GB2204690A

GB2204690A - A method of and apparatus for testing bond strengths

Info

Publication number: GB2204690A
Application number: GB08711166A
Authority: GB
Inventors: Geoffrey Richard Marr; Alfred Raymond Curtis; Graham Jackson
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1987-05-12
Filing date: 1987-05-12
Publication date: 1988-11-16
Anticipated expiration: 2007-05-12
Also published as: GB2204690B; GB8711166D0

Abstract

An ultrasonic transducer probe 11 is used to transmit sharp pulses of large bandwidth (1-30 MHz) through a single overlap bond between two adherends 12, 13. The signals reflected from the two interfaces between the adherends and the adhesive 14 are gated, digitised and passed to a computer 36. The computer determines the respective amplitudes of the first and second peaks of the signals corresponding to the reflections from the first and second interfaces, works out the ratio of the peaks in dB, and assigns a bond strength value to that dB value from a previously compiled look-up table. The transducer may be caused to scan the bond in a raster pattern corresponding to the raster of a TV monitor so as to produce a colour-coded bond strength map 44. <IMAGE>

Description

A METHOD OF AND APPARATUS FOR TESTING BOND STRENGTHS This invention concerns a method of and apparatus for testing bond strengths.

The non-destructive testing and evaluation of bonded joints is of increasing importance. Many materials currently used in the construction of e.g. aircraft and motor vehicles, such as carbon-fibre reinforced plastic composites, are seared together by means of adhesive bonded joints, e. single overlap joints.

The magnitude of the thickness of the adhesive is often very small, e.g. 0.1 mm. Defects in the bonds, such as voids, are known to lower the strength of the bond.

One known method of testing bonds involves ultrasonic techniques to analyse pulses transmitted through or reflected from the test specimen. The magnitudes of the signals reflected, respectively, from the front face of the specimen, from the interface between the rear face of the front component of the specimen and the adhesive, from the interface between the adhesive and the front face of the second component, and from the rear face of the second component vary as the reflection coefficients vary. The transmission of ultrasonic energy is reduced when the bond between the adhesive and the adherent is poor, resulting in an increase in the reflection coefficient and thus in a change of signal magnitude. But this known method is capable of providing qualitative bond strength indications only.

The present invention seeks to solve the problem of providing quantitative bond strength testing.

According to one aspect of this invention, there is provided a method of testing the strength of a bonded joint, comprising passing signals from a transmitter through said joint, measuring the amplitude of the signals reflected from each interface between the jointing material and the materials to be jointed, determining the peak amplitudes of said signals, computing the ratio of said peak amplitudes, and determining the strength of the bonded joint by comparing the value of said ratio with previously determined and stored values correlated to bond strength values.

According to another aspect of this invention, there is provided apparatus for testing the strength of a bonded joint, comprising a probe for transmitting signals through said joint and for receiving signals reflected from each interface between the jointing material and the materials to be jointed, means for determining the ratio of the peak amplitudes of the said signals, computing means for computing the ratio of said peak amplitudes, and comparing means for comparing the computed ratio with previously determined and stored values correlated to bond strength values.

In a preferred embodiment, the signals are sharp ultrasonic pulse signals of large bandwidth produced by a transducer.

The reflected signals are preferably rectified to produce a video signal and gated. The gated signals may be digitised and stored as data for analysis by a computer.

The transducer may be arranged to scan the specimen by an X Y digital drive means in a raster-like scan pattern, the values of bond strength may be determined at regular intervals along the scan and these values may then be used to produce a colour-coded map of bond strength on a TV monitor, utilising data from the computer.

The arrangement of gating the video signals preferably includes a first gate to filter out all reflected signals from faces or interfaces other than those from the joint under test; and a second gate which, advantageously, slows down the reflected signals to eliminate pulse to pulse variations at a particular point, for integration prior to digitisation.

The second gate 'open' time is controlled by comparing a sawtooth ramp voltage with a slowly varying reference voltage, the periods of these voltages differing by e.g. three orders of magnitude.

