Feature: Quality testing
resolution images, and they are also easier to interpret than with x-ray technology.” The testing is initially done on a new part, particularly for
aerospace and defence projects, which require very specific and unique specs. Parts can be all sizes – anywhere from one inch square to 30-by-30 inches. “If necessary, we follow-up with destructive testing too, but
only after using SAM to identify the precise location of the failure,” said Ager.
Superior non-destructive testing SAM uses the interaction of acoustic waves with the elastic properties of a specimen to image the interior of an opaque material. A transducer – the heart of a SAM system – directs focused sound at a small point on a target object. The sound hitting the object is either scattered, absorbed, reflected (scattered at 180 degrees), or transmitted (scattered at 0 degrees). By detecting the direction of scattered pulses and the “time of flight”, the presence of a boundary or object can be determined, as well as its distance from the transducer. To produce an image using SAM, samples are scanned
point by point and line by line. Scanning modes range from single-layer views to tray scans and cross-sections. Multi- layer scans can include up to 50 independent layers. The resolution of the microscopic image depends on the
acoustic frequency, material properties and the aperture of the transducer. Transducers perform such a critical role that manufacturers like PVA TePla design and manufacture an extensive range of different transducers for use in their contracted testing services. The frequency of the ultrasonic signals can be increased
to the GHz range, allowing detection of defects in the sub- micron range. At PVA, their contract testing services are performed only
on equipment the company manufactures, namely the 300, 302 HD2, 500 and 501 HD2 scanners, with scanning ranges of 200μm x 200μm to 500mm x 500mm using transducers of up to 400MHz. Given the critical role they play, in-depth knowledge of
the advanced software features available is another considerable benefit. “SAM testing houses that purchase equipment may only be
familiar with some features of the software,” said Logan. “As the manufacturer, we have to understand all the features and even fulfill customer requests for customisation, if needed.”
Correct interpretation The quality of the equipment and knowledge of its use are key factors in SAM testing, but operating a SAM system requires a trained technician and, even more importantly, experience configuring the equipment and interpreting its scans. Setting up the scans and interpreting the images is similar to a radiologist reading MRI scans. “Wouldn’t you want your doctor to have the clearest
38 March 2022
www.electronicsworld.co.uk
Figure 3: PVA TePla SAM investigates points of failure
image possible to make the best medical decision?” asked Logan. “When you realise that an undetected flaw can have a catastrophic impact, it’s the same for quality testing, too.” SAM system experts work with three different imaging
modes, A, B and C. Mode A is an X, Y or Z point, and provides information on all the echoes occurring inside a part, providing insight into the material, time-of-flight, amplitude and polarity. This scan must be interpreted appropriately to produce an accurate B or C scan that is shared with the customer. “An operator needs to interpret, focus and change the
volume of the A scan as well as adjust certain things for the image to be accurate. It is important to ensure that the image does not convey something false,” said Logan. “The smallest and thinnest parts increase the difficulty of interpretation because the echoes of the interface are really tiny and close together. It causes an overlap of echoes, and you end up having to adjust the frequencies used. As you go higher up in frequency, it is more difficult to manage the transducers.” Like other companies, California Brazing considered
bringing ultrasonic testing in-house, but to do it well it needs someone who can read the images and interpret them accurately. “It is not easy to find people who have that depth of
experience. We have a very good working relationship with PVA TePla. As a result, there is very little interaction needed because they are familiar with our process and types of products,” said Ager. California Brazing typically gets its results from PVA TePla
in a few days, which it then follows up if needed, say, to focus further on a particular area or at a certain level.
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