WO2004003541A1

WO2004003541A1 - Optoacoustic measurement of material properties by a two-point laser-induced generation of ultrasonic waves

Info

Publication number: WO2004003541A1
Application number: PCT/GB2003/002610
Authority: WO
Inventors: Gregory Adam Baker; Brian Cecil Moss
Original assignee: Aea Technology Plc
Priority date: 2002-06-29
Filing date: 2003-06-18
Publication date: 2004-01-08
Also published as: GB0215122D0; AU2003240106A1

Abstract

Ultrasonic waves are generated at a surface of an object (12) using a pulsed laser beam (18). The beam (18) is incident on the surface at two separate locations (14, 15) at a fixed separation (S). This may be achieved using a beam splitter (22) and a reflector (23). Large lenses are not required, so that the locations (14, 15) can be close together, for example 10 mm apart. Such an apparatus for generating ultrasonic waves may be combined with an apparatus for detecting ultrasonic waves, preferably a laser detector (30, 37). Ultrasonic wave velocities can therefore be measured in a non-contact manner.

Description

Optoacoustic measurement of material properties by a two-point laser-induced generation of ultrasonic waves

This invention relates to a method and an apparatus for generating ultrasonic waves at a surface of an object 5 using optical techniques.

The use of optical techniques using lasers to generate and to detect ultrasonic waves on a surface of an object is widely known. In one detection technique a

10 continuous laser beam is split into two, one beam being incident on the surface and the other beam following a reference path, and the resulting reflected beams are arranged to interfere with each other. Changes in the position of the surface cause a phase difference between

15 the two beams, so when the beams interfere the intensity changes. In another detection technique a laser beam is incident on the surface, and is then reflected to pass through a very narrow band filter (such as a Fabry-Perot etalon) ; movement of the surface changes the frequency of

20 the reflected beam, and so changes the intensity of the light emerging from the filter. Use of a pulsed laser source to generate short pulses of ultrasonic waves within a material is known too; for example in GB 2 172 106 B a pulsed laser is used to generate ultrasonic 25 waves, and a laser interferometer is used to detect ultrasonic waves.

In some applications the velocity of the ultrasonic waves may be related to properties of the material which

30 are of interest, and this velocity may be determined by measuring the time the waves take to propagate a known distance. Where the waves are surface waves, or Lamb waves, an obvious approach is to take this distance as the distance between the transmitter and the receiver. 5 However this can give inaccuracies when using a pulsed laser source, as such sources exhibit pulse-to-pulse positional variation. This problem can be overcome by using two separate laser detectors at spaced-apart positions, and the times of arrival at the two detectors can be compared. A way of achieving this, using a single laser beam that is split to be incident at the two positions, is described in GB 2 356 931 A.

According to the present invention there is provided a method for monitoring material properties of a homogeneous body by generating ultrasonic waves at a surface of the body, using a pulsed laser beam and means to cause the beam to be incident on the surface at two separate locations at a fixed separation, and detecting the ultrasonic waves at a different location on a surface of the body.

Preferably a laser detector is used to detect the waves. Such a detector may comprise a single continuous laser source arranged to be incident on the surface, means to detect the laser beam after reflection from the surface, and an interference device for monitoring changes in the reflected beam due to movement of the surface. The preferred interference device is a Fabry- Perot etalon.

Preferably the means for generating ultrasonic waves comprises a beam splitter to separate the pulsed laser beam into two pulsed laser beams, and means to cause the beams to be incident on the surface at the two locations.

Where it is desirable to provide a large stand-off between the optical components and the object (for example when taking measurements on steel in a rolling mill where the surface may be at 1200°C) , the split- detector system of GB 2 356 931 requires large diameter collection lenses to be used to preserve collection efficiency. This leads to the problem that the two positions at which the observations are made must be separated by a distance at least as great as the sum of the radii of the two collection lenses, which may be too great for satisfactory ultrasonic measurements because of attenuation of the ultrasound. With the present invention, in contrast, small lenses may be used to focus the laser beams onto the surface even where a large stand-off is used, so that the two beams (and therefore the two locations) can be quite close together even with such hot objects.

