GB2608413A

GB2608413A - Spatial-phase-shift shearography system

Info

Publication number: GB2608413A
Application number: GB2109462.8A
Authority: GB
Inventors: Gao Jianxin
Original assignee: Front Tech Ltd
Current assignee: Front Tech Ltd
Priority date: 2021-06-30
Filing date: 2021-06-30
Publication date: 2023-01-04
Also published as: GB202109462D0

Abstract

A spatial-phase-shift shearography system using a polarised image detector 18 comprises a beam splitter 22 for splitting a beam into two parts in different directions and two polarisers 24, 26 each for polarising by transmission one of the two parts and each for polarising by reflection one of the two parts. The polarised transmitted and reflected parts of the beam can then be interfered without the use of a quarter wave plate. The spatial-phase-shift shearography system of the invention does not employ circular polarisation and so does away with the need for a quarter wave plate.

Description

Spatial-Phase-Shift Shearography System The present invention relates to a spatial-phase-shift shearography system, more specifically to a spatial-phase-shift shearography system using a polarised detector, and to a method of testing a test piece.

Shearography is used for non-destructive testing by revealing defects in a test piece. Shearography involves the comparison of a speckle image produced from a test piece in a relaxed or non-deformed state with a speckle image produced from a test piece in a strained or deformed state. The comparison of the speckle images produces fringe patterns, which can be processed to reveal the possible presence of defects.

To quantify the fringe patterns, usually at least three phase shifted fringes of the surface are required to be under a single load condition. With prior knowledge, it becomes possible to produce a phase map by using two phase-shifted fringe patterns. To produce the phase shifting, temporal or spatial phase shifting may be used. Spatial-phase shifting is preferred if the object under investigation is in a dynamic condition (dynamic movement or vibration) which means that temporal phase-shifting will not work properly.

In spatial-phase shifting, several (usually three or four) phase-shift interferograms are obtained in different spatial positions simultaneously. A carrier-frequency based method is a common spatial phase-shift technique for shearography, but its phase map quality is usually poor. A polarised camera that is equipped with a pixelated mask in front of the image sensors, and preferably right in front of the image sensors, is particularly useful in achieving spatial phase-shift shearography. Pixelated PhaseCam interferometers from 4D Technology, and the Polarization CMOS Image Sensor from Sony (RTM), are suitable polarised cameras currently available.

In spatial phase-shift shearography using a polarised camera, a source of coherent light is diffracted from an optically rough test surface and is split by a beam-splitter into two parts. The split light is then sheared, for example by reflecting the two parts of the light using mirrors having different tilts to each other. The light must be polarised for spatial-phase shifting using a polarised camera. The two parts of spatially-shifted light are then interfered when they reach the imaging sensors of the polarised camera to produce spatially phase-shifted interferograms.

Using polarised light to achieve spatial-phase shifting typically requires a large and complex optical set-up The present invention seeks to provide a solution to these problems.

According to a first aspect of the present invention, there is provided a spatial-phase-shift shearography system without a quarter wave plate for imaging a test piece, the spatial-phase-shift shearography system comprising: a beam splitter for splitting coherent light from the test piece into a first beam part and a second beam part; a first linear polariser having non-zero reflectivity arranged so as to polarise by transmission the first beam part so as to form a first transmitted sub-part which is linearly polarised in a first direction, and arranged so as to reflect the first beam part so as to form a first reflected sub-part which is linearly polarised in a second direction which is perpendicular to the first direction; a second linear polariser having non-zero reflectivity arranged so as to polarise by transmission the second beam part so as to form a second transmitted sub-part which is linearly polarised in a third direction, and arranged so as to reflect the second beam part so as to form a second reflected sub-part which is linearly polarised in a fourth direction which is perpendicular to the third direction; a first reflecting surface arranged so as to reflect the first transmitted sub-part; and a second reflecting surface arranged so as to reflect the second transmitted sub-part; the polarisers and reflecting surfaces arranged so that without use of a quarter wave plate the first transmitted sub-part is directly interferable with the second reflected sub-part and the second transmitted sub-part is directly interferable with the first reflected sub-part, or the first transmitted sub-part is directly interferable with the second transmitted sub-part and the first reflected sub-part is directly interferable with the second reflected sub-part.

