In many industrial sectors, Structural Health Monitoring (SHM) is considered as an addition to Non-Destructive Testing (NDT) that can reduce maintenance effort during lifetime of a technical facility, structural component or vehicle. A large number of SHM methods is based on ultrasonic waves, whose properties change depending on structural health. However, the wide application of SHM systems is limited due to the lack of suitable methods to assess their reliability. The evaluation of the system performance usually refers to the determination of the Probability of Detection (POD) of a test procedure. Up to now, only few limited methods exist to evaluate the POD of SHM systems, which prevent them from being standardised and widely accepted in industry. The biggest hurdle concerning the POD calculation is the large amount of samples needed. A POD analysis requires data from numerous identical structures with integrated SHM systems. Each structure is then damaged at different locations and with various degrees of severity. All of this is connected to high costs. Therefore, one possible way to tackle this problem is to perform computer-aided investigations. In this work, the POD assessment procedure established in NDT according to the Berens model is adapted to guided wave-based SHM systems. The approach implemented here is based on solely computer-aided investigations. After efficient modelling of wave propagation phenomena across an automotive component made of a carbon fibre-reinforced composite, the POD curves are extracted. Finally, the novel concept of a POD map is introduced to look into the effect of damage position on system reliability.
A harmonic point source of sound was placed within the potential core of an air jet and the distortion of its sound field by the jet flow was measured. An axial intensity minimum was observed that increases with increasing velocity, frequency, or temperature. The investigation indicates that the cleft in the heart-shaded directivity pattern of subsonic jet noise can be attributed mainly to refraction.
The paper presents guided elastic waves and their identification and damage interaction in a CFRP plate. After the excitation of a fiber transducer, different elastic waves emerge in a plate. By using specially developed 3D laser scanning software it was possible to specify the different wave modes. These wave modes have been described concerning their propagating velocities and different motion components. The interaction of different wave modes with introduced impact damage (7J) is shown. In some expts., it was proven that impact locations can be derived from the detected Lamb waves. This work is continued to develop structural health monitoring systems (SHM) for selected aircraft components (e. g. stringer elements, panels)
Sound rays are traced numerically from a point source on the axis of a jet flow with realistically chosen velocity profiles. The directivity patterns computed from the ray paths have no cone of absolute silence, in contrast with analytic results for nonspreading jets. A related observation is that the surfaces of constant phase are ultimately spherical. Nevertheless, the computed axial refraction valleys are much deeper than those observed in jet noise studies. The difference is due to diffraction, which tends to offset refraction effects at all but the highest (ray-acoustic) frequencies.
The equations appropriate to the propagation of sound in a realistic jet flow have been solved numerically for the case of a sinusoidal point source on the axis of a subsonic jet. The use of slowly varying quantities (related to phase and amplitude) as independent variables nonlinearized the wave equation but facilitated the application of finite difference methods. The difference equations approximating the partial differential equation were solved by nonlinear block relaxation using a Newton-like method. The solutions approach the results of ray acoustics in the high-frequency limit. For the most part, the finite frequency results agree well with experimental directivity patterns for a point source in a jet; they lend further support to the view that the downstream valley in jet noise is due to refraction. The computations have yielded detailed phase and amplitude data throughout the sound field. Unexpected findings are that the distortion of the constant phase surfaces is slight and that the flow beyond 100 nozzle diameters continues to deepen the refraction valley significantly. [Work supported by the Mechanics Division, AFOSR, OAR.]
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