Plants and structural components that have been in operation for long periods need special attention from plant operators to ensure safe and reliable performance. Efficient and economic operation of these plants requires regular preventive maintenance. In general, it is more effective to monitor the material condition that could lead to a subsequent failure rather than detecting a defect after its initiation. Conventional non-destructive evaluation (NDE) methods, such as ultrasonics and radiography techniques, fail to detect incipient damage caused by loading history or changing microstructural conditions that could lead to failure. Hence, improved or new NDE techniques are needed to monitor microstructural changes to estimate both remaining life of the component and extent of material degradation. The magnetic Barkhausen technique is one such advanced preventive maintenance tool for industry.
Magnetic nondestructive evaluation techniques
Nondestructive evaluation plays a significant role in pre-service and in-service inspection of plant components. Plant maintenance personnel need advanced NDE techniques to help them counter challenges such as increasing maintenance costs, shorter downtimes and stricter environmental regulations.
Primary objectives of NDE methods in plant maintenance include:
- Detection and evaluation of critical defects.
- Monitoring and assessment of loads and residual stresses.
- Evaluation of microstructure changes during service.
Magnetic NDE techniques are relatively new inspection methods gaining popularity for evaluating ferrous structural components. Magnetic properties of steels are sensitive to microstructural changes induced by mechanical and thermal treatments. Hence, changes in magnetic properties can be used to evaluate the material condition. Magnetic NDE techniques, especially Barkhausen emission measurements, show excellent sensitivity to residual stress levels and changes in the microstructure.
Barkhausen emission technique is an easy-to-use, semi-quantitative technique for monitoring changes in near surface stress distribution or microstructure of ferromagnetic machinery components.
Barkhausen emission analysis
Ferromagnetic materials are full of small magnetic regions called domains. Each domain is magnetized along a certain crystallographic easy direction of magnetization, and domains are separated from one another by boundaries called domain walls. These domain walls move under the influence of an applied magnetic field. This movement of domain walls results in a change in magnetization within the material and will induce an electrical pulse in a pick-up coil. When the electrical pulses produced by domain movement are added, a noise-like signal called the Barkhausen effect or magnetic Barkhausen emissions (MBE) is generated, named after its discoverer Heinrich Barkhausen. Amplification of these signals produces audio/radio frequency noise, which can be observed on an oscilloscope or spectrum analyzer.
MBE has a power spectrum that extends to about 2 MHz, the amplitude of which is damped exponentially as a function of depth below the surface. The damping is attributable to eddy current damping experienced by the propagating electromagnetic fields the domain wall movement creates.
Measurement depth in ferromagnetic materials depends on the frequency range of the Barkhausen emission signals and material properties, such as conductivity and permeability. The measurement range for MBE varies between 0.01 to 1.5 mm from the surface for most ferromagnetic materials.
Theoretical aspects and practical applications of Barkhausen effect can be obtained by excellent review articles by Allessandro et al., , Tiitto , Matzkanin et al. , and Jiles .
Magnetic Barkhausen emissions depend on material properties
The MBE spectrum depends on the magnetic state of the sample--including chemical composition, microstructure, magnetic and thermal history, and applied or residual stress. The characteristics of the MBE signal depend most importantly, for a given sample, on microstructure and stress.
Elastic stresses in metals are known to influence the metal's magnetic domain structure. This phenomenon of elastic stresses interacting with domain structure is called "magnetoelastic interaction". And in materials with positive magnetic anisotropy (such as iron, cobalt, and low-alloy steels), it is this interaction that causes tensile stresses to increase the intensity of Barkhausen emissions while the presence of compressive stresses decreases signal intensity. Therefore, MBE signals can be used to deduce the nature and magnitude of the stresses in the material.
In general, MBE intensity decreases continuously with an increase in hardness. Higher hardness implies an increased number of defect pinning centers, which impede the movement of both dislocations and magnetic domain walls. Hence, harder materials exhibit lower levels of magnetic Barkhausen emissions. The compressive stress state of a shot-peened surface will yield a relatively low MBE output. If the MBE signal from a sample begins to rise, it may indicate the protective compressive surface stresses induced by shot peening are being converted to a potentially dangerous tensile condition.
In general, MBE analysis can provide information about the stress-state of a sample when the microstructure is known or fixed. Conversely, it can provide information about microstructure as long as the sample remains in a known state of stress. The technique usually cannot, however, provide useful information when the stress-state and the microstructure are both unknown.