The invention is described, purely by way of example, with reference to the accompanying schematic drawings, wherein: Figure 1 is a block circuit diagram of apparatus according to the invention, Figure 2 is a representation of the reflected signals (omitting the signal reflected from below the joint, as viewed), and Figure 3 indicates schematic signal amplitudes.

Referring to the drawings, the test equipment according to the invention uses a microcomputer to measure and analyse ultrasonic signals transmitted into and returned from both sides of a carbon-fibre reinforced plastic (CFRP) single overlap bond, 12, 13, 14, with a typical adhesive thickness of 0.2 mm. The computer is programmed to compute bond strengths from the ratio of the amplitudes of the two returns or reflections from each interface of the joint and the CFRP. The ratio is converted to dB and bond strength determined from a look-up table of strength vs dB.

The transducer 11 is a PVDF plastic film transducer producing high energy, heavily damped pulses of the order of 1-30 megahertz bandwidth. The transducer 11 has gold-plated electrodes (not shown) and a focussed probe. The position of the transducer 11 is controlled by a digital X-Y drive mechanism schematically illustrated at 16 with respect to the specimen 10 which is immersed in water 15 to improve the transfer of acoustic energy from the transducer 11 to the specimen.

The transducer 11 is driven by a pulse generator/receiver amplifier 20 such as is conventional with ultrasonic flaw detectors. It produces 100-200 volt dc pulses at a pulse repetition frequency in the range 1-10 kilohertz with a pulse width of less than 20 nanoseconds, in a typical arrangement.

The return signals (Figure 2) from the specimen 10 are received by the transducer 11 between 'transmit' pulses. The signals are then rectified to produce a video signal 22 of maximum 20 V. This signal 22 is passed through two gates 24, 26.

The first gate 24 is a time gate with variable start and variable width, permitting a selected portion of the received signal to be passed to the second gate 26. The purpose of the first gate 24 is thus to filter out the initial transmission signal into the specimen 10 and the returns from the top and bottom of the specimen 10 so as to concentrate on a 'window' of signals R2, R3 centred around the double peak return from the specimen 10.

The second gate 26 is arranged to open for a very short time, of the order of 10 nanoseconds, at a time after each pulse transmission which is itself a linear function of time. The time of opening of the second gate 26 is determined by comparing a schematically shown sawtooth ramp voltage 27 with a slowly increasing reference voltage. The period of the sawtooth waxteform is of the order of 200 milliseconds, whilst the period of the slowly varying reference voltage can be of the order of 10 minutes. In this way, the 10 nanosecond samples of the received signal gated by the first gate 24 are passed to the analogue-todigital (A/D) converter 30 where they are digitised and stored as data for analysis by a computer 36. The speed of the sawtooth ramp voltage 27 is adjustable, as is the width of the second gate 26.The overall effect of the second gate 26 is, therefore, to slow the received signal down, somewhat like a stroboscope, so that pulse to pulse variations in the received signal 29 at a particular point are smoothed and integrated before digitisation by the converter 30.

The A/D converter 30 converts e.g. 256 samples of the 'widowed' received signal during each scan operation by the second gate 26, i.e., during each complete cycle of the slowly increasing reference voltage and passes them to the data store 32 of the computer 36. The computer algorithm is arranged to look at the 256 stored data to determine (at 33) the amplitude of the first and second peaks of the signal corresponding to returns R2, R3 from the first and second interfaces of the bond, see Figure 3. The computer 36 then works out the ratio of the two peak amplitudes and expresses them in dB (at 34) and selects a bond strength value corresponding to that ratio from a look-up table designated by 35. The look-up 35 is compiled on the basis of a correlation between first and second peak ratios and bond strengths determined by other known methods, for example destructive mechanical shear strength measurements.

In a preferred embodiment, the transducer 11 is caused to scan the specimen 10 by the X-Y digital drive arrangement 16 in a raster-like scan pattern corresponding with the raster of a TV monitor 40, as indicated by the broken line 42, then values of bond strength determined at regular intervals along the scan may be used to produce a colour-coded bond strength map 44 on the monitor 40 fed with data along a bus 45 from the computer 36.

