DE1648632A1 - Acoustic test system - Google Patents

Description

enfanwfilfe

-7. March 1967 i? *** mm ruschke

D »pUng.HBK'2 AGULAR

8 Munich 27, Pienzenauar Sir. 2

North American Aviation Inc.

1700 East Imperial Highway, El Segundo, California

"Acoustic test system"

The invention relates to a method and apparatus for testing structures, liquids and gases using mechanical vibration or load waves. First and foremost, the invention finds its main application — for Detection of structures under the surface of solids or other structures, but it can also be used to determine chemical compositions of liquids and gases.

There is a particular need for the invention in connection with the manufacture of composite articles,

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such as layer structures and also complicated, thin-walled objects, etc., the light ones as an example Laminated panels include those used extensively in the manufacture of modern, high-speed aircraft and Spacecraft are used. While so the invention is used for a variety of different purposes and with different Materials and structure is useful, it will be used for illustration in connection with the review of plates of the type mentioned above,

Plates of the type mentioned typically comprise a pair of spaced apart outer plates, which with a low density core, such as metallic or non-metallic honeycomb material having a cell wall thickness down to 0.0254 mm. The outer panels are bonded to the core material on each side of the same by bonding processes, the diffusion bonding, Can use solder or adhesives.

The use of such panels in vehicles of the type mentioned requires extreme resistance and cohesion in the finished panel, since these are usually highly stressed parts of plug-in tool cells or vehicle superstructures. So evenly strong and reliable connections between the outer plates and the core material lying on them are essential, and it

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It is of vital importance that defects in all joints between the panel components are absolutely avoided. For this purpose, the careful and complete inspection of such panels is absolutely necessary in order to ensure that the Ensure the goodness and complete integrity of every component, not just at the time of the first Manufacture and periodically during storage or shipping, but also before each flight of the aircraft or of the spacecraft on which such components are mounted are.

Where plates of the type described above are installed by welding, there is an inevitable risk of damage the plate joints and other adverse effects in the base metal as a result of the effect the heat of sweat. The absence of such effects must be determined with certainty before the vehicle in which such panels are installed its May perform tasks. When a panel is fully installed and permanently in with adjacent components a vehicle is connected and only one side of the plate as a result of the mutual structural disturbances by the adjacent Vehicle components are accessible, a full inspection of the plates is extremely difficult and cannot be performed using normal prior art techniques that involve immersion,

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usual ultrasound examination, X-ray examination or others Exams are used that have full accessibility to both sides of the object under investigation require. In addition, where there is a loosened or disconnected point or another area of a connection in close contact but not actually connected is present, creating an actual gap between the flush surfaces that may lie deep within the plate and not close to the surface, which is accessible, the location and identification of the fault location cannot normally be identified by such general known inspection methods, such as X-ray devices etc. can be carried out.

The invention in this case utilizes non-destructive testing and inspection of multiple walls and otherwise light, layered or hollow structures of complicated shape through analysis of the acoustic Behavior of the checked structure. The acoustic behavior of assembled structures is mathematical Complicated and literally unpredictable by theoretical analysis because of the structural geometry and Changes in material composition, compression and mass distribution. However, it has been shown that a Impedance testing of such structures using the novel method described herein eliminates all of these interfering factors.

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avoids. Impedance testing, as used herein, refers to methods and means for detecting changes in load on an electrically energized acoustic transducer whose acoustic response is affected by the workpiece being tested. Conventional ultrasonic methods are unsatisfactory for testing the acoustic behavior of structures as described above. Complicated reflections, interference, mode conversions, and excessive damping at or above the high kilohertz range are major factors that make most of the ultrasonic test types unsuitable in this regard. When such a composite adhesive honeycomb building sheet is tested at high frequencies by impedance, adequate energy transfer is seriously impeded by the factors mentioned. This precludes satisfactory standing wave formation by the entire composite material, with the result that the test will only reveal the connection features between the interfaces that are closest to the source of the standing wave. In the case of high-damping layer structures, such as agglomerated cork, glass fibers or rubber layers, the sound energy can generally not propagate satisfactorily through the adhesive boundary surface. When a sound wave is one too. cannot reach the interface to be checked, it cannot be reflected by it either, and accordingly no change in wave impedance can be between a good one

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and a bad connection, and other means are necessary to check the connection. Impedance testers, which are known in the prior art, are only able to measure surfaces or areas close to them to check on surfaces, but not masses in their depth. Likewise, the impedance testers can the technology does not penetrate through highly dampening layers in order to produce a practical tea jcta "eige".

