Barkhausen Noise Measurements Research Articles

Self-Calibrating Stress Measurement System Based on Multidirectional Barkhausen Noise Measurements

The system presented in this paper enables automatization of the two-dimensional calibration process (determination of Barkhausen noise (BN) intensity dependence on in-plane components of strain). Then, using dedicated software created by the authors in LabVIEW environment, and with the help of two dimensional calibration data one can effectively determine strain and stress distribution i.e. magnitude and orientation of main strain/stress components relative to measurement direction. BN signal measurements are performed using an advanced, multidirectional Barkhausen noise (BN) measuring sensor and a measurement system dedicated for cooperation with it. The system uses a robust algorithm for the strain components determination based on calibration surfaces, instead of usually applied curves, thus taking the influence of normal strain component directly into account instead of treating it as a correction factor (if not completely neglecting). The originality of the system arises also from the fact that this is the first BN measurement system that is self-calibrating (i.e. automatically loads the calibration sample in a pre-programmed way, performs BN signal measurements and calculates calibration planes), provided that the user possesses enough of the investigated material for calibration sample preparation.

Calibration Method of Measuring Heads for Testing Residual Stresses in Sheet Metal Using the Barkhausen Method.

Among non-destructive testing methods, a group dedicated to the assessment of the state of residual stresses can be distinguished. The method of measuring residual stresses using the Barkhausen noise method has many advantages, as evidenced by the number of publications. The residual stresses in metal products are important for the further processing of such metal, such as laser cutting or bending. The results presented in this work are of an experimental nature, and the presented method of calibration of measuring heads shows how various research techniques can be used to correlate results. The research was carried out for structural steel due to the market share of this type of steel. The method can be used to measure the residual stresses in ferromagnetic metal products in order to assess their directions and quantify them. A prerequisite for the use of this measurement method is that the amplitude and geometry of the Barkhausen noise are adequately correlated to the specific values of the state of stress depending on the tested steel grade or other metals. In this study, a method for calibrating measuring sensors for the residual stress measurements is presented, as developed by the authors. The method involved conducting bending tests in both numerical modeling and experimental tests. During the bending tests, changes in the magnetic field (Barkhausen noise waveform) were recorded, taking into account the state of elastic stresses. Correlating the results of the numerical calculations and Barkhausen noise measurements made it possible to determine the quantitative values of the residual stresses in the steel sheets. Thanks to the method used, very accurate measurement is possible, and the obtained results are repeatable.

Evaluation of Welding Current Impact on Residual Stresses Status in Ship-Building Steel Weldments Using Barkhausen Noise

The paper intends to determine the relationship between heat input and the residual stresses distribution in shipbuilding steel weldments using the Magnetic Barkhausen Noise (MBN) technique. The rate of fusion during welding was controlled by amount and time of fusion for a given weld bead width. Barkhausen noise measurements were performed parallel to the weld bead at the back of the plates welded using variable welding currents along the line that crosses the weld bead. The heat affected zones were differentiated by a variation in MBN peak height. The high welding current was characterized by high MBN amplitude compared with the medium and the low currents. The results showed that the heat affected zone was narrower with high welding current and increases with decreasing current. The results indicate that residual stresses introduced in steel by welding could be mapped nondestructively by magnetic Barkhausen noise

Evaluation of Welding Current Impact on Residual Stresses Status in Ship-Building Steel Weldments Using Barkhausen Noise

The paper intends to determine the relationship between heat input and the residual stresses distribution in shipbuilding steel weldments using the Magnetic Barkhausen Noise (MBN) technique. The rate of fusion during welding was controlled by amount and time of fusion for a given weld bead width. Barkhausen noise measurements were performed parallel to the weld bead at the back of the plates welded using variable welding currents along the line that crosses the weld bead. The heat affected zones were differentiated by a variation in MBN peak height. The high welding current was characterized by high MBN amplitude compared with the medium and the low currents. The results showed that the heat affected zone was narrower with high welding current and increases with decreasing current. The results indicate that residual stresses introduced in steel by welding could be mapped nondestructively by magnetic Barkhausen noise.

