Non-destructive stress determination of steel elements in pre-stressed constructions using micromagnetic and ultrasound methods

Pre-stressed construction elements 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 pre-stressed steel elements of the structure, such as tendon ducts or cables. Access can often be provided. Then a non-destructive assessment of the current stress is possible. However, several points have to be taken into account before micromagnetic (MM) or ultrasound (UT) methods can be used for determining the current stress values. For both methods the determination of quantitative values requires a prior calibration. MM measurements were performed with the 3MA approach developed by Fraunhofer IZFP. 3MA is the acronym for Micromagnetic Multi-parameter Microstructure and stress Analysis. 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. UT for stress determination are based on very accurate time-of-flight (TOF) measurements. The consideration of the influence of texture of the steel has also to be considered. The paper explains how quantitative stress values can be determined based on MM and UT TOF data recorded in the field and laboratory. Furthermore, a MM monitoring example is described.