WO2006110089A1

WO2006110089A1 - Method and apparatus for assessing quality of rivets using ultrasound

Info

Publication number: WO2006110089A1
Application number: PCT/SE2006/000435
Authority: WO
Inventors: Tadeusz Stepinski
Original assignee: University Of Warwick
Priority date: 2005-04-14
Filing date: 2006-04-12
Publication date: 2006-10-19
Also published as: GB2425179A; GB0507574D0

Abstract

A method and apparatus are provided for non-destructive assessing quality of rivets using ultrasound. The rivet (10) is inspected by sending an ultrasonic continuous wave, at a predetermined frequency to the rivet (10) through a differential piezoelectric transducer (1) provided with a waveguide that is acoustically coupled to the rivet head. The rivet quality is evaluated by monitoring variations in electrical impedance or admittance of the said transducer (1) represented by both the phase and amplitude displayed in complex impedance plane and monitoring the difference between the impedance corresponding to the inspected rivet and the predetermined scatter of values corresponding to sound rivets. The method is preferably used for the inspection of self piercing rivets.

Description

METHOD AND APPARATUS FOR ASSESSING QUALITY OF RIVETS USING ULTRASOUND

TECHNICAL FIELD

The present invention relates in general to ultrasonic testing of rivets, and in particular to methods and apparatuses using narrow band resonance inspection.

BACKGROUND

Ultrasound has been widely applied for non-destructive inspection of material and structures for many years. In the most commonly used ultrasonic setup (pulse-echo mode) a piezoelectric transducer is excited by an electric pulse or a pulse train, and ultrasonic wave is transmitted into the inspected material with help of a special coupling agent. The ultrasonic wave is reflected from the material discontinuities (reflectors), such as, cracks, porosity or delamination, and the backscattered ultrasonic wave is received by the same or another transducer. The received echoes are assessed in the time domain; their arrival time is used to locate the discontinuity and its amplitude bears information on the reflector size. The detected discontinued can be resolved and characterised provided that the transducer has sufficient resolution in time and space, i.e. the pulse it generates is sufficiently short and its beam is sufficiently narrow. In many applications the use of high frequency focused transducers is required to achieve this goal. Inspection of spot welds is a typical example where sophisticated high frequency transducers have to be used to resolve echoes arriving from boundaries between thin sheets and interpretation of the acquired result requires special expertise (either human operator or a sophisticated self- learning algorithm) .

Disclosures of prior art apparatus and methods for spot joint inspection employ pulsed ultrasonic waves propagating into the joint either in pulse- echo or transmission mode. In the pulse-echo mode, according to the known prior art an ultrasonic transducer, which is either independent [1] or integrated into the joint producing means [2] sends, pulses to the joint and receives periodic echoes from the joint. The periodic pattern of the echoes is then evaluated to detect a partly connected spot. In the transmission mode, according to the known prior art, a transducer pair is employed where one of the transducers, used as a transmitter emits ultrasonic pulses that are received by the second transducer (receiver) placed on the other side of the joint. Joint quality is evaluated based on the amplitudes of the received pulses that have passed the joint or time of flight of the ultrasonic pulse. In the basic configuration, according to [3] a single pair of transducers emitting longitudinal waves is used. Other configurations are possible, as that proposed by [4] where both longitudinal and transversal waves are applied.

In some applications it is more feasible to analyse spectra of the ultrasonic signals acquired in a resonance test. Ultrasonic resonance spectroscopy (URS), which was invented by A. Migliori [5] utilizes information in the frequency domain obtained due to the constructive and destructive interference of elastic waves for non-destructive evaluation of inspected objects. In a resonance test an ultrasonic tone-burst with sweeping frequency is applied to an ultrasonic transducer and a resonance spectrum of the inspected structure is acquired. The acquired frequency spectrum contains primarily the information about material properties but it also may bear some information about the presence of flaws in the inspected structure. The transducers used for the URS test should be characterized by a wide linear frequency response and they should not affect resonance pattern of the measured object, which means that they should be weakly coupled to the inspected structure.

