Inverse magnetostrictive effect and electromagnetic non-destructive testing methods

Inverse magnetostrictive effect and electromagnetic nondestructive testing methods W. Polanschiitz The influences of mechanical stresses on magnetic properties are investigated. A qualitative and quantitative description of the reversible and irreversible components of the magnetostrictive effect in iron and carbon steels is presented. Applications of the effect in a new method for analysis of stress waves in mechanically stressed ferromagnetic metals, and also in an alternative strategy for enhancement of the signal-to-noise ratio of eddy current testing of tubes, wires and rods in mechanized lines are described, as is a non-destructive method for evaluation of the martensite content in the structure of wire rod. Keywords: non-destructive strictive effect, martensite

testing, stress waves, acoustic emission, magneto-

1

List of symbols area

0

Index

for reference

value

P

Mechanical

Pe

Value of internal

stresses

Ps

Maximum

pressure

R

Electric

S

Maximum

Cross-sectional

a

Calibration

C

Carbon

CL

Longitudinal

cs

Calibration

D

Diameter

S

Wall thickness

E

Energy

t

Time

E

Young’s modulus

ui

Induced

E

Energy density

V

Volume

f

Frequency

W

Localization

f PU

Lower limit of the natural

frequency

Distance

fPO

Upper limit of the natural

frequency

Factor of proportionality

G

Reject threshold

g

Martensite

H

Magnetic

L

Inductivity

Magnetostriction

I

Length

Reversible

permeability

A4

Magnetization

Magnetic

field constant

m

Magnetic

moment

Relative

N

Indicated

value

n

Number

of turns

Non-dimensional electromagnetic

content sound velocity factor

sound

126/86/040249-l

particle

velocity

voltage

probability

and/or

difference

coefficient

Elongation

This paper is dedicated to Professor Dr E. Krautz. my mentor, on the occasion of his eightieth anniversary.

NDT INTERNATIONAL.

resistance

Magnetomechanical

in the structure

field strength

0308-9

sound

Sign for change

content

initialvalue

stress

A

factor

and/or

0 $03.00

VOL 19. NO 4. AUGUST

1986

coefficient

permeability characteristic methods

P

Density

,

u

Electric

conductivity

I

*

Angular

frequency

I

0 1986

Butterworth

8 Co (Publishers)

of

the

Ltd 249

When electromagnetic methods are applied for nondestructive testing of materials, the results generally provide information on the conductivity and permeability of the sample and on the interaction volume which is dependent on the testing system. These primary influences on the signal are in turn determined by a number of properties concerning the composition, structure and geometrical state of the samples[~-3l, ie their metallurgical and physical qualities. The physical conditions acting on the sample, such as temperature, mechanical stresses and magnetic field, also have an effect on the primary influences to the signal. When applying different non-destructive electromagnetic testing methods, a high sensitivity, depending on the material to be tested and the target of the test, is aimed at for the detection of one of the secondary influencing factors, ie generally one or two primary influencing factors, with an optimum supression of the residual influences. The useful sensitivity and the useful signal-tonoise ratio of the electromagnetic method depend on the degree to which the selectivity can be increased. Typical examples are material sorting of ferromagnetic steels[4'~l, for which a high selectivity as to changes in the conductivity and permeability is to be aimed at, eddy current testing of austenitic steel tubes[61, for which the detection of changes in the interaction volume and in some cases in the conductivity, combined with a simultaneous reduction of the influences of structure-related discontinuities of the permeability, are decisive, and finally the magnetoinductive testing of wire ropePI as well as the electromagnetic leakage flux test[S,91, which show a maximum sensitivity to changes in the permeability and in the effective interaction volume and a low or negligible degree of sensitivity to conductivity effects. For improving the accuracy of electromagnetic testing methods and for expanding into new fields of application, the use of up-to-date technological facilities when developing or constructing test equipment with optimum selectivity of the primary signal parameters, on the one hand, and an application-related detection of the qualitative and quantitative relations between the primary and secondary signal influences concerned, which forms the basis of the process technology, on the other hand, are required. Three examples of the consequent technical utilization of special physical effects in electromagnetic testing methods are given in the following: the continuous noncontact temperature measurement of rollers and rolling discs with consideration to the dependence of the temperature on the electric conductivityll°l, the measurement of internal stresses[Ul, making use of the relationship between mechanical stresses and the Barkhausen steps at magnetization, and finally the simultaneous nondestructive measurement of hardness and the effective hardening depth on motorcar parts[ ~zl, making use of the dependence of the frequency on the effective interaction volume. This study covers practical applications of the inverse magnetostrictive effect during electromagnetic material testing. Starting with a physical description of this effect, which causes the influence of the mechanical stress condition on the relative permeability of magnetized ferromagnetic materials, the study deals with the practical significance of the effect in eddy current testing of wires, rods and tubes made of ferromagnetic steels in mechanical testing lines. Other applications described are an electromagnetic method for the detection of mechanical

250

stresses in the microstructure of ferromagnetic steels and a new non-destructive method for the detection of stress waves in ferromagnetic materials.

