The benefits of immersion ultrasonic inspection of composites
11/07/2022
Ultrasonic testing (UT) uses sound waves to detect cracks and defects in materials, including composites. Its ability to produce clearly defined images and indicate characteristics deeper than surface-level make it a highly regarded NDT method in industries such as oil & gas, aerospace and power generation.
When any new composite material is developed, the properties of the composite need to be carefully determined through qualification testing involving multiple specimens. Ultrasonic inspection uses electronic transducers to transmit high-frequency sound waves through the material. These sound waves bounce back signals that can reveal key characteristics of the material’s properties and indicate issues such as cracks, inclusions and other subsurface defects. UT can also be used to measure a material’s thickness, such as the wall of a pipe.
There are two key methods of ultrasonic inspection: contact and immersion ultrasonic testing (IUT), the latter being particularly applicable for the examination and qualification of composites.
As a laboratory-based inspection, IUT is especially useful for detecting smaller defects, from cracking to porosity. By submersing the composite, sound travelling from the transducer is enhanced, providing more accurate reporting of subsurface irregularities and flaws.
IUT can support a broad range of requirements as it can be applied to many composite materials. It can identify cracking, porosity, inclusions and delaminations and is especially useful in the oil & gas industry to identify incomplete penetration, lack of sidewall fusion and other weld strength defects.
Element’s global testing laboratories are home to some of the world’s most cutting-edge IUT equipment. The laboratories house a bespoke combined immersion and jet probe system, which has the option of using standard pulse-echo immersion techniques or through-transmission. The highly versatile system can inspect composite materials with flat and curved surfaces as well as varying geometries. The system’s evaluation software automatically records the results, which readily converts the ultrasound responses into A-, B- and C-scan images, offering cross-sectional and plan views of the results.
The system carries out conventional immersion inspection utilising an immersion manipulator with the composite located above a glass reflector plate mounted on a levelling table. This technique is ideal for thin monolithic components where sufficient signal is available to pass through the component twice.
Dense materials and those that dissipate sound to a large degree, such as glass-fibre composites, are more suited to the system’s ability to offer water jet through-transmission inspection. This has become the industry-standard method of inspection for such composites. It is also said to be the most suitable technique for inspecting buoyant composites and those that have been used in components subject to water ingress, where immersion would cause problems.
Previously, Element’s IUT specialists were asked to meet with a composite manufacturer that was producing a composite designed for a specific aerospace component for a leading aerospace manufacturer.
The manufacturer brought a reference panel, a known defect sample with seeded defects, to the meeting at Element’s laboratories. Element offered to inspect the panel using its immersion ultrasonic system, which immediately picked up all the known defects.
Following collaborative discussions regarding how Element would approach the project, Element’s team of IUT experts formulated and submitted a technique, also known as a data card, on how to inspect the specific component, a composite with an aluminium honeycomb sandwich in the middle. The aerospace manufacturer was then able to build this technique into its quality control test procedure.
Before the project commenced, the aerospace manufacturer undertook an in-depth audit of Element and its UKAS-accredited sites to ensure all the required checks and controls were in place, which the test-house passed with flying colours.
The project involved ultrasonic inspection of the whole panel, which included precured carbon/glass laminate, aluminium honeycomb, film and foaming adhesive and Nomex. Looking specifically for defects that may occur between the bonded layers within the material, it was performed using water jet through-transmission inspection, which is considered the best method to find core damage in honeycomb components. The project was carried out in several batches of panels, which each took two days to complete.
Prior to the inspection, each part was visually examined by Element for edge delaminations, surface irregularities, marks, wrinkles and resin residues to ensure that anything that may have inhibited the inspection or affected the results was noted in advance.
The ultrasound responses were recorded by the system and converted into C-scan images. The system’s ability to load multiple data files also meant it was easy to compare the different batches to a reference panel, with any suspect indications observed and assessed against a list of predefined acceptance criteria. It was agreed that any rejectable indications would be followed up with an additional A-scan pulse-echo inspection. On completion, Element then produced a detailed test report, which included all the results and C-scan images.
The project’s success meant that Element was asked to undertake a second project to produce and submit a technique for a different aerospace component. Element is currently working on a third project for the same manufacturer.
Element’s IUT services can be used to inspect a wide variety of composites and components of different size and shape. The IUT technique enhances the evaluation of the probability of detection provided by automated ultrasonic testing (AUT). Deliberately flawed seeded test welds are first examined by the AUT system under qualification, they are then inspected by IUT prior to physical sectioning for definitive confirmation of the size of the known flaws. The testing service is particularly relevant in identifying smaller defects and in clarifying clean areas due to the high-accuracy robotics and controlled immersive testing environment.
Element’s immersion ultrasonic testing capabilities include turntable immersion tanks for components up to six feet in diameter and immersion tanks for plate, bar and extrusion stock up to six metres long.
