Fracture mechanics properties of welded bridge steel from the 1960 s: experimental characterization and comparison with literature data

• Fracture mechanics properties were determined for steel from a 1960 s welded bridge. • Fatigue crack growth shows uniform Paris parameters and ΔK th across web and flange. • Near-threshold values lie within the upper scatter band of structural steel literature. • Fracture toughness tests at −30 °C reveal ductile behavior with pop-in initiation. • Results validate conservative guideline parameters for bridge integrity assessment. Fracture mechanics-based assessment methods are increasingly used to evaluate the residual lifetime and determine inspection intervals for existing welded steel bridges. Reliable application of these methods requires representative fatigue crack growth and fracture toughness parameters for steels used in existing bridge construction. This study experimentally investigated material extracted from a dismantled welded railway bridge built in the 1960 s, focusing on fracture mechanics properties. Specimens from the web and flange of a cross girder were tested for microstructure, tensile properties, fatigue crack growth behavior, and fracture toughness. The base material exhibits a predominantly ferritic-pearlitic microstructure, with moderate banding observed locally in the flange. Fatigue crack growth tests according to ASTM E647 yielded threshold values ΔK th of approximately 5 MPa√m and similar Paris-law parameters for all specimen locations and crack orientations, indicating uniform crack propagation behavior. No systematic influence of microstructural banding or crack orientation was observed. Fracture toughness tests performed at −30°C revealed significant macroscopic ductility alongside repeated pop-in events, which are characteristic of fracture initiation within the ductile–brittle transition regime. We evaluated fracture toughness at the onset of pop-in and assessed its validity according to ISO 12135 limits, which highlights the influence of specimen thickness and crack-tip constraint. The results suggest that stable crack growth is primarily governed by crack-tip plasticity and is largely insensitive to local microstructural variations. In contrast, fracture initiation is controlled by local constraint conditions. The experimentally determined parameters align with published data and guideline values, which supports their use, as well as the applicability of existing assessment curves, in fracture mechanics-based assessments of comparable welded railway bridges.