TWI Industrial Member Report Summary 1020/2012
By M Weston and C Nageswaran
Background
This report describes the development of a full matrix capture (FMC) ultrasonic array data acquisition technique and its associated signal processing algorithms. Due to the intensive processing involved with FMC, the primary focus was to improve the speed of inspection whilst maintaining a high quality image. This report specifically investigates the use of novel sparse array designs to improve image frame rate. A novel highly efficient algorithm which allowed for inspection through layered material using minimal processing power was also developed. For the first time methods to calibrate the image sensitivity of FMC were also suggested. Such calibration methods were adapted and developed from existing phased array technology.
Within industry there is a constant need for accurate defect sizing and characterisation of safety critical structures and components. Inspection using ultrasonic phased array is now well established, and has several advantages over conventional ultrasonic techniques, which relate to its ability to steer, and focus ultrasonic waves using a single transducer. Through beam steering and focussing, a single ultrasonic array transducer may perform the task which is usually performed using multiple conventional ultrasonic transducers. Electronic beam steering also minimises the required mechanical movement of the transducer, which can save time, improves flaw sizing accuracy, and is advantageous when access to a component is limited.
The FMC data acquisition process, and its associated imaging algorithms, form a relatively new ultrasonic array inspection technique. The fundamental principles of the technique have been developed within academic institutions since 2005. Here it has been shown to outperform existing phased array techniques in terms of lateral image resolution and also offers a far greater 'depth of field' due to its ability to generate fully focused imagery. The technology is now at a stage where it may be developed for commercial use.
FMC is a data acquisition process which captures A-scan (time-amplitude) data for every transmitter-receiver combination of elements in an array. The technique uses a "transmit on one and receive on all" data capture approach. Initially a single element in the array is used as a transmitter, while all elements then receive. This process repeats until all elements in the array have been fired. An in-house algorithm developed by TWI, known as Sequential Phased Array, is used to process the acquired data. This algorithm generates fully focused imagery using a 'delay and sum' beam forming approach.
Developments contained in this report extend on the work described in Industrial Members Report 966/2010 'Development of Ultrasonic Full Matrix Capture Technology to Improve Flaw Characterisation' (Weston, 2010).
Objectives
- Develop the FMC algorithms for contact mode inspection with or without use of a wedge to facilitate angled beam inspection and compare performance to existing phased array techniques, which are currently the industry standard.
- Implement novel sparse element array designs and determine the performance gains/losses in terms of image quality and speed of inspection.
- Develop novel techniques and procedures to calibrate inspection sensitivity for use with FMC.