无损金属磁记忆检测技术试验研究方法

Experimental Research on Metal Magnetic Memory Method K.Yao&Z.D.Wang&B.Deng&K.ShenReceived:12December2010/Accepted:9March2011#Society for Experimental Mechanics2011Abstract Metal magnetic memory(MMM)method is a novel,passive magnetic method for inspecting mechanical degradation of ferromagnetic components.To promote a further understanding of the relation between the magnetic characteristics and mechanical deformation,the normal spontaneous stray field component and its gradient of Q235-steel specimens were measured during uniaxial tensile and compressive loading processes.The results show that the normal spontaneous stray field component and its gradient are effective in capturing different deformation stages under tensions,but no detectable change can be found during the whole compressive loading pared with the amplitude of the normal spontaneous stray field component,the gradient is a more sensitive parameter.In addition,the result demonstrates that it is easy to differentiate macro-crack and plastic deforma-tion because the differences among measured spontaneous stray field signals are obvious.Moreover,various factors affecting test results were also considered.Keywords MMM method.Spontaneous stray field signal. Elastic deformation.Plastic deformation.Lift-off value IntroductionFerromagnetic materials consist of numerous magnetic domains.An internal or applied stress can change the macro-magnetic properties of ferromagnetic materials due to the magnetic moments rotation and domain wall displacement under mechanical loading[1,2].Such a magnetic/mechanical coupled effect is the physical basis for developing various magnetic non-destructive techniques for assessing mechanical properties of ferromagnetic structures. Traditional magnetic techniques[3–9],such as magnetic particle inspection(MPI),magnetic flux leakage(MFL),are effective in diagnosing fully developed defects,but provid-ing little information about the mechanical degradation and early damage of the objects under test.Meanwhile,the traditional magnetic techniques usually require magnetic saturation of the objects,which is time-consuming and even impractical on some conditions.Therefore,it is of great interest and necessary to develop more simple and effective non-destructive magnetic techniques.Recently,a novel magnetic testing method,named metal magnetic memory(MMM)method,was introduced[10,11] and has received much attention[12–16].The MMM method presents several advantages over other magnetic test methods[17].For example,it is a passive test method where the self-magnetic-flux-leakage(SMFL)signal of objects is measured.The Earth’s magnetic field instead of an applied strong field is used as the stimulus source.In addition,it is regarded to be effective in assessing the early damage as well as fully developed defects,and it is time-saving and the maximum measurement velocity can be up to several meters per-second.The MMM method has received great interest due to these primary advantages.However,up to now this method is still in its infancy.Theoretically,the developed magneto-elastic models[18–21]cannot provide a mechanism explanation of such a plastic-based magnetic/stress cou-pling phenomenon.Experimentally,no systematic data were collected to provide enough information about the relation between the SMFL signal and deformationfeature K.Yao:Z.D.Wang(*):B.Deng:K.ShenDepartment of Mechanics,School of Civil Engineering,BeijingJiao-Tong University,Beijing100044,Chinae-mail:zhdwang@Experimental MechanicsDOI10.1007/s11340-011-9490-3of ferromagnetic materials.The proposal of this paper is to present an experimental research on the MMM method.ExperimentalThe test material is Q235low-carbon steel,which is commonly used in engineering structures.Uniaxial tensile and compressive tests were performed.Figure 1(a)and (b)illustrate the shapes and sizes of tensile and compressive specimens,respectively.The thickness of the tensile specimen is 5mm,which is not labeled in Fig.1(a).As shown in Fig.1,small holes (1mm for tension and 2mm for compression)were machined in the middle of the specimens to produce a local stress-concentration during the loading process.The static tensile and compressive tests were performed with a home-made testing machine named SANS-100kN,whose load error is within ±0.5%.The normal component of SMFL signal,H p (y),was measured by a commercial EMS2003magnetic memory device.During the experi-ments,the specimen was firstly loaded to a pre-determined value,and then it was taken out from the grip holder.The measurement of H p (y)values was performed along the three dashed lines [see Fig.1(a)],which are about 1mm,3mm and 5mm away from the hole edge,respectively.After that,the specimen was loaded again