Tue, 12 October, 2021
TWI and Direct Conversion have been working together to develop next generation non-destructive evaluation solutions for carbon reinforced polymers (CFRPs) in the automotive industry.
CFRP parts are both light and strong, meaning there is less fuel consumption when they are used for automotive applications. However, the uptake of these materials has been undermined by difficulties in cost-effective inspection.
The TACOMA project uses advanced X-ray radiography technology to allow for rapid inspection, however the X-ray detectors can struggle to locate delamination in the plane perpendicular to the beam direction. To overcome this and provide a complete inspection solution, hybrid air coupled ultrasonic testing (ACUT) is utilised for the first time in the inspection of automotive composite components.
ACUT does not require a constant stream of water as a coupling medium (as is the case with conventional ultrasonic inspection methods). This offers an advantage for robotic systems as it can be deployed in environments where the water jet may pose a physical or inspection hazard. In addition, a robotic scanning system offers faster scans and complex scanning trajectories, creating a high inspection turnover for a wide range of varying parts.
However, application of ACUT poses a challenge due to the high percentage of energy that is reflected as a result of the impedance mismatch between the air and the component material. It is difficult to generate enough power for the signal to pass through the component at a detectable level, therefore large transducers and long input pulses are required, reducing spatial resolution in the scanning direction and the time resolution of the signal. Furthermore, a series of bespoke amplifiers and hardware filters are necessary to maintain a strong signal with low noise.
To verify the ACUT inspection setup, the project team completed three scans on three different parts of the same design. The parts were specifically designed and manufactured for the TACOMA project and included artificial defects. One of the parts had delamination defects, another had core crush defects and the last part was free on any defects so it could act as a control part. These samples were scanned to determine if the ACUT could locate the flaws. The scans used a custom designed circuit to power the transducers and collect and filter the data. A RITEC RAM 5000 SNAP wave generator was used to power the transducers, while the Picoscope 2000A was used to capture the ultrasonic data and a yoke was designed to mount the transducers onto the cobot.
Figure 1. ACUT setup with main components highlighted
In addition, a custom software solution was developed to facilitate data capture using the Picoscope and to interpret the robotic coordinate information in order to create a 3D visualisation of the inspection. Some initial data analysis code was developed to support the data review process. The results from the scanning showed that the delamination flaw could be detected in the component containing this flaw. The measured flaw width was 10mm wide at the side of the component, running along the edge.
Figure 2. C-scan results of CFRP part with delamination defect
Indications of flaws appeared in the crushed component, however, these could not be confirmed as flaws because their size was smaller than that of the transducer resolution. Further investigation is needed in order to improve these results. Finally, as expected, no flaws were found in the control component.
Figure 3. C-scan results of CFRP part with core crushed defect
This latest work has helped progress the project to find an inspection solution for the automotive industry as well as other potential applications in areas like aerospace, rail, power generation and sport.
You can get involved in this exciting new project, as we are now looking for industrial partners from the quality services or manufacturing industries who are interested in improvements in speed, image quality and detection capabilities for the inspection of CFRP materials and components.