The present invention provides an apparatus for analyzing a workpiece for plaguing and finding structural discontinuities therein with a probe for generating an acoustic standing wave pattern through a portion of said workpiece when energized done by an electrical signal, and for changing said signal in response to changes in according to said pattern, an electrical source for supplying current to said probe by means of a signal in a proportionate manner low frequency and reading devices for observing said changes in the signal.

The invention also provides a method for inspecting a workpiece with an accessible surface part with the steps of bringing a transducer into working relationship with said surface part, a bridge circuit with the transducer on one leg and one

* in Q mm

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Reactive load on the other leg, said bridge being electrically excited to create a standing wave from the to generate said transducer into the workpiece, the size of the imbalance in the output of the Bridge is observed, which is caused by the acoustic load on said transducer.

The apparatus and method of the invention permit testing of assembled structures through their entire mass, while they require access to only one side of this mass, which in the case of relatively thick ones Superstructures is extremely important.

The device and method according to the invention also allow an exact spatial location and an area definition for errors or structural discrepancies in compound Superstructures.

Further features and advantages of the invention emerge from the following description of one of the attached drawings schematic drawings illustrated embodiment.

Fig. 1 shows a side view in cross section of an exemplary one Embodiment of an immersion probe for Use with the device of the invention as described herein with a layer of liquid

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considerable strength, which acts as an energy connector between the probe and the workpiece to be checked serves j

FIG. 2 is a view corresponding to FIG. 1 Contact probe for use with the device according to the invention, wherein only a thin, liquid, connecting film is necessary that establishes the connection with the workpiece to be examined,

Fig. 5 shows a cross-sectional view of a lightweight Layer build-up plate in which at different Places loosened or not, connected places are present and at the same time typical readings are shown by the location of these unconnected places with the device after Invention can be identified,

FIG. 4 shows one corresponding generally to FIG. J5 View, but with a workpiece made up of layers of the same type instead of a layer structure made of different material,

Fig. 5 shows a simplified schematic circuit for generating a standing wave through a workpiece by an arrangement that differs from the embodiment differs according to Fig. 6 and 7,

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Pig. β shows a schematic block diagram of a circuit, which can be used to practice the method of the invention described herein to apply

Figure 7 shows a view of the Pig device. 6, but with a various illustrative embodiment of a circuit for carrying out the novel Procedure,

8 shows a view corresponding to FIGS. 7 and ^{6, but 1} with a third illustrative embodiment which deviates from the circuits shown in FIGS. 6 and 7, and _v

Fig. 9 shows in schematic form an exemplary embodiment of a circuit that is used for the bridge which is shown in block form in FIG. 6 can be used.

With reference to the drawings described above, and particularly to FIG. 1, it will be understood that the invention described herein utilizes a probe which can take the form as shown in either of FIGS. 1 or 2. The probe shown in Fig j 1 requires a Flussigkeitsver- ^'.: Binding with the workpiece as shown in Fig. 2

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Probe 3 is essentially a contact probe that directly against the workpiece surface, which is covered with a thin Film of a liquid bonding agent is covered.

