NO322717B1 - System and method for painting properties in or below the surface of a material, as well as its use - Google Patents

Description

This invention relates to a method and a system for measuring properties at, that is to say in or under, the surface of materials.

Material testing and non-destructive testing (NDT) of materials represents a subject area of ever-increasing importance. The use of slim and light constructions as well as the use of new and specially adapted materials require increasingly better tools for testing the various structures. Many of the traditional NDT methods are to a limited extent suitable for testing and troubleshooting in more modern construction materials, such as polymeric composite materials, and the need for new and improved test methods is increasing.

Testing of materials and detection of defects is useful and required in various contexts, both during development, production, product testing and during regular inspection of structures in use. Such testing can be time-consuming and expensive both because the equipment and the testing are expensive in themselves, and because the product to be tested is taken out of use during the testing. Typical examples of products where regular testing is a demanding and expensive process are in the aviation industry, shipping, and in many other contexts where high safety factors are required.

A technology that has begun to gain a foothold in the testing of materials, and especially composite materials, is electronic shearography. This is an optical interferometric method where small movements of the object's surface are measured over an entire surface (full field measurement). In the case of different loads on the object, surface defects and defects below the surface can affect the movement pattern of the surface, and thus these defects can be detected and recorded. The loading can be dynamic with the application of vibrations with a single frequency or many frequencies at the same time, or the loading can be static using various methods such as pressure or vacuum, heating, mechanical loading and more.

Description of a typical shearographic system

Different names are used for shearographic systems, such as shearography, electronic shearography, digital shearography, etc. Common to most systems today is that they use so-called phase shift techniques for calculating the surface movement, whether the movement is dynamic or static. The basic principle in shearography is that the object's surface is illuminated with one or more expanded laser beams or other coherent light sources, and that the illuminated object area is imaged onto a detector array, for example a ccd array using a lens system. Using optical components such as a beam splitter, the light wave from the object is split into two waves, either before it passes through the imaging system, or after the light has passed through the imaging system. In this way, you get two separate images of the object superimposed on each other on the ccd array, where one image may be slightly shifted laterally in relation to the other. Figure 1 shows a typical shearographic setup, based on a so-called Michelson's interferometer. The beam splitter BS splits the light into two waves, and one wave goes via mirror Ml and to the ccd array 5, while the other wave goes via mirror M2 and to the ccd array 5. The two beams each form a separate image of the object on the ccd the array, and by tilting or turning one or both mirrors M1, M2, the mutual displacement of the two images (= the shear) can be adjusted both in size and direction. An example of such a solution is given in EP 1099947.

In this way, it is achieved that each point on the ccd array 5 receives light from two neighboring points on the object surface. The light from these two neighboring points on the object surface interferes in the image plane, and this interference provides information about how much one point moves in relation to the other point.

This effect applies to every point in the entire ccd array, and a measurement is obtained that is sensitive to the gradient of the surface movement, while homogeneous displacements (displacements like a piston) are recorded to a small extent.

Figure 1 illustrates the known technique.

The fact that piston-like movements are not recorded means that the system is relatively insensitive to external noise sources and vibrations, unlike many other interferometric techniques. Noise signals and vibrations from the surroundings will tend to cause the object to vibrate at relatively low frequencies, and these low-frequency vibration modes have relatively low spatial gradients in the movement pattern, and thus they will be registered to a limited extent by the shearographic system.

When the two light waves interfere, this can be described by the intensity I(x,y) at the points (x,y) on the ccd array by the equation:

where Ix ( x, y) is the intensity in light wave 1 (picture 1) on the ccd array (x,y)

I2( x, y) is the intensity in light wave 1 (picture 2) on the ccd array (x,y)

<Xi( x, y) is the optical phase of light wave 1

a2 ( x, y) is the optical phase of light wave 2

The information about the object's movement (or the gradient of the movement) is given by the change in the optical phase difference between the two light waves, i.e. change of the term ( a^ x, y)- a2{ x, y)) in equation (1).

When so-called phase shift technology is used, this is typically done by moving one of the mirrors a small distance from or towards the beam splitter BS in Figure 1, so that a controlled change of the optical phase afay) or cc2( x, y) is obtained to a of the interfering rays. By doing this repeatedly, one can acquire several images (also called an interferogram) and use the intensity values I(x,y) in these so-called phase-shifted images to calculate the optical phase difference ( ax ( x, y) - a2 ( x, y )) in equation (1). In this way, numerical values for static deformations can be obtained, and the method can also be used to increase the contrast in strip images when measuring dynamic displacements (vibrations).

