Distinguishing Stress Concentration or Cracking in Ferromagnetic MaterialUsing Abnormal Magnetic Signals

Shi Changliang1,2, He Peng1,+, Dong Shiyun3 and Xu Binshi3

1State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China2Analytical and Testing Centre, Guangzhou Research Institute of Non-ferrous Metals, Guangzhou 510650, China3National Key Laboratory for Remanufacturing, Academy of Armored Forces Engineering, Beijing 100072, China

In the geomagnetic field, spontaneous magnetic signals in ferromagnetic materials can be induced by stress, which can be potentiallyapplied to estimate the damage degree. In this research, a method of distinguishing stress concentration or cracking in ferromagnetic material wasproposed. The normal component of magnetic field, Hp(y), was measured on the surfaces of ferromagnetic specimens with notch. The resultsindicated that the shapes of Hp(y) curves of stress concentration and cracking were different, there was abnormal magnetic peak in the Hp(y)curve of cracking. Meanwhile, the effect of lift-off on abnormal magnetic peak was studied, and the optimal lift-off value was also presented.[doi:10.2320/matertrans.M2012393]

(Received December 17, 2012; Accepted April 9, 2013; Published June 25, 2013)

Keywords: stress concentration, cracking, ferromagnetic material, abnormal magnetic peak, lift-off

1. Introduction

For the good mechanical properties, ferromagnetic materi-als are widely used in welding. Many ferromagnetic weldingcomponents are subjected to fatigue, causing lots ofcatastrophic accidents.1­3) Fatigue crackings are often ini-tiated from components’ surfaces which have stress concen-tration zone,4­6) so it is very important to use a method forevaluating the stress concentration degree. At present, a novelnon-destructive testing method named metal magneticmemory testing (MMMT) was created by Russian professorDoubov in 1997.7,8) The MMMT is based on magneto-mechanical effect. Under geomagnetic field, load can lead tothe generation of spontaneous weak magnetic signals withoutany other extra magnetic field. In stress concentration zones,the tangential component of stray field, Hp(x), has a maximumvalue, meanwhile, the normal component, Hp(y), changespositive-negative symbols and has a zero value. Therefore,the stress concentration zones can be detected by measuringthe Hp(y) values and their variable gradient without any otherextra magnetic field. Compared with other non-destructivetesting methods, such as ultrasonic testing, eddy-currenttesting and magnetic particle inspection, the MMMT has nouse for surface pretreatment and special magnetic equipment.It can be applied in inservice inspection, it can be effectivelyused to detect stress concentration zones and microdefects.

Lots of experiments on MMMT have been done bynational and international academicians,9­11) it is reportedthat there are some relationship between stress concetrationdegree and the gradient of Hp(y), and the Hp(y) signals ofstress concentration zone is similar with the signals ofcracking, but there are few investigations to find thedifference. In this paper, the tension to tension fatigue testof 18CrNi4A steel specimen with notches was carried outuntil cracking appeared, the normal component of strayfield, Hp(y), on surface of the specimen were detected. Thedifferences between Hp(y) signals of stress concentrationzone and cracking was discussed.

2. Experimental

The testing material is 18CrNi4A steel, which is a kindof case-hardened steel. Many factors, such as machiningprocess, heat treatment condition and transport situation,intensively affect the initial magnetic signal, so all specimenswere demagnetized by pulse demagnetization machine inorder to reduce the processing magnetization.

The shape of specimen is given in Fig. 1(a). Stressconcentration factor ¡ of the notched specimen is 5. Theirsurface roughness Ra was 3.2 µm. One horizontal line ABwas drawn on the surface of specimen before cracking,whose length was 60mm. Four parallel lines marked bynumbers d1, d2, d3 and d4 with 5mm intervals were drawn onthe surface of specimen after cracking.

3mm

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Fig. 1 Specimen shape and scanning lines: (a) before cracking (b) aftercracking.

+Corresponding author, E-mail: hithepeng@hit.edu.cn

Materials Transactions, Vol. 54, No. 7 (2013) pp. 1102 to 1104©2013 The Japan Institute of Metals and Materials

The dynamic tension tests were done with MTS810 servohydraulic testing machine, whose dynamic load error was«1.0%. Tension-tension fatigue tests of constant amplitude(sinusoidal waveform) were performed. The stress was900MPa, ratio R was 0.1, and load frequency f was 3Hz.Hp(y) values were measured by an EMS2003 intelligentmagnetic memory device. A probe was fixed on 3D electricalscanning platform made of non-magnetic materials andplaced vertical to the surface of specimen with differentlift-off values. The diameter of probe was 6mm. The end ofprobe was soft cushion to prevent scratching the samplesurface. The testing system is shown in Fig. 2.

In the fatigue testing, the specimen was positionedvertically between the upper and lower holders of the testingmachine. When the specimen was loaded to a preset cyclenumber, it was taken from the holders and laid on the platformin south (A) to north (B) direction. Then, the Hp(y) values ofscanning line were detected. After detection, the specimenwas loaded again to a more preset cycles, as well as the aboveprocedure was repeated until the cracking appeared. Then theHp(y) values of scanning line d1, d2, d3 and d4 were detected.