Grinding burn classification with surface Barkhausen noise measurements

Industrial Barkhausen noise (BN) measurements are commonly utilized for final quality control after machining operations such as grinding to point out grinding burns. Grinding burns might compromise the final use and fatigue lifetime of the ground component. The industrial BN method itself is based on a pre-determined threshold value of the BN rootmean-square value (RMS). Elevated RMS values indicate detrimental changes in the component. Usually, the evaluation of grinding burn severity is not carried out. In this study, real ground cylindrical samples were collected that were rejected based on an industrial quality control with a BN unit. A more detailed BN analysis was carried out for 41 individual grinding burn locations followed by X-ray diffraction based residual stress (RS) surface measurements and residual stress and diffraction peak full-width-at-halfmaximum (FWHM) depth profiles. K-means clustering was applied to profiles to label the data points related to grinding burns of different severity. Three classes of grinding burns were identified and verified by micrographs and hardness. A linear discriminant classification model was then identified between the surface BN measurement features and labeled data points. The classification results were reasonable with about 80 % classification accuracy at worst. They showed that the classes identified can be detected with the surface BN measurements. Thus, the approach presented in this paper shows great potential in the practical use of BN measurement where grinding burns can be detected and evaluated with a surface BN measurement.

Barkhausen noise emission in soft magnetic ribbons heat treated in an external magnetic field

This paper deals with Barkhausen noise emission in the soft magnetic ribbon produced by planar flow casting and subsequent heat treatment in an external magnetic field. Barkhausen noise measurements were carried out in the direction of the domain wall alignment as well as in the perpendicular direction. Barkhausen noise bursts exhibit three separate regions in all cases. The first region is associated with the nucleation and growth of reversed domains, whereas the strongest pulses can be detected in the second region in which the 180° domain wall motion plays the major role. The last region is associated with the annihilation of reversed domains. Hysteresis loops extracted from the Barkhausen noise also exhibit three separate avalanches. The first and third regions produce a high number of weaker pulses in contrast to the second region, which emits a lower number of very strong pulses.

Nondestructive microstructural characterization of austempered ductile iron

AbstractAustempered ductile iron (ADI) has been preferred in a wide range of applications due its unique combination of high strength, good ductility, wear resistance and fracture toughness together with lower cost and lower density compared to steels. Magnetic Barkhausen noise (MBN) measurement offers a better alternative to traditional characterization techniques by being fast and non-destructive. A simple linear regression using only one single independent variable cannot correlate the MBN with the microstructure of ADI, since its microstructure is multi-component. Multiple linear regression analysis (MLRA) was used to build a model that uses the characteristic features of microstructural constituents as input parameters to predict the MBN. For that purpose, Cu-Ni-Mo alloyed ductile iron samples austempered between 325 and 400 °C and for 45–180 min duration were used. The results show that MBN is most sensitive to the size and shape of acicular ferrite and retained austenite. Moreover, MBN is almost insensitive to the size, morphology and volume fraction of graphite particles. This indicates that retained austenite pins the domain walls more effectively than the graphite particles. Considering the results MLRA, MBN technique can be used to characterize the ausferritic microstructure of ADI.

Effects of normalizing heat treatment on the mechanical and magnetic properties of the puddled iron

