to develop into multi-sensor arrays that are resilient and flexible and ideal for use in the production-line robotics necessary to really speed up and reduce the cost of the inspection process.
Manufacturers need techniques to inspect the inner structure of their carbon fibre components and CFLUX is designed to do just that. The inspiration is the traditional eddy current non-destructive testing techniques that have been used for aluminium aircraft. These are fast and effective for finding hidden flaws but rely on the good conductivity of metals. Carbon fibre is 1000 times less conductive than aluminium, making eddy current testing impossible, until now.
The project partners are developing two non-contact, dry NDE techniques for carbon fibre. These techniques will allow thorough quality assurance of raw materials, using automation and advanced eddy current technologies to identify subsurface defects in larger manufactured components.
The project involves the design of demonstrator parts, which will be subjected to finite element analysis and mechanical testing with the NDE technologies integrated with automation via a ‘cobot’ and viewing / interrogation software.
TWI Technology Centre (Wales) has been using industrial robots for automated non-destructive testing (NDT) since 2012 and is continually working towards improving the existing setup. While research until now has focused on integrating NDT into the manufacturing process, TWI is also developing a more flexible automated inspection solution for smaller component inspection that can be used alongside a human operator. Cobots are easily programmable, can interface with external equipment and can position NDT sensors with six degrees of freedom. The cobot has a high-resolution force-torque sensor, which allows it to ‘feel’ and follow a surface while exerting a controlled force onto the surface. This is especially relevant for ultrasonic and potentially eddy-current inspections.
The Innovate UK-funded project has recently reached a milestone with a 3D sample being used for scanning tests. A plastic dummy part, enhanced with metal washers was used to produce positive signals that could be used to tune the eddy current response. This offline path planning was performed on RoboDK software before the system signals were integrated on TWI CFLUX software. The amplitude signals from the bespoke EtherNDE eddy current probes were plotted on a 3D mesh to create a CAD type model result on the screen.
This first breakthrough is a large step in creating a final system that will allow for the inspection of composite parts for the aerospace industry.
For more information: www.twi-global.com