Dependence of the high-frequency components of the magnetoelastic voltage on plastic deformation and stress amplitude during stepwise cyclic stressing of ferromagnetic material

The high-frequency components F _H of the magnetoelastic voltage u _B induced during cyclic stressing of ferromagnetic material has been investigated. The true root mean square value B _M of the high-frequency components was measured as a function of stress amplitude σ. The effect of strain ageing on the behaviour of B _M was also studied. It was found that B _M depends on the stress amplitude, plastic deformation, the dislocation arrangements and the mobility of individual dislocations. During successive stepwise increases in the stress amplitude it was found that the B _M value of F _H increases in the elastic and microplastic deformation range. B _M reaches a maximum at a stress amplitude corresponding to the onset of the macroscopic plastic deformation Δε_pl. In technically pure iron the onset of the macroscopic deformation range Δε_pl was measured to be 0.5 × 10⁻⁴. The B _M value of the high-frequency components F _H decreases when the stress amplitude exceeds the value σ_B corresponding to the onset of the macroscopic plastic deformation range. The decrease in the high-frequency components F _H was found to depend on the macroscopic plastic deformation range Δε_pl. The reason for the decrease in B _M is both reversible and irreversible plastic deformation. The stress amplitude σ_B corresponding to the onset of macroscopic plastic deformation was found to be close to the fatigue limit of the test material. Thus this relationship between the magnetic changes and plastic deformation can be used as a non-destructive method to measure the fatigue limit of ferromagnetic material. This was confirmed with several different kinds of structural steel.