The designated 1 probe in Fig. 1 comprises a housing 10 made of a suitable conductive material such as brass and of generally cylindrical form with a piezoelectric or optionally a magnetostrictive element 12, which also aligned generally cylindrical in shape and axially therein and is closed at one end by a closure part 14 and at the other end by a closure part 16. The parts 14 and 16 are preferably made of dielectric material such as lucite or the like. Electrical connections are attached to each end of element 12, such as indicated at 18 and 20, and are suitably connected for external connection through element 22 and housing 10. The workpiece to be examined, as shown in Fig. 1, may consist of a pair of metallic outer plates 2 and 4 which carry between them a lightweight core, such as honeycomb material 6, glued or otherwise secured between them . A positive connection is provided for the probe 1 and is denoted by 24.

In Fig. 2, the probe 3 can consist of a housing 28, which has one or more elements JO, which the magneto-

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strict part 12 according to Pig. 1, and the housing includes said elements by suitable terminations 32, which may consist of a dielectric such as lucite, epoxy or the like, at one end, as shown, while the other end 34 forms part of the housing 28 one piece with it. Appropriate electrical connections at 36 and 38 are connected to external means 40 for applying or receiving electrical signals from element JO. The workpiece 42 shown in FIG. 2 can correspond in type and construction to the workpiece as shown in FIG. 1 and as described above. The support surface 44 on the closure part 32 of the probe 3 can be on the upper surface of the workpiece k2. are present, as shown in FIG. 2, without the need for a liquid connection, as has been described in connection with the probe 1.

Fig. 3 shows typical inspection results in use of the new device when checking a workpiece 3C of relatively thin-walled construction with a large number of interconnected components, this connection being made with the aid of adhesive or mechanically connected connection points or through intermolecular exchange. The workpiece 50 comprises a pair of substantially parallel, spaced apart outer panels 52 and 54, one of which

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each with their inner opposite surfaces to intermediate layers 56 and 58, for example by soldering or the like. Are attached. Plates or layers 56 and 58 are fixed with their inner opposite tops to a core of relatively low density, which can consist of honeycomb material 60, for example. As shown in FIG. 3, the workpiece 50 has several structural defects of a type common to compound or complex workpieces of thin-walled construction. The defects shown in FIG. 3 can be characterized as loosened or disconnected locations, that is, areas between aligned surfaces or between elements of different density or bulk shape, such as the core and interlayers 56 and 58. The loosened or disconnected locations 62 and 66 represent areas between the aligned surfaces of the outer and intermediate layers 52-58 as described above, while the loosened or disconnected locations 64 and 68 represent areas of separation between the core material 60 and the intermediate layers 56 and 58 on either side thereof . Dissolved or unconnected bodies of both types can result from the fact that from the beginning, the components of the workpiece not sure ver together tied were as they should be attached to each other during manufacture of the workpiece, or they may consist of errors caused by temperature or

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Load stresses have arisen that occurred in the use of a part that was originally had been made flawlessly.

When checking the workpiece 50, as shown in Fig. J5, according to the principles of the invention as set out herein, it can initially be assumed that the workpiece for inspection purposes only on the outer surface 70 the outer plate 52 is accessible and that either the Probe 1 or Probe 3 can be used to perform the verification, choosing the probe from the size, the local position, the material properties and the mass distribution of the elements depends, which together form the workpiece. In any case, the readings are those obtained using the device described here are shown illustratively for each point in the workpiece by the display instruments 72-8Ο. The readings or advertisements shown here are only an example and do not represent any particular or precise values for each individual case. This is how the display instrument shows 72, which may consist of a milliammeter or other suitable device to detect changes in an electrical signal such as an oscilloscope to distinguish a portion of a workpiece 50 ^ the free of errors, and this indication would result from setting the probe at a point that is perpendicular

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sifa · 'υ ; ;

is aligned under the display instrument 72. The movement of the probe aligned vertically at a point below the indicating instrument 74 in Fig. j5 would be the detached or disconnected point 62 by a substantial change in the reading display, illustrative is shown as a maximum in Fig. J5 and from which it could be seen that the thus indicated detached or unconnected

~ Location is close to surface 70. In a similar way and way, the movement of the probe along surface 70 ^ cI would be in alignment with the indicating instruments 76/78 and 80 relatively minor changes the readout display as shown by representative values in each case, thereby making the identification and Location of the loosened points 64, 66 and 68 is made possible. For reasons that are more clear from the one given below As will become apparent from the description, the test results shown as an example in Fig. J represent a capability of the

The device according to the invention described herein is capable of determining changes in the acoustic impedance and not necessarily absolute values of the impedance. It will also be apparent to those skilled in the art that other information displays may be used, such as Lissajous patterns on a cathode ray tube, digital displays, etc., in lieu of the meter readings described with reference to FIG. 3>.

