DD258291A1 - NON-DESTRUCTIVE TESTING METHOD FOR HETEROGENIC MATERIALS - Google Patents

Abstract

The invention relates to a test method which particularly characterizes material regions close to the surface. The method is suitable for the quality control of applied layers on materials. The phase velocity of ultrasound waves is determined by the signal propagations excited in the sound receiver by the sound pulses except, as already known, at the beginning X0 and at the end Xmax of a given measurement distance, according to the invention at n further measurement positions defined by formulas 1 and 2 individually recorded, the associated phase spectra determined individually, group delay determined, carried out according to formulas 3 and 4, a phase correction, which makes it possible to determine the phase velocity according to formula 5. From the phase velocity, material parameters can be obtained mathematically or by graphical representation and setup of calibration curves.

Description

For this 1 page formulas

Field of application of the invention

The invention relates to the field of physics and relates to a test method for trouble-free quality control, in particular for near-surface material areas.

When using ultrasonic surface waves, the method allows the determination of such parameters as

Layer thickness of applied surface layers

Insertion depth of structural changes

Ε module of thin layers

Gradient of the elastic properties of applied layers.

When using plate or bar waves in sheets or wires is also suitable for the investigation of deformation textures, which have an anisotropy in the mechanical, electrical and magnetic properties of the material and to determine the plate or wire thickness. The invention is applicable to all materials containing heterogeneous surface areas e.g. by coating, alloying, fusing, diffusion, etc. and thus causing dispersion of sound propagation, i. a dependence of the phase velocity on the wavelength and the frequency. It can also be applied in the case of geometric dispersion, i. when the extent of the workpiece in one or two directions (plates or bars) is of the order of the sound wave length.

Characteristic of the known state of the art

Known is the possibility of non-destructive quality control of heterogeneous materials with the impulse-echo method (DRAllen and WHCooper, Ultrasonies International 83rd Conference Proceedings Halifax, 12.-14.JuIi 1983, Butterworths Sevenoaks, Kent 1983) and the impulse transmission method ( W. Sachse and Y. Pao, J. Appl. Phys. 49 [1978] 4320-27), in which the phase transit time of ultrasonic waves is determined by the Fourier transformation of the pulse profile. The pulse course is recorded at the beginning and at the end of a test section of length L. For both pulses, the phase spectrum Ip ₁ and cp2> are calculated separately with the aid of the Fourier transformation. Thus, for each frequency component f, which is contained in the pulse spectrum, the phase delay ί _φ = (φ _Ί - cp ₂ ) ²ⁿ f can be determined. The phase velocity can be determined by Cf = L / t φ. In heterogeneous materials, the phase velocity is frequency-dependent. The disadvantage of this method is that the correct phase velocity can only be determined when either operating at low frequencies, or the high-frequency pulse has low-frequency components, the frequency function of the pulse must have zero zeros up to the frequency zero point. These conditions can be realized in the rarest cases, since a US converter with these properties is difficult to produce. If the above requirements can not be realized, the method is only applicable if the phase velocity is approximately known or the measuring cell is smaller than the wavelength, but in the last-mentioned case very large measuring errors occur in the determination of the phase velocity. This method is therefore only applicable for routine examinations in which the sound propagation properties of the sample to be examined are approximately known and change only within narrow limits.

Object of the invention

The aim of the invention is the safe, economical non-destructive determination of material parameters such as layer thickness, Ε-modulus, gradient of elastic properties in near-surface material areas or deformation textures and thickness of sheets and wires, even with unknown composition of the samples to be examined.

-2- 258 291 Presentation of the Essence of the Invention

The invention has for its object to provide a test method in which can be determined with commercial equipment, the phase velocity of ultrasonic waves accurately and over a wide frequency range. According to the invention, the object is achieved in that the waveform which is excited in the sound receiver by the sound pulses, except, as already known, at the beginning X ₀ and at the end X _{max of} a predetermined measurement distance at η further, precisely defined and by the formulas. 1 and 2 determined measuring positions Xi to X _n between X ₀ and X _{max is} recorded individually, from which the Fourier transformation the associated phase spectra φ ₀ , φι biscp _n and cp _{max are} determined, the group delay t _gr - tg _r , tg _r - tg _r until t [J _r - tgV and t ™ ^x - tg _r , which requires the pulse maximum for passing through the individual distances from X ₀ to the individual measuring positions, a phase correction according to formulas 3 and 4 is carried out, which makes it possible to determine the exact phase velocity according to formula 5 and then graphed in a known manner either as a function of the frequency and the establishment of Eichku used by a computer program, eg to Newland (Phil. Trans. Roy. Soc. Lake. A245 [1952] p. 213-303) is used to determine such material parameters as Ε modulus, thickness of applied layers, etc. In the following, the invention will be explained in more detail by an embodiment.

embodiment

The inventive method is used to determine the thickness and the Ε-module of nickel layers, which are applied to WC-Co hard metal samples. First, the apparatus design will be described, which is composed of commercially available measuring technology and allows automation of the process. ultrasonic apparatus

