Investigating post-processing of phased array data for detection and sizing capabilities using incoherent compounding

Advanced non-destructive techniques integrate ultrasonic arrays with array controllers that perform full waveform capture (FWC). FWC is the term used to describe the acquisition of RF waveforms corresponding to specific transmit-receive beams from an array. With FWC, the full waveform is available for post-processing. However, FWC only allows rectification and gating during data acquisition and post-processing. If the correct information is not acquired the first time, inspection cost and time may be compromised. FRD (full raw data) collection works similarly to FWC, acquiring the time-domain responses from the excited phased array transmit-receive combinations. However, the transmit and receive apertures for FRD are much smaller than for FWC - typically one element each - and there are many receive locations for each transmit location. The main advantages of FRD collection are the facility for real-time imaging and to optimise features such as beam steering, aperture shading and focusing during the post-processing stage(1).

Compounding, a technique previously used for biomedical applications, has the potential to be applied as a novel technique in NDT. Incoherent compounding is a signal processing technique that involves recombination of echoes from different propagation paths. Incoherent compounding only considers the amplitude information from received signals. Therefore, the interpretation of received signals is simplified. A favourable application of this technique is crack sizing. Coherent processing, in which the amplitude and phase of received signals are considered, would be beneficial when enhanced resolution is necessary. Both techniques would be applicable to multi-layered structures commonly found within the aerospace industry.

Experimentally, using a B-scan, incoherent compounding can be demonstrated by mirroring the direct image onto the reflected image(2). This paper will discuss incoherent compounding using a linear phased array probe with FRD capture as an innovative post-processing method. Furthermore, the results confirm the sizing and location of a combination of side-drilled holes that mimic defects.