On the stress vs. Barkhausen noise relation in a duplex stainless steel

Various parameters of a Barkhausen noise burst were measured under applied stress in prestrained duplex stainless steel specimens. After quenching, the root mean square value of the Barkhausen noise amplitude was low because of the compressive macroscopic and interphase macrostresses in the ferrite phase. Prestraining increased the amplitude of the Barkhausen noise in all measuring directions due to reduction of macroscopic compressive residual stress and introduction of tensile interphase microstresses in the ferrite phase. Under applied stress, Barkhausen noise measured in the loading direction behaved similarly to ferritic steels: Barkhausen noise increased under tensile stress and decreased under compressive stress. In the direction transverse to the loading axis, the Barkhausen noise response was anomalous: Barkhausen noise increased or remained unchanged under macroscopic compressive strain in the transverse direction and decreased under tensile strain in the transverse direction. In unstrained specimens in the loading direction, applied stress vs. Barkhausen noise showed a linear relation under the applied stress range from −200 to 250 MPa. The stress sensitivity of the Barkhausen noise was, however, low. In the prestrained specimens, the stress sensitivity was higher than in the unstrained specimens, especially under applied compressive stress.     

Magnetic characterization of cold rolled and aged AISI 304 stainless steel

Magnetic easy axis predicted by the orientation distribution of the maximum amplitude of magnetic Barkhausen emission (MBE), which is obtained by magnetization in radial directions from the center of the specimens has been applied to determine the magnetic anisotropy on cold rolled and aged 304 SS in two sets of specimen. The maximum of the MBE has been found to orient along the rolling direction (RD) compared to the transverse direction (TD), indicating the presence of magnetic easy axis along the rolling direction for both sets. The strain induced martensite phase transformation has been determined using X-ray diffraction technique. The orientation distribution function (ODF) analysis has been carried out to obtain the crystallographic texture with cold rolling. ODF analysis revealed the 110 texture as the major. The magnetic anisotropy factor has also been determined with cold deformation and noticed that the strength of magnetic anisotropy decreases above 50% deformation for both the sets. Results have been explained considering two competitive effects, formation of crystallographic texture in the martensite phase and presence of compressive residual stresses along RD during cold rolling.     

Analysis of the stress dependent magnetic easy axis in ASTM 36 steel by the magnetic Barkhausen noise

The angular distribution of the magnetic Barkhausen noise energy is used to determine the influence of the applied stress in cold rolled ASTM 36 steel. In particular, the magnetic easy axis is predicted by the angular distribution of the magnetic Barkhausen noise energy, which, in turn, is obtained by magnetizing the samples in the radial direction from the center of the sample. The results indicate that the applied uniaxial stress gives origin to a continuous rotation of the magnetic easy axis. In addition, this study reveals that the jump energy distribution presents a clear angular dependence and is very sensitive to the anisotropic behavior of the magnetic Barkhausen noise activity.     

Influence of uniaxial plastic deformation on magnetic Barkhausen noise in steel

Magnetic Barkhausen noise (MBN) measurements were made on hot-rolled mild steel samples uniaxially deformed to differing magnitudes of plastic strain, to study the dependence of MBN activity on plastic strain. The results indicated an initial increase in MBN energy with increasing plastic strain followed by a decrease at higher plastic deformations. At still higher plastic deformations, the MBN energy was found to be almost independent of plastic strain. The results are explained in terms of different mechanisms of interaction of domain walls with dislocations, with increasing plastic strain. The behavior of MBN energy with plastic strain was found to be anisotropic and the angular MBN measurements indicated that the deformation-induced easy axis of magnetization changed direction with increasing plastic strain. At higher deformations, the MBN activity was largely controlled by the deformation-induced anisotropy, due to residual stress.     

Alternative magnetic parameters for characterization of plastic tension

The applicability of magnetic techniques (hysteresis and Barkhausen noise) in the detection of plastic deformation was comprehensively studied. Structural low-carbon steel subjected to uniaxial plastic tension was accurately measured for a wide range of residual strains and at different magnetization angles with respect to the stress direction. In addition to classical magnetic parameters, new quantities with better sensitivity-stability ratios were introduced on the basis of two-phase remagnetization caused by compressive residual stress. The magnetic methods had highest sensitivity in the deformation direction (hard magnetization axis). The parameters changed as cos² with rotation to the perpendicular easy magnetization axis, where the magnetic sensitivity was lowest. In this direction, only the root mean square value of Barkhausen noise considerably changed with the strain. The extremes of the Barkhausen parameters with respect to the magnetization angle did not exactly correspond to the hysteresis extremes (real easy and hard magnetization axes) in that there was a shift of about ±10°.     