to a higher pre-determined stress,and the above procedures were repeated until the specimen was failed.Results and DiscussionsFigure 2(a)and (b)present the experimental tensile and compressive results of the force versus displacement,respectively.According to the results,the tensile specimen was yielded at about 12kN,and then experienced a plastic deformation.When the applied load reaches 16.5kN,it was broken.Meanwhile,the compressive specimen was yielded at about 75kN,and then experienced a significant plastic deformation without much increase of the applied load.No critical broken point was measured during this process.Figure 3shows the variations of spontaneous magnetic signals,H p (y),along the three measured lines under different tensile loads,where the variable,d ,is defined as the lift-off value of the sensor representing the distance between the sensor and the specimen.In the initial state without the applied load [see Fig.3(a)],the H p (y)signal does not keep a constant value and shows an approximately linear change along the test lines.It is due to the re-arrangement of the domain structure during the specimen fabrication.During the elastic deformation stage [Fig.3(b)and (c)],all measured curves become more regular and are linearly coincident along the loading direction.Such a phenomenon is due to the piezomagnetic effect [22].It is known that an applied stress can lead to the reorientation of magnetic domains along the tensile direction or perpendic-ular to the compressive direction [23].Obviously,the results in Fig.3(b)and (c)indicate a magnetic ordering state to some extent after the elastic deformation.F o r c e (k N )Displacement (mm)F o r c e (k N )Displacement (mm)Tension(b)(a)Fig.2Tensile and compressive force-displacement curvesTensionCompression(a)(b)Fig.1Geometry and dimen-sions of the specimen (in mm)Exp MechDifferent from that of the elastic deformation,the H p (y)signal in the stress concentration zone departs from the linear change and decreases more quickly when the specimen is plastically deformed [Fig.3(d),(e)and (f)].For example,small non-linear changes of the H p (y)value along the three lines were shown in the stress concentration zone in the case of 12kN.When further increasing the load to be 15kN,obvious non-linear changes along lines 1and 2were measured in the stress concentration zone.When the force reaches 17kN,all of the three measured lines show non-linear change in the stress concentration zone.The above result clearly indicates the stress-concentration location and plastic-deformation development.Figure 3also shows further interesting results.The change of the H p (y)value in the test range (−30~+30mm)increases with the increase of elastic deformation [Fig.3(a-c)],0H p (y ) (A /m )H p (y ) (A /m )H p (y ) (A /m )H p (y ) (A /m )H p (y ) (A /m )H p (y ) (A /m )X (mm)X (mm)X (mm)X (mm)X (mm)X (mm)(a) (b)(c) ()Fig.3Variations of H p (y)signals on the three measured lines of the specimen during the tensile testExp Mechbut decreases with the increase of plastic deformation[Fig.3 (d–f)].Such phenomena can be explained by the different acting mechanisms of the elastic and plastic deformations on the domain structure of ferromagnetic materials.As mentioned above,elastic deformation promotes the domain-wall movement and magnetic-moment rotation along the tensile stress direction.However,the plastic deformation leads to the accumulation of the dislocation density.The dislocation plays the role of pinning sites to block the movement of domains and decrease the magnetization[24,25].Hence,the elastic deformation increases the amplitude of H p(y),but the plastic deforma-tion decreases it.Figure4presents a comparison of H p(y)values along three measured lines at F=15kN,d=1mm.Significant non-linear change of H p(y)signal in the stress concentration zone can be observed along line1.However,the non-linear change becomes weak with the increase of the distancebetween the measured lines and the hole edge(lines2and 3).This result clearly indicates the maximum stress concentration location along the perpendicular loading direction.In engineering practices,we can determine the maximum stress concentration location by comparing the results along different measured lines.Figure5presents a comparison of H p(y)signals when the specimen is plastically deformed and broken.After theHp(y)(A/m)X (mm)Fig.5Comparison of H p(y)signals on the plastic deformation andfractureHp(y)(A/m)Hp(y)(A/m)Hp(y)(A/m)X (mm)X (mm)X (mm)(a)(c)(b)Fig.4Comparison of H p(y)signals on the three measured lines at F=15kN,d=1mmExp Mechfracture,two broken parts were taken carefully from the testing machine,respectively,and then connected up.The results in Fig.5show that the H p (y)curve in the case of the