Method for determining the sound velocity in a basic material, particularly for measuring the thickness of a wall

Translation of description of equivalent WO 2004017021 A2

Title: Method for determining the speed of sound in a Basismat_erial, especially for a wall thickness measurement

The invention bezielit to a method for determining the sound velocity Cb in a base material using an ultrasonic probe comprising a transmitter element, a receiver element and a Vorlaufkörper-, the leading body a) has ein_e coupling surface, with the probe to the base material can be coupled , b) receives the receive crystal and the transmit crystal and c) has a sound velocity Cv, the transmit crystal and receive crystal are each inclined to one another and obliquely ausgeirichtet the coupling surface, so that a main transmission direction of the transmitting transducer and a main receiving direction of the receiving transducer intersect below the coupling surface, transmit crystal and receive crystal a IMittenabstand K from each other, the transmitter element a center distance Ds from the coupling surface and the receiver element a distance De from the coupling surface has, in which method an ultrasonic pulse front transmit crystal is generated, passes through the precursor body into the base material, there causes a creeping wave and of this part on the probe tip reaches the receive crystal and a entsprechen_de device.

The determination of the sound velocity Cb is a prerequisite for being able to determine the wall thickness of the base material. While it is known to determine the wall thickness of a base material by multiple reflection of a pulse on a EintrittsflεLche and a rear surface of the base Mate arial, but this method is sufficiently reflective and thus smooth surfaces, in particular a sufficiently smooth back surface advance, thus it to multiple reciprocating Herläufen comes in the base material. In rough back surfaces, this method can not be applied, but one is dependent on a single reciprocation. can be found via the sound velocity Cb then the wall thickness.

From US 6,035,717 a method and an apparatus. zmr determining the thickness of a coated base material known. In this method, the sound velocity Cb of Ba ^ is ismaterials initially measured the uncoated base material for the determination, there is a pulse of a transmit crystal acoustically irradiated by the leading body in the base material where a creeping wave is generated by the back decouples a portion and is received by the receiving transducer. In that regard, there is Ü conformity with the invention.

The path of this pulse is accepted but now defined as fixed according to US S, 035, 717th The inventors of this US patent where, so it seems, well aware that this assumption of a geometrically fixed path along the main beams certain TJngenauigkeiten in determining the SchallgeschL brings windiness Cb. He therefore proposes the practical instructions to keep the distances between the two oscillators as small as possible from the coupling surface. Thus, the determination of the sound velocity in the base material is gemauer in fact, the inaccuracy is in other words reduced. Now has a test head with a short sound propagation path of the lead body has the disadvantage that little Ivlaterial the lead body is taking place in any practical examination wear lead body available, so the probe needs to be replaced sooner than a test head with greater standoff.

This is where the invention. It has set itself the task of the process according to US 6,035,717 A to the effect<img id="imgf000004_0001" he="5" wi="45" file="imgf000004_0001.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="no" /> that the sound velocity Cb is determined in the base material precisely that thus precisely the thickness of a Schick.! on this base material can be determined, and that a probe can be used, "the one for the Practice may have sufficiently thick Vorlaufstreckze.

This object is achieved starting from the features mentioned, and this including the fact that the shortest Sclhalllaufzeit measured Tdead and the sound velocity Cb is determined in the base material over that way, which provides between transmit crystal and receive crystal depending on the speed of sound, the shortest total travel time T tot.

In this method, the fact is considered that the way the pulse by the leading body, takes along the Oberflächie of the base material (as surface acoustic wave) and back through the Vorlaufkiörper, in addition to the previously known Grossen K, Dv mnd Cv is affected (K = center distance the contact surfaces of the transducer, Dv = Mi^~ttelpunktsab- was the KontaktfLä.che a vibrator from the paddock area) by the sound velocity Cb. Is this in comparison with the sound velocity Cv in the leading body is relatively large, so the proportion of running distance Sb along the Oberflä.che of the base material is relatively large. However, if the sound velocity Cb in the base material is relatively small, so is the running distance Sb of the surface wave in. Base material is relatively short, the running tracks Sv within the lead body on the other hand are longer. Similar conditions also in the light refraction between different optical media, such as water and air, before. Also in this case, the geometrically shortest route is not the shortest time path for a L / ot pulse.

The performance of the invention is now to have realized that the detection of the shortest duration T tot of the ultrasonic pulse and optimizing all möglieben sound propagation paths ___ru to that SchaLUaufweg way that provides the shortest total travel time as a function of Cb, a precise indication of the sound velocity Cb in base material supplies. The invention are thus the actual paths zrugrunde which traverses an acoustic pulse. It makes no assumptions überr the way, as is the case in the US 6,035,717 A. The error of this known Messverfalirens and apparatus are Datier ss erfindungsgemέ avoided.