Generally, there are two types of resonance test, global and local. The global test provides synthetic information about the entire inspected part, and therefore, it can be applied to relatively small parts where vibrations can be excited in the whole inspected volume [6]. The local test is more suitable for large structures, for instance in, aerospace applications, where local structure condition is of interest and only a selected part of the structure is to be excited. Local ultrasonic resonance spectroscopy has been used mainly for the inspection of aerospace structures, for example for bond testing.

A fundamental limitation of resonance inspection (especially, in global test) is its sensitivity to factors that are unessential for the test, such as, variations in dimensions or material properties that may mask the effect of smaller defects. This problem can be solved either by employing sophisticated self-learning algorithms for spectrum classification or reducing the information volume by using relatively narrow signal frequency band and using simple signal features, such as, frequency shift of a single resonance peak.

In the narrowband resonance inspection (NBRI) of a structure its surface is scanned with a resonant transducer and the transducer frequency response is monitored in a narrowband. Since a piezoelectric transducer is an electromechanical device the parameters of its electrical resonance depend on its mechanical load, i.e., the acoustical impedance of the inspected material. Thus, in this kind of test the structure's spectrum is not measured directly but it influences the resonance parameters of a narrowband transducer. Defect detection can be performed by an operator observing simple features of the acquired spectra, such as, a shift in the transducer's resonance frequency.

The resonance frequency monitoring is applied in Bond Tester from Fokker, [7]. However, serious difficulties may be encountered when applying the frequency shift based NBRI to attenuating materials or spot joints. Resonance of a probe coupled to an attenuating structure is not only shifted in frequency but also spoiled in the sense that the resonance peak broadens up so much that the accurate detection of the resonance frequency may be difficult. Another way of extracting information about the inspected structure using a narrowband piezoelectric transducer is sensing variations of its electrical impedance caused by the varying conditions of the inspected structure. The electrical impedance for a predefined frequency in the vicinity of transducer's resonance is relatively easy to measure using some kind of vector voltmeter.

This idea has been introduced by R. Botsco [8] in application to bond testing. Operator's job is quite simple - it consists in observing a phasor corresponding to transducer's electrical impedance in complex impedance plane when scanning the inspected structure. The defect is detected if the phasor moves outside an area, typically a rectangle, assigned to a sound material during calibration.

SUMMARY

In all disclosures of prior art apparatus and methods for NBRI, important disadvantages remain. A first one is that of poor stability, typically due to the influence of ambient temperature on the piezoelectric transducer. Ambient temperature underlies large variations of resonance characteristics of a piezoelectric element, mainly its resonance frequency, which results also in the respective variations of its electric impedance. Another disadvantage is a strong dependence of the measurement result on the quality of acoustical coupling between the transducer and the inspected material.

Although some of the aforesaid disclosures of prior art instruments and methods describe the use of NBRI based on the impedance measurement, e.g. [8], none of them is concerned with a method or apparatus for assessing quality of rivets, possibly due to the above mentioned problems.

An object of the present invention is thus to provide improved methods and devices for non-destructive inspection of rivets. A further object of the present invention is to provide rivet ultrasonic testing devices and methods, offering an improved coupling behaviour to the investigated rivet. Yet another further object of the present invention is to provide rivet ultrasonic testing devices and methods, having improved temperature stability.

The above objects are achieved by devices and methods according to the enclosed patent claims, and in particular when used according to the use claims. The present invention is in general words a non-destructive method and apparatus that utilizes ultrasound for assessing rivets. The present invention employs a frequency domain ultrasonic method. The present invention furthermore utilizes a narrow band resonance inspection (NBRI) based on measuring complex valued electrical impedance of a piezoelectric transducer for assessing quality of an investigated rivet. A piezoelectric element of the transducer has a mechanical resonance frequency and is excited by an alternating voltage having a predetermined frequency in a vicinity of the mechanical resonance frequency. The active transducer element is provided with a buffer rod, acting as a waveguide between the piezoelectric element and the rivet to be investigated.

