WO2003044517A1

WO2003044517A1 - Method and apparatus for non-destructive testing of materials

Info

Publication number: WO2003044517A1
Application number: PCT/AU2002/001611
Authority: WO
Inventors: Suszanne Thwaites; Laurence Dickinson
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2001-11-23
Filing date: 2002-11-25
Publication date: 2003-05-30

Abstract

The present invention provides a method of testing a material sample for defects. The method comprises the steps of utilising a probe with actuator to apply mechanical energy to the material sample and measuring an electrical input impedance associated with the probe or the actuator to determine defects in the material sample. The present invention also provides an apparatus for testing a material sample for defects. The apparatus comprises a probe with actuator, a means for applying an electrical signal to the probe or the actuator, a means for measuring the input impedance of the probe or the actuator and a means for analysing the input impedance to determine defects in the material sample

Description

METHOD AND APPARATUS FOR NON-DESTRUCTIVE TESTING OF

MATERIALS

Field of the invention

The present invention relates to a method and apparatus for the non-destructive testing of materials and particularly, though not exclusively, to a method and apparatus for testing of panel-like structures for mechanical defects.

Background of the invention

Non-destructive testing (NDT) may be used to test composite structures, such as aeroplane panels, for mechanical defects. Defects can be caused by stress on materials or mechanical damage from the impact of objects on panels or defective manufacture.

The ability to test these materials in si tu is very important. Regular testing is obviously required for safety, particularly in the aeroplane industry, and it is impractical to disassemble aircraft or parts of aircraft to carry out testing. With composite structures in particular defects may also include failure of adhesion or delamination.

One preferred method of NDT is to utilise a probe to excite the test material structure with vibrations (generally, but not exclusively, acoustic and near acoustic frequencies or near ultrasonic are used for NDT) and detect a response. The response can be indicative of whether the test sample is faulty or not "defected" . Usually the response is compared with a response from similar reference material or structure, which will usually be an undetected material or structure. One of the most popular known NDT systems utilises the pitch/catch impulse test. A typical pitch/catch probe comprises two spring-loaded contact tips which are held in contact with the test sample. These are equipped with two actuators, one of which generates a mechanical vibration within the above frequency range and the other of which detects the response. The detector measures the response of the test sample at its contact point. The propagation of the disturbance from the driver to the detector is influenced by the nature of the intervening structure and in particular, by any damage or anomaly in this region.

Such NDT systems have the disadvantage that they are relatively complicated and expensive devices. Further, defects can only be detected from the analysis of response signals which may often be of relatively poor signal-to- noise ratio which limits the detection sensitivity. It would therefore be desirable to be able to provide a simpler and more sensitive method and apparatus for the detection of defects in such test materials or structures.

Summary of the invention

The present invention provides a method of testing a material sample for defects, comprising the steps of:

• utilising an actuator of a probe to apply mechanical energy to the material sample and

• measuring an electrical input impedance associated with the actuator or the probe to determine defects in the material sample.

The invention may also be defined in terms of an apparatus for testing a material sample for defects, the apparatus comprising: • a probe including an actuator,

• a means for applying an electrical signal to the probe or the actuator,

• a means for measuring the input impedance of the probe or the actuator and

• a means for analysing the input impedance to determine defects in the material sample.

The above-defined method and apparatus have the advantage that only one actuator is required which simplifies instrumentation and detection procedure and therefore reduces costs. Conventional NDT involves the analysis of the response signal - the analysis of the impedance associated with the input signal is entirely novel. The input signal usually is stronger than the response signal and therefore the detection sensitivity may be improved.

Preferred Features of the Invention The method preferably comprises the additional steps of

• positioning the probe relative to a material sample such that a vibration of the actuator can be translated to the material sample and

• applying an ac voltage signal to the actuator.

