Early detection of material fatigue in structural steel by means of optical full-field strain measurements
In order to maintain a functioning infrastructure, it is important to be able to describe and predict the service life of bridge structures. Due to the constantly growing volume of traffic, these structures are subject to an equally increasing load. In steel structures, material fatigue is often one of the main causes of failure. The damage caused by material fatigue can for the most part only be detected in an advanced state. Here it is necessary to investigate procedures that enable early detection of the damage so that maintenance and repair measures can be planned and implemented at an early stage.
Material fatigue describes the mechanical behavior of a material under repeated cyclic loading. For metallic materials the damage resulting from fatigue can be divided into three phases. In the phase of crack initiation, dislocation movements and slip band formation occur in the metal structure. This process is called crack nucleation and it leads to microcracks, which grow steadily during the phase of stable crack propagation and ultimately result in a macrocrack. When a macrocrack reaches a critical length, the third phase begins, the unstable crack propagation, in which the growth rate of the crack increases and ultimately the residual fracture of the component occurs. Conventional methods for damage detection and assessment in structural testing can usually detect damage at the earliest when a macrocrack is already present. However, especially in structures with a high number of load cycles, the crack initiation phase is decisive for the majority of the service life.
For this purpose, the research project will investigate to what extent early detection of damage is possible by means of full-field optical strain measurements. In the first step, various fatigue tests will be carried out on samples made of mild steel S355. The full-field strain measurements are carried out with an Electronic Speckle Pattern Interferometer (ESPI). Local changes in the strain fields should enable a statement to be made about the occurrence of damage in the phase of crack initiation. Furthermore, tests are carried out to quantify the earliest possible time of damage detection.
Publications within the framework of the RTG:
Publications in peer-reviewed scientific journals:
J. Müller, M. Grabowski, C. Müller, J. Hensel, J. Unglaub, H. Kloft, K. Thiele and K. Dilger. Design and parameter identification of Wire and Arc Additively Manufactured (WAAM) steel bars for use in construction. In: Metals 2019, 9(7), S. 725ff, 2019.