Non-destructive evaluation of the micro residual stresses of IIIrd order by using micro magnetic methods

Nanoscale coherent precipitates and the corresponding micro residual stresses play a dominant role in the strengthening process of materials. At present, there exists no experimental method for measuring micro residual stresses of IIIrd kind non-destructively. In the frame of the present work, it will be shown that micro-magnetic measurement techniques based on the tensile loading dependent maximum Barkhausen noise amplitude can be used for the analysis of micro residual stresses (MRS) of IIIrd kind (coherency residual stresses). For this purpose, Fe–Cu-alloys with well-defined contents of Cu-precipitates were produced and investigated.     

Measurement of residual stress in steel using the magnetic Barkhausen noise technique

Surface residual stresses on a structural beam steel sample were evaluated using a non-destructive technique based on the measurement of surface magnetic Barkhausen noise (MBN). MBN measurements were performed using a high resolution probe consisting of a small magnetic read head mounted between the poles of a ferrite U-core magnet. Applied stress magnitudes were correlated to MBN energy levels for calibration purposes. MBN energy measurements were done at various locations on the steel sample. The magnitude of the residual stress component along the sample beam axis was evaluated across the width of the beam using these calibration curves. The range of sensitivity of the MBN signal to stress as defined by the calibration curves was limited by the direction of easy magnetization. The effect of stress on the MBN signal was interpreted in terms of the active 180° domain wall population. The validity of the residual stress results was confirmed experimentally using conventional methods: cutting and sectioning and hole drilling. Possible sources of residual stresses and their distribution on the structural steel specimen are discussed.     

Relation between residual stress and Barkhausen noise in a duplex steel

The relation between residual stress and Barkhausen noise (BN) was evaluated in a duplex stainless steel. The residual stresses had been created to the test piece by local heating. Rms value, position of the maximum of the BN burst, its full width half maximum, power spectrum and pulse height distribution were characterised. All the parameters depended to some extent on the residual stresses and none of them was totally independent on the other parameters. The best parameter for the residual stress evaluation was the rms value of the BN amplitude. The rms value varied according to the total stress (macrostress+homogeneous microstress) in the ferrite phase. The stress component perpendicular to the direction of the magnetization had also some influence. Based on the results it is possible to evaluate the residual stresses of the ferrite phase of duplex stainless steels by the BN method.     

Magnetic Barkhausen measurements for evaluating the formation of Lüders bands in carbon steel

Localized inhomogeneous plastic deformation phenomenon was experimentally investigated in a structural steel by using the magnetic Barkhausen noise measurements. ANSI 1050 steel plates, throughout oriented in the rolling transversal direction, were selected additionally inducing different strains. Scanning of the magnetic Barkhausen energy (MBN_energy) shows the formation of Lüders bands as well as the displacements in the material deformation zone. The MBN_energy angular dependency on the applied stress and on the anisotropy coefficient were determined as well as the presence of Lüders bands in samples oriented according to the rolling direction.     

Residual Stress Relaxation of Shot-Peened Deformation Surface Layer on Duplex Stainless Steel Under Applied Loading

The relaxation of residual stress in shot-peened surface layer of duplex stainless steel S32205 under static and cyclic loading was investigated. The results reveal that the compressive residual stress is relaxed under applied tensile stress. The relaxation of residual stress in longitudinal direction is more obvious than that in transverse direction in both austenite and ferrite. When the applied stress is beyond the yield strength of the materials, the relaxation of the residual stress becomes drastic. Under cyclic loading, the residual stress relaxation occurs fast in the first few cycles, it then becomes stable gradually. A model was used to quantitatively calculate the residual stress under cyclic loading with different applied tensile stresses. The relaxation behavior is determined by the applied loading, the number of cycles, dislocation density, and the residual stress gradient. The relaxation behavior difference under cyclic loading between ferrite and austenite is discussed.     

