Keeping it simple

Failure however, is rarely an option, so Sonatest has developed a range of equipment to simplify verification and speed up processing.
While for some, the use of carbon fibre as an alternative to metal components is nothing new, the rate of change in terms of the specific materials, grades and processing methods means that manufacturers will always have a requirement to evaluate a high percentage of their products. Failure modes can still be unpredictable so consistency is paramount to confidence, especially when it comes to flight critical structures. Ultrasound technology is one of the more traditional, tried and trusted methods of testing and forms the basis of any quality management process. In terms of using the technology however, Sonatest's aim is to keep things simple as far as the user is concerned.

Typical problems stem from the manufacturing process, such as uneven resin distribution for foreign objects which often fragments the film that the raw material is supplied with. Resin starved areas can lead to porosity whereas too much resin can cause internal stress. It is also important to consider the effects of damage once the part is made, particularly in handling which can be easily overlooked. Sometime called Barely Visible Impact Damage (BVID), cracks can propagate under the surface after a ‘knock', spreading out as it passes down the layers. Machining and drilling can also cause delamination.

Sonatest's RapidScan 2 is a proven product designed to meet the requirements of most applications in terms of composite and aerospace testing. This ultrasonic NDT system, has been in the field for a few years now but has recently been joined by the Radius WheelProbe, designed specifically for curved structures and large areas, which can be easily implemented for MRO operations. The key benefit in both cases is the roller concept, which provides a flexible, high signal transmission interface between the sensor and the part, also ensuring that wear and tear is kept low. This close, consistent contact also means that only a little water is required for the system to function, removing the need for couplant and immersion techniques.

Simple though this may be, the important thing is its capability, and to this end it is on the software side, where bringing meaningful outputs from data is the key. With much of the older technologies, a good understanding of phased array theory is required but the Sonatest range requires only 2-3 days training. Designed to be easy to interpret, the C-Scan is displayed using a colour spectrum to represent data from different levels within the component, or in this case, different plies, and is capable of scanning 200mm a second to a depth of 40mm, depending on material.

Multiple C-Scans can also be viewed simultaneously to clarify the type of defects in a part. For example, in a consistent material, the user would get a large amplitude signal. A material containing pores that may otherwise be too small to see will scatter the beam, weakening the signal, and there would be a threshold strength to pass or fail the part. Delaminations within the structure are typically characterised by an attenuated echo as well as separate echoes bouncing off the defect. Multiple scans can then be built up to provide a topographical image of the consistency of the overall structure. Consequently, the RapidScan and WheelProbe range can be used to build an accurate picture of a wide variety of bonded materials with variable response characteristics.

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