The ac voltage signal may be one of a plurality of ac voltage signals applied successively to the actuator. In this case each of the ac voltage signals may have a different frequency within a range of frequencies. The frequency range may be up to 50 to 70 kHz, and may be between 1 and 20 kHz. The frequency range preferably is between 3 and 15 kHz. The ac voltage signal preferably has a frequency that sweeps through the frequency range .

Alternatively, the ac voltage signal may be a single ac voltage signal having a range of frequencies such as broadband excitation or a noise signal. In this case the step of analysing the input impedance to determine defects may comprise frequency filtering or Fourier analysing the signal .

In the above-defined apparatus the actuator preferably comprises a piezoelectric material and most preferably a bimorph piezoelectric material . The piezoelectric material may be a titanate of Perovskite structure. The piezoelectric material preferably comprises Lead Zirconium titanate (PZT) , but may also comprise any other piezoelectric material.

The actuator may be suspended by a biasing means which is supported by a probe housing. The biasing means may be a spring. The biasing means preferably is a low load helical spring. A contact pin may be attached to the actuator and which, in use, functions to translate a vibration of the actuator to the material sample. The contact pin preferably is composed of a material that has a mechanical impedance similar to that of the material sample. The contact pin preferably is of an elongate shape having a larger and a smaller end. The larger end of the pin preferably is in contact with the actuator.

Brief description of the drawings

Features of the present invention will become apparent from the following description of an embodiment thereof, by way of example only, with reference to the accompanying drawings, in which; Figure 1 shows a schematic diagram of an apparatus in accordance with an embodiment of the present invention,

Figure 2 shows a diagrammatic representation of a probe with actuator according to an embodiment of the present invention, and

Figure 3 shows ac voltage frequency versus input impedance plots for different test samples.

Detailed description of preferred embodiments Referring to Figure 1, an apparatus in accordance with an embodiment of the present invention is illustrated. The apparatus, generally designated by reference 1, comprises a probe with actuator 2 with a means 3 for applying an AC voltage to the actuator 2. The probe with actuator 2 generates a vibration in the test sample 4 which may be an aeroplane panel . The probe with actuator is connected to a PC computer 5 which functions to control the probe as well as to process and analyze the data. Figure 2 shows a diagrammatic representation of the probe with actuator. The actuator 20 is suspended by a low load helical spring 21 which is supported by a probe housing 22. Pin 23 is attached to the actuator and functions to translate the vibration of the actuator to the test sample 4. The pin 23 comprises a portion having a rounded shape. The apex of rounded portion is in contact with the sample 3 and therefore the contact area is relatively small compared to the overall diameter of the pin 23. The pin 23 is composed of a material that has a mechanical impedance similar to that of the sample 3. The actuator 20 is composed of a material that has a high piezoelectric coefficient such as PZT.

When testing the sample 4 for defects, an ac voltage having an amplitude of a few volts is applied to the actuator. In this example, the ac voltage has a frequency that increases gradually from 5 to 30 kHz. The voltage amplitude is kept constant and the current to the probe is measured while the frequency of the AC signal gradually increases from 5 to 30 kHz. The application of the ac signal as well as the measurement of the current and the voltage involve the usage of suitable data acquisition cards which are incorporated into the computer 5 or an external system. A suitable software routine can then calculate the input impedance as a function of the frequency and can display it a as plot on a computer monitor. Alternatively, a "broadband" ac signal may be applied having a frequency bandwidth of approximately 5 to 30 kHz.

Figure 3 shows examples of input impedance measurements for 30 an undamaged aeroplane panel and, 31 and 32, for aeroplane panels which have structural defects. The input impedance is dependent on the vibration characteristics of the sample which will be different for an undamaged sample and for a sample that has structural defects. As can be seen from the figure, for the damaged panels the measured impedance has a larger intensity within the frequency range of approximately 5 to 13 kHz.

Therefore, the input impedance measured as described above, is a measure for structural damages of a material sample .

