Metallurgical and mechanical characterization of the historical tie-rod of Duomo di Milano

Tie-rods have an important structural role in most of the vaulted constructions due to their capacity to balance the horizontal thrusts. However, their reliability in ancient buildings is often impaired by the manufacturing technology and the incidence of defects. One paradigmatic example is represented by Duomo di Milano, whose hand forged elements date back to the first half of the 15th century and have been the object of the metallurgical characterization herein presented. The occasion arose from the replacement of a deeply fractured tie-rod, two cross-sections of which were analyzed in details: one close to the breaking surface and the breaking surface itself. The investigations carried out on the first sample were aimed to characterize the material, while those performed on the second one were finalized to estimate the causes of the failure. Stereoscopy, light optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and Vickers hardness tests were employed to analyze the local chemical composition, microstructure and mechanical properties of the material. A good agreement among the results of these analyses was found. The insight into the quality of this wrought iron is of remarkable significance as concerns the representativeness of laboratory mechanical tests and the potential of diagnostic techniques for in-situ inspection.