Nondestructive Mechanical Characterization by Small Punch Test and Statistical Fractography for the Determination of the Residual Lifetime of Old Steel Bridges

The purpose of this work is to present an innovative method of structural integrity assessment dedicated to steel bridges. According to the European Commission, at least 30% of steel bridges are more than 100 years, and most of them were not designed to support today’s increased traffic intensity. However, this cyclic solicitation gives rise to fatigue induced damage of vital bridge elements that ultimately may lead to their failure. The structural integrity assessment of these elements by fracture mechanics requires the mechanical properties of the material aged by environmental and mechanical conditions. However, destructive sampling for conventional testing is seldom not viable. A patented technology based on the statistical fractographic analysis of the surface of a millimetric fractured punch test specimen opens new perspectives to address this issue. It provides the fracture properties (toughness, cohesive strength…) of a material in a “quasi-nondestructive” manner, and it can be combined with the determination of the elastoplastic properties through the analysis of the force versus displacement response of the punch test specimen. Such a procedure provides a full characterization of the residual mechanical properties of an aged material that can then be used as input data for predictive fracture mechanics-based models of residual lifetime. We believe that such original approach may help local authorities involved in assessment of fatigue-prone bridges to take appropriate decisions.