An approach for the detection and characterisation of fatigue-damage in mild steel with non-destructive methods
The design of safe and efficient structures requires a realistic fatigue-life-prediction of its components. Due to many load cycles the fatigue survey becomes crucial for traffic steel structures in many cases. Structures, which have been built just a few decades before, have to be surveyed frequently. Additionally, the traffic load has been increased dramatically at the same time. Often, the load history and therefore the occurring damage are insufficiently known. This fact leads to uncertainties in the determination of the residual service life.
Fatigue damage is physically divided into three periods. It starts with the crack initiation. Here, dislocations and the resulting slip bands originate first discontinuities. Hence, micro cracks grow during the phase of the stable crack propagation leading to macro cracks. The technical crack at the components surface delimits the design life. It defines the transition between micro- and macro crack growth as well. The evolution of the macro crack could be described by fracture mechanics. As soon as the crack reaches a critical length, the phase of unstable crack propagation is present. This phase is characterized by a ductile residual fracture of the component.
In the context of the research programme as a part of the GRK 2075, a fundamental understanding for the experimental determination of a components damage and its temporal classification of the current state on the S-N- curve should be developed. In this framework, notched tension specimens consisting of mild steel S355 will be conducted to fatigue tests. During these tests, full-field, highly sensitive strain measurements will be performed by a speckle interferometry system (ESPI) on the specimen's surface. With the alteration of the strain field in the vicinity of the notch while cumulating load cycles local micro damaging could be deduced. The investigations focus on detection and characterisation of the processes until the formation of a macro crack.
Publications within the framework of the RTG:
Publications in peer-reviewed scientific journals:
K. Ritter, J. Unglaub and K. Thiele. Untersuchung der Mikroschädigung in Baustahl S355 mit ESPI. Bautechnik (im Druck), 2020.
Conference contribution with publication in conference proceedings:
K. Ritter and K. Thiele. Zur frühen Detektion von Ermüdungsrissen mithilfe der Speckle-Interferometrie. Tagungsband des 21. DASt-Forschungskolloquium, 2018.
K. Ritter and K. Thiele. Monitoring Micro-damage Evolution in Structural Steel S355 using Speckle Interferometry. Proceedings of the 7th International Conference on Fracture Fatigue and Wear, Ghent, Springer Verlag, 2018.