Further advantages and features of the invention result from the remaining claims and the following. Description nicrit limiting to-understand embodiments of the invention, which are explained in more detail with reference to the drawing hereinafter, besides, also the inventive method will be explained. In the drawings:

Figure 1: A basic representation in side view of a probe with two oscillators, which is coupled to a base material, the individual links of the entire path are shown,

Figure 2: The illustration shown in Figure 1 with marked running tracks, sound velocities, etc.,.

3: a Da_rstellung as Figure 1, but has now also the base material, a thin layer (a coating), for example, a color, a metallic coating or plastic coating, Uncl

4: a view similar to Figure 1, but now with two additional transducers for measurement of wall thickness.

The probe shown in Figure 1 has a specially shaped, substantially prismatiscrien leading body 20. This has a et&gt; ene coupling surface 22, also called active surface and "this opposite bevels on which a transmit crystal 2^&lt;= 4 and a receive crystal 26 are held, in particular cemented. Both oscillators 24, 26 are identical in construction. They are inclined to one another and also inclined .to coupling surface 22 arranged. In this arrangement will be discussed in more detail below.

A central vertical line, ie a perpendicular extending from the contact surface of Sctiwin- gers with your leading body 20 and through the center of the contact surface of the transducer line runs in a particular ^"Angle to the coupling surface 22, this angle (90 ° -cιv) and is for both oscillators 24, 26 is equal. Further, the respective bisectors are in the same e^"bene, namely in the plane of FIG. 1.

This may a^~uch be other words, the two oscillators 24, 26 are mirror-symmetrical to a symmetry is industry level 32. They are so inclined for coupling surface 22 that is coupled to a base material 34 to which the Vorlaufkorper 20 via suitable, per se known means, a Oberflä-chenwelle produced 35, which will be discussed in detail.

A contemplated substantially along the plane of symmetry 32 Tremn- layer 36 ensures that a direct crosstalk (crosstalk) between transmit crystal 24 and receive crystal is inhibited 26th

The Mittelsenkreclαten listed are usually a Haixpt- ray, so a prime beam 38 and a main receiving beam 40 together.

The sound velocity Cv in the leading body 20 is known. the distance K between the Flächenmittelp is known nlcten the two Scttwin- ger 24, 26. Finally, the distance of the Flächerimittelpunktes of the transmit oscillator 24 from the coupling surface 22 and the distance of the center of the receiving transducer 26 from the coupling surface 22 are bestimmtiar and thus known. But due to the symmetry^"both the value of Dv. Using only these requirements, it is now possible to Schiallgeschwindigkeit Cb to determine the base material 34th In a further step, you can then the thickness, ie the wall thickness Db of Basi materials determine 34th

If the sound velocity Cb in the base material 34 is about as big ss as the sound velocity of steel, is the shortest way of a sound pulse from the transmitting transducer 24 to receive crystal 26 the following: The Impmls runs along the main end beam - then as Oberflächienwelle 35 in base material 34 and finally again along the main receive beam 40 in the receive crystal 26. This path is shown in dashed lines in Fig. 1, it runs along the main beam end 38 and the Forggensee pfangsstrahl 40.

But even if the sound velocity Cb in Ba_sismaterial 34 klein_er than that of steel, so the sound path will be used as much distance inn outside of the lead body 20, the length of the distance Sb, which is realized by the surface wave 38 in the base material 34, is short. This case is illustrated in Figure 1 by a dotted illustrated S challlaufweg 42nd

Is the other way around, the sound velocity Cb in the base material is greater than that of steel, the Sclnallstrecke Sv within the lead body 20 is short in favor of a longer running distance Sb as surface acoustic wave 35. This case is by in Figure 1, shown the dashed acoustical 44th

For ease of illustration, only the complete acoustic path is represented by dashed lines in Figure 1, which runs along the main beams 38, 40 '. Ivian detects that the running distance Sb of the surface wave 35 is a function of the speed of sound Cb in the base material 34 and is also dependent on the sizes constant K, Cv and Dv. Erfϊndungsgemä_ß the sound velocity Cb is &lt;= 4 obtained by optimizing the associated sound propagation path in the base material. 3 It is the sound path basis, the liefext the shortest total travel time T tot as a function of to be determined sound velocity Cb.

Although now decreases the amplitude of the sound pressure, the greater is the angle to the main beam. If one measures but only the signal with the shortest total duration T tot, so it is within certain limits on the amplitude of the received signal independently. Ideally, the oscillators 24, 26 would be spherical radiator, but this is not the case would be. Innerh_alb occurring in practice, speeds of sound itself makes the influence of the non-spherical emission of the crystals 24, 26 are not as noticeable, that one would have to take this into account "and evaluate specially. The orientation of Scbwinger 24, 26 is ideally done for an average value of the speed of sound Cb (eg for steel Cb about 6000 m / s).