Inverse magnetostrictive effect in steels With ferromagnetic materials, a direct relation between the geometric lattice parameters and the magnetic state can be observed, resulting from the spin-orbit coupling which gives rise to a simultaneous rotation of spins and the non-spherical electron clouds as an effect of magnetization. Due to this spin-orbit coupling, the collective alignment of the electron spins is frequently accompanied by a simultaneous geometrical distortion of the elementary lattice cell during magnetization; this distortion can be macroscopically detected as changes in length, ie magnetostriction031. Changes in magnetostriction depend on the magnetic field strength in a complex way, and also on the reversible components of the lattice distortions resulting from mechanical stresses. This is illustrated in Figure 1, which depicts the relation between the external magnetic field and the magnetostriction coefficient[~4]

x(H) [p =

(:(m - :o)/tol r

(1)

of polycrystalline iron. According to the principle of Le Chatelier, the magnetostrictive effect is reversiblet151, ie the magnetic state is dependent on the lattice distortions caused by external and internal influences. This means that the magnetization state of a ferromagnetic material, if magnetized, can be changed by mechanical stresses. This effect has been variously designated as the inverse magnetostrictive effect, the magnetomechanical effect1161,and, according to a proposal made by Lee[~71, the Villari effect, Figure 2 is a schematic of the influence of mechanical stress on the magnetization of ferromagnetic materials with positive magnetostriction. It can be inferred that according to the character of the magnetization processes prevailing with different field strengths, reversible and irreversible (entropical) components can be distinguished for the inverse magnetostrictive effect. Thus, trials to derive the magnetization changes in the inverse effect from the magnetostriction characteristic in a direct analogy by means of one single coefficient showed no satisfactory resultPgl. In general, the thermodynamic formulation for the inverse magnetostrictive effect is as follows: AX(H, (p)) ---r.-Am(H, e(p))

(2)

The reversible components of the magnetization change depending on the stress can in particular be defined as follows[Ul:

(d

/dp) I,-, "-- (dl/dZ-Z)I.

(3)

For practical applications, these types of representation can rarely be used. In this case, it is expedient to divide it into processes with steady changes in magnetization due to stationary stresses, corresponding to the irreversible component, and into processes with discontinuous magnetization changes due to unsteady changes in stress, corresponding to the reversible component.

NDT INTERNATIONAL. AUGUST 1986

be considered that changes in the relative permeability caused by mechanical stress show a distinct maximum in a narrow range of magnetic field strength; the location of the maximum has to be determined for any material separately; and b) it turned out that changes in permeability caused by steady mechanical stresses (ie internal stresses) can be largely compensated for by adequate magnetization, if they become apparent as interfering effects. This is used especially in eddy current testing of rolled products of ferromagnetic steels.

10

M e c h a n i c a l stress

~

-100

(N mm - 2 )

I

o ,< E" o

For determining the influences of the reversible components of the magnetomechanical effect, a special formalism has been developedllgl. Analogously to the characterization of magnetization components dependent on the field strength at dynamic magnetic reversal by help of the reversible permeability P'A, a magnetomechanical coefficient 8 is introduced, which describes the electromagnetic induction effect in the case of dynamic mechanical stress changes and thus the reversible components of the magnetomechanical effect. Starting from relation (3) and from Faraday's law, it can be shown that the electric voltage induced from a magnetized rod with / >>d with changes in the mechanical stress into a surrounding coil can be described as follows:

--10

u

0

o

10

-5

50

U i

-10

5

10

15

20

25

30

Magnetic field intensity (kA m - 1 ) Fig. 1 Dependence of the magnetostriction coefficient of polycrystalline e-iron on magnetic field strength, at different mechanical stresses (according to Kurzuzar114]I

=

A/xr =

A/Zr ( H , p )

with

can be used for practical application; this dependence represents the correlations given in relation (2) if effects of second and higher orders are ignored. The respective correlation which can be used for the practical electromagnetic measurement of mechanical stresses occurring on and in samples must be determined on the basis of experiment. Naturally, the composition and heat treatment of the material must be considered. Figure 3 shows the results of measurements of the change in the relative permeability of a ferritic steel with a C-content of 0.08% in a normalized heat treatment condition. Figure 4, which shows the dependence of the relative permeability of the same material on the field strength at a mechanical stress p = 0, illustrates the absolute amount of the effect. In spite of the evidence of the qualitative correlation between both effects, according to relation (2), the significance of the irreversible components can also be clearly inferred from the comparison between Figures 1 and 3. The results shown raise two points for practical electromechanical testing: a) when utilizing the effect, it has to

NDT INTERNATIONAL

. AUGUST

1 986

(4a)

~.Lo n ~5 dp/dt

This relationship defines the magnetomechanical coefficient with the dimension [ A m 2 s2 kg-1]. As can be inferred from Equation (4a), the induced voltage represents the amount of discontinuous mechanical stress changes in the material as well as their derivative with respect to time. If sensor elements other than the surrounding coil, eg yoke coils, are used for measuring changes in magnetization, a sensor-specific calibration factor Cs has to be included, resulting in the following: Ui

In the case of static mechanical stresses occurring on ferromagnetic samples, the dependence