In preferred embodiments, two transducer elements are used in a differential configuration, preferably an electric bridge circuit, whereby one transducer element acts as a reference element.

An advantage of the present invention is that by moving the inspected structure into the transducer's far field, the disturbing effect of variations in acoustical coupling is substantially reduced. Furthermore, the differential configuration eliminates the influence of ambient temperature. Such a configuration thus enables achieving a substantially higher sensitivity than that obtained from a single element transducer. A proper choice of the test frequency and a phase sensitive detection furthermore enables suppressing effects of some irrelevant variables, such as, variations of the acoustical coupling or the shape of the inspected rivet. BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

Fig. 1 is an embodiment of an ultrasonic testing device according to the invention;

Fig. 2A is an embodiment of a piezoelectric transducer according to the invention; Fig. 2B is another embodiment of a piezoelectric transducer according to the invention, having a single piezoelectric element;

Fig. 2 C is yet another embodiment of a piezoelectric transducer according to the invention, having a spatially separated piezoelectric element pair;

Fig. 3 illustrates a principle of the narrow band resonance inspection; Fig. 4 illustrates the KLM model of a piezoelectric transducer in thickness mode;

Fig. 5 shows diagrams illustrating impedance of the transducer in the air and after coupling it to aluminium and steel, respectively;

Fig. 6 is a flow diagram of main steps of an embodiment of a method according to the present invention; and

Fig. 7 is an illustration of a measurement result display.

DETAILED DESCRIPTION

It is worth mentioning that probe characteristics are crucial for the NBRI test, particularly in the present invention, since the shift of its resonance frequency indicates conditions of the inspected structure. Therefore, highly resonant piezoelectric probes are preferred for this kind of test.

A basic test setup used for the NBRI is shown in Fig. 3. A piezoelectric transducer is arranged in contact with a tested piece 25 so that the ultrasonic wave generated by the transducer is transmitted into it. Electrical impedance of a piezoelectric transducer can be calculated using the KLM model [9] taking into account acoustical loads on both surfaces of its piezoelectric element. From Fig. 4 it is seen that the KLM model includes a transmission line for modelling the mechanical side of the transducer and a transformer converting the mechanical side to the electrical side. The transmission line of a thickness equal to thickness of the transducer's piezoelement t is loaded with an acoustical impedance of a test object Zm on one side while on the other side its load consists of a high loss material (backing) Zb. A transformer connected to the midpoint of the transmission line transforms the acoustical . impedance Z_a seen at this point to electrical impedance that is connected in series with the capacitor Co and the inductance X₁ to form the input impedance ZKLM- Thus, input impedance of the transducer is a series coupling of the transducer capacitance Co, the reactance X₁ and the impedance transformed from the secondary side:

where:

CD²Z_n

Z_1>r denotes acoustic impedances seen from the middle of piezoelectric element, respectively on the left and right hand side, Zm is acoustical impedance of the inspected material, Zb is acoustical impedance of backing and z₀ = pcA is acoustical impedance of the piezoelectric material used for the transducer. If the transducer consists of a dielectric material characterized by the material constants S33 and b.33, with thickness t and electrodes having surfaces A, its the resulting capacitance is c₀ = -^d and its resonance occurs

at frequency _{ω =} ^πc For frequencies close to _ωo the impedance transformed to the electrical side will be ₂ ~ ^Z° Thus, close to the resonance, impedance of such

" z_{i +}z_m transducer can be expressed as:

where z. +z = z, +Z +ΔZ = z_k +ΔZ denotes variations in the acoustical load of the transducer due to variations in the inspected material.

Assume that the transducer designed for material inspection is included into a linearised bridge together with another similar dummy transducer. The active transducer is loaded with the impedance of the tested material Z₁₁₁ = z_{m +} AZ₁₁₁ while the dummy by a reference piece with impedance z,,, . Then the unbalance signal measured at the bridge output will be:

"» ω² {Z_bm Z_{bm +} AZ_m ) ω² {z_bm(Z_bm + ΔZ_m))

where K is transfer function of the linearised bridge.