Alternatively the means for generating ultrasonic waves may comprise means for causing the pulsed beam to be incident first at the first location and then at the other location. This may be achieved for example by moving the laser itself, or by moving a mirror. Alternatively the generating beam may be arranged to alternate between the two locations. Although this approach does not overcome the problems arising from pointing error (the positional variation between pulses) this error may be insignificant, particularly with thick objects .

It will be appreciated that any detector of ultrasonic waves may be used, but that it is particularly beneficial to use a laser detector, as it enables material property monitoring to be performed in a non- contact manner, for example on material which is too hot to touch, for example during casting of metals, or on a moving specimen. Furthermore the generator generates a wide range of different wave modes, so that the detector may be arranged to detect any type of surface movement, but is particularly beneficial when monitoring ultrasonic waves which pass along the surface, for example surface waves (Rayleigh waves), or Lamb waves. The invention is particularly suited to monitoring material properties of a sheet of metal, even where the sheet is hot; unless the sheet is thin, both bulk waves and surface waves are generated.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawing which shows a diagrammatic plan view of an apparatus for generating and detecting ultrasonic waves.

Referring to the drawing, an apparatus 10 is shown in which the arrival times of ultrasonic surface waves in a specimen 12 are observed at a location A, the waves being generated simultaneously at two positions 14 and 15 at a separation S. Ultrasonic waves are generated by a laser 16 arranged to emit a pulsed light beam 18; for example the laser 16 may produce pulses of energy 80 mJ and of duration 10 ns at a pulse repetition frequency of for example 20 Hz. The light beam 18 is passed through a half wavelength plate 20 and then a polarising beam splitter 22, so generating two beams marked 1 and 2. The polarising beam splitter 22 reflects light with a vertical plane of polarisation, and transmits light with a horizontal plane of polarisation, and by adjusting the angle of the half wavelength plate 20 the relative intensities of the beams 1 and 2 can be adjusted. The beam 2 is reflected by a prism 23 to be parallel to the beam 1, and the beams 1 and 2 are then focused onto the surface of the specimen 12 by converging lenses 24, so as to be incident on lines 14 and 15 about 1 mm wide (and 10 mm long) . Consequently each laser pulse produces very sharp ultrasonic pulses in the specimen 12 which include frequencies above 1 MHz. For detecting ultrasonic waves, the apparatus 10 comprises an argon ion laser 30 producing a continuous beam 32 of light. The laser 30 incorporates an etalon within the optical cavity to ensure it operates in a single longitudinal mode, so it emits a single frequency (at a wavelength of 514 nm) . This beam 32 is passed through a plane polarising plate 33 so it is horizontally polarised. It is then incident on the surface of the specimen 12 at location A, via a polarising beam splitter 34, and a quarter wave plate 35. The beam 32 reflected from the surface of the specimen 12, after returning through the quarter wave plate 35 is vertically polarised, and so is reflected by the beam splitter 34. The beam 32 is then reflected by a prism 36 to pass through a confocal Fabry-Perot etalon 37, to be incident on a photo diode 38. The photo diode 38 provides electrical signals to an electronic detector, which in this embodiment is a cathode ray oscilloscope 40. The detector 40 is also provided with trigger signals from the generating laser 16.

Every pulse of light generated by the laser 16 generates pulses of ultrasonic waves at the two positions 14 and 15 which propagate across the surface of the specimen 12, passing through location A in succession. Hence the oscilloscope 40 will display two pulses at a time separation equal to the time the wave takes to propagate between those locations. The propagation time between positions 14 and 15 can therefore be measured accurately, and is unaffected by any displacement of the laser beam 18.

