Guided Wave Displacement Validation for SHM Applications Using Air Coupled Ultrasonic Scanning Technique

Structural Health Monitoring with guided waves is a promising technique for online testing of metal or composite panels. However, wave propagation and interaction in complex structures poses a hard challenge for signal validation due to reflections, refractions or mode conversions. In the past different measuring and imaging techniques were developed which allow a deeper analysis and understanding of wave behavior. One known measuring technique for guided waves is the laser vibrometry. Its most important advantage is the possibility of a three dimensional scanning of surface deformations using different scanning directions. However, this method requires a reflection alignment with films or painting and a high averaging rate due to a low signal-noise ratio. These disadvantages increase measurement costs and can be avoided using an ultrasonic scanning technique. Here, compression waves emitted by leaky waves are measured with a contactless ultrasonic sensor. The necessary measurement equipment is comparatively cheaper and scanning cycles are faster. The wave emission of leaky waves depends on the specimen, the fluid between the specimen and a sensor, the leaky wave mode, its in-plane propagation direction and frequency. All these parameters can influence a wave emission decisively. They have to be deducted from the air coupled ultrasonic scanning data in order to reconstruct the three dimensional displacements of a specimen surface. This paper presents all these necessary alignments for isotropic and anisotropic specimens. Based on the derived methods a validation of wave behavior in any homogeneous specimen is possible without undesirable emission influences. All used proceedings are based on analytical calculations and numerically determined dispersion curves and do not require finite element simulations. Based on the computed deformation information different kinds of application are possible. One of them is the design and optimization of virtual sensors for Structural Health Monitoring. A further possibility is the usage of guided waves as a Non Destructive Testing method for impact detection in situations, where no transmission or impulse-echo investigations are possible.