Assuming that AZ₁₁₁ « z_m we finally get an approximate expression for bridge unbalance in a narrow frequency band close to transducer's resonance:

υ^K^^f- (4)

⁰³ ^ hm

Eq. (4) illustrates principle of narrowband spectroscopy where a piezoelectric transducer excited with its resonance frequency is used for transforming an acoustic impedance of an inspected structure to the electric impedance that can be directly measured using a vector voltmeter. Although Eq. (4) is accurate only for the resonance frequency _ωil = — _> the principle presented

above is valid also for other frequencies. This is illustrated by Fig. 5 where the impedance plot of a IMHz transducer in air can be compared to that of the transducer loaded with aluminium and steel, respectively. The left diagram illustrates the real part of the impedance for three situations. The curve 100 corresponds to a transducer in contact with air, curve 101 a transducer in contact with steel and curve 102 a transducer in contact with aluminium. The right diagram illustrates the imaginary part of the impedance for three situations. The curve 103 corresponds to a transducer in contact with air, curve 104 a transducer in contact with steel and curve

105 a transducer in contact with aluminium. It should be noted that the impedance plots were simulated using KLM model for an idealized transducer made of PZT27. For a real transducer the impedance in the air would be much closer to that of the loaded transducer.

Fig. 5 also illustrates problems encountered when using only resonance frequency shift as a significant feature for inspection, which has been used in prior art. The precise resonance frequency of a loaded transducer is in general difficult to detect, especially, that in practical situations the impedance plot may be far from smooth. In contrary to pure resonance frequency measurements, the complex valued electrical impedance can be reliably measured for any frequency and since it is a complex valued variable it yields two parameters that can be used in the similar manner as in eddy current technique. By combining the totally available information from both the real and imaginary parts, more accurate information is possible to deduce.

From Fig. 5 it is also obvious that a high Q-value of the resonant transducer is beneficial as well as exciting the transducer in the vicinity of the resonance. A load variation causes different responses at different frequencies, and the responses are generally high closer to the resonance frequency. Fig. 1 shows an apparatus 20 according to one preferred embodiment when applied to non-destructive assessing quality of rivets. The apparatus 20 comprises an ultrasonic transducer 1 having a multitude of mechanical resonances. The ultrasonic transducer 1 is connected to a bridge circuit 12 in an electronic circuit 21 and furthermore in the present embodiment arranged in contact with a head of a rivet 10 to be inspected. In a typical case, the rivet 10 is a so-called self piercing rivet used as spot joint between metal sheets 23, 24 of a test object 25. The test object can typically be a part of a vehicle or an aircraft.

Ultrasonic waves generated by the transducer 1 are thus transmitted into the rivet 10. The bridge circuit 12 that is fed from a source of alternating voltage 11, which in the present embodiment comprises an ultrasonic frequency that excites mechanical vibrations in the transducer 1 in a vicinity of its mechanical resonance. The bridge circuit 12 produces an input signal for an impedance evaluation circuit 13 evaluating the transducer's complex valued electrical impedance at the excited frequency. The complex valued impedance, being a phasor in an impedance plane can be rotated in a coordinate rotation circuit 14. A result of rotation, being a phasor in impedance plane, is displayed at a display 15. The display 15 is provided with means for presenting a resulting complex valued result in a two- dimensional area enabling determining if a measured complex valued result falls within a predetermined sub-area of a whole measurement range. A signal corresponding to one component (vertical or horizontal) of a phasor presented in the display 15 is fed to a threshold detector 16. The detector 16 produces a logical output if the input signal exceeds a preset value. By rotation 14 and threshold detecting 16 the influence of undesired quantities, such as, acoustical coupling, that result in a different phase that that of a response to the desired quantities, such as, rivet quality, can be suppressed while the latter can be enhanced at the same time.