The ultrasonic waves to be detected may be in the frequency range 1-15 MHz. Preferably the etalon 37 incorporates a piezoelectric tuning device to ensure that the peak intensity from the argon laser 30 (514 nm) is to one side of the transmission peak and about half way down the peak (so as to maximize the rate of change of transmission with frequency) , and a feedback circuit is preferably provided to maintain this optimum sensitivity despite any vibrations of the apparatus 10. Such vibrations would typically be no more than 1 kHz, which is well below the frequency of the ultrasonic waves. The distance S between the positions 14 and 15 is set by adjusting the position of the prism 23, and might be in the range 10 mm - 20 mm, the distance between the position 15 and the location A might also be about 20 mm.

Bulk waves, both shear and compression, are generated if the specimen 12 is thick, as opposed to Lamb waves in thin sheet specimens. If, as shown, the laser 16 is incident on the same side of the sheet 12 as the detecting beam 32, the times of arrival at A of singly reflected bulk waves, tl and t2 , enable an unknown thickness of a sheet to be eliminated, so that wave velocity can be determined in terms of the beam separation S and the distance (A-15) between the generating and detecting beams. By observing also the arrival times of Rayleigh waves, or of triply-reflected compression waves, the unknown distance (A-15) can also be eliminated.

It will hence be appreciated that to obtain accurate values of wave velocity we need a very accurate measurement of the beam separation S. This may be obtained by using a test plate, whose thickness is accurately known or may be measured for example with a micrometer. From observed arrival times tl and t2 of singly-reflected bulk waves in this test plate, the separation S can be determined.

Not only do such measurements enable the wave velocity of bulk waves to be determined, but also the thickness of the sheet 12. Measurements of wave velocity may be related to other material properties, such as temperature, as may measurements of thickness. Additional information may also be obtained by measuring the arrival times for surface compression waves .

In attempting to measure the transit time between position 14 or 15 and location A, there has been found to be a time delay between the generation of the pulse from the laser 16 and the triggering of the oscilloscope 40. This delay is typically a few tens of nanoseconds and may be due to the risetime of the electronics, and differences in cable lengths. Clearly this time delay must be determined if ultrasonic velocities and specimen thicknesses are to be measured.

It will be understood that the apparatus 10 may differ from that described, for example the continuous laser 30 may be replaced by a different type of single- frequency laser, for example a diode-pumped neodymium/yttrium aluminium garnet laser, and the pulse generating laser 16 may also be of different types such as Q-switched flashlamp-pumped neodymium/yttrium aluminium garnet, neodymium/glass or neodymium/vanadate .

In a further modification the beam splitter 22 may be replaced by a reflecting prism (not shown) , combined with a linear actuator to move the prism in and out of the beam. With the prism in the beam, the beam 18 follows route 2 and is incident at position 14, whereas when the prism is out of the beam, the beam 18 follows route 1 and is incident at position 15. A separate measurement of transit time would be taken for each position of the prism (and so of the wave generation) . Although this modification does not enable pointing errors to be eliminated, it may be applicable where pointing errors are small, and particularly with thick specimens for which the pointing error may be less significant .

Alternatively, a single laser beam may be rapidly switched between two positions on the surface of the specimen using a circular disc spinning about an axis through its centre, with reflecting segments alternating with segments cut away so the light beam is unaffected.

Claims

1. A method for monitoring material properties of a homogenous body (12) by generating ultrasonic waves at a surface of the body, using a pulsed laser beam (18) and means (22,23) to cause the beam to be incident on the surface at two separate locations (14,15) at a fixed separation, and detecting the ultrasonic waves at a different location (A) on a surface of the body (12) .

2. A method as claimed in claim 1 wherein a beam splitter (22) is used to separate the pulsed laser beam (18) into two pulsed laser beams, and means (23,24) are used to cause the beams to be incident on the surface at the two locations (14,15) .

3. A method as claimed in claim 2 wherein the means to cause the beams to be incident on the surface include two cylindrical lenses (24) .

4. A method as claimed in claim 1 wherein the pulsed beam is caused to be incident first at the first location

(14) and then at the other location (15) .

5. An apparatus for monitoring material properties of a homogenous body (12) by a method as claimed in any one of the preceding claims.