In Fig. 4 it is shown that the work designated by 82

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is comprised of a plurality of separate members 84 interconnected by any suitable means to form a composite structure or layer structure having a top surface 86 and a bottom surface 88 with undesirable defects such as detached or unconnected Positions 90, 92 and 94 as shown in the drawing. Using probes 1 or 3 with the fluid communication system as discussed above for each of the probes, representative readings, intended only to show the general relationship between indications of error-free construction and the various error ranges, are illustrated in FIG. 4 by the gauge readings indicated by 96, 98, 100 and 102 are designated. As indicated above in connection with FIG. J, a Lissajous reading can be obtained in place of the gauge readings of FIG combine one ad. The gauge reading does not provide a means of knowing which parameter has changed, or how much each parameter may change in the resulting signal as the probe passes along surface 86, for example, but it does provide a very satisfactory

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Providing, simple and cheap method of obtaining inspection information in most cases.

The means by which probes 1 or 3 provide the final data discussed in connection with Figures 5 and 4 are discussed below. It is the function of the probes and associated circuit components to provide means for determining the specific acoustic impedance of the workpiece, as illustratively shown in Figures 1 through h , by using a standing wave capable of acoustically sounding the workpiece to penetrate through its entire thickness between its outer surfaces. The standing wave pressure at any given point can be expressed mathematically as the phasor sum of the transmitted and reflected wave pressures traveling in opposite directions in the composite component on a single path. The pressure for a load wave propagating through a damping medium can be represented as follows:

sin w (t - - £ -)

or by converting it into a complex setting notation (phasor notation): (l)

P = Pe ^Z e ° ^KZ c '
χ ο

P.
χ ο

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The general expression for the standing wave pressure:

c '+ Pe ο r

where P represents the transmitted wave component and P represents the reflected wave component. The other symbols are defined as follows:

P = acoustic wave pressure at point x, Dyn per cm χ = a point in an acoustic wave field P = acoustic wave pressure at reference point o, - ^vv *

p Dyn each. cm

e = natural (Naperian) logarithmic base =

2.71828 ...

Q. - Coefficient of acoustic attenuation, db per cm

j = phasor operator

U) = angular velocity, radians per second = »21Γ f

f = acoustic wave frequency in Hertz

t »time in seconds

c = acoustic speed cm per second

2 ρ »acoustic wave pressure, dynes per cm

2 P. »transmitted acoustic wave pressure, dynes per cm

2 P = reflected acoustic wave pressure, dynes per cm

u = »particle speed in the acoustic field cm per second

Po _β static density of the medium, grams per cm- ⁵

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σ = denotes partial differentiation

ρ Z - specific acoustic impedance, grams per cm - sec.

0 = reactive part of Z "

= Reflectivity constant (pressure ratio) β = change in phase angle, radians

λ = acoustic wavelength cm d = distance in cm

2 Z = characteristic impedance, grams per cm - second.

In order to minimize the acoustic attenuation problem that can arise in the assembled workpieces, a low frequency standing wave is preferred and in most cases essential. The use of low frequency in the foregoing context essentially ^{eliminates the attenuation term, e "ctx} throughout the derivative. This close approximation reduces equation (2) to the following specialized form:

χ
P - ^p t ^{e ω} " ^{C + P} r ^ec ^)

The particle velocity at any given point in the standing wave is proportional to the pressure and can be expressed by the ratio
1

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be achieved. .