If the two polarisers are perpendicularly arranged so that the first direction is perpendicular to the third direction and the second direction is perpendicular to the fourth direction, then the first transmitted sub-part is directly interferable with the second reflected sub-part and the second transmitted sub-part is directly interferable with the first reflected sub-part. If the two polarisers are parallelly arranged so that the first direction is parallel to the third direction and the second direction is parallel to the fourth direction, then the first transmitted sub-part is directly interferable with the second transmitted sub-part and the first reflected sub-part is directly interferable with the second reflected sub-part.

Since each combination of polariser and mirror produces two orthogonally polarised beams, no quarter wave plate is required since the two orthogonally polarised beams from a first combination of polariser and mirror can be interfered with the two orthogonally polarised beams from a second combination of polariser and mirror. In fact, if a quarter wave plate is provided to circularly polarise the different linearly polarised beams, which usually must be placed 45° with regard to the two orthogonally positioned polarising light beams, then this will complicate and potentially invalidate phase calculations.

As such, the arrangement of the present invention provides a simpler and smaller spatialphase-shift shearography system. This may provide different advantages, for example 10 permitting the portability of spatial-phase-shift shearography systems.

Preferably, the first and second linear polarisers may be configured so that the first and third directions are parallel and the second and fourth directions are parallel so as to polarise the respective transmitted beam sub-parts in parallel directions to each other and the respective reflected beam sub-parts in parallel directions to each other. The light reflected by a polariser is defined as s-polarised light, and the light transmitted by the polariser is defined as p-polarised light. The reflected s-polarised light beam sub-parts from the two polarisers are parallel with each other and can interfere to produce a fringe pattern. Similarly, the transmitted p-polarised light beam sub-parts from the two polarisers are parallel with each other and can interfere to produce another fringe pattern.

Alternatively, the first and second linear polarisers may be configured so that the first and fourth directions are parallel and the second and third directions are parallel so that the first transmitted sub-part and the second reflected sub-part are polarised in parallel directions, and the second transmitted sub-part and the first reflected sub-part are polarised in parallel directions. In this case, the s-polarised light from one polariser is parallel with the p-polarised light from the other polariser. The reflected light from one polariser can be interfered with the transmitted light of the other polariser, and vice versa.

Beneficially, the beam splitter may be a partially silvered mirror.

Advantageously, the first and second linear polarisers may be wire-grid polarisers. These may permit for improved reflection therefrom.

In a preferable embodiment, the first linear polariser and the first reflecting surface may be arranged in a direction relative to the beam splitter which is perpendicular or substantially perpendicular to a direction in which the second linear polariser and the second reflecting surface are arranged relative to the beam splitter. As such, the beam splitter is arranged to split the beam in perpendicular directions.

Advantageously, the first and second reflective surfaces may be at or adjacent to the first and second polarisers respectively. As such, each reflective surfaces may be right 5 behind the respective polariser.

Beneficially, the first and second polarisers may be polariser plates.

In a preferable embodiment, the spatial-phase-shift shearography system further comprises: a source of coherent light for producing a beam which is diffractable from a test surface of the test piece; and a polarised detector for receiving the first and second 10 transmitted sub-parts and the first and second reflected sub-parts.

The coherent light may only be diffractable from optically rough test surfaces, and so the test piece is required to have an optically rough surface.

Preferably, the source of coherent light may be a laser.

Advantageously, the spatial-phase-shift shearography system may further comprise a 15 beam expander for expanding the beam produced by the source of coherent light.

