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A possible scenario is proposed to put into evidence how the combined effect of fatigue at very high number of cycles and corrosion could have been responsible for the failure of one of th...

Size effects on crack growth are of paramount importance on the fatigue problem. Paris’ and Wohler’s fatigue curves exhibit scale effects, which can be explained in the framework of incomplete self-similarity and fractal geometry concepts. In particular, scaling laws are found for the main fatigue parameters, that is, the fatigue threshold ΔKth and the fatigue limit Δσfl. The fatigue threshold increases with the crack length, whereas the fatigue limit decreases with the specimen size. Eventually, the proposed models are positively compared to experimental data available in the literature.

On August 14, 2018, a few spans of the cable-stayed viaduct crossing the Polcevera river (Genoa, Italy) collapsed, causing tens of fatalities along with considerable material damage and hundreds of people displaced. The viaduct, as well as many others belonging to the national road network, was built in the second half of the last Century, and has been in service for over fifty years. In the present paper, a possible scenario is proposed to put into evidence how the combined effect of fatigue at very-high number of cycles and corrosion could have been responsible for the sudden failure of one of the strands and the subsequent collapse of the so-called balanced system conceived by the designer Morandi. The analysis accounts for an actual estimation of the heavy lorries traffic and load spectrum, as well as the European standards prescription for the fatigue damage accumulation assessment. In addition, the effective construction phases of the viaduct are considered. The structural analysis is carried out by means of analytical models, in order to simplify the structure complexity without prejudice to the description of the most relevant aspects of the structural behaviour. The purpose of the present study is not to identify responsibility among the different actors involved with the infrastructure collapse, aspect to be addressed by the Italian magistracy. On the other hand, the main goal is to warn the scientific community and the public administrations that the combined effects of low amplitude fatigue and corrosion can be dangerously underestimated, and that the existing asset of last Century bridges deserves special attention in this respect.

Fatigue life assessment can be carried out according to two well-known approaches. The first is based on the so-called Paris’ law that express the sub-critical crack growth rate as a function of the stress-intensity factor range. The second is based on Wohler’s curve that relates the applied stress range with the number of cycles to failure. Both approaches exhibit scale effects, which can be explained in the framework of dimensional analysis and intermediate asymptotics concepts. More recently, the application of fractal geometry concepts provided an alternative way to obtain similar scaling laws. In this new framework, it is possible to conceive that the propagating crack of the Paris formulation is characterised by an invasive fractal roughness, whereas in the Wohler context the material ligament is represented with a lacunar fractal, in order to account for cross-sectional weakening due to inherent defects. In particular, scaling laws are found for the fatigue threshold \(\Delta K_{th} \) and fatigue limit \(\Delta \sigma _{fl} \). The former increases with the crack length, whereas the latter decreases with the specimen size. The fractal formulations for the two approaches are presented and put in comparison. Subsequently, the size effects represented by the two fractal formulations are compared with some recent results available in the literature.

The present paper investigates the combined effect of corrosion and fatigue at very-high number of cycles on the recently collapsed Polcevera Bridge. Since the construction, in the mid-sixties, the viaduct has experienced a dramatic increase in the heavy lorries traffic. Although the amplitude of stress oscillation in the strands was limited, it is assessed that the number of larger load amplitude cycles reached eighty million, out of almost half a billion of total vehicles that crossed the bridge. Due to the aggressive environment, the degradation of the bridge has developed much faster than expected. It is likely that, already at the beginning of the Eighties, the effectiveness of the prestressed concrete covering of the strands was vanishing in some sections close to the antenna, both in terms of protection and local stiffness. As a result, the levels of corrosion detected in the strands could have been sufficient to trigger the brittle failure of one of the stay cables, and the subsequent collapse of one of the self-standing structural systems. Besides to figure out the possible collapse mechanism of the Polcevera Bridge, the authors wish to rise the attention of the scientific community on the rather underestimated phenomenon of very-high cycle corrosion fatigue in existing civil infrastructures.