Sound source is a pulsed laser that excites surface wave pulses on the sample surface that are received by a piezoelectric transducer mounted on the sample surface. The laser pulses excite surface wave pulses having a wide frequency spectrum, therefore the choice of the transducer determines the frequency range in which the measurement is made. In the application, the frequency range is 33 to 39 MHz, the converter thus has a center frequency of f _M = 36 MHz, which corresponds to a period T _M = 27.8 ns. Displacement

The sample with the converter located on it is placed on a cross table which can be displaced perpendicularly with respect to the incident laser beam with a micrometer screw and thus the individual distances X between the sound source and the sound receiver required in the method according to the invention can be set with high accuracy of ± 1μηη

-Zeitmessung

The time measurement is carried out by means of a digital oscilloscope with connection to a digital computer, as well as a box office integrator can be used with connection to a digital computer. The duration of the sound wave with the frequency

f = - is composed of two parts. The first part is the group delay t _gr , the time between the impact

of the laser beam at the measuring position X and the occurrence of the maximum sound pulse at the transducer. The second part consists in the phase shift of the wave with respect to the pulse maximum T φ (f). The phase spectrum φ (f) is calculated with the Fourier transform from the pulse curve, with the time zero point of the transformation lying in the pulse maximum.

The transformation zero point is thus identical to the end of the group delay.

The execution of the test method according to the invention now proceeds as follows:

The nickel coated WC-Co sample has a gauge length of X _max - X ₀ = 10 mm. First, the group delay and the phase spectra are determined at the beginning X ₀ and at the end of the measuring path X _max . The group delay tg _r and t ™ * are 4.364 and 8.636ms. The positions X ₁ and the number η of the additional measurements between X ₀ and X _max are made from the accuracy of the time and distance measurement technique used, the period T _{M of} the center frequency of the spectrum and the group delay t t * * tgr, which the sound impulse traverses the measuring path X _max - X ₀ required, determined according to the formulas 1 and 2. The following values are used:

Period T _M = 27.8ns

Δχ = 1 μηι

X _max - Xo = 10mm

These values used in formula 1, give the first additional measuring position at a distance of X ₁ - X ₀ = 0.08mm. The next additional measuring position is determined by inserting the values in formula 1 with X ₂ - X ₀ = 1.2 mm. The number of additionally required measuring positions is obtained when X, - X ₀ > (Xmax - Xo) from η = i - 1 Since in the present case X ₃ - X ₀ = 15.5 mm already greater than the predetermined measuring distance between X ₀ and X _max , only two additional measurement positions are required. The group delays for the additional measurement positions are t _gr = 4.415ms t _gr = 4.878 με

The Fourier transforms of the pulse waveforms recorded at X ₀ , Xi, X ₂ and Xm _3x give the phase values for the center frequency f _M = 36 MHz: φ _ο = 2.5 rad φ, = 0.5 rad φ ₂ = 0 , 45 rad • <Pmax = -1.70 rad

From the measured values for the X, -. X ₀ , t _gr and cp, can be calculated according to the formulas 3 and 4, the phase correction m _{n +} i. The result is m ₃ = -36. Thus, according to formula 5, the phase velocity over the entire measuring path X _max - X ₀ determined

become. It is 3063 m / s. With a sample pitch of 1 ns and a number of 4096 sample points, the discrete Fourier transform provides the phase components φ for 49 frequencies in the range of 33 to 39 MHz. Thus, except for f _M = 36 MHz for another 48 frequencies, the phase velocity in this frequency range can be calculated according to formula 5, which is plotted in the form of a curve as a function of the frequency. From the graph, the Newland calculation program (Phil. Trans. Roy. Soc. See, A245 [1952] p.213) determines the thickness of the 38Mm nickel layer and the 203GPa E modulus.

List of formulas

^x max- ^x O ( ⁷ m

, max _ , Q

-? - Δ * \ - 3ΔΧ

Δχ = accuracy of the distance measurement At = accuracy of the time measurement T _m = period of the center frequency

for i = 1 to η taking into account mi = 0

^χ Ί ~ ^ _0

_{c β} ^X max ~ ^X Q

max_.O ^f gr

C = phase velocity on the section X] -X ₀

C = phase velocity over the entire measuring path X _013x - X ₀

mi = phase correction on the leg Xj - X ₀

η = number of additional measuring positions between X ₀ and X _ma x

tgr = tgr = group delay on the leg Xj - X

Vf = phase of the center frequency

Claims (1)

Destruction-free test method for heterogeneous materials by determining the phase velocity of ultrasound waves, which is graphically represented as a function of the frequency and / or used by a computer program for determining material characteristics, characterized in that the Signalveriäufe that are excited in the sound receiver by the sound pulses, except on At the beginning of the measuring path X ₀ and at the end of the measuring path X _max , at η further, precisely defined measuring positions X ₁ and X _{n determined} between formulas 1 and 2 are recorded individually between X ₀ and X _max , with the Fourier transformation the associated phase spectra cp _{0 /} Cp ₁ to φ _π and cp _{max are} determined, the group delays t _gr - tg _r , tgr - t | _r * t * _r - t ° _r to tgV - t ° _r - t ° _r and t "gT - tg _r can be determined that the pulse maximum for passing through the measuring section X ₁ - requires X _0, carried out a phase correction according to formulas 3 and 4: - X _0, X ₂ - X ₀ to X _n - X ₀ and X _max and the phase velocity is determined according to the formula 5.