Beneficially, the spatial-phase-shift shearography system may further comprise an aperture and a lens.

Preferably, the spatial-phase-shift shearography system further comprises a stressing means for deforming the test surface. The stressing means may otherwise be referred 20 to as a loading means.

According to a second aspect of the invention there is provided a method of testing a test piece using a spatial-phase-shift shearography system according to the first aspect of the invention, the method comprising the steps of: a) producing a reference speckle image of a test surface of the test piece in an un-deformed state; b) deforming the test piece using the stressing means so that the test piece and test surface is in a deformed state; c) producing a testing speckle image of the test surface in the deformed state; and d) comparing the reference speckle image and the testing speckle image.

Preferably, the test piece may be a wind-turbine blade. However, it will be appreciated that other test pieces may be considered such as engine turbine blades or other parts of 30 an aeroplane.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a first embodiment of a spatial-phase-shift shearography system according to a second aspect of the invention, including an image shearing device in 5 according to a first aspect of the invention; Figure 2 shows a fringe pattern from a test speckle piece produced by the spatialphase-shift shearography system of Figure 1; Figure 3a and Figure 3b show two fringe patterns extracted from pixels of a mask used with the detector of the stereography system with 00 and 900 polariser masks in the 10 fringe pattern of Figure 2, and a direct current component in the fringe patterns has been removed; Figure 4 shows a demodulated image of the two fringe patterns of Figures 3a and 3b; Figure 5 shows a phase denoised image of the demodulated image of Figure 4; and Figure 6 shows an unwrapped phase image of the phase denoised image of Figure 5.

Referring firstly to Figure 1, there is shown a spatial-phase-shift shearography system 10 which is imaging a test piece 12. The system 10 preferably includes a source of coherent light 14, a polarised detector 18, and a processing device 20, such as a computer or computing device, communicatively connected to the detector 18. The system 10 includes a beam splitter 22, a first polariser 24, a second polariser 26, a first reflective surface 28, and a second reflective surface 30. The system 10 as a whole does not have a quarter-wave plate or other means of converting linearly polarised light into circularly polarised light.

The beam splitter 22 is for splitting coherent light which is diffracted from the test piece 12. The beam splitter 22 may, for example, comprise a partially silvered mirror or partially reflective surface which is positioned at a 45-degree angle or substantially 45-degree angle to the reflected path of the beam. Wien the diffracted light is incident on the beam splitter 22, a first part transmits through the mirror and part is reflected from the mirror.

Since the beam splitter 22 is positioned at 45-degrees to the path of the diffracted coherent light, the two parts are split in orthogonal directions.

The first polariser 24 is arranged to receive a first part of the beam which is split by the beam splitter 22. For instance, the first polariser 24 may be arranged so as to receive the part of the beam which is transmitted. The detector 18 has a plurality of main axes. An axis of the first polariser 24 is parallel or perpendicular to one of the main axes of the polarised detector 18. For example, when the polarised detector 18 is positioned horizontally, the axis of the first polariser 24 can be along the horizontal or vertical direction.

The second polariser 26 is arranged to receive a second part of the beam which is split by the beam splitter 22. For instance, the second polariser 26 may be arranged so as to receive the part of the beam which reflects from an interface of the beam splitter 22. As such, the first polariser 24 is arranged in a direction relative to the beam splitter 22 which is perpendicular or substantially perpendicular to a direction in which the second polariser 26 is arranged relative to the beam splitter 22.

The linear polarisers 24, 26 have non-zero reflectivity and preferably are wire grid linear polarisers The first polariser 24 is arranged to polarise the first beam part by transmission in a first direction, and the second polariser 26 is arranged to polarise the second beam part by transmission in a second direction. The first and second directions may be the parallel or perpendicular. Preferably, the first and second directions are parallelly arranged. In this way, the first polariser 24 and the second polariser 26 may preferably both polarise transmitted light, i.e., p-polarised light, which may be perpendicular to the page from the perspective of Figure 1.