3. Results and Analysis

Figure 3 shows the magnetic signals of specimen fromline AB at different cycles with lift-off 0.5mm. The resultsindicate that magnetic signals sharply change and thegradient «k« intensely increase near the notch along thescanning line. It proves that there is close relationshipbetween stress concentration degree and the gradient ofHp(y). At the same time, the gradient «k« of the abnormalsignals go up with the increase of loading cycles approachingto a stable value until cracking appeared at 4300 cycles.When the cracking happened, the gradient «k« suddenlyincreased and the abnormal magnetic peak of Hp(y) appeared.

In order to study the character of Hp(y) curves of cracking,the magnetic signals of specimen after cracking from line d1,d2, d3 and d4 were detected with lift-off 0.5mm, shown inFig. 4. Line d1 and d2 go through the stress concentration zoneof cracking, while line d3 and d4 go through the cracking. It isindicated that the magnetic signals of cracking and stressconcentration both sharply change and their gradient «k«intensely increase, but there are abnormal magnetic peaksonly in the magnetic signals of cracking. Therefore, theabnormal magnetic peak is the key to distinguish the magneticsignals of stress concentration or cracking. Because the

diameter of probe is 6mm, the efficient detecting zone isabout 80%, in the experiment, the minimum value of crackingwhich can be detected in this method just appears along lined3, the value is 0.2mm inspected by visual microscope.

The results could be explained by the theory of interactionenergy in ferromagnets.12) When test temperature is farbelow Curie point, total free energy of ferromagnet includesmagnetocrystalline anisotropy energy Ek, stress energy E·

and demagnetization energy Ed provided that total freeenergy is equal to internal energy as eq. (1):

E ¼ Ek þ E· þ Ed ð1ÞStress energy E· is far greater than Ek and Ed under theapplied axial tensile stress. When the applied load reachedto the ultimate strength, fracture happened and stress energyreleased immediately. Meanwhile demagnetization energyincreased dramatically to recover system balanced state. Sothe abnormal magnetic peak occurred in the fracture zone andthe amplitude of magnetic signals changed sharply.

It is known that the lift-off affects the magnetic signals ofHp(y), but there are few research to study the relationshipbetween lift-off and magnetic signals, which is veryimportant for the application of MMMT. In this paper, the

(a)

(b)

Fig. 2 Testing system: (a) MTS-810 fatigue testing machine (b) EMS2003MMM detector.

Fig. 3 Magnetic signals of specimen from line AB at different cycles.

Fig. 4 Magnetic signals of specimen after cracking from line d1, d2, d3and d4.

Distinguishing Stress Concentration or Cracking in Ferromagnetic Material Using Abnormal Magnetic Signals 1103

magnetic signals of specimen from line d4 with differentlift-off were detected, shown in Fig. 5.

Figure 5 shows that the value and gradient «k« of Hp(y) bothdecrease with the increase of lift-off value. When the lift-offis between 0.0 and 1.0mm, there is abnormal magnetic zonenear cracking, meanwhile, abnormal magnetic peaks exist inthe zone. When the lift-off is between 1.0 and 20.0mm, theabnormal magnetic zone exists, but the abnormal magneticpeaks disappear. While the lift-off is higher than 20.0mm, the

abnormal magnetic zone and abnormal magnetic peaks bothdisappear, the Hp(y) curve becomes linear. As we all known,the gradient «k« and abnormal magnetic peak are the keyparameter for expressing the stress concentration degree andcracking, therefore, the lift-off should be less than 1.0mmin practical testing, otherwise, the magnetic signals will bedistortion. Meanwhile, the lift-off should not be too low,inducing the friction of probe. The value of lift-off should bedecided by the roughness of surface.

Thus a simple and fast method for detecting of stressconcentration degree and cracking is proposed. The gradient«k« and abnormal magnetic peak are the key parameter forexpressing the stress concentration degree and cracking. Thelift-off should be less than 1.0mm and decided by the rough-ness on the surface of components in practical application.

4. Conclusions

There is close relationship between stress concetrationdegree and the gradient of Hp(y). When the crackingappeared, the gradient «k« intensely increased and theabnormal magnetic peak of Hp(y) occurred, which is thekey to distinguish the the magnetic signals of stressconcentration or cracking. The theory of interaction energyin ferromagnets can be used to explain the effect of abnormalmagnetic peak on cracking. Meanwhile, the suitable value oflift-off in practical testing is proposed. The lift-off should beless than 1.0mm and decided by the roughness on the surfaceof components in practical application. However, there is alot of basic experimental work to apply the novel methodin practical detecting because of many influencing factors,such as load type, extra magnetic field and heat treatmentconditions. Further more work will focus on quantitativelyestimating the stress concentration degree by magneticsignals and excluding the effect of influencing factors.

Acknowledgements

This work was financially supported by the NationalNatural Science Foundation of China (Grant No. 50735006and Grant No. 50975287).

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S. Changliang, H. Peng, D. Shiyun and X. Binshi1104