In the mid-19th century, with the advent of the industrial revolution, the puddled iron was the main metallic material produced on a large scale to meet the existing demand in the manufacture of large riveted structural components, such as railway bridges, thus representing a technological advance in this period. Due to the incipient metallurgical knowledge of the time and the fact that corrective maintenance may have been carried out during the lifetime of the components, such as localized heating and/or welding that might not be suitable for this type of material, the study of this class of material becomes important since many of the structures made with it are still in operation. Furthermore, its heterogeneous characteristics, inherent to its manufacturing process, make it difficult to characterize it and there is little information about the status of the microstructure and mechanical properties of these structures, making the application of non-destructive characterization techniques appealing in this scenario. Thus, the present work is based on the characterization of the effects of the microstructure on the mechanical and magnetic properties of samples of puddled iron taken from a component in service for approximately 150 years and compared when subjected to a normalizing heat treatment. The results denote a ferritic microstructure of heterogeneous grain size containing numerous coarse nonmetallic inclusions that result in low tenacity values at room temperature both in specimens obtained from samples as received and normalized, although in the thermally treated condition a higher value of ferritic grain size, on average, was obtained which promoted a slight decrease in the hardness values. These phenomena were characterized by magnetic measurements of magnetic Barkhausen Noise (MBN) allowing the distinction of an easy magnetization axis aligned with the conformation direction of the evaluated components. Therefore, a great potential is exhibited in the non-destructive characterization of this class of material, although several standards are required to obtain a calibration curve with the evaluated properties.

Sub-Surface Analysis of Grinding Burns with Barkhausen Noise Measurements

Barkhausen noise (BN) measurements are commonly used for surface characterisation. However, often there is also a need to verify the sub-surface region because detrimental tensile stresses may be present after different manufacturing steps. Especially in a grinding burn, the surface stress may be compressive, but it changes quickly into tensile stress below the surface. The aim of this study was to find out whether regular surface-sensitive BN measurement is also sensitive to the stresses below the surface caused by grinding burns. More specifically, the aim was to study the relationship between BN features and sub-surface stresses and to identify a model that estimates sub-surface stresses. Real samples were collected from an actual process. The samples were cylindrical samples manufactured from commercial alloyed AISI/SAE L6 steel that was through-hardened prior to grinding. Barkhausen noise measurements were carried out for 42 grinding burn locations followed by X-ray diffraction-based residual stress surface measurements and residual stress depth profiles. Depth information was obtained through step-by-step electrolytic removal of thin layers. The stress profiles were pre-processed through interpolation and averaged stress was calculated as a function of depth below the surface. Correlation analysis was carried out to evaluate the relationships between BN features and stress at different depths and among BN features. The main outcome of the analysis was that BN measurement is dominated by the sub-surface tensile stresses rather than the compressive stress at the surface. It was also noticed that BN features form two groups, corresponding to average Barkhausen activity and magnetising field strength leading to maximum Barkhausen activity. Models for stress at different steps were identified systematically. The performance of the models for sub-surface stresses was reasonable with R2 values of around 0.85 and root mean squared error (RMSE) values of around 95 MPa. Based on the results, it is concluded that BN measurement provides information about sub-surface stresses and that stress can be evaluated through straightforward modelling, allowing fast detection of grinding burns.

Characterization of the Fatigue Behaviour of Low Carbon Steels by Means of Temperature and Micromagnetic Measurements

Investigations on low carbon (non- and low-alloy) steels were conducted in form of load increase tests (LIT) and constant amplitude tests (CAT) to find the correlation among material behaviour, mechanical load, and the type of NDT method. With the help of preprogrammed load-free sequences, the thermal impact on magnetic Barkhausen noise (MBN) measurement can be avoided, so that the cyclic deformation properties of material responses can be interpreted more precisely. The results indicate differences between the change in temperature and the MBN-derived variable during LITs and CATs regarding the demonstration of the incubation stage and the cyclic hardening behaviour.

Magnetic properties and Barkhausen noise evolution during FeSiCuNbB nanocrystalline material aging

The use of compact electrical systems in aircraft applications and the automotive industry is promoted to increase energy efficiency. The reduction in mass and volume of electrical systems exposes magnetic components to high temperatures and degrades their properties over time. This paper investigated the thermal aging of different grades of FeSiCuNbB nanocrystalline materials. Aging evolution was analyzed by monitoring macroscopic magnetic properties. Furthermore, magnetic Barkhausen noise measurements were performed to investigate the domain pattern microstructure and domain wall movements during aging.

