USE OF STRAIN-MEASUREMENT METHODS FOR RECORDING ACOUSTIC EMISSION

Yu. S. Vorob'ev, Ao V. Kolodyazhnyi, and V~ Vo Voronin

UDC 531.781

At present acoustic emission is being widely used in studying the mechanical character- istics of varioUS materials. Piezoelectric transducers are normally used as the receiving transducers of the acoustic emission signals. Together with their advantages, in particular high sensitiVity to displacements, they possess certain significant disadvantages, including strong nonlinearity of the amplitude--frequency characteristic, complex calibration, and the

capacity of recording only the acceleration of the sample surface and not its deformation.

In the use of a number of characteristics of materials and also in studying crack growth high sensitivity is not always necessary. For example, the amplitudes of displacements of the points of a sample surface in opening of a crack are on the order of 10'5-10 -6 m [i]. Impulses with such an amplitude may be successfully recorded by strain measurement methods with the use of standard semiconductor strain gauges.

Let us evaluate the sensitivity of the proposed method of recording acoustic emission. The sensitivity of strain gauges using a potentiometer measuring circuit is determined using the equation [2]

(1)

where Svmax is the maximum voltage sensitivity, V; Rg, strain gauge resistance, ~; S, gauge factor; and Igmax, maximum current through the strain gauge, A.

With the use of a type KTD-2A strain gauge with a base of b = 2.10 -3 m, a gauge factor of S = 200, a current through the gauge of Igmax = 2"10 -2 A, and a gauge resistance of Rg = i00 ~ we will have Svmax = 400 V.

Let us determine the minimum amplitude of the deformation signal which may be recorded with the use of strain gauges with a level of noises on the input of the amplifier of Umi n = 5.10 -6 V:

Ami. Umi. b >S---;-~-~-m~' (2)

where Amin is the minimum amplitude of the acoustic emission deformation sample, m; b, gauge base, m; and Umin, level of noises on the amplifier input, V. From Eq. (2) we find

bUmin (3)

After calculation we obtain Ami n > 2.5.10 -11 m.

TherefOre, the sensitivity of strain gauges is completely sufficient for recording acoustic emission signals from growing cracks in the investigated material.

In addition, the primary advantage of semiconductor strain gauges is the recording not of acceleration but of deformation of the sample surface. This openswider possibilities for inVestigating the stressed and deformed state in the zone of a developing defect in a test sample~

The acoustic emission was recorded with the use of a type KTD-2A silicon semiconductor strain gauge in pure bending of a 400 • i0 x i0 mm U8 steel beam with a triangular cut in the center (Fig. i).

The Institute of Problems of Machine Building of the Academy of Sciences of the Ukrainian SSR, Khar~kov. Translated from Problemy Prochnosti, No. 8, pp. 126-128, August, 1982. Orig- inal article submitted June 4, 1981.

1144 0039-2316/82/1408-1144507.50 �9 1983 Plenum Publishing Corporation

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Fig, i Fig. 2

Fig. i, Method of testing a steel beam in bending: i) test sample; 2) stationary support; 3) movable support to which the load is ap- plied; 4) micrometer; 5) type KTD-2A strain gauge.

Fig. 2. Circuit for recording acoustic emission inpulses: PB) power block; R) supplementary resistance; SG) strain gauge; A) amplifier; MO) memory oscillograph.

To the moveable support 3 is applied a force in the direction shown by the arrow. The deflections of the beam are recorded by the micrometer 4. The type KTD-2 gauge is cemented with BF-2 cement to the surface of the beam at a distance of 3-10 -2 m from the center of the beam.

The circuit for recording the acoustic emission impulses is shown in Fig. 2.

A UIP-2 universal power source was used for powering the sensor with direct current. The gauge was connected to a i00 V voltage through a i0 k~ resistance, which provided a current through the gauge of 10 -2 A. To the gauge was connected an alternating current amplifier with a level of noises of 5 ~V and a factor of K = i000 and to the output of the amplifier a type $8-2 memory oscillograph. Scanning of the oscillograph was started by the input signal. The strain gauge factor was 140.