Furthermore, assuming the polarised detector 18 has a polarised pixelated mask with four orientations (0°, 45°, 90° and 135°), then the axis of the first polariser 24 can be along 0°, 45°, 90° or 135°, and the second polariser 26 is either parallel or perpendicular to the first polariser 24. Preferably, polarisers 24 and 26 have parallel axes.

The light transmitted by the first polariser 24 may be referred to as a first transmitted sub-30 part and the light transmitted by the second polariser 26 may be referred to as a second transmitted sub-part.

The first and second polarisers 24, 26 also reflect light which is polarised. For example, the first polariser 24 is arranged to polarise the first beam part by reflection in a third direction and the second polariser 26 is arranged to polarise the second beam part by reflection in a fourth direction. The third direction is perpendicular to the first direction, and the fourth direction is perpendicular to the second direction. The third and fourth directions are thus preferably parallel. As such, the first and second polarisers 24, 26 may preferably both polarise reflected light, i.e., s-polarised light, which may be parallel to the page from the perspective of Figure 1.

The light reflected by the first polariser 24 may be referred to as a first reflected sub-part 10 and the light reflected by the second polariser 26 may be referred to as a second reflected sub-part.

The first reflected sub-part and second reflected sub-part return along the same path as the first and second beam parts respectively due to the orientation of the surfaces of the first polariser 24 and the second polariser 26.

Whilst parallel orientation of the axes first and second polarisers is preferred, it will be appreciated that the polarisers may in fact have transversely, perpendicularly or orthogonally arranged axes so that the first and second directions are perpendicular to each other, the third and fourth directions are perpendicular to each other, the first and fourth directions are parallel with each other, and the second and third directions are parallel with each other.

The first reflective surface 28 is arranged to reflect the first transmitted sub-part, and the second reflective surface 30 is arranged to reflect the second transmitted sub-part. The first and second reflective surfaces 28, 30 are mirrors. The reflective surfaces 28, 30 are preferably arranged perpendicularly or substantially perpendicularly to each other. Whilst described as being perpendicular or substantially perpendicular, to produce image shearing, one of the reflective surfaces may be tilted. For example, the second reflective surface 30 may be tilted.

The source of coherent light 14 is preferably a laser. The source 14 is arranged to emit coherent light onto the test piece 12 so that light is diffracted towards the beam splitter 30 22.

The spatial-phase-shift shearography system 10 may further include a beam expander or a diffuser for expanding the coherent light emitted by the source of coherent light 14.

The test piece 12 may be positioned on a test-piece receiver or surface, for example.

The spatial-phase-shift shearography system 10 may further include an aperture and a 5 lens between the test piece 12, or test-piece receiver, which may be for focussing the reflected light from the test piece 12 towards the beam splitter 22.

The detector 18 is preferably a pixelated polarised camera, for example pixelated PhaseCam interferometers from 4D Technology or the Polarization CMOS Image Sensor from Sony (RTM).

The spatial-phase-shift shearography system 10 is used as part of a spatial-phase-shift shearography testing system. The testing system also comprises a stressing means for deforming the test piece 12. Such deformation may be for example provided by heating the test piece 12. As such, the heating means may be a heater such as a lamp. Alternatively, the deformation may be provided by mechanically straining the test piece 12.

In use, the test piece 12 is positioned on the test-piece receiver and is initially maintained in a relaxed and/or non-deformed state. To produce a speckle image, the test piece 12 is required to have an optically rough surface. The laser light is emitted from the laser, may be expanded by the beam expander, and is diffracted from the test piece 12 towards the beam splitter 22. The diffracted laser light may pass through the aperture and lens.

A beam of laser light diffracted from the test piece 12 is thus received by the beam splitter 22. The first beam part transmits through the beam splitter 22 towards the first polariser 24, and the second beam part is reflected by the beam splitter 22 towards the second polariser 26.

