GB2365976A

GB2365976A - Apparatus for testing of materials comprising a megnetostrictive actuator

Info

Publication number: GB2365976A
Application number: GB0006474A
Authority: GB
Inventors: William John Metheringham; Peter Stuart Holiday; Allan Matthews; Stephen Downey
Original assignee: Newlands Technology Ltd
Current assignee: Newlands Technology Ltd
Priority date: 2000-03-18
Filing date: 2000-03-18
Publication date: 2002-02-27
Anticipated expiration: 2020-03-18
Also published as: GB2365976B; GB0006474D0

Abstract

Apparatus for use in the testing of materials comprises a magnetostrictive actuator (1) connected to control means (13-16) for supplying a cyclic actuating signal thereto and for recording the number of cycles performed by the actuator. The apparatus can perform impact testing or load cycling testing,simulating actual load cycling but in a much reduced time period.

Description

<Desc/Clms Page number 1> APPARATUS FOR TESTING OF MATERIALS Field of the Invention This invention relates to apparatus for the testing of materials, in particular thin film coatings on materials and ceramic materials.

Background to the Invention Fatigue testing of materials is an important way of determining the ability of materials to perform the functions for which they are intended. For example, hard thin film coatings such as titanium nitride are used as anti-wear and anti-friction coatings on mechanical components, and need to be tested for their resistance to impact and for their adhesion to the component.

A problem with conventional impact testing is that it is typically achieved by the use of hydraulic or pneumatic loading devices, and these have a relatively slow cycling time. Tests may be conducted by applying a predetermined load for varying numbers of cycles, and examining the test workpiece at the end to determine the effect of the repeated impacts or loadings, or running for a predetermined number of cycles, each run being at a different fixed loading. In this way a fatigue curve can be built up for a particular material. Since some tests may require as many as 106 or even 108 cycles, each test can take many days to complete, and so building up a fatigue curve for the material can involve a substantial cost.

Ceramic materials are used for a wide range of applications, one example of which being in piezo-electric devices. While impact testing may not be appropriate for such materials, it may be desirable to determine the ability of the materials to survive repeated cycles of compressive loading.

It has now been found that, with the use of a magnetostrictive actuator accurate and repeatable test results may be obtained in the impact testing of thin film coatings and the like in a very much shorter time than with the conventional tests. It is believed that advantages will also be gained in the compressive testing of ceramics and other materials.

Summary of the Invention The invention provides apparatus for use in the testing of materials, which comprises a magnetostrictive actuator connected to control means for supplying a cyclic actuating signal thereto and for recording the number of cycles performed by the actuator, The device preferably comprises a probe directly attached to the actuator. For impact testing, the probe may comprise a ball of tungsten carbide or like hard material. A suitable size of ball is 6mm diameter, although other sizes may be used. Other shapes of probe end may be employed. For example, in the compressive testing of ceramics, it may be desirable to employ aProbe having a generally flat end.

Preferably, the device includes means for measuring the static or dynamic force applied to the material to be tested, and the control means ma,y be arranged to monitor a change in force indicating failure of the material/coating. The measuring means is suitably a load cell, which may be incorporated in the probe or in the mounting thereof. or which may alternatively be provided in the support for the workpiece being tested. Alternatively, or additionally, the displacement of the :Dall nnaX be monitored, and again an increase in displacement could be monitored to indicate failure of the coating, for example.

Since magnetostrictive actuators can be operated I at high frequencies, the testing operation can be greatly accelerated. For example, the impact frequency will typically be of the order of 20OHz, enabling tests involving around 106 cycles to be completed within about 2 hours. For some tests, lower frequencies may be desirable, and in some circumstances very low repeat rates may be employed.

The apparatus of the invention enables a very wide range of forces to be applied. The pattern of application of force may be varied by varying the wave form of the driving signal supplied to the actuator, for example square wave, sine wave or a series of peaked pulses. The ability to controi the force application precisely permits accurate modelling of the conditions which may be experienced in use by the material being tested. For example, an anti-wear coating on a cam in an internal combustion engine is subjected to a Pattern of impact and loading which is not accurately represented by the simple repeated striking the surface with a probe. The use of a magnetostrictive

actuator in the apparatus of the invention permits precise control of the application of force to a coated sample, thereby enabling the conditions experienced in the engine to be accurately reproduced during the test. It will also be seen that the waveform applied to the actuator need not remain constant throughout the test, but may be varied The magnetostrictive actuator is suitably one comprising a rod of Terfenol-D material, an alloy of the rare earth elements dysprosium and terbium with iron, with a chemical composition DyO.7TbO.3Fe1.95. The invention is not, however, limited to the use of any particular magnetostrictive material.

In the case of non-impact testing, the probe is held in contact with the work- piece to be tested with an initial small biasing force, and the force applied is then cycled at the driving frequency.