The sensor consists of two components. First is a yoke or "horseshoe" electromagnet, which can be driven by a bipolar wave (typically a triangular waveform) having a peak amplitude capable of driving the sample over a significant percentage of its hysteresis loop. The second component is a sensitive pickup coil, typically consisting of a large number of turns wound on a ferrite core.
Barkhausen signals acquired by the ferrite-cored surface sensor are fed to a low-noise, filtered amplifier. The sensitivity of the technique to stress levels at different depths in the specimen can be changed by varying the frequency of the excitation signal and by limiting the bandwidth of the detected signal. Because Barkhausen signals originating near the surface of the sample are richer in high-frequency components than signals originating deeper in the material, bandwidth filters provide a way to estimate the depth from which the MBE activity is occurring. Also, higher frequency drive fields penetrate less deeply into the substrate and are used for analyzing near surface regions.
The Barkhausen voltages can be digitized and analyzed to determine the number of pulses, effective intensity and root mean square (RMS) voltage of the Barkhausen pulses.
Effective preventive maintenance for plant components involves monitoring for:
- Nature and amounts of residual stress levels.
- Fatigue damage.
- Creep damage.
- Radiation damage.
- Grinding burns (damage).
- Variations in coating thickness.
- Depth of case hardening.
- Surface hardness.
Its strong dependence on microstructure and stress makes magnetic Barkhausen emission technique an attractive preventive maintenance tool. The measurements usually can be made quickly, it generally requires no special surface preparation, and can be conducted through thin layers of dielectric or non-magnetic coatings and electroplate. Special-purpose probes can be fabricated easily to conform to curved surfaces.
Surface modifications (whether intentional or not) frequently will cause measurable changes in the MBE spectrum of a material. Such surface modifications may be induced by grinding, shot peening, fatigue, radiation damage, heat treatment or case hardening. The Barkhausen technique has been used to estimate radiation damage , the depth of hardening imparted by conventional surface hardening techniques  and residual stress . MBE measurements have also been used to determine the thickness of various electrical or non-electrical coatings . Coating thickness is monitored by the changes in the number of Barkhausen pulses; a decrease here indicates an increase in coating thickness. The measured magnetoelastic parameter (equivalent to the RMS voltage of the Barkhausen signal) in the region of a weld (Figure 3a) was high before stress relieving. After annealing, the magnetoelastic parameter dropped, indicating a decrease in the residual stress levels in the weld region. The figures indicate data taken at different depths. MBE showed a large increase with increasing tensile stress and marginal decrease with increasing compressive stress. These examples indicate the applicability of MBE technique to detect microstructural changes caused by various material degradation processes.
MBE analysis is generally not capable of giving absolute information about the properties of an unknown sample, but that changes in the spectrum of a previously categorized sample provides a powerful tool for detecting material degradation of machinery components on the shop floor. To take advantage of MBE measurements as a viable NDE tool, one has to compare the data acquired from the component with the data from a calibrated sample. Alternatively, a threshold value can be established for either the number of Barkhausen pulses or maximum amplitude of RMS voltage. Unexplained changes or values above the threshold value may indicate that the part has been subjected to excessive heat, grinding burns, stress or damage that could lead to failure.
Structures and components that have been studied by MBE analysis include pipelines, bearing races, engine blocks, gears, cylinder heads, rollers, turbine blades, crankshafts, railroad wheels and rails, automotive body panels and hydraulic cylinders.
What it means
Barkhausen emission analysis is a relatively inexpensive nondestructive evaluation method for monitoring changes in the condition of ferromagnetic components during service. However, a drawback of magnetic measurements is that, in general, magnetic properties change in response to not only stress but also to changes in the material's microstructure. Hence, magnetic measurements cannot be compared for samples of even similar materials if they have undergone different heat treatment processes or been subjected to varying service environments. Careful planning, however, can help take advantage of magnetic measurements.
Magnetic properties can be evaluated on components made of the same material with similar composition, microstructure, and manufactured under similar processing conditions. In addition, a combination of bulk and surface magnetic properties greatly enhances the reliability in the measurements. Therefore, magnetic measurements could be used for characterizing components in a production environment. The changes in the magnetic properties of these components could be correlated with changes in microstructural conditions. Preventive maintenance steps can then be taken to avoid microstructural conditions that could lead to potential catastrophic damage. More research is needed in correlating the changes in MBE parameters with microstructural changes associated with specific damage mechanisms. With advances in instrumentation and further research, magnetic Barkhausen emissions technique could become an effective in-service monitoring technique.
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