Innovative X-ray diffraction and micromagnetic approaches for reliable residual stress assessment in deep rolled and microfinished AISI 4140 components

The residual stress state in the subsurface is a key element of surface integrity. It is well known to have a significant impact on a component's properties in terms of fatigue behavior and resistance to wear and corrosion. For this reason, adjusting residual stresses during manufacturing is a major challenge in modern production engineering, to improve and ensure a component's fatigue strength. In this context, hydrostatic deep rolling of the workpiece surface using adapted parameters enables the targeted induction of compressive residual stresses into subsurface layers. Due to specific properties regarding subsurface and topography for functional components in tribological applications, a further machining operation by microfinishing following deep rolling seems to be purposeful. In particular with regard to the production of components exposed to periodic load changes when used, the process combination can enable a substitution of the typically required conventional subsurface zone hardening. With the aim of economical process design, the corresponding parts can be manufactured with significantly reduced time and costs. Efficient and well-founded methods for monitoring the resulting influence on the subsurface zone properties are essential for a reproducible and target-oriented process design. The prevailing method for the non-destructive assessment of residual stresses in both academia and industry is X-ray diffractometry using the sin2 ψ-method. However, this method is time-intensive and requires complex instrumentation. Thus, efforts have been undertaken in past decades to develop alternative methods for the efficient and reliable characterization of residual stresses. In this research, the applicability of the cos α-method in X-ray diffractometry and a micromagnetic approach for residual stress assessment was investigated, analyzing deep rolled and microfinished AISI 4140 specimen conditions. In addition to the diffractometric and micromagnetic measurements, metallographic and topographic analyses of machined surfaces were carried out. Deep rolling was found to induce significant compressive residual stresses of up to −1100 MPa. After microfinishing of the deep rolled surfaces, favorable compressive residual stresses remain in the subsurface, reaching approximately up to −750 MPa. Based on this, the production of tailored surfaces with respect to a suitable combination of topography and subsurface is possible. For all surface states investigated, a good agreement between the two approaches in X-ray diffraction was found. Magnetic Barkhausen noise (MBN) measurements prove to be well applicable for an efficient and holistic assessment of surface integrity in the subsurface of deep rolled and microfinished AISI 4140 specimens.

In-Process Measurement of Barkhausen Noise for Detection of Surface Integrity during Grinding

The Barkhausen noise (BN) analysis is a method increasingly used for the post-process assessment of thermo-mechanical surface damages from grinding and has several advantages compared with the established nital etching method. In-process measurement of the BN has not been used industrially yet, but the basics have already been developed and promise time savings by avoiding time spent on inspections after grinding. Furthermore, it bears potential for the optimization of grinding processes and, in perspective, a process control. In the present work, the suitability of in-process BN analysis for the detection of thermo-mechanically influenced near-surface regions was assessed. Case-hardened workpieces were ground, and BN signals were related to the properties of the surface and subsurface area, in particular residual stresses, microstructure and surface hardness after grinding. The results show a clear dependency of BN on surface layer properties that allows for an in-process detection of detrimental changes in the surface state. Special attention was paid to the differences between in-process and post-process measured signals, and the suitability of the different measurement parameters for in-process detection was investigated.

Design of advanced micro-magnetic sensors for various applications

The state of the microstructures determines both the mechanical properties (hardness, yield strength, elongation, etc.) and micro-magnetic properties (magnetic Barkhausen noise (MBN), incremental permeability (IP), etc.). The non-destructive nature of the micro-magnetic testing technology facilitate it be a valuable tool for quality control in the manufacturing of ferromagnetic components. In the implementation of micro-magnetic testing, the functions and the geometry of the sensors need be customized for specific target properties and components. In this paper, four types of advanced or multifunctional sensors are presented and applied for micro-magnetic testing. First, a multifunctional sensor is developed for flat surface. Unique intermittent magnetizing method using superimposed magnetic fields of high and low frequencies enable the sensor to measure the magnetic Barkhausen noise, incremental permeability, eddy currents, tangential magnetic field and main flux. Second, to conduct magnetic Barkhausen noise testing at the inner corner of V-shape bending plate, a miniaturized sensor with U-shape roller magnetizer is designed to perfect contact with the corner region. Third, read head of a magnetic tape player is revised to act as high-lateral-resolution (up to 500μm) sensor for magnetic Barkhausen noise measurement in ferromagnetic film. Fourth, a slender probe is developed and embedded into an anti-bulking fixture that is clamped onto the flat specimen. With the slender probe together with the special fixture, the dependency of the magnetic Barkhausen noise on the compressive and tensile stress is first experimentally observed in a single flat specimen under continuous load.

