Critical aspects of experimental damage detection methodologies using nonlinear vibro-ultrasonics

Nonlinear ultrasonics has become a major research focus in non-destructive testing (NDT) in recent years. Techniques using higher harmonics or cross-modulation of two excitation frequencies have been developed and have shown to be extremely sensitive and to allow measuring defects that are undetectable using conventional ultrasonic NDT. This paper reports on the application of nonlinear vibro-ultrasonics to detect delamination damage in composite laminate beams. In addition, measurement and data analysis requirements are evaluated to ensure optimal sensitivity, reliability and robustness of nonlinear vibro-ultrasonic damage detection systems. The importance of the individual components of the measurement chain is demonstrated, and a measurement system is described that has a dynamic range of more than 120 dB and 24-bit amplitude resolution, which enables the measurement of sideband frequencies with unprecedented sensitivity and accuracy. The influence of the selection of input variables such as excitation frequencies and input voltages on the sensitivity of the system is also demonstrated. It highlights the challenges of reliable damage evaluation in the case of materials and structures with unknown damages. The conclusion of the paper is that nonlinear vibro-ultrasonics has great potential as NDT technique with unmatched sensitivity, but that measurement systems and experimental parameters and processes have to be selected and optimised with extreme care to ensure that damage detection results are reliable