GB2221991A

GB2221991A - Ultrasonic testing of metal-matrix composite materials

Info

Publication number: GB2221991A
Application number: GB8918653A
Authority: GB
Inventors: Ronald Leslie Smith
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1988-08-16
Filing date: 1989-08-16
Publication date: 1990-02-21
Anticipated expiration: 2009-08-16
Also published as: GB8819441D0; GB8918653D0; GB2221991B

Abstract

The mechanical properties of a metal matrix composite material e.g. a bar 12 depend upon both the proportion of filler and the proportion of voids or pores. The filler proportion and the porosity can be assessed non-destructively by causing ultrasonic waves to propagate through a specimen, and measuring both the wave velocity and attenuation in the specimen. These measured values are compared (for example graphically) with those obtained with material of known filler volume fraction and porosity, and hence the unknown values determined. The apparatus 10 includes a tank 14 containing water 16 (as a coupling fluid). The tank has leaky seals 18. Two transducers 20, 22 are provided and a computer 24 is used to provide the results. <IMAGE>

Description

Quality Assurance This invention relates to a method and an apparatus for assessing the quality of a material non-destructively, in particular the quality of a metal-matrix composite material.

Metal matrix composite materials consist of a metal matrix incorporating a filler material consisting of metallic or non-metallic powders, fibres, filaments or whiskers, whose presence affects the mechanical properties of the resulting composite. One method for making such materials is described in GB 2 172 825 A. The mechanical properties (such as the moduli of elasticity) of the composite are affected by both the filler volume fraction and the porosity, so it is generally important to ensure the filler material is distributed uniformly throughout the metal matrix and the porosity is uniform, so that the mechanical properties are also uniform.

An object of the invention is therefore to provide a non-destructive method for assessing the filler volume fraction and the porosity of a specimen of a metal matrix composite material.

According to the present invention there is provided a method for assessing non-destructively the filler proportion and the porosity of a specimen of a metal matrix composite material, the method comprising causing ultrasonic waves to propagate through the specimen, measuring the velocity of the waves and measuring the attenuation of the waves in the specimen, and from those measured parameters determininq the filler proportion and the porosity.

The measured parameters are compared to measurements made with material of known filler proportion and porosity.

The filler proportion is usually calculated in terms of the filler volume fraction, though the proportion by weight could also be determined. The porosity is also generally calculated as the volume fraction of voids, and since these voids are typically only a few micrometres in size this is often referred to as microporosity. By attenuation is meant the logarithm of the ratio of the ultrasonic signals at two spaced-apart positions in the specimen along the propagation path, per unit distance apart (in dB/m); in practice the ratio may be taken of the received signal to the transmitted signal, or to the received signal when the specimen is not present.

The specimen is preferably immersed in a liquid such as water to ensure good coupling of ultrasound into and out of the specimen. Ultrasonic waves may be arranged to propagate into the specimen and be reflected back by a surface of the specimen so as to be detected at the same location from which they were transmitted, or may be arranged to propagate straight through the specimen.

An apparatus for performing the above method is also provided.

The invention will now be further described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 is a cross-sectional view, partly diaqrammatic, of an apparatus for quality assurance of a metal matrix composite material bar; and Figure 2 is a graphical representation of the relationship between wave velocity, wave attenuation, filler volume fraction, and microporos ity.

Referring to Figure 1, there is shown an apparatus 10 for assessing the quality of a long bar 12 of metal matrix composite material of thickness 100 mm. The bar 12 passes through a tank 14 of water 16 with leaky seals 18 at each end. There are two ultrasonic transducers 20 and 22 in the tank 14, one above the bar 12 and one below it, immersed in the water 16, and each connected to a power supply unit and computer 24. At regular intervals as the bar 12 passes through the tank 14 the power supply 24 enerqises one transducer 20 to transmit a pulse of ultrasound into the water 16 and through the bar 12, to be detected by the other transducer 22. Both the arrival time and the amplitude of the detected ultrasonic pulse are determined, from which the computer 24 can determine the ultrasonic wave velocity (v) in the bar 12 and the wave attenuation (a) in the bar 12.Hence the filler volume fraction (f) and the microporosity (p) can be determined, as discussed below.

Referring now to Figure 2, this represents graphically the variation of wave velocity (v) and wave attenuation (a) with filler volume fraction (f) and with microporosity (p), for a particular metal (e.g. an aluminium alloy) as matrix and for a particular filler (e.g. silicon carbide powder).

The graph is generated from experimental measurements of velocity and attenuation on samples of material of known filler volume fraction and microporosity. The point marked O indicates the values of wave velocity and wave attenuation for the metal alone with no filler and no microporosity. As the filler volume fraction f increases (leaving microporosity p unchanged) the wave velocity v and the attenuation a both increase. As microporosity p increases (leavinq filler volume fraction f unchanged) the wave velocity v decreases and the attenuation a increases.

It will he appreciated that by measuring the values of velocity v and attenuation a at different positions along the bar 12 of Figure 1, it is possible by use of such a calibration graph or an equivalent computer look-up table to determine the filler volume fraction f and the microporosity p at each position along the bar 12.

Hence the quality of the bar 12 can be assessed non-destructively.

It will be appreciated that the exact shape of the graphical relationship shown in Figure 2 will depend upon the nature of both the metal forming the matrix, and the filler material. In general it will be necessary to generate a calibration graph by preliminary measurements on specimens of known different values of filler volume fraction f and microporosity p, for the particular matrix-filler combination under test. The numerical values will also depend to some extent upon the frequency (or wavelength) of the ultrasonic waves, and when generatinq the calibration graph it is therefore advisable to measure the frequency dependence of the attenuation, in order to determine the optimum frequency at which measurements should be taken. The preferred frequency range is 15 to 50 MHz.

Claims

1. A method for assessing non-destructively the filler proportion and the porosity of a specimen of a metal matrix composite material, the method comprising causing ultrasonic waves to propagate through the specimen, measuring the velocity of the waves and measuring the attenuation of the waves in the specimen, and from those measured parameters determining the filler proportion and the porosity.

2. A method as claimed in Claim 1 wherein the measured parameters are compared to values of those parameters obtained with specimens of known filler proportion and known porosity.

3. A method as claimed in Claim 2 wherein the comparison is performed numerically, by a computer means.

4. A method as claimed in Claim 2 wherein the comparison is performed graphically.

5. A method for assessing non-destructively the filler proportion and the porosity of a specimen of a metal matrix composite material substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

6. An apparatus for assessing non-destructively the filler proportion and the porosity of a specimen of a metal matrix composite material, the apparatus comprising means for causing ultrasonic waves to propagate through the specimen, means for measuring the attenuation of the waves, means for measuring the velocity of the waves, and means for determining from those measured parameters the filler proportion and the porosity.

7. An apparatus for assessing non-destructively the filler proportion and the porosity of a specimen of a metal matrix composite material substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.