If this is applied to equation (5) then • surrendered

U = -

U = 5

+ f

' O

The complex relationship of sound to particle speed is called the specific acoustic impedance, Z an defined at any given point in the acoustic transmission line. The specific acoustic impedance in an acoustic transmission line or in a circuit is analogous to the electrical impedance in one electrical circuit and is expressed mathematically as follows:

^Z s =

J (UX

j U) X

V - C ^{+ P} r ^{e c} X -JuJ x ι3 ^ω . C. - ^P r ^e C.

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^Z s "Po °

The P _o c part of equation (6) contains the resistive or real part of the impedance, while 0 represents the reactive or imaginary part of it. Since 0 contains acoustical information about the mutual interaction of the transmitted and reflected components of the standing wave, it would be the impedance factor that is influenced by changes in the voltage that lie somewhere in the entire depth of the workpiece. Change changes, which indicate defects in the workpiece, cause phase and amplitude changes in the reflected wave components. It is necessary to take into account, and distinguish between, relative phases and amplitude changes that occur when a wave is reflected back from a termination point or an end point. The amplitude can be expressed as P _r -2 «. and the

Phase shift can be expressed as -e. The resulting general expression for 0 becomes

_Λ + jß + jkd -Ä _o - j / 9 - jkd

0 =! _! If (7)

h jkd _ -α - jß -

where d is the distance between the end and the point of impedance measurement and k - ^ - - ^ -J-. The mathematical one

J.

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Checking equation (7) shows that this is exponential Fraction defines a hyperbolic tangent in the following way:

0 = tanh

* o ♦ y (/

(8th)

The general expression for the specific acoustic impedance along the transmission line of a standing wave is:

^Z s =

^fcanh

(9)

Equation (8) defines the impedance behavior for a flat sine wave propagating through a medium exhibiting negligible loss of attenuation. In connection with the device according to the invention described here, it is assumed that the damping losses in the assembled workpieces of thin-walled construction or layer structures of thin elements, such as in Pig. 3 and 4 are relatively small at lower frequencies and this assumption is proven by the successful results obtained with such devices. The specific acoustic impedance, which brings about an interaction between sound pressure and particle speed , is essentially a measure of mechanical energy scattered per cycle.

It is a significant feature of the inventive concept described

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Idea that a small diameter piezoelectric transducer in combination with the electrical circuit means as described herein is capable of measuring the specific acoustic impedance. The workpiece ground impedance is sensed as a converter charge impedance. By means of electroacoustic conversion, the acoustic stress effects are measured in terms of electrical impedance. The theory discussed above permits testing of the entire workpiece mass from just one surface by changing the electrical impedance resulting from material defects of the type mentioned above. The main requirement in the practical implementation of the method based on the above theory is that the frequency of the initial signal must be sufficiently low to bring the attenuation to a minimum and thus lead to a standing wave that travels between the outer surfaces or across the various Boundaries of the workpiece is formed that is to be checked. This minimal attenuation effect allows the reactive or imaginary part of the impedance to be expressed as:

tanh

iß + 2jT_d_)

(10)

takes effect. As a result, any end change, such as that produced by loosened or disconnected points in the context described above, is detectable anywhere within the structure, that of the standing wave

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is penetrated. Equation (8) shows that Z is periodic as a puncture from either the end distance d or the acoustic wavelength fluctuates. The acoustic impedance cycles through the same Set of values for each half-length change in the end distance (neglecting attenuation), whereby causing the same impedance value to occur for more than one end distance within a workpiece. This undesirable condition can, however, be switched off by selecting a frequency in such a way that the entanglement the impedance value does not take place or does not occur in an inspection level. The periodic nature of the acoustic impedance can then act as a clear benefit for indicating the relative distance between the test surface, such as surface 70 in Figure j5 and the defect such as the loosened or disconnected spot 68 in the same figure. The susceptibility to errors can be achieved by making minor adjustments in frequency or phase and using certain test reference standards increase.