--

=

-

(4b)

Cs]-£o n 8 d p / d t

The magnetomechanical coefficient & which depends on the composition and heat treatment of the material as well as on the magnetic field strength, can be determined on rods subjected to oscillating mechanical load, which are placed in the field of a magnetizing coil by means of the voltage induced into a surrounding measuring coil, using the following relation:

p=+p M

f

tO •~ D .N

p=o

c

o o

Magnetic field intensity, H

Fig. 2 Schematic of the connection between mechanical stresses and the change of magnetization in ferromagnetic materials with positive magnetostriction

251

coefficient decreases: this is due to a reduction in the reversibility of the elementary magnetization processes as a result of the increase in the density of anchorages of the local magnetic states with an increased perlite content in the structure.

0.10[

For practical use in electromagnetic NDT methods, the results given in Figure 5 imply that the reversible component of the inverse magnetostrictive effect, which becomes effective in the case of dynamic changes in the mechanical stress, can be utilized or must be taken into consideration as an interfering effect to an increasing extent with increasing soft magnetic behaviour of the materials examined, and generally with the increasing magnetization.

:i

Practical applications Reduction of interfering effects during current testing of f e r r o m a g n e t i c materials I

I

I

I

50

100

150

200

250

Mechanical stress (N mm -2) Fig. 3 Variation of the relative permeability with mechanical stress at different magnetic field strengths (in kA m-1) for carbon steel with 0.08% C

U i

=

-

tXonS~po

(5)

coswt

Figure 5 illustrates the dependence of the magnetomechanical coefficient on the magnetic field strength for carbon steels with C-contents of 0.05 - 1 weight % with normalized structure. Especially from the curve for steels with carbon contents of 0.05 - 0.4 %, the correlation between the height of the magnetomechanical coefficient and the degree of reversibility of the magnetization processes prevailing in the respective magnetization stages can be clearly inferred. With increasing field strength, the magnetomechanical coefficient increases with increasing magnetization, which is mainly influenced by reversible processes at first. With the influence of the reversible processes decreasing, the magnetomechanical coefficient decreases with increasing field strength and reaches a relative minimum approximately in the transition to saturation which is determined by irreversible rotating processes, With further increasing field, the magnetomechanical coefficient obviously increases up to a saturation value, which is due to the increased component of the reversible rotating processes. With increasing carbon content of the steels, the magnetomechanical

eddy

During eddy current testing of ferromagnetic materials, especially when testing rolled and drawn products, such as tubes, bars, rods and wires, in the course of production and finishing lines, the inverse magnetostrictive effect occurs as an interfering effect in two ways. With its irreversible component it causes variations in the relative permeability of the material to be tested at localized internal stress fields, which may result especially from locally variable chemical composition, from structural inhomogeneities and from cold working. With its reversible component it may cause the induction of interfering signals which result from elastic vibrations or stress waves of the material under test. Elastic vibrations occurring in the material under test may arise during operation from machining processes, such as deburring or sawing, and from transport processes, eg when the material under test knocks against guiding and transporting elements. The interfering effects, which are due to the irreversible component of the inverse magnetostrictive effect resulting in localized fields of stress on the material, can be successfully reduced or suppressed by applying the methods suggested by F6rsterl2q. The main method is the magnetization of the testing zones but also, with a phasesensitive evaluation, the phase angle can be optimized in correlation with the testing frequency. With appropriate magnetization, the relative permeability decreases, and, as can be inferred from Figure 3, the dependence of the

3.0

600

"E ~

A3

2.0

4o0 e~

~"~ 1.0 ~ 200 m

o

0

I

5

I

10

I

15

I

20

0 25

Magnetic field intensity, H (kA m - 1 ) Fig. 4

Relative permeability as a function of the magnetic field strength for carbon steel with 0.08% C

252

10

20

30

40

50

Magnetic field intensity, H (kA m - 1) Fig. 5 Dependence of the magnetomechanical coefficient on the applied magnetic field strength for carbon steel with 0.05% - 1.0% C in normalized structural condition

NDT INTERNATIONAL. AUGUST 1986

relative permeability on the stress, caused by the irreversible component of the inverse magnetostrictive effect, greatly decreases. Apart from the inverse magnetostrictive effect, the magnetization is also advantageous for setting a defined magnetization state in the material under test: thus, locally magnetized zones with permeability variations (the former are due to eg transportation by magnetic rollers) can be equallized.

frequently used for practical testing, ie centring devices arranged on both sides of the testing zone, the natural frequency of the material tested, and thus the main frequency of the interfering signals induced by the inverse magnetostrictive effect, lies between a lower frequencyfpu resulting when clamped on one side and supported on the other and an upper frequencyfp o resulting in the case of clamping on both sides.

When carrying out practical tests with surrounding coils, especially when checking for hole-shaped defects and when using drilled holes as test defects as is the case eg with non-destructive leakage tests carried out on pipes, it is not always expedient to apply m a x i m u m magnetization. This is because, with the interfering signals decreasing due to permeability variations above the value of the magnetic field strength, the former depending on the shape and size of the hole-shaped defects, the signal level decreases simultaneously, possibly even more than the background noise. This is connected with the equalization of the local discontinuities of magnetization along the defects. Therefore, with the inclusion of the effective signal-to-noise ratio for the defects to be detected and the test defects used, the magnetic field strength has to be optimized.

