Micromagnetic and ultrasound methods to determine and monitor stress of steel structures

Prestressed constructions have been extensively used since the 1950s for infrastructure and large-span constructions. During maintenance renovation and after events causing damages, it is often important to know the current stress on the prestressed steel elements of the structure, such as strands or cables. Limited access can often be provided. Then a nondestructive assessment of the current stress is possible. However, several points have to be taken into account before micromagnetic or ultrasound testing methods can be used for determining the current stress value. For both methods, the determination of quantitative values requires a prior calibration. Micromagnetic measurements were performed with the micromagnetic multiparameter microstructure and stress analysis approach developed by Fraunhofer IZFP. It has different sensitivities for microstructure and stress state, but disturbing influences related to microstructure, geometrical effects, temperature, etc. also have to be taken into account. Ultrasound testing for stress determination is based on very accurate time-of-flight (TOF) measurements. The influence of texture of the steel also needs to be considered. Electromagnetic acoustic transducers allow the generation of shear horizontal wave modes, which are optimal to fulfil this requirement. A further advantage is that no couplant is needed and even a small lift off (e.g., due to a coating) is allowed. The paper explains how quantitative stress values can be determined based on micromagnetic and ultrasound testing TOF data recorded in the field and laboratory. Furthermore, a micromagnetic monitoring example is described. Two bundles of bridge cables of a suspension bridge were instrumented with micromagnetic sensors. A loading test was used for verification. The long-term monitoring shows stress changes due to traffic and the renovation of the bridge as well as temperature-induced stress changes in the bridge cables.