fracture is significantly different from that of the plastic deformation.Only a little non-linear change in the local stress concentration zone appears during the plastic defor-mation process,but H p (y)changes positive –negative symbols and has a zero in the maximum stress concentra-tion zone after the fracture.Moreover,the amplitude of the H p (y)signal at failure is about one order larger than that of the plastic deformation.The MMM method was firstly proposed by Prof.Doubov in 1997and two basic criteria were introduced [10]:the tangential component H p (x)reaches to the maximum value,the normal component H p (y)changes positive –negative symbols and has a zero in the maximum stress concentration zone.Prof.Doubov also reported extensive engineering test results based on the MMM method [26].However,most of the detected defects in Prof.Doubov ’s reports are cracks,not the plastic deformation.When the specimen is plastically deformed,some recent experimental reports [27–30]confirmed that there is no peak-peak change of H p (y)signals in the maximum stress concentration zone.Therefore,the results of Fig.5at least indicate two points.First,the different characters of H p (y)curves in the cases of the plastic deformation and fracture might be due to different deformation mechanisms of the domain structure in ferromagnetic materials,and thus more accurate criteria for inspecting the plastic deformation are required in the MMM method.Second,it is easy in engineering practices to differentiate the plastic deformation from maroc-crack based on the different features of H p (y)curves.The lift-off value is a crucial parameter affecting MMM test results.Figure 6shows the effect of lift-off value on H p (y)signals when the specimen is plastically deformed.As the lift-off value,d ,is 1mm,significant non-linear changes of H p (y)signals in the stress concentration zone can be found along the three lines.However,further increase of the lift-off value leads to weaker non-linear changes.When the lift-off value increases to be 5mm,non-linear changes are hardly detectable.Figure 7shows the effect of the lift-off value on H p (y)signals when the specimen is broken.Obviously,large lift-off values decrease the peak-peak amplitude and increase its width.The detailed results of the peak-peak amplitudeH p (y ) (A /m )H p (y ) (A /m )H p(y ) (A /m )X (mm)X (mm)X (mm)(c) Fig.6Comparison of H p (y)signals on different lift-off values after the plastic deformationExp Mechand width with different lift-off values are shown in ually we pay more attention on the peak-peak amplitude of H p (y)signals than its width in the MMM method.In fact,the peak-peak width of H p (y)signals can also provide vital information in characterizing the features of structure defects.For example,previous numerical simulations [31,32]have confirmed that the peak-peak width is sensitive to the size of defects,but insensitive to the defect location (surface or inner defects).Figure 9presents a comparison of the H p (y)amplitude and its gradient when the specimen is plastically deformed,where the gradient is calculated via the amplitude divided123481216w (m m )Line 123400800120016002000H p (y ) (A /m )LineWidth Amplitude(a)(b)Fig.8Width and amplitude of peak-peak H p (y)signals on the three measured lines after the fractureH p (y ) (A /m )X (mm)-800-600-400-2000200400600H p (y ) (A /m )X (mm)-600-400-200200400H p (y ) (A /m )X (mm)(a)(c)(b)Fig.7Comparison of H p (y)signals on different lift-off values after the fractureExp Mechby the distance of two adjacent measured points along the axis of the pared with the amplitude of the H p (y)signal,the gradient seems to present a more clear indication about the maximum stress concentration loca-tion.Similar results can be found in Fig.10.Hence,the gradient of the H p (y)signal might be a more useful parameter in the MMM method compared with the H p (y)amplitude.It is known that the SMFL signal developed on the surfaces of ferromagnetic materials is due to the irreversible movement of the domain structure under the action of stress and the Earth ’s magnetic field.However,the Earth ’s magnetic field is a vector field with a fixed direction.Theoretically,the influence of the Earth ’s magnetic field along different test directions is different.Thus,it is an interesting topic that how to compare the test data collected in different directions and present a unified result.Figure 11shows the curves of the H p (y)amplitude and its gradient under different directions for a same specimen,where E-W-line1denotes the test result of line 1along the east –west axis and S-N-line1denotes the test result of line 1along the south-north axis.Since the Earth ’s magnetic field is a fixed vector field,its influence can be