Barkhausen effect in steels and its dependence on surface condition

Micromagnetic Barkhausen signals from magnetic materials originate from the discontinuous changes in magnetization under the action of a changing applied magnetic field.Barkhausen emissions that are detected by a sense coil come predominantly from a surface layer. In iron based materials this layer is about 500 μm thick. The Barkhausen signal is affected by changes in material microstructure and the presence of residual stress, since these affect the dynamics of domain wall motion. The selective attenuation of high frequency components of the Barkhausen signal due to eddy currents in electrically conducting materials is used to evaluate changes in material condition at different depths inside the material.Barkhausen measurements on specimens subjected to different thermal treatment during surface conditioning procedures are presented. Also presented for comparison are analysis of the material condition using x-ray diffraction for assessment of residual stress, and microhardness measurements which evaluate the surface microstructure condition. The results show that Barkhausen emissions can be utilized to evaluate changes in the surface condition of materials

Effects of surface condition on Barkhausen emissions from steel

Temperature changes during mechanical processing such as grinding of steel parts can cause phase changes in the microstructure. Thermal shock during the process can give rise to localized surface residual stress. The net result can be reduced wear resistance and fatigue life leading to early failure during service. Effective methods for the detection of such damage are necessary. Barkhausen emissions, which arise from discontinuous motion of domain walls, are sensitive to microstructual changes that affect domain dynamics. Detected Barkhausen signals are predominantly from a surface layer about 200 μm thick, those from deeper being attenuated due to eddy currents. An analysis of the detected signals can provide an indication of the surface condition of the material.Barkhausen signals from parts ground under controlled conditions were found to be dependent on the grinding process conditions. The signal changes were consistent with residual stress measured by x‐ray diffraction and with hardness measurements that are indicative of changes in microstructure

Convenient synthesis of bis(benzo)-fused BEDT-TTF and higher homologues: new electron donor molecules for organic charge transfer salts and `molecular rulers`

A convenient synthesis of bis(benzo)- and bis(naphtho)-fused bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) derivatives, employing the [4+2] cycloaddition reaction of oligo(1,3-dithiole-2,4, 5-trithione) with appropriate alkenes as a key step, is presented

Parametric Imaging of Surface Hardness Using Barkhausen Technique

Magnetic Barkhausen technique has been used to parametrically image the effects of thermal exposure on the surface condition of industrial components. Wear and fatigue resistance of industrial components are primarily determined by the mechanical condition of the surface layer. Standard hardness measurement techniques cannot easily be used to assess the condition of components in service because they are not well adapted to in situ measurements and are essentially destructive in nature. Barkhausen signals are sensitive to changes in surface hardness because the altered microstructure affects the dynamics of domain wall motion and consequently the Barkhausen signal. This paper presents the results of Barkhausen technique used to image the surface hardness on a flat plate material.</p

Magneprobe: A Computerized Portable System for Non Destructive Evaluation of Surface Conditions in Ferritic Components

Several techniques are now available for testing specimens for imminent failure modes. These include ultrasonic, eddy current, radiography and thermal imaging techniques for detection of cracks; X-ray, low angle neutron diffraction and magnetic techniques for the detection of residual stresses and microstructural. While techniques such as ultrasonics and eddy currents lend themselves to portability and show excellent sensitivity in detection of micro-cracks that have already formed, their sensitivity to the stages leading to formation of the micro cracks viz. changes in microstructure and residual stress patterns is not adequate for repeatable and reliable measurements. X ray and low angle neutron diffraction methods on the other hand permit absolute measurements of the residual stress profile in the specimen, however these are not easily portable and are necessarily destructive in nature because they require a specimen to be cut from the component in order to perform these examinations.</p