In a preferred embodiment, the first and second polarisers 24, 26 are arranged to polarise transmitted light in parallel polarisation directions to each other, hence the polarisation of the reflected light is also parallel to each other since the transmitted light and reflected light of a given polariser have perpendicular polarisation.

The p-polarised light transmitted by the first polariser 24, forms a first transmitted sub-30 part. The s-polarised light reflected from the first polariser 24 forms a first reflected sub-part. The first reflected sub-part is reflected back towards the beam splitter 22. The first transmitted sub-part is reflected by the first reflective surface 28 back towards the beam splitter 22.

The p-polarised light transmitted by the second polariser 26 forms a second transmitted sub-part. The s-polarised light reflected by the second polariser 26 forms a second reflected sub-part. The second reflected sub-part is reflected back towards the beam splitter 22. The second transmitted sub-part is reflected by the second reflective surface 30 back towards the beam splitter 22.

The first transmitted sub-part and the second transmitted sub-part may therefore 10 interfere at the beam splitter 22. The second reflected sub-part and the first reflected sub-part may therefore also interfere at the beam splitter 22.

However, if the two polarisers are perpendicular to each other, the first transmitted subpart and the second reflected sub-part may therefore interfere at the beam splitter 22. The second transmitted sub-part and the first reflected sub-part may therefore also 15 interfere at the beam splitter 22.

The interference of the linearly polarised light occurs without a quarter wave plate, and therefore no quarter wave plate is required for the spatial-phase-shift shearography system 10.

The interfered light is then detected by the pixels with either 00 or 90° polarisation direction on the polarised detector 18. For those pixels with 45° and 135° polarisation, all the four polarisation lights will have a projection on them which are essentially the same but with low fringe contrast when image subtraction is performed. This means that although there are four polarisations in the pixels of a polarised camera, only two phase-shifted shearography fringe patterns can be extracted with high contrast by image subtraction (the 0° and 90° phase-shifted fringe patterns). The fringe patterns from pixels with 45° and 135° polarisation may be also used to provide additional conditions for improving the phase calculation results via suitable algorithms such as principal component analysis.

The detector 18 thus produces a reference speckle image which is received by the 30 processing device 20.

The test piece 12 is then deformed via the stressing means and a test speckle image is produced in a similar or identical way as the reference speckle image. The reference speckle image is also received by the processing device 20.

The reference speckle image and the test speckle image are then compared which 5 produces a fringe pattern. Such an example is shown in Figure 2.

By arranging all the pixels with the same 0° and 90° polarisation angles two phase-shifted fringes patterns can be produced, as shown in Figure 3a and Figure 3b. A direct current component of the fringe pattern is removed in Figures 3a and Figure 3b.

The two images are then demodulated using various phase extraction algorithms such 10 as Gram-Schmidt (GS) orthonormalization or principal component analysis (PCA). Such a demodulated image is shown in Figure 4.

The demodulated image is then denoised using Windowed Fourier Filtering (WFF). Such a denoised image is shown in Figure 5.

The denoised image is then unwrapped. Such an unwrapped image is shown in Figure 15 6.

Whilst light is described, it will be appreciated that other coherent radiation may be considered.

Whilst wire grid linear polarisers are described, it will be appreciated that other types of linear polarisers may be considered assuming that reflection from the linear polariser is 20 possible.

It is therefore possible to provide a shearography system using a polarised camera without involving circular polarisation which would conventionally require a quarter wave plate. The utilisation of light, which is reflected from a polariser, as well as which is transmitted by the polariser, results in orthogonal polarised light produced from a single polariser.

The words 'comprises/comprising' and the words 'having/including' when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The embodiments described above are provided by way of examples only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined herein.

