Advanced Virtual Source Aperture Imaging for Non Destructive Testing
By E Hoyle and M Weston
Phased array (PA) is one of the most commonly used and accepted ultrasonic testing (UT) methods in the non-destructive testing (NDT) industry. An array of ultrasonic elements is used to focus sound at given locations by applying a time delay to the transmission and reception time of each element. Due to the nature of the technique, there is little computational requirement associated with testing with PA, and therefore high inspection speeds can be achieved. However a drawback of the technique is that focussing is reduced further away from the physical focussing point of the beam.
Full matrix capture (FMC) is another established technology in ultrasonic inspection for NDT. FMC was developed to overcome the limited focussing ability of PA. Here each element of the ultrasonic array emits sequentially while simultaneously receiving on all individually, building up what is commonly known as the full matrix of data. A post processing algorithm known as the total focussing method (TFM) is used to synthesise a phased array focussing point in every single pixel of the image. In short, this technique eliminates the focussing shortcomings of PA, making flaw detection and recognition easier for the user. However, the processing burden of the TFM algorithm makes the inspection slow compared with PA.
Virtual source aperture (VSA) and plane wave imaging (PWI) are supplementary techniques to FMC being developed to improve imaging rates by combining phased array (PA) like transmission with FMC reception. The TFM algorithm is used to generate an image. The advantage of using VSA/PWI is that the A-scan signal to noise ratio (SNR) is improved allowing a similar image to be generated using fewer transmissions, increasing the rate of inspection.
This report describes work carried out to develop our understanding of VSA and PWI, and the new techniques developed in this area.
- VSA and PWI algorithm were implemented as part of the TWI Crystal software for fast TFM based ultrasonic imaging. An inspection rate of up to 250fps was achieved, a factor of 3-14 times faster, depending on setup, than the original FMC algorithm (18fps), with only a small compromise in image quality.
- Multimode imaging algorithms were developed. This allows for mode converted sound to be accounted for in the TFM algorithm, giving the user greater imaging capability.
- A Gradient Method for calculating times of flight for the TFM algorithm was developed. This allowed for faster immersion scanning, or for inspection using conformable wedges.
Lower surface breaking flaw imaged using VSA half skip and self-tandem.
Top surface breaking flaw imaged using VSA self-tandem single mode and VSA self-tandem multimode.