Fig. 7 illustrates such a display of results. The two axes represent the real and imaginary parts, respectively, of a complex impedance measure. By calibration or determined in any other way, it is known that measured values within a certain area 26 correspond to faults-free objects with a certain probability. In the illustrated case, the measurement 27 comes with a high probability from a fault-free rivet, while the measurement 28 corresponds to a defect one. The area 26 is in the present embodiment a rectangular area centred in the origin of the axes, since irrelevant information is compensated for before the displaying. However, any size and shape of the area 26 can be used, depending on the particularities of the measurement equipment and the objects to be investigated.

Fig. 2A shows an ultrasonic transducer probe 1 according to a preferred embodiment of the present invention. The ultrasonic transducer probe 1 comprises two piezoelectric elements 2 and 3 having electrodes 4a and 4b that enable applying an electrical field within the elements. The piezoelectric element 2 is at a first side 18 provided with a matching layer 5 facilitating transmission of ultrasonic waves into a buffer rod 6 at a first end 19, and in turn, to an inspected joint which it is arranged in contact with. The matching layer 5 has preferably a thickness essentially equal to one quarter of a wavelength of sound at the mechanical resonance frequency. The matching layer 5 is arranged for matching an acoustical impedance of the piezoelectric element 2 to an acoustical impedance of the buffer rod 6. Piezoelectric element 3 is a reference element and is in the present embodiment arranged in contact with a reference object 7, i.e. acoustically coupled thereto, at one side and with a damping material 8, which is common for both piezoelectric elements, at the other side. The damping material 8 typically provides high acoustical losses.

All the elements 2 - 8 are in the present embodiment arranged in a common house 9 which is open on one side to enable contact of a second end 22 of the buffer rod 6 with an inspected joint. Electrical contacts 17a, 17b are provided that enable connecting the electrodes 4a and 4b to the bridge circuit 12 (Fig. 1). The buffer rod 6 can be constituted in different ways. Typically, the rod material is selected to cause a low acoustical loss. In one embodiment, the rod is made of a solid material, e.g. plexiglas.

By having two piezoelectric elements (2, 3), preferably connected to a bridge circuit, one can be used as a reference element, which makes it possible to compensate, e.g. for temperature shifts. If the reference object 7 is selected carefully, e.g. an object having similar properties as a fault-free object to be inspected, any output signals from the bridge circuit will correspond to actual deviations from an ideal inspected object.

The method and device is also possible having only one piezoelectric element. Such an embodiment is illustrated in Fig. 2B. In such an embodiment, the evaluation has to be altered somewhat accordingly. In one embodiment, the bridge circuit is kept and instead of a reference piezoelectric element, an electric circuit having corresponding electrical behaviour is used as a reference. The evaluation means then has to consider deviations in the behaviour between the electric reference circuit and the piezoelectric element. In another embodiment, the complex valued impedance of the single piezoelectric element is measured directly, and any compensation has to be performed in the evaluation means.

When using a reference piezoelectric element, the configurations may also be different in different embodiments. Fig. 2C illustrates an ultrasonic transducer probe 1 having two piezoelectric elements 2, 3. However, in this embodiment, each element 2 and 3 has its own house 9 and backing 8 and thus constitute separate probe portions. The piezoelectric elements 2, 3 are, however, still connected to the bridge circuit, as in Fig. 2A. Preferably, the two portions are exposed for similar conditions, such as temperature. By having two separate portions, it is possible to position the reference portion against an identical object to the investigated one, which is known to have no faults. Fig. 6 shows a flow diagram of main steps of an embodiment of a method according to the present invention. The procedure starts in step 200. In step 210, an ultrasonic vibration is excited in a first piezoelectric element. The piezoelectric element has a mechanical resonance frequency in the vicinity of the frequency of the excited vibration. In step 212, the vibration is transferred from the first piezoelectric element to an object to be tested, and in the substep 213 this transfer is performed by guiding the vibration through a buffer rod. In step 214, a quantity representing a complex valued electrical impedance of the piezoelectric element is measured. Finally, in step 216, the complex valued electrical impedance is evaluated. The procedure ends in step 299.

