[3C3] Progress in reliable detection of near-surface reflectors when inspecting anisotropic and isotropic materials using the total focusing method
L Bergbreiter¹,², J Grager², A Narr², H Mooshofer¹ and C Grosse²
¹Siemens AG, Germany
²Technical University of Munich, Germany
Load-bearing components made of composite laminates of several centimetres thickness, for example in wind turbine blades, are frequently used in the energy sector. These components are usually tested with conventional ultrasound techniques. A typical approach to increase energy penetration depth is testing with lower frequencies. This leads to a decrease of sensitivity and consequently to a reduced detectability of small defects compared to higher frequencies, especially for defects close to the surface. Another possibility is to use a high-excitation voltage or gain to improve penetration, but this also leads to a much more pronounced initial pulse with saturated or clipped A-scans resulting in a loss of information. Consequently, the defects close to the surface are often indistinguishable to the initial pulse and the flaw is overlooked. In comparison to conventional ultrasonic testing, the total focusing method (TFM) shows a higher resolution of near-surface defects using the same frequencies. The TFM can be adapted to anisotropic media by consideration of the direction-dependent wave propagation. Therefore, sound paths not perpendicular to the surface, which show less clipping, can be used for imaging. In this contribution, the approaches to improve the detectability of defects close to the surface in carbon fibre-reinforced plastic (CFRP) and aluminium using full matrix capture (FMC) and TFM are discussed.
As a result, in CFRP, flaws with a depth of 0.9 mm and above can be detected. The presented methods also improve the signal-to-noise ratio of near-surface defects in the TFM reconstructions up to 4 dB. The first approach filters the FMC pulses in the wavenumber-frequency domain, which reduces the aforementioned disturbances in the time domain signals and thus improves the detectability of near-surface defects. The second approach is based on a maximum angle in the reconstruction step, which reduces the entries of the information matrix based on location. This procedure is similar to taking the directivity function of each array element into account. Therefore, only time signals with a high signal-to-noise ratio are considered.
As a result, in CFRP, flaws with a depth of 0.9 mm and above can be detected. The presented methods also improve the signal-to-noise ratio of near-surface defects in the TFM reconstructions up to 4 dB. The first approach filters the FMC pulses in the wavenumber-frequency domain, which reduces the aforementioned disturbances in the time domain signals and thus improves the detectability of near-surface defects. The second approach is based on a maximum angle in the reconstruction step, which reduces the entries of the information matrix based on location. This procedure is similar to taking the directivity function of each array element into account. Therefore, only time signals with a high signal-to-noise ratio are considered.