When a workpiece of the type described above, such as an adhesive bonded honeycomb material , is impedance tested at high frequencies, adequate energy transfer is severely hampered. This prohibits a satisfactory standing wave formation

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the entire workpiece therethrough, with the result that the test is only information that arise on the joint interface in the vicinity, said near side is the only surface that is accessible for inspection purposes, such as by the surface 70 in FIG. J > illustrated. When applied to highly dampening layer structures such as agglomerated cork, fiberglass or rubber sheets, the acoustic energy may never and normally does not propagate through the workpiece to the joint interface. If the wave is not reflected from the bonded interface, no indication of the presence or absence of a proper bond is obtainable. If this condition is expressed mathematically as equation (6) and it is found that P _r = 0, then it results

- 0

z _s = r _o c, 0 = 1 (12)

This means that there is a free wave state (ie without boundary conditions) and that the sound energy is only radiated in a pure resistance. The reactive or imaginary component of 0 is absent.

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This special case of Z ^ is called the characteristic Impedance defined for a material, expressed mathematically as:

^Z c = P o ^c

In summary, the theoretical procedure as derived above, and which forms the basis for a successful implementation of the method as described herein, comprises measuring the specific acoustic impedance over the entire thickness of the workpiece and not the characteristic impedance. Defects, such as loosened or disconnected points, as shown for example in FIGS. 3 and 4, which are locally detected somewhere through the entire thickness of the workpiece, act as acoustic end points and thus change the impedance value. The use of lower frequencies for the initial signal in the device as described herein is a key factor in achieving the necessary impedance response, including the novel result of the invention in this case. A piezoelectric probe, such as probe J5, coupled to suitable electrical circuitry as suggested in Figures 5 through 9 is used to generate the required acoustic standing wave and, at the same time, to monitor the resulting impedance. As explained above, the inspection result obtained in this way only includes the ability to detect acoustic irapedance changes or modifications.

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len and not necessarily absolute values.

With reference to FIGS. 5 to 9 it is shown that several different electronic circuits can be used in combination with the probes 1 or 3 in order to carry out the method according to the invention described here in practice. For example, FIG. 6 shows that the probe is connected to a bridge circuit 110 which may take any of several forms well known in the art, one of which is shown in FIG. When the element ^ O in the probe 3 is a magnetostrictive element, the bridge circuit 110 is of the usual resistance-inductance type, and when the element j50 is a piezoelectric element, the bridge circuit 110 is of the usual resistance-capacitor type, as in Fig. 9 shown. In the circuit as shown by FIG Amplitude for
To create excitation of the probe. The signal from oscillator 114 is fed to the probe through bridge circuit 110 which also receives a response from the probe and functions to provide a comparison of the probe input and output signals and the resulting comparison

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The resultant to be fed to the amplifier 116. The amplifier 116 can take any of numerous forms well known in the art and operates merely to create a signal of sufficient strength to allow signal changes by appropriate Data display devices can be observed, such as the milliammeter 118 or the like.

Referring to Fig. 5, it is shown that the oscillator 114 is connected to the element 30 of a probe which has an additional crystal or a magnetostrictive element 134, which is structurally defined or otherwise Manner is connected to the element 30, so that both parts 30 and 134 vibrate together. This is how the Output from element 134 through the amplifier circuit 136 is passed into the signal source 114 to the input signal to element 30 as determined by suitable means such as the II8 milliammeter will. Changes in current indicated by the II8 milliammeter can be used to record changes in determine the stress on the crystal 30, which results from loosened or disconnected points in one result in a work-related workpiece. Depending on the crystal features and the acoustic properties of the workpiece together with operational values of the electrical ones Components that are in the circuit of FIG

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are used, the arrangement shown in this way can suffice, to provide useful indications of the presence or absence of workpiece defects from the gauge 118 without the use of a bridge circuit.

The diagram shown in Fig. 7 represents a further illustrative embodiment of a device for carrying out the method according to the invention using an fm signal generator adapted to pass an appropriate low frequency signal to the probe J> through the self-balancing bridge switching network 122, the output of which is passed to a detector and reader 124, which can take any of several suitable forms well known to those skilled in the art.

