IND02 Detection of cracks with Rayleigh waves

Rayleigh waves are surface waves that propagate along the free boundary of a solid. They represent a combination of longitudinal and transversal particle shifts. They can be used to detect surface faults. In the experiment, a test block with defined cracks as surface faults is investigated. The Rayleigh waves are produced by mode conversion from longitudinal waves by means of a 90° probe, where a special attachment with a comb-shaped structure is used. The velocity of the Rayleigh waves is ascertained as a difference calculation from time of flight measurements for different probe spacing. The reflection or transmission amplitude of a Rayleigh wave can be set in relation to the crack depth in the case of crack depths in the range of their wavelength. By comparing transmission amplitudes without and with crack, the crack depth can be estimated.

Attenuation in dependence of crack depth

For the aluminium test block, a Rayleigh wave velocity of approx. 2920 m/s was ascertained. For the probe frequency of 1 MHz, a wavelength of approx. 2.92 mm results. For the determination of crack depth, the probes acting as transmitters and/or receivers were each placed at a distance of 5 cm away from the investigated crack. In the diagram, the attenuation of the transmission signal is entered for different crack depths; the crack depth is here entered in relation to the wavelength. For the investigated crack depth range, there results an almost linear correlation between crack depth and attenuation of the transmission amplitude, corresponding to the exponential amplitude decrease of the Rayleigh wave with the penetration depth.