Residual stress relaxation in typical weld joints and its effect on fatigue and crack growth

Many factors influence the fatigue and crack growth behavior of welded joints. Some structures often undergo fairly large static loading before they enter service or variable amplitude cyclic loading when they are in service. The combined effect of both applied stress and high initial residual stress is expected to cause the residual stresses relaxation. Only a few papers seem to deal with appropriate procedures for fatigue analysis and crack growth by considering the combined effect of variable amplitude cyclic loading with residual stresses relaxation. In this article, some typical welded connections in ship-shaped structures are investigated with 3-D elastic-plastic finite element analysis. The effect of residual stress relaxation, initial residual stress, and the applied load after variable amplitude cyclic loading is revealed, and a formula for predicting the residual stress at hot spot quantitatively is proposed. Based on the formula, an improved fatigue procedure is introduced. Moreover, crack growth of typical weld joints considering residual stresses relaxation is studied.     

Magnetic Barkhausen emission technique for detecting the overstressing during bending fatigue in case-carburised En36 steel

The effect of bending fatigue at different maximum stress levels on the magnetic Barkhausen emission (MBE) has been studied in case-carburised En36 steel specimens. The low frequency MBE profile has been measured after unloading the specimen at different number of fatigue cycles. It has been found that, beyond 1000 MPa, the MBE peak height decreases just after few thousand cycles and the percentage reduction in the MBE peak increases with maximum bending stress level. The reduction in MBE peak at lower stresses (<1400 MPa) is attributed mainly to the effect of residual stresses becoming more compressive below the surface due to the application prior tensile stress. At higher stresses (1500 MPa), the variation in the MBE peak also indicates the effect of cyclic hardening and softening with progressive fatigue cycles. The MBE profiles measured after monotonic loading and unloading with different maximum stress levels also show similar reduction in the MBE peak with increase in pre-stress level. However, at higher stresses (>1400 MPa), the cyclic loading shows larger reduction in the MBE peak than the monotonic loading. This is attributed to the effect of cyclic microplasticity induced enhancement of dislocation density in addition to the residual stress modification. This study clearly shows the MBE technique can be used to detect the maximum stress level seen by the specimen beyond 1000 MPa. Any overstressing of this case-carburised steel beyond the fatigue limit of 1150 MPa can be easily detected from the percentage reduction in the MBE peak. Since the crack propagation stage is insignificant in these hard steels, the detection of any overstressing using the MBE technique would be very useful in assessing and preventing the impending catastrophic failure.     

On the role of residual internal stresses and dislocations on Barkhausen noise in plastically deformed steel

The dependence of Barkhausen noise on elastic and plastic deformations, achieved in tension and in compression, has been investigated both in Armco iron and a low carbon steel. These materials exhibit quite different behaviours, especially with regard to the effect of plastic deformation: a tensile plastic deformation (>1%) induces a marked increase in Barkhausen noise for Armco iron while it induces a steep decrease in the low carbon steel. The comparison between the tensile and compressive behaviours, as well as between the elastic and plastic regimes of deformation enables us to attribute these effects to two underlying mechanisms, i.e. effect of residual internal stresses through magneto-elastic coupling and dislocation–domain wall interaction. In Armco iron, the latter mechanism seems to have the strongest influence on the Barkhausen noise, while in the low carbon steel the influence of residual internal stresses prevails.     

Characterization of angular dependence of macroscopic magnetic properties in ASTM 36 steel using magnetic Barkhausen noise

Angular-dependent magnetic Barkhausen noise measurements were performed on ASTM 36 steel samples for various values of applied uniaxial stress. The maximum differential permeability and the coercive field are obtained from this Barkhausen signals. The angular dependence of these macroscopic magnetic properties is studied. The tension for which the magneto-elastic energy balances the anisotropic energy is estimated. The results are discussed based on the domain wall theory.     