Figure 6 shows the respective relationsl2Zl for the determination Offpu andfpo for rod-shaped and tubular test samples. In most casesfp u is less important as far as commonly used testing frequencies of eddy current testing are concerned, whereasfp o is frequently of the same order of magnitude as the testing frequencies selected,

Since the interfering signals caused by the irreversible component of the inverse magnetostrictive effect frequently show a significant phase angle difference from the useful signals of defined defects, the signal-to-noise ratio can be frequently increased by optimizing the evaluated phase array, by selecting a testing frequency with a m a x i m u m phase angle difference.

However, as can be inferred from the relation between the field strength and the magnetomechanical coefficient (see Figure 5), the interfering signals caused by the reversible components of the inverse magnetostrictive effect can be further reduced by using a lower magnetic field strength. If, due to the material, the amplitudes of interfering signals caused by the irreversible component of the inverse magnetostrictive effect, ie by localized stress fields in the material, are low, eg in the case of carbon steels with low and average C-contents and with normalized or softannealed structures, it may even be sufficient to carry out the tests without magnetization of the testing zone.

Whereas compensation of the interfering effects by means of mechanical stress fields in the material had already been in use when the industrial utilization of eddy current processes was first developed, the possibility of compensating interfering effects by means of variable mechanical stress changes due to the propagation of elastic waves in the material to be tested has not been sufficiently utilized. When suppressing the interfering effects by the reversible component of the inverse magnetostrictive effect which are caused by dynamical stress changes on the material under test, a largely sophisticated and partly contrary strategy needs to be adhered to, taking into account the physical conditions prevailing. For this process, most importantly the level of the elastic vibrations in the testing zone has to be lowered. This can be done by damping the conduction of vibrations between transport and guiding devices and the material under test by selecting a testing zone far from the vibration-exciting elements and by a mechanical guide with a minimum number of mechanical degrees of freedom in the area of the testing zone. The next step is the provision of a m a x i m u m gap between the frequency band of the mechanical vibrations in the material under test and/or of the resulting voltage signals induced into the testing coils and the testing frequency or the frequency band of the primary and secondary signal filtering respectively. When adequately optimizing the frequency range for testing, the value of the natural frequency of the material in the testing line has especially to be taken into account: the natural frequency depends on the physical properties, the shape and the dimensions of the material to be tested as well as on the type, dimensions and contact pressure of the guiding devices. With the type of guide in testing lines which is most

NDT INTERNATIONAL . AUGUST 1986

For reducing the interfering influences of the reversible components of the inverse magnetostrictive effects it is expedient to select a testing frequencyf i> 3Jlp.. o It is not possible to reduce the interference level determined by the reversible component of the inverse magnetostrictive effect by phase-sensitive evaluation and optimizing the frequency-correlated phase angle as the respective voltage inductions have no constant relation to the measuring signal of the eddy current testing as far as the frequency and the phase position are concerned.

Application of this strategy is illustrated by the optimization of the eddy current test process for checking weld quality in a production line for section tubes. In this line. quadratic and rectangular sections with outside widths from 40 to 101 m m and wall thicknesses o f ~ 3 - 5 m m are welded. They are mainly of carbon steels with carbon contents of 0.08-0.25%. For eddy current testing of the weld, a flat coil arrangement with differential connection is used. For magnetizing, a yoke electromagnet is available which is closed in a magnetic circuit by the material under test. In the course of operational tests, an analysis of the interfering signals showed that the Natural ency

r~

fPu

(~ J

f

section

Rod

Tube

(3ED2 /27r214p) 112

(E/4 12p)112

(3E(D2_Ds) 7r21p)112

(E/4 12p) 1/2

Fig. 6 Lower and upper limits of the natural frequency of material in testing lines, with mechanical guides on both sides within the testing zone (for tube and rod cross-sections)

253

background noise caused by the irreversible components of the inverse magnetostrictive effect could be neglected as compared with the interfering signals caused by the reversible components. The elastic vibrations of the material to be tested are caused by the transportation process and by the cutting process, using a flying saw; both sources can be influenced to a small extent only. The mechanical guide for the section tubes in the testing zone gives an upper limit for the natural frequency fpo of ~ 4 kHz. So, 22 kHz was selected as the testing frequency. An amplitude evaluation was also selected. The selectivity1231 of the test was determined by comparing the results of eddy current testing and metallographic examination of defects with different magnetization of the testing zone as a parameter. In this trial, the signal-tonoise ratio of weld defects in the form of cracks on the external surface was entered as a function of the metallography measured crack depth to eliminate any dependence on the amplification factor. On this basis, the response curves for a detection probability of 0.5 were determined. Figure 7a shows the results of this evaluation with a magnetization of the testing zone according to a field strength of 20 kA m-L As a comparison, Figure 7b depicts the response characteristic resulting from a test without magnetization. It turns out that the defect detection capacity in this case, in which, due to the magnetic uniformity of the welded

material, the influence of the reversible component of the inverse magnetostrictive effect dominates, is higher

.o"

W=0.5

o

3 -

o

o ...0__._-o

°°~ e-

2 -

--L

g

O

f

o

o

o

:

o

o

-20 kA m-1

m

!