FIG. 8 shows the probe and oscillator arrangement as represented by FIG. 5 and denoted by 126 is connected to a variable filter 128 and amplifier I30 to the reading device 132 which consists of a measuring instrument or a cathode ray tube, all as described in detail above.

Referring to FIG. 9, one embodiment may include a Bridge circuit 110, which is suitable, for example, for use in the circuit of FIG. 6, the one shown

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Take shape. This circuit is described using the following symbols:

W ₁ is the transformer winding of terminals 5-6 of transformer T ₁ .

W ₂ is the transformer winding between terminals 7 and 8 of transformer T ₁ .

Ep is the bridge excitation voltage in the Wp windings.

E ₁ is the bridge excitation voltage in the W, windings ".

Z ₁ is the impedance, consisting of Rw + fL · + R, _ in parallel with the impedance of the capacitors Cp + S ₂ . + S ^, where the switch S, is in position B. The resistance and capacitance provided by this impedance are both variable so the impedance of Zp can be adjusted to balance the bridge and bring the voltage at Rg to zero. Should probe J5 become conductive, switch S- at A is set in such a way that the impedance Sh and Sj- are conducted to the probe in order to make a bridge compensation possible.

Z ₂ is the impedance, which consists of the resistors R ₁ + R _{2 in} parallel with the probe 3, the switch S, being in position B. Should the probe 3 become inductive, the switch S _{5 is set} to position A,

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whereby the impedances S ₂ , and S then appear in parallel at Z ₂ and thus to a new impedance Z ' _? will.

Z _QA is the impedance of the bridge in end terminals 0 and A, with resistor Rg removed from the circuit.

E _R is the signal voltage developed in resistor Rg.

Rg is the bridge load over which the bridge unbalance voltage can be measured.

Su is a decade of 100 millimicrofarad capacitors.

S _n - is a decade of 1000 millimicrofarad capacitors. D.

Sp is a switch that is used to change the values of the load resistive elements at Z, and Zp. The resistances

R ₁ and Rr- are removed from this switch. 15th

t R ₁ and Rr- Ψ 15

Z is the change in the impedance of the bridge leg Z ₂ as a result of the probe disturbance, which is caused by errors in the tested material.

With reference to the symbols above, it should be understood that the open circuit condition is represented as follows can:

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^E 0A

Open circuit

ζ ₊ λζ

^2Z 1 ^Z l + ^Z 2 The following relationships can be derived from this:

E = E R _r j AZ . R r ^ j 2AZR R OA R + Z _0A Z ₁ + Z ₂ R + Z ₁ (Z ₁ + Z ₂ ) (2R

(16)

2AZR Λ ZR

(2R + Z ₂ ) Z ₂ (2R

That through Pig. 9 and described above can be described as comprising a transducer 5 connected to one leg of the bridge, while a reactive load, represented by C ₂ , S ^ and S ^ , is connected to the other leg of the bridge connected is. The reactive load is balanced by manual adjustment in order to achieve a balanced bridge condition, the probe being in working relationship with a fault-free part of the workpiece during operation. Thereafter, imbalance signals are obtained from line I58 when the acoustic load on element 30 in probe j5 is changed, such as by a loosened or disconnected spot.

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Having regard to the overall working relationship between the parts described above, it is important to point out that a specific acoustic impedance for the entire cross-sectional thickness of the workpiece through the inventive thought is delivered. It means that a sufficiently low frequency must be used to to switch off essentially the entire damping and thus a standing wave of significant amplitude in the entire cross-sectional thickness of the workpiece or the part of it that is to be checked. Relatively high frequency signals, as they are commonly used to create a characteristic Determining acoustic impedance cannot create a standing wave of the kind used to run the inventive method as described herein is necessary. In addition, a successful work participates the method according to the invention as described herein, a careful selection of the transducer is necessary in order to achieve the to achieve the necessary performance characteristics in order to achieve a suitable standing wave pattern. So press some crystals or piezoelectric elements, when an electric current is supplied to them, collapse perpendicularly, some warping sideways, others change their dimensions in other symmetrical or non-symmetrical ways, depending from the initial shape, construction, material, or assembly that is used to make the element in a probe to keep. It is clear that the converter so chosen is around

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generate a standing wave for the practical implementation of the new method according to the invention and the energy signal characteristics, which are used in connection with it, are chosen in such a way that errors of the type sought are specific significantly change acoustic impedance, which is reflected in the loading of such a transducer.