In conclusion, a method and apparatus are provided for non-destructive assessing quality of rivets using ultrasound. The rivet is inspected by sending an ultrasonic continuous wave, at a predetermined frequency to the rivet through a differential piezoelectric transducer provided with a waveguide that is acoustically coupled to the rivet head. The rivet quality is evaluated by monitoring variations in electrical impedance or admittance of the said transducer represented by both the phase and amplitude displayed in complex impedance plane and monitoring the difference between the impedance corresponding to the inspected rivet and the predetermined scatter of values corresponding to sound rivets. A simple threshold detection circuit is also provided for phasors with a preset phase. The method is preferably used for the inspection of self piercing rivets.

The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims. REFERENCES

[1] US 4,208,917. [2] US 6,297,467. [3] US 4,099,045.

[4] US 5,920,014. [5] US 5,062,296. [6] A. Migliori and J. L. Sarrao. "Resonant Ultrasound Spectroscopy", John

Wiley & Sons, Inc., 1997. [7] US 5,303,590.

[8] US 4,215,583.

[9] R. Krimholtz, D.A. Leedom and G. L. Matthaei, "New Equivalent Circuits for Elementary Piezoelectric Transducers", Electronic Letters, vol. 6, pp. 398-399, 1970.

Claims

1. Ultrasonic rivet testing device (20), having:

^■ultrasonic transducer probe (1) having a first piezoelectric element (2); a said ultrasonic transducer probe (1) having electrodes (4a, 4b) arranged for enabling exciting of said first piezoelectric element (2); means (11, 17a, 17b) for providing a continuous alternating voltage of a predetermined frequency to said electrodes (4a, 4b); and electronic circuit (21) connected to said ultrasonic transducer probe (1); said electronic circuit (21) being arranged for measuring a quantity representing a complex valued electrical impedance of said first piezoelectric element (2) for association with a rivet quality, characterised in that said first piezoelectric element (2) has a mechanical resonance frequency; said predetermined frequency being in a vicinity of said mechanical resonance frequency; said ultrasonic transducer probe (1) further comprises a buffer rod (6) conducting ultrasonic waves; a first end (19) of said buffer rod (6) being joined with a first side (18) of said first piezoelectric element (2); and a second end (22) of said buffer rod (6) being arranged for enabling acoustic contact with a rivet (10) to be tested.

2. Ultrasonic rivet testing device according to claim 1, characterised in that said buffer rod (6) being joined with said first side (18) of said first piezoelectric element (2) by a matching layer (5); said matching layer (5) being arranged for matching an acoustical impedance of said first piezoelectric element (2) to an acoustical impedance of said buffer rod (6).

3. Ultrasonic rivet testing device according to claim 2, characterised in that said first piezoelectric element (2) has a plate or disc shape; said matching layer (5) having a thickness essentially equal to one quarter of a wave-length of sound at said mechanical resonance frequency; and a second side of said first piezoelectric element (2) being joined with a material (8) having high acoustical losses.

4. Ultrasonic rivet testing device according to any of the claims 1 to 3, characterised in that said buffer rod (6) comprises a solid material of low acoustical loss.

5. Ultrasonic rivet testing device according to any of the claims 1 to 4, characterised in that said ultrasonic transducer probe (1) further comprises a second piezoelectric element (3), having a resonance at said mechanical resonance frequency, whereby said electronic circuit (21) being arranged for measuring a quantity representing a vector difference of complex valued electrical impedances of said first (2) and second (3) piezoelectric elements.

6. Ultrasonic rivet testing device according to claim 5, characterised in that said piezoelectric elements (2, 3) of said ultrasonic transducer probe (1) are comprised in a common house (9).

7. Ultrasonic rivet testing device according to claim 5 or 6, characterised in that said electronic circuit (21) comprises a bridge circuit (12) producing a vector difference of complex valued electrical impedances of said first (2) and second (3) piezoelectric elements.

8. Ultrasonic rivet testing device according to claim 5, 6 or 7, characterised in that said second piezoelectric element (3) is acoustically coupled to a reference object (7).