/

0

/

I

I

I

I

I

0.2

0.4

0.6

0.8

1.0

a

I 1.2>1.2

Depth of crack, t (mm)

W=O.5 _

O

o

o~

'~ 2

o O

O

H = 0 kA m - 1

/ /

f

b

I

I

I

I

f

0.2

0.4

0.6

0.8

1.0

I 1.2>1.2

Depth of crack, t (mm)

Fig. 7 Signal-to-noise ratio of crack-type weld defects during eddy current testing of welded section tubes: a -- magnetic field strength 20 kA m - l ; b - - without magnetization of the testing zone

254

without magnetization than with magnetization. With magnetization in a field of 20 kA m -1 the minimum defect depth detectable under operational conditions (signal-tonoise ratio = 6 dB, W = 0.5) is ~0.45 mm, whereas it falls below 0.30 mm without magnetization. Moreover, without magnetization the interference is reduced to an extent that during the cutting time a signal fade-out can be omitted, due to which the testing reliability is increased.

Electromagnetic stresses

determination

of

mechanical

With electromagnetic methods, the inverse magnetostrictive effect can be utilized for determining mechanical stresses occurring on and in ferromagnetic materials. These methods, by means of which the changes in permeability caused by mechanical stress can be detected in a non-destructive and non-contact way, are useful for operational checks on structural elements and for the insepction of work-pieces and semis. They are particularly suited for the detection of microscopic and macroscopic internal stresses occcurring in ferromagnetic steels and nickel-based alloys. As an example, a method for showing the existence of natural stresses in wire rod caused by martensite content is described. This method was developed for production and runout checking in a heavy-duty wire rod mill1241. When wire rod made of carbon steels is cooled, martensite may form in the structure. The martensite particles produce highly localized internal stress fields, and therefore affect the properties as to further processing and application. If their structural content exceeds a preset value, martensite particles represent a serious quality defect. Up to now, the martensite content in the structure has only been checked metallographically. This metallographical method has three essential disadvantages: high expenditure of labour, high expenditure of time which leads to a delay in the decision upon quality, and decreased significance of the results due to investigation of one cross-section only. The starting point for a nondestructive method according to which these disadvantages can be avoided is the electromagnetic measurement of the internal stresses caused by the martensite particles, which may reach values of ,--200 N mm-q251. Due to the irreversible component of the inverse magnetostrictive effect, internal stresses cause local changes in the relative permeability, which, with integral measurement, contribute to the change in the relative permeability determined on the sample, according to the structural part of the martensite particles. Figure 8 shows the relative permeability of the welding wire material SG2 measured at a magnetic field strength of 2 kA m -1, as a function of the martensite content. Changes in the relative permeability can be determined electromagnetically by simply applying the method of the self-exciting RL-oscillations1261. In this process, the probe coil and the sample together form an inductance switched into an RL-oscillator. The natural frequency of the oscillator therefore depends on the permeability of the sample material. Figure 9 is a schematic of such a measuring arrangement. In this process, the primary measured quantity is the frequency of the oscillator. It basically depends on the positive feedback, which is determined by the wiring of the oscillator, and is inversely proportional to the coil inductance. Apart from the number of turns, the indue-

NDT INTERNATIONAL. AUGUST 1 9 8 6

ing, the samples are magnetized with the external field strength defined. For this purpose, the measuring arrangement comprises two magnetization coils which surround the sample in magnetic parallel on both sides of the measuring coil. The magnetization field strength is selected such that a state in the magnetic transition range is set.

600 Steel grade SG 2 H=2kA m- 1 e~

y

550

m

E e~

-$ 500 er

I

450 0

I

I

1

I 2

1

I 3

I

When applying the method under operating conditions, the possibility of errors occuring due to geometrical deviations and changes in conductivity caused by changes in chemical composition has to be taken into consideration. The magnitude of the errors can be estimated by applying the relation of similarity according to F6rsterPl in a form corresponding to the H-theoreml~l:

I 4

Martensite content, g (%)

1-IET =

Fig. 8 Relative permeability of the welding wire material SG2 as a function of martensite content; magnetic field strength 2 kA m -1

tance L of the coil depends on the permeability and the conductance of its core[ 141. With constant conductance and constant magnetic field strength H, a change in the relative permeability Aktr causes a change in inductance: ,,- tZo AtxrA/llH,~

(6)

where A is the cross-section and 1 is the magnetically effective length of the sample under test. As can be inferred from Figure 3, the stress-dependence of the relative permeability can be expressed through a factor of proportionality which depends on the sample composition, by:

#r = / ~ o + aPl H

(7a)

in certain ranges, with the magnetic field strength defined. Accordingly, when the martensite content g is low, the changes in permeability due to the stress fields of martensite particles statistically distributed in the microstructure can be expressed as follows: ~rm = ~ro +

gapdH

(7b)

where Pc is the mechanical stress at the martensite particles. The frequency of the oscillating circuit can be represented by a proportional value N in the applied arrangement. Thus. including Equations (6) and (7a), the dependence of the indicated value on the martensite content is as follows: N ----((1/No) + g a ) - l l H , (7

(9)

For the diameter tolerances admissible for wire rod (+3.5%), the following indication errors are expected: 0.5% for the martensite content; ~ 0.2% for the changes in conductivity to be assumed on the basis of the analysis margins; and 0.1% for the changes in conductivity due to sample temperature variations of ~ _2.5°C. This is confirmed by experimental error analyses. The reject threshold of N for a defined martensite content is determined on the basis of comparative measurements of the indications given and the martensite contents determined metallographically.

Figure 10 shows such a comparison. It can be inferred that, with an admissible martensite content o f g + = 1.5% and adhering to a reject threshold of G = 0.96 N o, the significance for the correct identification of samples with too high a martensite content is 100% and for the correct identification of samples to be accepted more than 80%. To avoid a pseudo reject, the testing instructions specify that all samples on which too high a martensite content is indicated in the non-destructive test have to be checked metallographically. In the current example (worst case test), this applies to ~ 35% of all samples. Due to the considerably reduced time required for electromagnetic determination of the martensite content, a time saving of more than 60% as compared with an exclusively metallographical determination can be achieved, with consideration to the confirming checks required.

Electromagnetic

method

The significance of the measured values depends on the magnetic state of the sample. Therefore, during measur-

for

determining

elastic

waves

Due to the reversible component of the inverse magnetostrictive effect, changes in the stress condition of ferromagnetic materials lead to changes in magnetization,

~

(8)

where N o is the indicated value of a sample with the same composition and basic structure (and thus conductivity) without martensite in the structure, and a is a system calibration factor depending on the sample base composition, on the magnetization state and on the measuring geometry (A, l) as well as on the measuring system quantities. With the magnetizing field strength defined, the calibration factor a is determined at samples with the same composition, basic structure and dimensions, with martensite contents set by different heat treatment.

NDT INTERNATIONAL. AUGUST 1 9 8 6

1If Dzcr/to/~r

I Sl:,ecimen

Indicating instrument

IIIIIIII ~ Probe

. . . . .

J

I

Q'

1I I

t!

Fig. 9 Schematic of the electromagnetical process for non-destructive indication of martensite content in wire rods

255

integral part of the sensor element. The information content of the signals is chiefly determined by the elastic waves in the interaction volume and by the frequency content of the latter; the frequency content, in turn, is determined by the natural frequency of the sample. Thus, the method is a very narrow-band one and, moreover, largely insensitive to coupling effects. However, this electromagnetic method is less sensitive than SEA; but this disadvantage can be partly compensated for by the increased signal-to-noise ratio.

I

1.05

i

o 1.00 .9

t

I I I

,I

: I I •

~- 0.95

|

e¢-

z

._~

i

0.90 I

,g

I

2

I

3

I

,

;

;

I

7

Martensite content, g (%)

Fig. 10 Comparison of the indications of the non-destructive method and the metallographical determination of the martensite content

To be able to judge the sensitivity of the electromagnetic method, it is necessary to estimate the energy content of the oscillation pulses which cause the measurable voltage pulses in the receiver. The energy density E of an oscillation pulse in an elastic solid of density p is as follows: (10a)

if+ = pS2/2

which can be measured by means of electromagnetic sensor elements such as coils or yoke probes. This has enabled the development of an electromagnetic method (magnetomechanical method MMM), according to which material deterioration processes in the structure of stressed components and stressed structural elements can be detected and analysed. Material deterioration due to local plastic deformations, crack formation and crack propagation occurs with discontinuous energy release, which leads to the propagation of elastic waves in a wide frequency range. As is generally known, this is utilized in SEA (sound emission analysis) and SWA (stress wave analysis) for detecting and tracing component deterioration, especially for the operational supervision of structural elements which are relevant to structural safety116-191. Changes in stress occurring in ferromagnetic materials during the propagation of elastic waves can also be detected electromagnetically. This is possible with a relatively simple arrangement as shown in Figure l I, consisting of surrounding coils with rod-shaped structural elements. For this purpose, the sample is surrounded by a probe arrangement in a section which need not necessarily coincide with the deteriorated section. This probe arrangement consists of two field coils in a Helmholz arrangement, between which a measuring coil is provided. For the magnetization of the sample material, the field coils are supplied with direct current with minimum ripple. In the cast of discontinuous changes in stress on or in the sample, voltage pulses are induced into the measuring coil according to Equation (4a), which reproduce the amplitude and frequency of the oscillation pulses by which they are caused. These voltage pulses show a characteristic which is similar to that of the signals examined in SEA In this arrangement, the voltage inductions are displayed on an oscilloscope. To improve the properties for further processing, the voltage pulses are amplified and filtered in a band-pass filter. These conditioned signals are transmitted to a counter with a variable threshold level, the pulse sum content of which is recorded by an xt recorder. As compared with SAE, this method is different in a number of properties, apart from being non-contact. The advantages of this electromagnetic method are the considerably reduced experimental expense required and the reduced susceptibility to interfering signals, eg friction noises. The advantages can lead to considerable cost savings compared with SEA. The increased signal-to-noise ratio of the electromagnetic method is mainly due to the fact that the sample forms an