The j eweilige way the sound pulse takes the shortest total duration T tot, is thus a function of Sch_allgesch windiness Cb and further dependent on the known values ​​of K, Dv and Cv. If not klappsymmetrischem build the different Mittelpunktsentfer- have openings of the transmitting transducer 24 and the Ernpfangsschwingers 26 are considered by the coupling face 22nd

According to Figure 1 propagates the ultrasonic Tom transmit crystal 24 via a first path segment Sv until bin for base material 34 made, for which he needs the time Tv. There is a creeping wave 35 is generated. She has indicated in Figure 2 length Sb. This length is traversed Tb in time. Of the creeping a portion reaches the receiving transducer 26 via a path due to the symmetry has the length and Sv for the time TV is needed.

Wanted by distance Sb between the starting point and the end point of the creeping wave or surface wave 35 for longit dϊnale wave len:

In the following general formulas are established for the propagation of sound. Here only the shortest total travel time T tot is respected. To determine this, it is necessary to consider the total running distance from the transmitting transducer 24 to receive crystal 26th As constant over time, at least for the short duration of the measurement, and as already known are assumed K = Mittelp nktsentfernung the probes 24, 26; Dv = center distance of the crystal 24, 26 from the coupling surface 22 and Cv = sound velocity in the leading body.

A emitted from Sendeschvwinger 24 Ultraschallimpiαls effected in the base material 34 niclit only a surface wave 35, but also other waves, the longitudinal surface wave 35 has the shortest travel distance and the shortest total travel time T tot.

According to Figure 2 applies:

(1) S_b = K -2K_b;

K

(2) tano;_v = - ^b- - K_b = D_v tana_v;

(3) S_b = K -2D_v tε a_v; (1/2)

Different sound propagation paths differ in α entry angle. This is obtained by assuming a shortest possible total duration T tot:

<img id="imgf000010_0001" he="32" wi="54" file="imgf000010_0001.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="no" />

(7) cosαr_v = -z; → S_v = ^v-

S_v cos a_λ (8) T_v = ~ ^^; (5/7)

C "cosof ,,

(9) _T = ^2D_{* +}K-2D_vtana_V (4/8/3/6]) C_v cosa_v C_b

&lt;¹⁰&gt; ^τ-^'χ^D- ^ - ^y' ⁽⁹⁾

Wanted: a minimum of the linear function Tdead (v). This can, for example, on the first derivative with respect to the "angle are determined αv, the first derivative must αv for a certain angle to be zero, the second derivative must be positive:

(11) ^d dead.^av) _{= Q} .DELTA.a_v

<img id="imgf000011_0001" he="11" wi="144" file="imgf000011_0001.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="no" />

(13) ^^ - = 0_;→ sin .alpha_v= - ^ - _v= Asin (^)_; (12)

Considering now (3), one realizes that the sound propagation path Sb depends on the two sound velocities Cv and Cb:

(14) S_b =_JST-2E&gt;_vtan (arcsin (^)); (3/12)

Since you accept as a constant K, Dv and Cv ann, this means: Sb = f (Cb). The following equation (15) now describes the relationship between the measured total running second Tdead and to be determined sound velocity Cb:

<img id="imgf000011_0002" he="23" wi="147" file="imgf000011_0002.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="no" />

This is a clear link zwischien the total duration T tot and to be determined sound velocity Cb reached. All other Sizes in the equation (15) are known and constant.

Thus, over the entire duration Tdead clearly determining the sound gesc windiness Cb in the base material possible. With this knowledge now, the wall thickness Db of the base material can be determined.

Figure 3 shows the Anordmαng from the above previous figures, j edoch is now an additional layer 46, a so-called coating on the base material 34. This layer has a thickness Ds. They should be determined by the sound velocity Cs layer. Since this is u is known, as it was before first ermiittelt the speed of sound.

Figure 3 again shows only the sound propagation path with the shortest total duration T tot. While it can find other spreads instead, for example, a surface wave is also at the coupling surface 22 generated to facing surface of the coating 46, this is but arrive in time to the surface wave 35 in the base material 34th This means that the speed of sound C^"b in the base material 3-4 sufficiently larger "than the sound velocity Cs must be in the layer 46. In practice, this is usually met. The base material is typically a metal, the sound velocities are at 4500-7000 m / s. The layer 46 is typically a plastic, a color and dergleictien, the sound velocities are typically between 2000 to 3000 ms. in the event that the speed of sound Cs in the layer 46 is relatively large, for example, the Schiclnt is a metal coating on a base material made of plastic, the Schictit from a metal higher speed of sound than the ESasismate- rial is such. B. coating in Ag, Au base material in must be used in accordance with the considerations previous set, by simply changing the layer 46 is used as the base material.