Claims

Claims 1 A spatial-phase-shift shearography system without a quarter wave plate for imaging a test piece, the spatial-phase-shift shearography system comprising: a beam splitter for splitting coherent light from the test piece into a first beam part and a second beam part; a first linear polariser having non-zero reflectivity arranged so as to polarise by transmission the first beam part so as to form a first transmitted subpart which is linearly polarised in a first direction, and arranged so as to reflect the first beam part so as to form a first reflected sub-part which is linearly polarised in a second direction which is perpendicular to the first direction; a second linear polariser having non-zero reflectivity arranged so as to polarise by transmission the second beam part so as to form a second transmitted sub-part which is linearly polarised in a third direction, and arranged so as to reflect the second beam part so as to form a second reflected sub-part which is linearly polarised in a fourth direction which is perpendicular to the third direction; a first reflecting surface arranged so as to reflect the first transmitted subpart; and a second reflecting surface arranged so as to reflect the second transmitted sub-part; the polarisers and reflecting surfaces arranged so that without use of a quarter wave plate the first transmitted sub-part is directly interferable with the second reflected sub-part and the second transmitted sub-part is directly interferable with the first reflected sub-part, or the first transmitted sub-part is directly interferable with the second transmitted sub-part and the first reflected sub-part is directly interferable with the second reflected sub-part.
2 A spatial-phase-shift shearography system as claimed in claim 1, wherein the first and second linear polarisers are configured so that the first and third directions are parallel and the second and fourth directions are parallel so as to polarise the respective transmitted beam sub-parts in parallel directions to each other and the respective reflected beam sub-parts in parallel directions to each other.
3 A spatial-phase-shift shearography system as claimed in claim 1, wherein the first and second linear polarisers are configured so that the first and fourth directions are parallel and the second and third directions are parallel so that the first transmitted sub-part and the second reflected sub-part are polarised in parallel directions, and the second transmitted sub-part and the first reflected sub-part are polarised in parallel directions.
4. A spatial-phase-shift shearography system as claimed in any one of the preceding claims, wherein the beam splitter is a partially silvered mirror.
5. A spatial-phase-shift shearography system as claimed in any one of the preceding claims, wherein the first and second linear polarisers are wire-grid polarisers.
6 A spatial-phase-shift shearography system as claimed in any one of the preceding claims, wherein the first linear polariser and the first reflecting surface are arranged in a direction relative to the beam splitter which is perpendicular or substantially perpendicular to a direction in which the second linear polariser and the second reflecting surface are arranged relative to the beam splitter.
7 A spatial-phase-shift shearography system as claimed in any one of the preceding claims, wherein the first and second reflective surfaces are at or adjacent to the first and second polarisers respectively.
8. A spatial-phase-shift shearography system as claimed in any one of the preceding claims, wherein the first and second polarisers are polariser plates.
9 A spatial-phase-shift shearography system as claimed in any one of the preceding claims further comprising: a source of coherent light for producing a beam which is diffractable from a test surface of the test piece; and a polarised detector for receiving the first and second transmitted subparts and the first and second reflected sub-parts.
10. A spatial-phase-shift shearography system as claimed in claim 9, wherein the source of coherent light is a laser.
11.A spatial-phase-shift shearography system as claimed in claim 9 or claim 10, further comprising a beam expander for expanding the beam produced by the source of coherent light.
12. A spatial-phase-shift shearography system as claimed in any one of the preceding claims, further comprising an aperture and a lens.
13. A spatial-phase-shift shearography system as claimed in any one of the preceding claims, further comprising a stressing means for deforming the test surface.
14 A method of testing a test piece using a spatial-phase-shift shearography system as claimed in claim 13 and without involving circular polarisation, the method comprising the steps of: a) producing a reference speckle image of a test surface of the test piece in an un-deformed state; b) deforming the test piece using the stressing means so that the test piece and test surface is in a deformed state; c) producing a testing speckle image of the test surface in the deformed state; and d) comparing the reference speckle image and the testing speckle image.
15. The method as claimed in claim 14, the test piece is a wind-turbine blade.