9. Ultrasonic rivet testing device according to any of the claims 1 to 8, characterised in that said electronic circuit (21) comprises evaluation means (13) for evaluation of electrical impedance of said ultrasonic transducer (1) at said predetermined frequency.

10. Ultrasonic rivet testing device according to claim 9, characterised in that said electric circuit (21) comprises a coordinate rotation means (14) enabling rotation of phasors obtained from said evaluation means (13).

11 Ultrasonic rivet testing device according to claim 10, characterised by threshold detecting means (16) comprising means for determining if a component of said rotated phasor exceeds a preset level.

12. Ultrasonic rivet testing device according to any of the claims 1 to 11, characterised by a display (15) connected to said electronic circuit (21), said display (15) being arranged for presenting complex valued electrical impedances or quantities derived therefrom in a two-dimensional area.

13. Ultrasonic rivet testing device according to claim 12, characterised by evaluation means (13) comprising means for determining if said measured complex valued electrical impedance or a quantity derived therefrom falls within a predetermined subarea (26) of said two-dimensional area.

14. Ultrasonic rivet testing device according to any of the claims 1 to 13, characterised in that said means for providing a continuous alternating voltage comprises a sine tone generator (11).

15. Method for ultrasonic rivet testing, comprising the steps of: exciting (210) an ultrasonic vibration in a first piezoelectric element (2); said ultrasonic vibration having a predetermined frequency; transferring (212) said ultrasonic vibration to a rivet (10) to be tested; measuring (214) a quantity representing a complex valued electrical impedance of said first piezoelectric element (2); and evaluating (216) said complex valued electrical impedance for association with a rivet quality, characterised in that said predetermined frequency being in a vicinity of a mechanical resonance frequency of said first piezoelectric element (2); said step of transferring (212) comprises guiding (213) said ultrasonic vibration through a buffer rod (6) between said first piezoelectric element (2) and said rivet (10) to be tested.

16. Method for ultrasonic rivet testing according to claim 15, characterised by the further step of: exciting an ultrasonic vibration in a second piezoelectric element (3); said ultrasonic vibration having said predetermined frequency; said second piezoelectric element having a mechanical resonance at said mechanical resonance frequency; said step of measuring (214) being arranged for measuring a difference of complex valued electrical impedances of said first and second piezoelectric elements (2, 3), respectively.

17. Method for ultrasonic rivet testing according to claim 16, characterised by the further step of: transferring said ultrasonic vibration of said second piezoelectric element (3) to a reference object (7).

18. Method for ultrasonic rivet testing according to any of the claims 15 to

17, characterised by the further step of rotating phasors obtained from the evaluating step.

19 Method for ultrasonic rivet testing according to claim 18, characterised by the further step of: determining if a component of said rotated phasor exceeds a preset level.

20. Method for ultrasonic rivet testing according to any of the claims 15 to 19, characterised by the further step of: displaying complex valued electrical impedances or quantities derived therefrom in a two-dimensional area.

21. Method for ultrasonic rivet testing according to claim 20, characterised by the further step of: indicating if a measured complex valued electrical impedance or a quantity derived therefrom falls within a predetermined sub-area (26) of said two-dimensional area.

22. Method for ultrasonic rivet testing according to any of the claims 15 to 21, characterised in that said rivet to be tested is a spot joint between metal sheets.

23. Method for ultrasonic rivet testing according to claim 22, characterised in that said rivet is a self piercing rivet.

24. Use of a method for ultrasonic rivet testing according to any of the claims 15 to 23 for testing a rivet connecting metal sheets (23, 24).

25. Use according to claim 24, characterised in that said metal sheets (23, 24) are parts of a vehicle.

26. Use according to claim 24 or 25, wherein said metal sheets (23, 24) are parts of an aircraft.

27. Method substantially as hereinbefore described with reference to the accompanying drawings.

28. Ultrasonic testing device substantially as hereinbefore described with reference to and as shown in the accompanying drawings.