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The maximum sound particle velocity S is linked with the maximum sound pressure Ps by (10b)

S = Ps/P ct,

where c L is the longitudinal velocity of sound. Accordingly, the energy of an oscillation pulse released in a sample with volume V which is clamped on both sides is (10c)

E = V p ~ / 2 p c~,

With consideration to the amplitude of the voltage pulses A U i, and in accordance with Equation (5), the amount of energy released during the formation of the oscillation pulse is as follows: E = (V/2p) (Ui/CLO~/t o 3n) 2

(11)

With the inclusion of the order of magnitude of the magnetomechanical coefficient for steels and the limit parameters admissible for the design of the coils and the signal amplifiers, the minimum energy of an event which can be recorded is~ 10-12 J. The efficiency and potential of this method for the detection and monitoring of material deterioration can be

Sensor coil Probe

+-?-+

Mechanical element

P

[oi,loopa]

I

I CU:uen:I I

Recorder

Counter ]

I

Fig. 1 1

Schematic of the electromagnetic method for detection of stress material (magnetomechanical method, MMM)

w a v e s in f e r r o m a g n e t i c

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shown by tracing the process of stress corrosion cracking on prestressing steels. During this examination, special attention was paid to the possibility of correlation between the signal curve determined and the stress corrosion cracking reactions as well as to the possibility of detecting the initial stages of deterioration. Wires made of the materials 54SiCr6 (heat-treated, R m = 1830 N mm -2) and 80Mn2 (patented, drawn, tempered, R m = 1860 N mm -2) were stressed according to the guidelines for approval and supervisory tests carried out on prestressing steels* in an acid corrosion medium under creep conditions at 0.8 R m. For the examination, the measuring arrangement as depicted in Figure 11 was used. The measuring probe was mounted outside the corrosion zone at a distance x >~ 1000 mm. The filtering was adjusted to the natural frequency of the stressed sample: the recording threshold for the signals was set according to an energy content of the elastic oscillation pulses of 10-s J. The recorded pulse sum shows a characteristic which corresponds to the deterioration curve (Figure 12a). When inserting the corrosive conditions (ts), the pulse rate is 0.05 min -1. The pulse sum increases up to a time t u with small discontinuities. At tu, the pulse sum increases gradually at a mean rate of I min-L At a time t o, the pulse rate increase is decelerated to '~0.1 min-L Figure 14b 15

El

x

10--

g .Q

Z

I 0

1

30

I ts

a

a

I

I

50

I

I 90

70

I tB I 110

Time, t (h)

600

4OO-o.

"6

200--

14

b

Summary

The inverse magnetostrictive effect is the dependence of the magnetization of ferromagnetic materials on mechanical stresses. For practical purposes it is advantageous to describe the inverse magnetostrictive effect in terms of its reversible and its irreversible components. The irreversible component can simply be presented as the dependence of relative permeability on mechanical stress. The reversible part can be described by a coefficient 8 (magnetomechanical coefficient) which represents the connection between variations of mechanical stress in ferromagnetic samples and electromagnetic induction into coil elements. The magnitude of the inverse magnetostrictive effect and its dependence on the magnetization state in carbon steels has been investigated. Practical examples of implications of the inverse magnetostrictive effect with electromagnetic non-destructive testing methods are given in:

en

E z

The metallographic test shows the existence of individual pits of average depths of less than 0.05 mm and partly with edges with sharp notches. These pits are filled with corrosion products in most cases. With samples that are subjected to an eddy current test immediately prior to the failure of the sample, these localized indications occur again, the number of which does not considerably increase between t > t o and t < t b. The amplitude of the eddy current indication increases considerably, which, together with the metallographic finding of the increasing depth of the pits, confirms the assumption that the processes which take place between tu and t o and which are observed by applying the MMM cause the formation of nuclei for failures under stress corrosion cracking conditions. With both materials, the stabilities of which differ considerably under stress corrosion cracking conditions (and thus, considerable differences also occur in tb), fracture is observed in a period 4t m < t b ( 10/m. The results gained from these examinations show that this method, which is based on the inverse magnetostrictive effect, enables an early detection of material deterioration under stress corrosion cracking conditions, for which only a relatively small number of instruments and information parameters to be evaluated is required.

f

0

shows a record of the pulse sum curve over this deterioration period. The flat curve to t o is interrupted by short discontinuities from time to time, which increase shortly before the failure of the sample at t b. The sample failure proper is characterized by the occurrence of pulses of increased energy (1> 10-7 J), which are determined with counting rates of up to "~100 m i n - ' . As can be inferred from the comparative eddy current test and the metallographic inspection of samples which were checked for surface defects before and after tu, the increase between tu and to (mean value time tm = (tu + to)/2) indicates the primary stages of deterioration of the sample. Samples examined after t u show characteristically localized eddy current indications.