eliminated by differenti-ating the H p (y)amplitude.According to Fig.11,it is clearly that the test direction affects the amplitude of the H p (y)signal,but no significant influence on its gradient.All of the above results are based on the tension.When the specimen is compressed,the variations of H p (y)signals along the loading direction are shown in Fig.12.Different from the tensile results,H p (y)signals change linearly during the whole loading process.No significant change can be found even if the applied load is higher than the yield strength of the material (~75kN).Hence,the H p (y)signal and its gradient cannot provide the information about when and where to be plastically deformed in the case of the compression.Previous reports [33,34]have confirmed that the compression in the field direction makes the magnetization of ferromagnetic materials harder,but the tension makes it easier.Such phenomena have been explained by magneto-elastic theory that the tension primarily affected the-300-200-100100H p (y ) (A /m )X (mm)510152025d H p (y )/d x (X 103A /m 2)X (mm)Amplitude(a)(b)Fig.10Amplitude and gradient of H p (y)signals on the three measured lines at F=17kN,d =1mmH p (y ) (A /m )X (mm)510152025d H p (y )/d x (X 103A /m 2)X (mm)Gradient(a)(b)Fig.9Amplitude and gradient of H p (y)signals on the three measured lines at F =15kN,d =1mmExp Mechmovement of 180°domains,but the compression mainly affected the movement of 90°domains.However,so far reported magneto-plastic models are based on the pinning theory.Obviously,such theory cannot explain the different results of the tension and compression in this study.New magneto-plastic theory is required.Another indication based on our test results is that new parameters are needed to evaluate the deformation state of the compressed objects since the H p (y)signal and its gradient cannot provide enough information in that case.H p (y ) (A /m )X (mm)-60-30306090H p (y ) (A /m )X (mm)-120-80-4004080120H p (y ) (A /m )X (mm)-90-60-300306090120H p (y ) (A /m )X (mm)(a)(c)(b)(d)Fig.12Variations of H p (y)signals on the three measured lines of specimen during the compressive test-400-300-200-1000100H p (y ) (A /m )X (mm)3691215d H p (y )/d x (X 103A /m 2)X (mm)Amplitude(a)(b)Fig.11Measurement directions affecting the amplitude and gradient of H p (y)signals on the three measured lines at F =14kN,d =3mmExp MechConclusionsExperimental results were shown in this study to capture the features of the normal spontaneous stray field component when ferromagnetic specimens were deformed from elastic, plastic deformations and up to fracture.The H p(y)amplitude and its gradient were measured.Various factors,e.g.loading modes,test location,lift-off value,were considered.Conse-quently the following results were obtained:1)The variations of H p(y)amplitude and its gradient canclearly differentiate the different deformation stages and determine the maximum stress concentration location on the condition of the tension.During the elastic stage,theH p(y)amplitude changes linearly along the tensiledirection.When the deformation proceeds into the plastic stage,the H p(y)amplitude changes quickly in the local stress concentration zone and a non-linear curve is observed,which causes a peak change of H p(y) gradient in the maximum stress concentration location.After the fracture,H p(y)changes positive–negative symbols and has a zero in the maximum stress-concentration location,and its value is about one order larger than that of the plastic pared with the H p(y)amplitude,its gradient seems to be more sensitive to the deformation state.2)The test location and lift-off value of the sensor havestrong influences on test results.More significant change of the H p(y)signal can be observed when the measured point is close to the maximum stress concentration location.Hence,the maximum stress concentration location can be determined by comparing the testing results along different measured lines.Meanwhile,a higher lift-off value decreases the test sensitivity in the MMM method.During the plastic tensile deformation stage,the non-linear change of theH p(y)amplitude is decreased with increase of the lift-off value.After the fracture,higher lift-off values decrease the peak-peak amplitude of the H p(y)signal and increase its width.3)Different from tensile test results,no detectable changeabout the H p(y)signal can be found when specimens are compressed from the elastic to plastic deformation.Such phenomenon cannot be explained by the present accepted pinning theory.New magneto-plastic theory should be proposed.Moreover,new criteria in the MMM method are required to evaluate the compressive deformation state of ferromagnetic materials. 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