An advantage of the device of the invention is that magnetostrictive actuators can be operated at high frequency for long periods of time, but have the flexibility to be operated at a wide range of frequencies and to be able to respond precisely to a drive waveform, thus enabling modelling of the conditions experienced by materials in actual use. Magnetostrictive actuators can be made of relatively small size, while still applying a useful testing force.

While the apparatus has been described with reference to fatigue testing involving cyclic loading at a relatively high frequency, it can also be used for other types of test, for example a single-strike indenting test using, for example, a diamond indenter, or even for controlled scratch testing in conjunction with lateral movement of the test material, where it may be desirable to apply judder to the indenter, i.e. a low-frequency cyclic loading of the indenter as it is dragged across the workpiece by lateral movement thereof relative to the indenter. Another example of low frequency testing is the testing of soft metallic materials such as solders, where the load is repeatedly applied and removed at relatively long intervals, for example 15 minutes.

Brief Description of the Drawings In the drawings, which illustrate an exemplary embodiment of the invention: Figure 1 is a diagrammatic side elevation of test apparatus according to the invention;

Figure 2 is a diagram illustrating the control and monitoring system for the device shown in Figure 1; and Figure 3 is a diagram illustrating a magnetostrictive actuator suitable for use in the apparatus of the invention.

Detailed Description of the Illustrated Embodiment The apparatus comprises a base 5 set on vibration isolation supports 6 and having upstanding therefrom two uprights 4 carrying a cross-member 2 whose vertical position can be adjusted using screw-threaded adjusters 3 in the uprights 4. The cross- member 2 carries the magnetostrictive actuator 1. The base 5 carries a test sample table 8 provided with a mechanism 7 for providing fine adjustment of the height of the test sample 10, which is mounted on a force transducer 9.

The actuator I carries a probe extending vertically downwards and having mounted at the free end thereof a tungsten carbide ball 'I I serving as indenter. In an alternative arrangement, a force transducer 12 is mounted between the ball 11 and the probe end instead of providing a transducer between the table 8 and the test sample 10.

In use, the sample 10 is mounted on the table 8 and the position of the actuator is adjusted using the adjusters 3 until the ball is close above the sample 10. Fine adjustment of the position of the sample relative to the ball is then carried out using the mechanism 7 on the table 8 so that, for example, the ball is just above the sample surface at the rest Position thereof so as to strike the sample repeatedly during operation of the actuator. Fine adjustment may be required to compensate for expansion in the actuator due to temperature increases in prolonged use, for example. The fine adjustment may be carried out manually or may be automatically controlled using a feedback control system, for example as described hereinafter with reference to Figure 2.

Referring to Figure 2, the apparatus includes a data acquisition and control sXs- tern 13 connected to the force transducer 12 (or 9) via a signal conditioning unit 14. The control system 13 also provides control signals to the fine table adjustment mechanism 7 to control its position relative to the actuator, and to a function generator 15 connected to the actuator I via an amplifier 16.

Figure 3 shows the construction of a suitable magnetostrictive actuator for use in the apparatus of the invention. The actuator comprises a Terfenol-D rod 20 mounted in a casing 21. A force transmitting member 22 is mounted at one end of the rod 20 and projects through the casing 21, a compression spring 23 between the member 22 and the inside of the casing 21 serving to pre-load the rod 20 to the desired degree. Permanent magnets 24 mounted between flux closures 25 surround the rod and provide a magnetic bias to the desired degree, while a driving coil 26 is mounted between the magnets and the rod, The coil is electrically connected to the signal generator, as hereinbefore described, while the force transmitting means carries the probe which engages the workpiece in use.

Claims

CLAIMS 1 Apparatus for use in the testing of materials, comprising a magnetostric- tive actuator connected to control means for supplying a cyclic actuating signal thereto and for recording the number of cycles performed by the actuator.
2. Apparatus according to Claim 1, comprising a probe directly attached to the actuator.
3. Apparatus according to Claim 2, wherein the probe comprises a ball of tungsten carbide-
4. Apparatus according to Claim 1, 2 or 3, comprising force measuring means for measuring the static or dynamic force applied to the material to be tested.
5. Apparatus according to Claim 4, wherein the control means is arranged to monitor a change in force indicating failure of the material being tested.
6. Apparatus according to Claim 4 or 5, wherein the force measuring means is a load cell.
7. Apparatus according to Claim 6, wherein the load cell is incorporated into the probe or means mounting the probe.
8. Apparatus according to Claim 6, comprising a support for the workpiece to be tested, the load cell being incorporated in the support.
9. Apparatus according to any preceding claim. comprising means for measuring the displacement of the probe. b
10. Apparatus according to any preceding claim, wherein the control means is arranged to provide a cyclic actuating signal having a frequency of the order of 20OHz.
11. Apparatus according to any preceding claim, wherein the control means comprises means for varying the waveform of the cyclic actuating signal.
12. Apparatus for use in the testing of materials, substantially as described with reference to. or as shown in, the drawings.