One of the oscillograms of the acoustic emission impulses obtained is shown in Fig. 3. The scanning of the oscillograph beam is 50 ~sec/div. The oscillograms make it possible to calculate the amplitude of the acoustic emission deformation signal using an equation based

on Eqs. (i) and (3):

b.Uout A----- 1~g .Ig.S.K ' (4)

where Uout i s t he a m p l i t u d e o f t h e s i g n a l on t h e o u t p u t o f t h e a m p l i f i e r i n V ( t h e Uout d e - t e r m i n e d from the o s c i l l o g r a m shown i n F i g , 3 i s 75-10 - a V).

After calculations we obtain A = 1.07,10 -I~ m.

From the oscillograms it is also possible to determine the frequency of the emitted

oscillations (in this case it is 61 kHz).

The acoustic emission was also recorded with the use of strain measurement apparatus in strength tests in combined static and impact loading of high voltage line rod insulators of

electrical porcelain.

The combined loading of the insulators was done on a stand making it possible to smoothly change the static bending and the parameters of impact loading of a cantilever fastened insu- lator. The static bending was changed by regulating the pressure in the hydraulic drive systel and the impact load was created by dropping a 5 kg weight from a height of 0.6 m on to the

free end of the insulator.

The resistance strain gauges recording the dynamic deformation of the sample and the acoustic emission were located at a distance of 0.3 m from the pinched end of the insulator.

To record the dynamic deformations type FKP3-100 foil resistance strain gauges connected in a measuring bridge circuit and apparatus for recording the dynamic deformations developed in the Institute of Problems of Hachine Building of the Academy of Sciences of the Ukrainian SSR with an operating frequency band of 70-2,105 Hz were used.

The information on dynamic deformation of the sample was fed to the upper beam of a type

$8-2 oscillograph.

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Fig. 3 Fig. 4

Fig. 3. An oscillogram of acoustic emission impulses in bending of an St.U8 steel beam.

Fig. 4. Oscillograms from the outputs of the deformation channel (upper beam) and the acoustic emission channel (lower beam) in load- ing porcelain insulators with a load far from the failure load (a) and close to the failure load (b).

The acoustic emission was recorded with the use of a type KTD-2A silicon semiconductor resistance strain gauge using the circuit shown in Fig. 2. The operating frequency band of the channel was 50-2000 kHz, The information was fed to the lower beam of the oscillograph.

Scanning of the oscillograph was started by an accelerometer located in the zone of im- pact,

Figure 4 shows oscillograms obtained at various levels of statodynamic loading.

Both channels of the apparatus record a qualitatively similar picture of impact deforma- tion of the part with a static bending moment of 0.9.104 N.m and dropping of a 5 kg weight from a height of 0.6 m (Fig. 4a). The differences in the oscillograms are caused by the frequency characteristics of the apparatus channels.

Figure 4b shows oscillograms with a bending moment of 1.8.104 N-m.

The parameters of impact loading were maintained at the previous level. The oscillograms obtained with such loading parameters differ sharply from those shown in Fig. 4a, In the spec- trum of the signal from the semiconductor strain gauges there appearshigh frequency components indicating the appearance of acoustic emission. This is caused by the fact that with an in- crease in the level of stresses applied to the test sample development of internal defects causing emission of acoustic emission impulses recorded by the apparatus starts in it. In both cases the scanning of the oscillograph was the same, 1 msec/div.

Therefore, recording of acoustic emission with the use of semiconductor resistance strain gauges is possible not only in static but also in dynamic strength tests of materials, which opens additional possibilities for the use of strain measurement methods of recording acoustic emission for nondestructive testing of parts. The use of the proposed method provides a linear amplitude--frequency characteristic in a range of several hundreds of kilohertzs and also makes it possible to analyze the deformation (and not the acceleration) of samples caused by acoustic emisslono

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The proposed method of investigation does not require special apparatus and specially developed sensors, which significantly broadens the area of use of nondestructive testing methods with the use of acoustic emission.

I.

2.

LITERATURE CITED

V. Ao Greshnikov and Yu. B. Drobot~ Acoustic Emission [in Russian], Standartov, Moscow (1976). M. N. Din and N. P. Raevskii, Semiconductor Strain Gauges [in Russian], Energiya, Moscow (1965).

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