Evolution of the residual stress state in a duplex stainless steel during loading

The evolution of micro- and macrostresses in a duplex stainless steel during loading has been investigated in situ by X-ray diffraction. A 1.5 mm cold-rolled sheet of alloy SAF 2304 solution treated at 1050°C was studied. Owing to differences in the coefficient of thermal expansion between the two phases, compressive residual microstresses were found in the ferritic phase and balancing tensile microstresses in the austenitic phase. The initial microstresses were almost two times higher in the transverse direction compared to the rolling direction. During loading the microstresses increase in the macroscopic elastic regime but start to decrease slightly with increasing load in the macroscopic plastic regime. For instance, the microstresses along the rolling direction in the austenite increase from 60 MPa, at zero applied load, to 110 MPa, at an applied load of 530 MPa. At the applied load of 620 MPa a decrease of the microstress to 90 MPa was observed. During unloading from the plastic regime the microstresses increase by approximately 35 MPa in the direction of applied load but remain constant in the other directions. The initial stress state influences the stress evolution and even after 2.5% plastic strain the main contribution to the microstresses originates from the initial thermal stresses. Finite element simulations show stress variations within one phase and a strong influence of both the elastic and plastic anisotropy of the individual phases on the simulated stress state.     

Orientation dependence of elastic properties and internal stresses in sub-microcrystalline copper produced by equal channel angular pressing

The orientation characteristics of the elastic properties of sub-microcrystalline copper produced by equal channel angular pressing (ECAP) were studied by measuring the velocities of the longitudinal and transverse ultrasonic waves in different spatial directions. It was shown that the effect of an “anomalous” reduction in the elastic moduli observed after ECAP treatment reflects a strong spatial anisotropy of the material. Internal stresses arising in copper as a result of ECAP processing were determined by means of X-ray diffraction line broadening analysis. An appreciable anisotropy of the internal stresses was also found. Possible mechanisms responsible for the anomalies of the elastic properties are discussed.     

线能量对TC4钛合金激光焊接残余应力和变形的影响

利用有限元分析和实验测试，研究了TC4钛合金平板激光焊接线能量对变形和残余应力的影响规律，并通过焊缝金相实验分析了线能量与焊接残余应力和变形的内在关系。结果表明：钛合金激光焊接产生的纵向残余拉伸应力约700MPa～850MPa，而横向残余拉伸应力只有50MPa～80MPa。激光焊接线能量增加时，纵向残余应力拉伸区域变宽，峰值应力降低，而横向残余应力随线能量的增加而升高。在临界焊透规范以上焊接时，角变形随线能量的增大而减小，但横向收缩变形增大。试件被完全穿透焊接时，线能量对角变形的影响作用降低。     

Fatigue properties of rolled AZ31B magnesium alloy plate

Fatigue strength, crack initiation and propagation behavior of rolled AZ31B magnesium alloy plate were investigated. Axial tension-compression fatigue tests were carried out with cylindrical smooth specimens. Two types of specimens were machined with the loading axis parallel (L-specimen) and perpendicular (T-specimen) to rolling direction. Monotonic compressive 0.2% proof stress, tensile strength and tensile elongation were similar for both specimens. On the other hand, monotonic tensile 0.2% proof stress of the L-specimen was slightly higher than that of the T-specimen. Moreover, monotonic compressive 0.2% proof stresses were lower than tensile ones for both specimens. The fatigue strengths of 107 cycles of the L- and T-specimens were 95 and 85 MPa, respectively. Compared with the monotonic compressive 0.2% proof stresses, the fatigue strengths were higher for both specimens. In other words, the fatigue crack did not initiate and propagate even though deformation twins were formed in compressive stress under the cyclic tension-compression loading. The fatigue crack initiated at early stage of the fatigue life in low cycle regime regardless of specimen direction. The crack growth rate of the L-specimen was slightly lower than of the T-specimen. Consequently, the fatigue lives of the L-specimen were longer than those of the T-specimen in low cycle regime.     

Characterization of texture and residual stress in a section of 610 mm pipeline steel

Gas pipelines are inspected for defects such as corrosion. The most commonly used nondestructive inspection tool uses the magnetic flux leakage (MFL) technique. The MFL signals depend on the magnetic behaviour of the pipe, which is sensitive to its microstructure and crystallographic texture as well as both residual and applied stresses. Here a section of commercial X70 pipeline is characterized using microstructural examination, X-ray diffraction (to determine crystallographic texture) and neutron diffraction (for residual stress measurement). The results correlate well with the manufacturing steps used for this type of pipe. Magnetic characterization is also performed using magnetic Barkhausen noise measurements, which reflect the magnetic anisotropy in the pipe and thus the MFL signal. These results do not correlate simply with crystallographic texture and residual strain results, but this is not unexpected given the complex nature of the material and its stress state.