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Lamb waves are widely used for non-destructive evaluation of material parameters as well as for detection of defects. Another application of Lamb waves is quality control of adhesive joints. Researchers are currently investigating shear horizontal and zero-group velocity modes for characterisation of the adhesive bonding strength. In a new approach, Lamb wave mode repulsion is used to obtain the coupling strength between different layers to characterise the adhesive bonding strength. The modes of the individual layers become coupled in the multi-layered systems forming particular regions, the so-called mode repulsion regions. This study investigates these modes and their interaction in two-layered plate-like structures with varying coupling strength both numerically, with the Scaled Boundary FEM, and experimentally. [ABSTRACT FROM AUTHOR]

Austenitic stainless steel welds as well as dissimilar metal welds with nickel alloy filler material, used in safety relevant parts of nuclear power plants, still challenge the ultrasonic inspection. The weld material forms large oriented grains that lead, on the one hand, to high sound scattering and, on the other hand, to inhomogeneity and to the acoustic anisotropy of the weld structure. The ultrasonic wave fronts do not propagate linearly, as in ferritic weld joints, but along the curves, which depend on the specific grain structure of the weld. Due to the influence of these phenomena, it is difficult to analyze the inspection results and to classify the ultrasonic indications, which could be both from the weld geometry and from the material defects. A correct flaw sizing is not possible. In an ongoing research project, different techniques to improve the reliability of ultrasonic testing at these kinds of welds are investigated. In a first step (in the previous research project) two ultrasonic inspection techniques were developed and validated on plane test specimens with artificial and realistic flaws. In the ongoing project, these techniques are applied to circumferential pipe welds with longitudinal and transverse flaws. The technique developed at the Federal Institute for Materials Research and Testing (BAM) in Germany uses a combination of ray tracing and synthetic aperture focusing technique (SAFT). To investigate the unknown grain structure, the velocity distribution of weld-transmitting ultrasound waves is measured and used to model the weld by ray tracing. The second technique, developed at the Fraunhofer Institute for Nondestructive Testing (IZFP) in Germany, uses Sampling Phased Array (Full Matrix Capture) combined with the reverse phase matching (RPM) and the gradient elastic constant descent algorithm (GECDM). This inspection method is able to estimate the elastic constants of the columnar grains in the weld and offers an improvement of the reliability of ultrasonic testing through the correction of the sound field distortion. The unknown inhomogeneity and anisotropy are investigated using a reference indication and the special optimization algorithm. Both reconstruction techniques give quantitative inspection results and allow the defect sizing. They have been compared to conventional ultrasonic testing with techniques that are state of the art for components in nuclear power plants. The improvement will be quantified by the comparison of the probability of detection (POD) of each technique. [ABSTRACT FROM AUTHOR]

In this contribution we present a novel thermo-acoustic approach for the generation of broad band airborne ultrasound and investigate the applicability of resonance-free thermo-acoustic emitters for very short high pressure airborne ultrasound pulses. We report on measurements of thermo-acoustic emitter consisting of a 30 nm thin metallic film on a usual soda-lime glass substrate, generating sound pressure values of more than 140 dB at 60 mm distance from the transducer and compare the results with conventional piezoelectric airborne ultrasound transducers. Our experimental investigations show that such thermo-acoustic devices can be used as broad band emitters using pulse excitation. [ABSTRACT FROM AUTHOR]

This paper presents an automated system for the evaluation and visualization of structural vibrations on the surface of solid materials in an ultrasonic frequency range using a single head laser Doppler vibrometer. For this method, the vibrometer performs a continuous scan of the surface driven by a two-axes robotic system. Scan and measurement parameters can be adapted to the requirements of geometry, frequency and resolution. Data processing techniques are employed to improve the signal-to-noise ratio and minimize the effect of inconvenient laser beam reflections caused by speckles. [ABSTRACT FROM AUTHOR]

In this paper the Scaled Boundary Finite Element Method (SBFEM) is applied for the simulation of Lamb waves in cracked plates. This method is highly advantageous to study the interaction of different Lamb wave modes with cracks as the crack is not discretized and no refinement is required around the crack tip. Numerical examples are presented for the reflection of the fundamental symmetric and antisymmetric modes from cracks of different depth. The spatial Fourier transformation is employed to calculate the amplitudes of reflected Lamb wave modes. The results reveal possibilities to obtain details of the crack geometry in non-destructive testing and structural health monitoring applications. [ABSTRACT FROM AUTHOR]