Online Monitoring of Fatigue in the LCF and HCF Range by using Micromagnetic NDT at Plain Carbon and Austenitic Stainless Steel.
G. Dobmann, M. Lang, I. Altpeter, IZFP, Germany
ABSTRACT
Austenitic stainless steels are in widespread application in the chemical as well as nuclear industry, mainly because of their high toughness and intensitivity against corrosion attack. However, under static as well as fatigue load the material has the tendency to response with localised phase transformations from the non-magnetic g- to the martensitic and ferromagnetic a'-phase. The process starts localised 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 (1.4551) fatigued under HCF- as well as LCF-conditions, at ambient temperature and 320Cº as well as plain carbon steel. The magnetic results show a clear early indication of damage. A special devise was developed to follow in-situ 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 total strain and clearly reveals cyclic softening and secondary 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 strain and reflects the load history up to the break.
Publication Source: 3nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, November 14-16, 2001, Seville Spain. Publisher: Tecnatom s.a. - [Homepage]
