Analysis of residual stresses on ferromagnetic materials
Barkhausen noise
Principles of the Barkhausen Noise method
The Barkhausen Noise method for measuring residual stress is applicable exclusively to ferromagnetic materials (iron, cobalt, nickel and their alloys). As is known, in these materials the magnetic domains are generally oriented in random directions.
Conditions
In this condition the global magnetization is zero. The application of a magnetic field, or a mechanical deformation, determines a change in the structure of the domains (shift of the boundaries that separate them or rotation of the magnetization vectors), which in turn modifies the global magnetization of the piece.
In particular, for materials with a positive magnetostrictive constant (iron and cobalt), the application of a tensile stress along one direction favors the orientation of the domains in the same direction (sketch b), while a compressive stress favors the orientation in perpendicular direction (sketch c).
The application of the magnetic field in turn tends to orient the domains in the direction of the field (sketch d), thus adding its effect to that of the tensile stress and subtracting it from the compressive stress, applied in the same direction.
Induced alterations of the surface tension state
An example are the alterations in the surface tension state induced by grinding burns, which are also easily detectable with X-ray diffraction or with the Barkhausen Noise methodology.
As regards residual austenite, its presence, even in small percentages (5%), can cause deformations that make the piece unusable. An example is ball bearing races and injector needles for diesel engines. Detecting their presence allows you to optimize heat treatments.
The equipment used by 2 Effe Engineering for the measurement of residual stresses and for the determination of residual austenite allow the absolutely non-destructive analysis of samples of any size and also allow measurements in the field. The Eng. Marconi, for example, carried out the measurement of the tensions, caused by casting and mechanica
Main
Applications
- determination of the quantity of residual austenite on bearings and parts of injectors for diesel engines
- detection of residual stresses on gear wheels
- detection of residual stresses on automotive engine parts (camshafts, connecting rods, crankshafts, rocker arms)
- relief of residual stresses induced by deep drawing (housewares and structural parts)
- detection of existing tensions in operation on gas pipelines
- detection of tensions in operation on large tensile structures
- measurement of the efficiency of shot peening and rolling treatments on components subjected to fatigue
- detection of residual stresses in castings (parts of cast iron machine tools and aluminum automotive castings)
- detection of voltages induced by welding (laser and electron beam)
- research of a correlation between residual stresses and fatigue resistance of aluminum alloy car rims
- optimization of machining parameters for chip removal in order to improve the fatigue resistance of mechanical components
- detection of residual tensions on helical and leaf springs
- search for critical areas resulting from the application of operating loads (armouring and aeronautical sectors)
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