On the microstructural stress and strain behavior determination of metallic materials based on electromagnetic phenomena

摘要

The objective of this thesis has been to quantify residual stresses at the grain level of metallic materials determined as micro-residual stresses using magnetic Barkhausen noise, whereby a micro-residual stress mapping method based on magnetic Barkhausen noise (RESTMAB) is proposed. Compared to conventional electromagnetic testing, the process of referencing is significantly simplified when the RESTMAB is used. The RESTMAB is based on two characteristics: a) the fact that the behaviour of the magnetic Barkhausen noise under stress (called MBN(σ)) curve shows an extremum which shifts when a stress is applied and which can be directly correlated to the MRS in the direction of stress, and b) that the MBN(σ) curve shows a linear range before the extremum that is constant. The interpretation of the results is discussed based on a micro-structural model of a single crystal to explain the physical reason for the MBN(σ) curves. To this end, magnetic hysteresis and magnetostriction curves were measured in situ. The results show that the activation of new magnetic easy axes due to stress anisotropy is the main reason for this behaviour. The change of the domain structures was measured in situ using magnetic force microscopy and the activation of new magnetic easy axes as a result of stress anisotropy could be confirmed. A Barkhausen noise and eddy current microscope was extended with the RESTMAB such that local MRS distributions could be determined for the crack tips of fatigued samples.