Experimental and numerical investigation of the interaction of the first four SH guided wave modes with symmetric and non-symmetric discontinuities in plates

The interaction of the SH0, SH1, SH2 and SH3 guided wave modes on a metal plate with a thickness discontinuity is numerically and experimentally investigated. Two different geometries were evaluated, namely symmetric and non-symmetric discontinuities, relative to the plate longitudinal mid-plane. Experiments were performed with periodic permanent magnet array EMATs as transmitters and receivers. Mode separation in transmission and reception was experimentally and numerically performed by dual transduction and by modal decomposition post-processing techniques, respectively. The reflection and transmission coefficients at the discontinuity for each of the investigated SH modes was calculated. It has been experimentally confirmed that when interacting with symmetric discontinuities, only modes that share the same symmetry as the incident mode are created by mode conversion, whereas mode conversion to modes of different symmetry can occur with non-symmetric discontinuities. Experimental and numerical data show good agreement, revealing that the higher the order of the incident mode, the more complex the behaviour of the reflection coefficient is, as a function of the discontinuity depth. For the same incident mode, symmetric discontinuities impose less complexity than non-symmetric ones

Method for measuring the chromatic dispersion of an optical fibre and apparatus for implementing such method

The apparatus according to the invention comprises a coupler (30) intended to couple between each other two monomode fibres presenting four branches (31, 32, 33, 34). The branch (31) is connected to a branch (36). The branch (33) is the reference fibre and forms with a converging optics (39) and a plane mirror (41) a delay line. The branch (32) is aligned at (37) with the fibre to be measured (38) associated to a mirror (42). The light reflected by the mirrors (41 and 42) forms interference fringes which are used to determine the chromatic dispersion of the fibre to be measured (38)

Mode selectivity of SH guided waves by dual excitation and reception applied to mode conversion analysis

SH guided waves, generated by periodic permanent magnet arrays have been used previously in non-destructive evaluation of metal plates and pipes. When an SH guided wave interacts with a defect or change in sample thickness, the incident SH wave may undergo mode conversion. Analysis of mode conversion is complicated, due to the interference of several propagating modes in the received signal, that can often temporally overlap. This paper proposes a mode selection technique to help understand the interaction of SH guided waves with changes in sample thickness. Using an understanding of the propagation characteristics of the guided waves, SH guided waves are sequentially generated and detected on both surfaces of the plate, capturing four distinct waveforms. By superposition of the detected signals, symmetric modes can be clearly separated from antisymmetric modes in the processed, received signals. For this method to work well, the transducers used should have very similar responses and be precisely positioned on exactly opposite positions either side of the plate. Finite element simulations are also performed, mirroring the experimental measurements, and the results correlate well with the experimental observations made on an 8 mm thick plate with a region of simulated wall thinning machined into the sample

Interaction of SH guided waves with wall thinning

This paper investigates through experiment and finite element modelling, the interaction and mode conversion phenomenon of SH0 and SH1 guided wave modes on a metal plate with machined wall thinning. Quantitative analysis was performed by calculating the reflection and transmission coefficients at the leading and trailing linearly tapered edges, for incident SH0 and SH1 modes. Several geometries were evaluated by varying the taper length and depth. Experiments were performed with periodic permanent magnet array EMATs as transmitters and receivers, generating a single SH mode, whilst both SH0 and SH1 are received. Experimental and numerical data show good agreement, revealing that the interaction of SH guided waves with such defects is complex when mode conversion arises. The values of the reflection and transmission coefficients are non-monotonic along the thinning depth and edge angle ranges. The quantitative results provide insight into the capabilities and limitations of guided SH wave measurements for simple corrosion type defects, indicating that with current capabilities, inspection of real defects will be limited to screening type measurements rather than detailed quantification of the defect region

Strain sensitivity enhancement of broadband ultrasonic signals in plates using spectral phase filtering.

The focused signal obtained by the time-reversal or the cross-correlation techniques of ultrasonic guided waves in plates changes when the medium is subject to strain, which can be used to monitor the medium strain level. In this paper, the sensitivity to strain of cross-correlated signals is enhanced by a post-processing filtering procedure aiming to preserve only strain-sensitive spectrum components. Two different strategies were adopted, based on the phase of either the Fourier transform or the short-time Fourier transform. Both use prior knowledge of the system impulse response at some strain level. The technique was evaluated in an aluminum plate, effectively providing up to twice higher sensitivity to strain. The sensitivity increase depends on a phase threshold parameter used in the filtering process. Its performance was assessed based on the sensitivity gain, the loss of energy concentration capability, and the value of the foreknown strain. Signals synthesized with the time–frequency representation, through the short-time Fourier transform, provided a better tradeoff between sensitivity gain and loss of energy concentration