Mechanical and magnetic properties of TRIP690 steel strengthened by strain-induced martensite

The sensitivity of the induced magnetic signals to microstructures makes it possible to characterize nondestructively the mechanical properties of ferromagnetic materials. In this study, utilizing the hysteresis loop measurement and the magnetic Barkhausen noise measurement, the correlation between the mechanical and the magnetic properties of low-alloy multiphase transformation-induced plasticity (TRIP) steel after plastic deformation was investigated, then the strength of the TRIP steel after plastic deformation was characterized. The results showed that, as the strain level increased from 0 to 0.2, the martensite volume fraction of TRIP690 steel increased from 0.78% to 3.64% which contributed to the enhancement of mechanical properties and the change of magnetic properties, and in particular, hardness and tensile strength had linearly increased by approximately 42% and 20%, respectively. Meanwhile, the peak value of the magnetic Barkhausen noise signal (MBNS) increased linearly with the increasing strain levels, the coercivity increased rapidly and then slowly, while the susceptibility decreased exponentially. By theoretically deriving the effects of the ferromagnetic phase volume fraction, dislocation density and residual stress of the TRIP690 steel on susceptibility and coercivity, the variation of the MBNSs under the influence of phase transformation, dislocation and residual stress was analyzed. The above non-destructive magnetic technique makes it possible to evaluate the mechanical response of the TRIP steel after plastic deformation, which allows it to be further applied to the quality evaluation of steel parts in stamping, such as body-in-white for the automotive industry.

Surface Integrity Characterization by Barkhausen Noise Measurement after Milling Processes with End Mills

In aerospace industry, safety-relevant parts are subject to strict testing guidelines to guarantee part functionality. Depending on the manufacturing process, the surface integrity of the part can be modified undesirably. In addition to common rim zone analyses, such as nital etching or X-ray diffractometry, Barkhausen noise measurement offers new potentials for a non-destructive surface integrity inspection. In this work, the influence of a 5-axis gear end milling process on the rim zone was investigated. A stable correlation between Barkhausen noise and material hardness and residual stress was determined.

Microstructure analysis of single-lip deep hole drilled bores by electron backscatter diffraction and magnetic Barkhausen noise

The surface integrity is crucial for the capability of machined parts; therefore, a detailed characterization of the obtained microstructure is very important for the assessment of machining processes. In this paper, the microstructure of single-lip deep drilled specimen, especially the formation of white etching layers (WEL), is considered by microscopic and micromagnetic investigations. Due to its asymmetrical nature, the single-lip deep hole drilling process induces severe plastic deformations in the bores surface. These severe deformations lead to a desirable fine-grained microstructure and the induction of beneficial compressive residual stresses. When too hard drilling parameters are chosen, the formation of a WEL can occur. These layers tend to be brittle and have tensile compressive stresses. The microstructures resulting from varied process parameters are characterized by light and electron microscopy as well with electron backscatter diffraction. These findings were correlated to the non-destructive magnetic Barkhausen noise measurement with a tailored bore sensor. This correlation enables an efficient and reliable evaluation of the process-induced microstructure and associated surface integrity.

