Localization of defects in pipes using guided waves and synthetic aperture focussing technique (saft)

In order to monitor offshore foundations of wind turbines permanently, durable sensors and sensor systems are needed which are well suited for the harsh environmental conditions. Additionally, the restricted offshore repair opportunities and therefore the urgent need for high reliability have to be taken into account. For the monitoring of welded seams in these applications areas, sensor rings were tested and qualified. The present paper introduces a technique to localize cracks in cylindrical structures as they are used for offshore foundations. The proposed technique is based on the use of elastic waves propagating in hollow cylinders. The so-called guided waves are used in a variety of wave modes. Every wave mode induces a different interaction potential with a crack depending on frequency and elastic stress components. Using the Finite difference method, the time-dependent elastic problem is solved by the simulation softwareWave3000Plus to determine well-suited wave modes and frequency domains for the interaction of waves with the expected cracks. Subsequently, measurements using a cylindrical test structure were performed to verify the simulation results. Varying wave modes provide differing main oscillation directions and require the usage of shear transducers and thickness crystal transducers. Furthermore, the synthetic superposition technique is approved by experiments on cracks with varying depth.