The tie rods in the Duomo are structural elements, subject to monotonic loading and cyclic loading. Non-destructive testing is used to find cracks and determine their geometry. Lamb waves are applied to determine the presence of cracks in the tie rod and to estimate their positions. The cracks are located using eddy current testing. Finally the geometries of the cracks are determined using active infrared thermography. Experimental testing in the field of fracture mechanics is done to determine the fracture toughness of wrought iron. Combining the geometry of the crack with the fracture toughness of the material, leads to an estimation of the critical crack size.

The tie rods are made of wrought iron. The material properties are very heterogeneous. A tie rod is made out of multiple wrought iron bars of 1-1.5 meters. These bars are welded together under an angle of 27°. Cracks of two damaged tie rods are found to be under such an angle. The search for cracks will be concentrated on inclined cracks with a repetition of 1-1.5 meters.
Lamb waves is the first technique applied. An ultrasonic tomograph A1040 MIRA is used for testing. The modelling of Lamb waves travelling in tie rods and the signal processing are based on the possibilities of the MIRA. The dispersion behaviour of Lamb waves is modelled to understand the wave propagation in tie rods and the influence it has on the testing done with the MIRA. Using this knowledge, proper testing settings are found. Subsequently, signal processing is considered. The main objective of this part is to be able to identify cracks present in the tie rods.

The position of the cracks in the tie rod is determined using eddy current testing. Commercial eddy current probes are too sensitive to discontinuities to be applied to tie rods. Therefore, new probes are developed to overcome this drawback. Two probes are designed, both for the horizontal face and for the vertical face of the tie rod. The vertical face of the tie rods are best accessible. Therefore, on-site testing is done with the probe specially designed for that face.

Active infrared thermography is implemented to establish the geometry of the crack. A finite element analysis has been performed to model the propagation of heat through a tie rod. This is followed by experimental testing in the laboratory carried out on a broken tie rod. Finally, the results of the modelling and of the experimental testing are compared.

The last part has a fracture mechanical nature. The fracture toughness of the material is determined assuming both linear-elastic and elastic-plastic material behaviour. Also the fatigue behaviour is evaluated. The values obtained from experimental testing have been applied to a simplified model of the crack geometry to determine the critical crack size of the tie rod, for monotonic loading, and the threshold crack size, for cyclic loading. Fatigue life is predicted taking the threshold crack size as the initial crack size and the critical crack size as the final crack size.