By Jane Allwright and Mark Sutcliffe
There is a major industrial need to detect ‘closed’ defects, such as fatigue cracks, kissing bonds and other tightly closed cracks, which may be invisible to currently available NDT inspection techniques. A crack is regarded as being ‘closed’ to ultrasound if it is sufficiently tight that ultrasonic waves are transmitted through it, rather than being reflected from it. The majority of ultrasonic inspection techniques rely on the assumption of linear wave propagation, and the derived properties such as linear superposition, time reversibility and reciprocity. However, fatigue cracks and certain other ‘closed’ defects may exhibit nonlinear responses due to contact between the surfaces, friction or ‘clapping’. Therefore, nonlinear ultrasonic approaches are being developed. These may have the potential to detect such flaws even in the presence of a much stronger linear reflector. However, such nonlinear approaches have not yet become widely available within the NDT marketplace.
The nonlinear approach investigated in this work has the advantage that it uses standard ultrasonic phased array equipment combined with a post-processing technique.
- A nonlinear imaging technique has been successfully implemented and experimentally proven by application to a fatigue crack.
- Through trial-and-error in experimental variable selection, a very good nonlinear response was achieved, allowing image reconstruction of a fatigue crack in an aluminium sample that was not previously visible using ultrasonic testing.
- The nonlinear image significantly improved the clarity with which the fatigue crack was imaged, compared with the linear FMC image.
- The fatigue crack tip produced a significantly nonlinear response.
- Due to experimental variable sensitivity, slow acquisition speed and high computational processing requirements, it is not currently recommended that the technique is adopted for industrial applications. Specifically the literature provides no guidance as to time delay selection, which if incorrectly selected would yield no detectable improvement in response compared with the linear technique.
- Results were extremely sensitive to the choice of parameters (for example those defining the time window for data collection). The optimum choices are not obvious, and vary according to the sample being tested.
- Further work would be required in order to develop rigorous criteria for identifying cracks from the nonlinear ultrasonic data or images.
Fatigue crack specimen