Claims

CLAIMS 1. A method of testing the strength of a bonded joint, comprising passing signals from a transmitter through said joint, measuring the amplitude of the signals reflected from each interface between the jointing material and the materials to be jointed, determining the peak amplitudes of said signals, computing the ratio of said peak amplitudes, and determining the strength of the bonded joint by comparing the value of said ratio with previously determined and stored values correlated to bond strength values. 2. Apparatus for testing the strength of a bonded joint, comprising a probe for transmitting signals through said joint and for receiving signals reflected from each interface between the jointing material and the materials to be jointed, means for determining the ratio of the peak amplitudes of the said signals, computing means for computing the ratio of said peak amplitudes, and comparing means for comparing the computed ratid with previously determined and stored values correlated to bond strength values. Amendments to the claims have been filed as follows CLAIMS

1. A method of testing the strength of a bonded joint, comprising passing signals from a transmitter through said joint, measuring the amplitude of the signals reflected from each interface between the jointing material and the materials to be jointed, determining the peak amplitudes of said signals, computing the ratio of said peak amplitudes, and determining the strength of the bonded joint by comparing the value of said ratio with previously determined and stored values correlated to bond strength values.

2. A method according to claim 1, wherein the signals are sharp ultrasonic pulse signals of large bandwidth produced by a transducer.

3. A method according to claim 1 or 2, including rectifying the reflected signals to produce a video signal and gating them.

4. A method according to claim 3, including digitising the gated signals and storing them as data for analysis by a computer.

5. A method according to any preceding claim, including arranging the transducer to scan the specimen by an X-Y digital drive means in a raster-like scan pattern, and determining the values of bond strength at regular intervals along the scan.

6. A method according to claim 5 when dependant from claim 4, including using the said values to produce a colour-coded map of bond strength on a TV monitor, utilising data from the computer.

7. A method according to claim 3 or any claim dependant therefrom, wherein gating of the signals is performed by providing a first gate to filter out all reflected signals from faces or interfaces other than those from the joint under test; and a second gate for slowing down the reflected signals to eliminate pulse to pulse variations at a particular point, for integration prior to digitisation.

8. A method according to claim 7, including controlling the second gate 'open' time by comparing a sawtooth ramp voltage with a slowly varying reference voltage, the periods of these voltages differing by at least one and at most four orders of magnitude.

9. A method according to claim 1, substantially as herein described with reference to and as shown in the accompanying drawings.

10. Apparatus for testing the strength of a bonded joint, comprising a probe for transmitting signals through said joint and for receiving signals reflected from each interface between the jointing material and the materials to be jointed, means for determining the ratio of the peak amplitudes of the said signals, computing means for computing the ratio of said peak amplitudes, and comparing means for comparing the computed ratio with previously determined and stored values correlated to bond strength values.

11. Apparatus according to claim 10, wherein the signals are sharp ultrasonic pulse signals of large bandwidth produced by a transducer.

12. Apparatus according to claim 10 or 11, wherein the reflected signals are rectified to produce a video signal and gated.

13. Apparatus according to claim 12, wherein the gated signals are digitised and stored as data for analysis by a computer .

14. Apparatus according to any of claims 10 to 13, wherein the transducer is arranged to scan the specimen by an X-Y digital drive means in a raster-like scan pattern, and the values of bond strength are determined at regular intervals along the scan.

15. Apparatus according to claim 14 when dependant on claim 13, wherein the said values are used to produce a colour-coded map of bond strength on a TV monitor, utilising data from the computer.

16. Apparatus according to claim 12 or any claim appended thereto, wherein the arrangement of gating the signals includes a first gate to filter out all reflected signals from faces or interfaces other than those from the joint under test; and a second gate which is effective to slow down the reflected signals to eliminate pulse to pulse variations at a particular point, for integration prior to digitisation.

17. Apparatus according to claim 16, wherein the second gate 'open' time is controlled by comparing a sawtooth ramp voltage with a slowly varying reference voltage, the periods of these voltages differing by at least one and at most four, preferably three orders of magnitude.

18. Apparatus according to claim 10, substantially as herein described with reference to and as shown in the accompanying drawings.