Below is the Schtalllaufweg shown in Figure 3 as the shortest maturity considered off. The entrance angle is αv to αs changed in the layer 46th The running tracks the volume of layer 46 resulting from Figure 3, they amount Ss. The associated sound running time of Ts.

Then, for the shortest total travel time:

(16) T_dead= 2 (T_v + T_s + T_b;

<img id="imgf000013_0001" he="11" wi="52" file="imgf000013_0001.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="yes" />

A

(18) cosor_v= - ^;

S, A = - cos ,,

D "

(19) τ_v= - (17/18)

C_v cosαr_v In the layer 46, the conditions are correspondingly applies to the layer 46:

(20) τ_s = D (19)

C "cosor_

(21) r - ^ύbc "

(22) S_b= K 2 (K_bK +_s);

(23) tanα_v = -; -&gt; K_s = D_v tanor,

<img id="imgf000013_0002" he="11" wi="71" file="imgf000013_0002.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="yes" />

(25) S_b= K 2 (D_S tanα. D +_v tan _v); (22/23/24)₂6) _τ - ^{κ ~ 2}(^Ds ^tanQf_ + A ^tm v). (21/25)

There are now available for all elements Tdead: (27) T_tt= 2 (- ^ + ^D'-_{) +} K-2 (D_sta_{s +}D_vtzncQ_ (16/19/20/26) ' "" C_vc sa_v C_sc sa C_b^J

(28) T ^{2&gt; v} I ^2D' _l^{K 2JD}*^tmas ² anα_v, ₍₂₇₎^; "" C "cosa" C.cosα. C₆ C₆ '

(29) tanc. ,,

Γ "= - + 2 (O ι_ tan # _

C "cosor ,, -⁾+ A⁽ 0); (28)

C_ cos. ^"cT

The total duration T tot is now not only (as in (15)) is a function of the entrance angle αv, but also a function of the entrance angle αs an d can be represented as follows:

(30) T_dead( _va_s) = ^ + 2 (f_l( _v) + F₂( _s) -); (29)

^ b

If the Tdead function (αv, αs) has a .Minimum, this can be stated as again via the first derivative with respect to the h&gt; eiden angles. The first derivatives must be 0:

(31) M = θ! d_v

(32) ^ _⁾₌ O! there_s

(33) IW _{= (} £ 5 ¹ - _{) = 0} (31/29) as_v C_vcos a_v C_bcos a_v

(34) ^ - ^ = (,_→ __ = ±_{: →}sin .alpha_p= ^; → α = £ aιcs_n(^^,); (33)

^ V ^ b ^ v ^ b ^ b ^ b

(35) _{= JDΛ}_ ^ _ ^l_{) = I0 (}32/29)

9e._ C_ cos a_s C_b cos ÖΓ_

_{/ ß \} sinα_ 1 _ sin ". 1 _ C_. ._{-X \}

(36) - *.-__ = 0; → - ^ = -; sιnα → _ = -; - "α_ = arcsm (- ^); (35)

The results (34) and (36) are now employed in the equation (29) this yields: (37)

<img id="imgf000015_0001" he="23" wi="173" file="imgf000015_0001.tif" img-format="tif" img-content="drawing" orientation="portrait" inline="no" />

(29/34/36))

Using a suitable measuring instrument, for example, the device DMS 2 of the Applicant, the shortest total travel time can be measured Tdead. About it the thickness Ds of the layer 46 and / or k ann, the thickness Db of the base material 34 are now determined when the sound velocities Cs and Cb are known. So you can determine with the help of (27) Cb at a -unbeschichteten point of the specimen. With an additional Schwinger pair for thickness measurement with the oscillators 48, which are identical, is acoustically irradiated steep, see Figure 4. A Rücl ^ wall echo produced whose maturity to the maturity of e&gt; s is reduced. From this maxi calculated with known sound velocity Cb the thickness Db of the base material _

A second, temporally subsequent echo obtained from a rear wall 50 of the base material 34. From the time difference between the two Eclxos and the previously measured sound velocity Cb in the base material 34 may be the thickness Db are determined. The thickness Db may also be obtained as the difference of this echo of the rear wall 50 for entry echo taking into account the speeds of sound, minus the thickness Ds sends 46th

From Ctleichung (37) the following relationships are recognized and clearly:

CC

1) - - and - * - must be less than 1; 'C_b C_b

2) C_s may be less than C_v be -