EDDY CURRENT NON-DESTRUCTIVE TESTING OF HISTORICAL METALLIC TIE-RODS

Defects reduce the area of the resistant cross-section and produce stress concentration affecting the capacity of load bearing elements. Therefore, their detection is of fundamental importance in the assessment of structural integrity. Regarding historical metallic components, established Non-Destructive Techniques (NDT) are not suited to this purpose because the morphological features of these elements (i.e. heterogeneities, inclusions, corrosion, and surface roughness) boost the problem of false indications. Hence, the applicability of methods commonly used to inspect modern metallic elements has to be carefully evaluated and studied. In this paper, the application of eddy current testing (ET) on the historical metallic tie-rods of Duomo di Milano is presented. Preliminary laboratory inspections were carried out on an original tie-rod of the cathedral characterized by a visible defect. The involved parameters (frequency, coil size and shape) were adjusted in order to reduce the effects of surface roughness and to increase the detectability of main discontinuities. Moreover, the operational improvements required by in situ application were recognized, allowing achieving a more efficient setup configuration. Interesting information on the manufacturing process were revealed, including a regular set of equally spaced welded joints. A systematic inspection campaign was then performed in situ on seven active tie-rods. ET was carried out by using a very efficient customized probe. It was properly designed for this application in order to increase the penetration depth of the electromagnetic field and to enhance the sensitivity to welded joints compared to other minor defects. Each indication was classified in terms of magnitude of the recorded signal and documented via a detailed photographic survey. The customized probe allowed recognizing efficiently just the most significant discontinuities, casting the base for more detailed inspections aimed at assessing the residual load bearing capacity of these historical structural elements.