On the use of differential permeability and magnetic Barkhausen Noise Measurements for Magnetic NDT Applications

Mapping microstructural to macroscopic magnetic properties is challenging yet necessary for the development of novel non-destructive testing techniques based on magnetic measurements. The macroscopic properties of a material reflect the underlying magnetization processes and the microstructural configuration as a result of the history of the material. Magnetic nondestructive evaluation techniques are needed for the health monitoring in critical structures and the quality monitoring of materials used in components, such as transformers and motors. In this work, we assess the usability of metrics related to AC magnetometry and magnetic Barkhausen Noise through measurements on several series of samples of various grain sizes and levels of plastic deformation. AC magnetometry is preferred to magnetic Barkhausen Noise as it provides more repeatable and reliable metrics than mBN which suffers from higher uncertainty. Also, differential permeability curves are preferred to hysteresis loops because they can be obtained directly through ac magnetometry and provide several types of information, such as the peak value, the coercivity, the overall shape, the emergence and location of multiple peaks which are related to stresses. In the case of stress-dependent magnetization processes, both differential permeability and mBN results are consistent, regardless of the way strain is applied, and corroborate results previously reported. However, permeability measurements are inconclusive on the effect of grain size on magnetization processes while the mBN voltage increases monotonically with decreasing grain size, in both types of samples. The mBN energy associated with each applied field step is proposed as a complementary measure for the assessment of the contribution of the irreversible processes in a material where the permeability measurements are inconclusive. Finally, the experimental observations have been used to set up micromagnetic calculations by modeling a plastically deformed material as a soft matrix with grains enclosed by boundaries with high transverse anisotropy which act as pinning centres and prevent further rotation of the magnetization and its alignment with the applied field thus reproducing the typical phenomenological features of plastically strained materials.

Measurement Potential of the Barkhausen Effect for Obtaining Additional Information on the Component Condition in Manufacturing

AbstractThe measurement of Barkhausen noise is one of the non-destructive testing methods which allows the use within the production line and within the cycle time at a high production volume. The aim of the present study was to answer the question, whether it is possible to extract the informations that the Barkhausen noise includes, concerning work-piece conditions, from the signal characteristic and more important assigning these findings. Therefore, soft machined and heat treated shaft components made of the ferromagnetic material Cf53 (1.1213) were analyzed to find characteristics in the Signal that allow to separate clearly an increase in temperature of the tested area from a change in the microstructure. For this purpose the shafts were analyzed at higher temperatures (up to 80 °C) and after an additional annealing process (to change the microstructure specifically). Both investigated situations (higher temperature and modified microstructure) showed different characteristic in the Barkhausen signal, thus an assigning is possible. Metallographic investigation and hardness measurements has been carried out to support the results.

Mimicking Barkhausen noise measurement by in-situ transmission electron microscopy - effect of microstructural steel features on Barkhausen noise

A relationship between microstructural steel features and an outcome of the Barkhausen noise (BN) measurement was studied. Two different microstructures, martensite and pearlite-ferrite were used. Commonly, BN is linked directly to the sample hardness. A BN outcome from both martensite and pearlite-ferrite was, however, similar even though martensite has three times higher hardness. To reveal the connection between microstructural features and BN, a typical industrial BN measurement was mimicked by in-situ transmission electron microscopy (TEM). Martensite needed higher field strength to move domain walls (DWs) than pearlite-ferrite. In martensite, DWs gathered to areas with high dislocation density. Fe3C lamellae in pearlite were strong pinning sites. DWs perpendicular and parallel to martensite laths started to move with the same field strength value. In pearlite, DWs perpendicular to lamellae started to move before the parallel ones. The RMS envelope of ferrite-pearlite starts earlier than that of martensite due to soft ferrite. Magnetically harder pearlite probably caused “a tail” and the envelope ends almost at the same time with martensite. . Nevertheless, similar peak width values were found for both samples. Martensite and pearlite have a lot of strong pinning sites, dislocations and Fe3C, respectively. Fe3C density is not as high as dislocation density. Ferrite has strong pinning sites only at low incidence, but as known, huge BN information volume compared to martensite and pearlite. This resulted in the similar pulse count from martensite and ferrite-pearlite.