The operation of the device can be described as follows:

While the method and structure as described above are used to check many different workpiece shapes' and can be used for various purposes, its operation in relation to workpieces is influenced by the Type of lightweight plates described, as shown in Figs. 1 and 2 for illustration. The review necessarily begins with the calibration of the system, which may take any of the forms shown in Figs. 6,7 or 8 and illustratively described in connection with the shape shown in FIG. the required calibration begins with making a suitable connection between the probe and the Werkswüuk, such as by immersing both in a liquid in the case of probe 1 or by bringing the probe directly into contact with the workpiece, with a slight liquid compound film is coated, such as glycerol, in the case of probe J. In any case, an initial

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Reading from the system on an area of the connector which is substantially free of defects or otherwise structurally acceptable. After that will be Defective areas of the same workpiece identified by changes in the reading results that are greatly affected by reject the initial calibration display. As stated above, the system described herein is not in fc primarily intended to measure absolute fourth mechanical impedance, but only to provide relative readings, whose evaluation on the part of the operator requires a certain amount of experience. Converter frequencies in the range from 2.5 to 80 Kdlohertz have been found to be useful in inspecting workpieces of the type described above Proven type, including soldered connections between the outer panels, core cross-sections and intermediate layers. Since the test results to a large extent depend on the fact that only one significant factor is changed for each reading while all of the others Factors are kept constant, it is significant that the same transducer and the operating factors, including electrical signal continuity and transmission efficiency of the probe to the workpiece connection during the total relative movement of the transducer over the workpiece surface is maintained.

While the individual components of the overall system like it

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in Pig. 5 to 9 of this case is able to are known in the art and may take many different forms in the practice of the invention, it is the exact combination of such components in the proportions shown above and claimed below, on which the achievement of the objects of the invention depends. Accordingly, the structural combination and that Methods described herein are intended as an illustration only and could be changed and modified to to achieve the same objects without thereby departing from the scope of the invention as in the following Claims is drawn.

209815/0222

BAD ORIGINAL, _

Claims (1)

Patent claims: Apparatus for analyzing a workpiece to detect and locate structural interruptions therein, characterized in that it comprises probes for creating an acoustic standing wave pattern through a portion of said workpiece when excited by an electrical signal and said signal to change in response to changes in said pattern, an electrical source for energizing said probe by a signal of relatively low frequency and reading means for observing changes in said signal. 2. Apparatus according to claim 1, characterized in that the probe has a transducer to the acoustic Generate standing wave and send an output signal, if the activation takes place by an acoustic wave and a comparator device for comparing the input and output signals, the reading means observing the results of said comparison allowed. J5. Apparatus according to claim 2, characterized in that the transducer includes a magnetostrictive element .. ₅₆ _ 209815/0222 BATH ORIGINAL and the comparison means an impedance-inductance bridge. 4. Apparatus according to claim 2, characterized in that the transducer contains a piezoelectric element and the comparison device comprises an impedance-capacitance bridge. 5. Apparatus according to any one of claims 1 to 4, characterized in that said electrical Source has a variable sweep frequency oscillator. 6. A method for checking a workpiece which has an accessible surface part, characterized in that that a transducer is brought into working relationship with the surface part, a bridge circuit with the transducer on one leg and a reactive load on the other leg, after which said bridge is electrically excited to generate a standing wave from the transducer into the workpiece, after which the size of the Imbalance in the output of the bridge is observed, which is caused by the acoustic load on the said transducer will. 20381S / 0222