Further data processing may involve the application of electronic filters or filtering software routines such as fast Fourier analysis. The probe may also be combined with a computer-mouse- like device. A lateral movement of the probe across the sample can then be correlated by the computer with a change in a x-y coordination. Together with input impedance measurements for every position of the probe, it is then possible to create a map of structural damages of the sample. Further details regarding the creation of a map of structural damages are described in international patent application PCT/AU02/00501 filed on 19 April 2002 by The Commonwealth Scientific and Industrial Research Organization.

The present invention has been described above as being particularly suitable for use with non-destructive testing of composite materials such as used in aircraft construction. It will be appreciated, however, that the NDT method and apparatus of the present invention may be used for testing other types of materials where it is suited, and is not limited to the testing of composite materials, and is not limited to the testing of aircraft structures. Other types of materials may include metallic or non-metallic materials having different structures including honey-comb structures. The apparatus of the present invention is implemented utilising a conventional PC. It will be appreciated that it may be implemented utilising any computing system or even hardware, and is not limited to being implemented using a PC. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. For example, instead of keeping the voltage amplitude constant during testing of a sample and measuring the probe current, the probe current may be kept constant and the voltage may be measured. Further, drive and data acquisition electronics may form an integral part with the probe and an external data processing system, such as a PC, may not be required. The device is also not limited to the actuator as herein described may include any suitable type of mechanical actuator.

It is to be understood that the references that are made to previous patent applications does not constitute an admission that these patent applications form a part of the common general knowledge in the art, in Australia or any other country.

Claims

CLAIMS :

1. A method of testing a material sample for defects, comprising the steps of:

2. The method as claimed in claim 1 comprising the additional steps of steps of:

• applying an ac voltage signal to the actuator.

3. The method as claimed in any one of claims 1 or 2 wherein the ac voltage signal is one of a plurality of ac voltage signals applied successively to the actuator.

4. The method as claimed in claim 3 wherein each of the ac voltage signals has a different frequency within a range of frequencies.

5. The method as claimed in claim 4 wherein the range of frequencies is between 1 and 20 kHz.

6. The method as claimed in claim 5 wherein the range of frequencies is between 3 and 15 kHz.

7. The method as claimed in any one of claims 4 to 6 wherein the voltage has a frequency that sweeps through the frequency range .

8. The method as claimed in claim 2 wherein the ac voltage signal is a single ac signal having a range of frequencies .

9. The method as claimed in any one of the preceding claims wherein the step of measuring the input impedance associated with the actuator or the probe to determine defects comprises frequency filtering.

10. The method as claimed in any one of claims 1 to 8 wherein the step of analysing the input impedance to determine defects comprises Fourier analysing the signal.

11. An apparatus for testing a material sample for defects, the apparatus comprising: • a probe including an actuator,

• a means for applying an electrical signal to the probe or the actuator,

• a means for measuring the input impedance of the probe or the actuator and • a means for analysing the input impedance to determine defects in the material sample.

12. The apparatus as claimed in claim 11 wherein the actuator comprises a piezoelectric material.

13. The apparatus as claimed in claim 12 wherein the piezoelectric material is bimorph.

14. The apparatus as claimed in claim 12 or 13 wherein the piezoelectric material comprises a titanate of Perovskite structure.

15. The probe as claimed in claim 14 wherein the piezoelectric material comprises Lead Zirconium titanate

(PZT) .

16. The probe as claimed in any one of claims 11 to 15 wherein the actuator is suspended by a biasing means which is supported by a probe housing.

17. The probe as claimed in claim 16 wherein the biasing means is a spring.

18. The probe as claimed in claim 17 wherein the biasing means is a low load helical spring.

19. The probe as claimed in any one of claims 11 to 18 wherein a contact pin is attached to the actuator and which, when in use, functions to translate a vibration of the actuator to the material sample.

20. The probe as claimed in claim 19 wherein the contact pin is composed of a material that has a mechanical impedance similar to that of the material sample.

21. The probe as claimed in claims 19 or 20 wherein the contact pin is elongate having a larger and a smaller end.

22. The probe as claimed in claims 21 wherein the larger end is in contact with the actuator.