On-line monitoring for installed piping in Nuclear Power Plants (NPPs), as well as for Oil & Gas and other kind of plants, is crucial to early detect local ageing effects and locate single defects before they may result in critical failures. All the actions able to prevent failures are of great value especially if non-invasive and allowing an In-Service Inspection (ISI). In particular the Long Term Operation (LTO) and Plant Life Extension (PLEX) may be invalidated from radiation, thermal, mechanical stresses besides their own ageing. Hence on-line monitoring techniques are of much interest especially if they assure the required safety levels and at the same time are simple and cost-effective. Guided Waves (GW) satisfy these requirements since they are structure-borne ultrasonic waves that propagate themselves without interfering along the same pipe structure, which in turns through its geometric boundaries serves as a confining structure for the GW used to test its integrity. The frequencies used for GW testing extend up to 250 kHz, thus allowing a long-range inspection of pipes (tens of meters in favorable circumstances). The experimental conditions (e.g. temperature, complex piping structure, wall thickness, materials) have to be considered since they strongly affect the results but GW generated through magnetostrictive sensors are expected to overcome such issues due to their robustness and positioning ease. In this paper, new experimental tests conducted using the proposed methodology for steel pipes having different types of structural complexity are described.

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