On-Line Monitoring of Fatigue in the LCF and HCF Range by Using Micro-Magnetic NDT at Plain Carbon and Austenitic Stainless Steel

Austenitic stainless steels are in wide-spread application in the chemical as well as nuclear industry, mainly because of their high toughness and insensitivity against corrosion attack. However, under static as well as fatigue load the material has the tendency to response with localized phase transformations from the non-magnetic g- to the martensitic and ferromagnetic a'-phase. The process starts localized at positions of higher stress intensity, i.e. at microstructure inhomogeneities like non-metallic inclusions and carbonitride precipitates. Magnetic sensitive sensors like SQUID or GMR are suitable to already sense small contents of the martensitic phase in the bulk volume. The report is to the Ti-stabilized austenitic stainless steel X6 CrNiTi 18 10 (1.4541) fatigued under HCF-as well as LCF-conditions, at ambient temperature and 320°C as well as plain carbon steel. The magnetic results show a early indication of damage. A special device was developed to follow insitu the fatigue damage accumulation in the servo-hydraulic machine. The sensor is based on giant magneto resistors (GMR) in a Wheatstone-bridge circuit in combination with an eddy-current instrument. At low eddy-current frequencies - in order to establish a large current penetration - the GMR-sensor provides a reasonable signal to noise ratio to measure eddy-current effects as function of the load cycles. Magnetic permeability changes are observed by the martensitic structure development and by residual stress effects whereas the electrical conductivity is influenced by the changing dislocation density and arrangement. The GMR-impedance is following the parallel measured plastic strain and reveals cyclic softening and cyclic hardening in stress-controlled experiments. Even in multiple step tests with stochastic amplitude variations in the mechanical load the GMR impedance is directly correlated with the total accumulated plastic strain and reflects the load history up to the break. Comparable behavior is also observed at specimens of plain carbon steel of the grade Ck45