The parallel-sequential field subtraction technique for coherent nonlinear ultrasonic imaging

Jingwei Cheng, Jack N. Potter, Bruce W. Drinkwater

Research output: Contribution to journalArticle (Academic Journal)

8 Citations (Scopus)
209 Downloads (Pure)


Nonlinear imaging techniques have recently emerged which have the potential to detect cracks at a much earlier stage than was previously possible and have sensitivity to partially closed defects. This study explores a coherent imaging technique based on the subtraction of two modes of focusing: parallel, in which the elements are fired together with a delay law and sequential, in which elements are fired independently. In the parallel focusing a high intensity ultrasonic beam is formed in the specimen at the focal point. However, in sequential focusing only low intensity signals from individual elements enter the sample and the full matrix of transmit-receive signals is recorded and post-processed to form an image. Under linear elastic assumptions, both parallel and sequential images are expected to be identical. Here we measure the difference between these images and use this to characterise the nonlinearity of small closed fatigue cracks. In particular we monitor the change in relative phase and amplitude at the fundamental frequencies for each focal point and use this nonlinear coherent imaging metric to form images of the spatial distribution of nonlinearity. The results suggest the subtracted image can suppress linear features (e.g. back wall or large scatters) effectively when instrumentation noise compensation in applied, thereby allowing damage to be detected at an early stage (c. 15% of fatigue life) and reliably quantified in later fatigue life.

Original languageEnglish
Article number065002
Number of pages10
JournalSmart Materials and Structures
Issue number6
Early online date3 May 2018
Publication statusPublished - Jun 2018


  • fatigue crack growth
  • noise compensation
  • nonlinear coherent imaging
  • ultrasonic phased array

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