,

II

I

,o 120

I 22

I

I 24

i)

An integrated strategy for enhancing the signal-tonoise ratio of eddy current testing in mechanized testing lines with a high level of mechanically generated interfering effects, showing the application of this strategy to eddy current testing in a welding line for section tubes.

ii)

A new method for non-destructive determination of microstructural internal stresses used for a

Time, t (h)

Fig. 12 a - - characteristic pulse sum curve when examining samples under stress corrosion conditions applying the magnetomechanical method; b - - example of a record in primary stages of stress corrosion *Institute of Civil Engineering of Berlin Technical University

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257

p r o c e d u r e to i n d i c a t e the m a r t e n s i t e c o n t e n t in roiled wire rod. iii)

A n e w m e t h o d for n o n - c o n t a c t d e t e c t i o n o f stress w a v e s in r o d - s h a p e d s t r u c t u r a l e l e m e n t s m a d e o f ferromagnetic materials (magnetomechanical m e t h o d ) , d e m o n s t r a t i n g the sensitivity o f this m e t h o d by w h i c h the p r i m a r y stages o f stress c o r r o s i o n c r a c k i n g in steel c a n be detected.

10 11 12 13 14 15

References 1 Fbrster, F. and Stambke, K. 'Theoretische und experimentelle Grundlagen der zerst6rungsfreien Werkstoffpriifung mit Wirbelstromverfahren'Zf Metallkunde 45 (1954) p 206 2 F6rster, F. 'Eddy current test principles' in McMaster: Non Destructive Test Handbook Ronald Press, New York (1959) 3 Libby, H.L. "Introduction to electromagnetic non-destructive testing methods' Wiley Interscience, London, Sydney, Toronto (1971) 4 Beeker, E.A. and Vogt, M. 'Wirkungsweise und Anwendungsm6glichkeiten eines elektromagnetischen Geffige- und Verwechslungspriifgerfites' Materialprit'fung 15 ( 1973) p 182 5 Polansehiitz, W. M6glichkeiten der zerst6rungsfreien Verwechslungsprfffung" Proc2nd European Conf on NDT, Vienna (1981) 6 Davis, T.J. "Multifrequency eddy current system for inspection of steam generator tubing' EPRI-Report NP-1621 (1982) 7 Wait, J.R. "Electromagnetic methods in nondestructive testing of wire ropes" Proc IEEE 67 (1979) p 892 8 Stumm, W. 'PriJfung von geschweissten Rohren im Produktionsprozess mit einem magnetischen Streuflussverfahren" Neue Fachberichte 13 (1975) p 237 9 Luz, H. "Ein neues Verfahren zur Priifung von ferromagnetischen Rundstfiben' MaterialpriJfung 21 (1979) p 282

16 17 18 19 20 21 22 23 24 25 26

Keller, P. 'Ber0hrungslose Temperaturmessung' Drucksch. CH-IG 112 271 D/BBC (1979) Tiitto, S. 'On the influence of microstructure on magnetization transitions in steel" Acta Polytechnica Scandinavia Appl Phys Ser Helsinki (1977) Jiikel, T.'Verfahrensparameter beim magnetinduktiven Pri~fen der H~irtetiefe" Maschinenrnarkt 86 (1979) p 103 Kittel, C. "Physical theory of ferromagnetic domains" Rev Mod Phys 21 (1949) p 541 Steinbuch, K. 'Taschenbuch tier Nachrichtenverarbeitung" Springer-Verlag, Berlin, Vienna, Heidelberg (1967) Pawlowski, J. 'Die Ahnlichkeitstheorie in der physikalischtechnischen Forschung' Springer-Verlag, Berlin, Heidelberg, New York (1979) Arrington, M. "Introduction to acoustic emission testing' Metrology and Inspection 10 (1976) p 1430 Rettig, T.W. and Felsen, M.J. 'Acoustic emission method lot monitoring corrosion reactions' Corrosion NACE 32 (1976) p 121 Battle, P. 'Stress wave emission: defining its capabilities'Metals and Materials 3 (1976) p 37 Vahavidolos, J. 'SWE a tool for nondestructive testing" Tooling and Production 41 (1975) p 41 Carr, W.J. 'Principles of ferromagnetic behaviour' inMagnetism and Metallurgy Vol 1 ed A.E. Berkowitz and E. Kneller, Academic Press, New York (1969) F6rster, F. Materialp~fung 4 (1962) p 397 Mende, S. and Simon, H. Physik Wilhelm-Heyne-Verlag, Mfinchen (1974) Polanschiitz, W. Materialp~fung 22 (1980) p 364 Polanschiitz, W. Materialp~fung 26 (1984) p 222 Denis, S., Simon, A. and Beck, G. HTM 1 (1982) p 18 Lambeck, M. Materialpmfung 21 (1979) p 268

Author T h e a u t h o r is with Voest A l p i n e A G , P e t e r T f i n n e r strasse 15, A - 8 7 0 0 L e o b e n , Austria.

Paper received 30 December 1985

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