Shearography is typically used for the detection of defects in polymer-based composite materials such as honeycomb materials, laminates and other bonded structures. Typical defects detected are delaminations, debonds, core breaks, etc. The most typical test methodology with shearography is based on vacuum loading, where you measure static movement of the surface while changing the pressure against the object's surface. Such testing can be done inside tight spaces or containers where the air pressure can be changed, or vacuum bells can be used that are held against the surface of the object and where a shearographic measuring head is integrated into the vacuum bell.

Shearographic testing with dynamic loading is also used, and a typical method is based on the use of an entire band of frequencies ("white noise"), where areas where there are defects are made to vibrate while defect-free areas vibrate to a lesser extent or with less rash.

Description of the invention

This invention includes equipment and a method for measuring vibrations with shearography, where the sensitivity of the measurement is significantly improved compared to conventional shearography. This increase in sensitivity allows defects in the material to be detected when loading and vibrating the object at a single frequency at a time. Thus, it is an object of this invention to provide an improved system and method for measuring properties, and in particular defects, in or under the surface of a material.

More specifically, the above objectives have been achieved as stated in the requirements.

The invention also includes an averaging technique for static measurements with shearography, so that the resolution of defects is significantly increased compared to traditional shearographic measurements.

The invention will be described in more detail below with reference to the attached figures, which illustrate the invention by means of examples.

Figure 1 illustrates shearography measurements according to the known technique

Figure 2 illustrates a preferred embodiment of the invention.

Figure 3-5 illustrates different measurement results.

Figure 2 shows a principle sketch of the invention, where a case is shown where a shearograph principle based on a Michelson's interferometer has been chosen. Light is projected onto the object 2 from a suitable light source 1 with a sufficient coherence length to achieve interference between the two branches in the interferometer, and where part of the light 3 is reflected or scattered towards the interferometer. The invention implies that one Ml or both mirrors Ml and M2 in the interferometer can be moved in small steps towards or away from the beam splitter BS controlled by a control signal 7. According to the invention, one or both mirrors will be impressed 9 with a vibration movement or other periodic movements in the direction towards/from the beam splitter BS, where both the step displacement and the vibration are preferably controlled by the system's computer. In addition, one or both mirrors can be tilted so that the shear can be adjusted both in size and direction.

The object is vibrated using a shaker 6 or another device, and the vibration is preferably controlled by the system's computer with a signal 8. The computer can adjust the frequency and amplitude both for the object excitation and for the mirror, and at the same time also control the relative vibration phase between the object and the mirror vibration. The invention is based on a recording algorithm where both mirror and object are vibrated with the same or approximately the same frequency, and where the phase between the two vibration signals of object and mirror is varied according to a given algorithm.

The object is illuminated with one or more expanded laser beams, and the invention also includes a unit 10 which makes a geometrical change in the lighting geometry. When laser light is reflected from a normal rough surface, the reflected light will contain so-called speckle noise due to the light's high coherence. When a geometrical change is made in the lighting geometry, this speckle pattern is changed, and by acquiring many such images with changed or decorrelated speckle patterns and then summing them together (average), this speckle noise can be completely or partially averaged out. The change in the lighting geometry can be done in different ways, for example by letting the laser steel or laser beams pass through one or more inclined glass plates 10, and where the glass plate(s) are tilted or rotated around an axis 11 so that the illumination point (point source) is moved (apparently) in the room, for example along a circle. The illumination geometry can also be changed by physically moving the laser(s), by using mirrors that can be tilted or rotated, by small movements of the entire optical head, or by small movements of the entire object. It is also possible to perform the speckle averaging by, for example, changing the position of an aperture in the imaging system.

One of several alternative recording and calculation algorithms is as follows:

1. The object is vibrated with a certain frequency, for example 2000 Hz, but other frequencies can also be used.

2. The mirror is vibrated with the same frequency and with a given amplitude

3. 4 images are obtained and digitized, where one mirror has been moved a length L/8 in the direction from or towards the beam splitter between each image taken. L is the wavelength of the laser light. The four phase-shifted images are named Bl, B2, B3 and B4. 4. The following size (image) is calculated: Il = sqrt((Bl-B3)<*>(Bl-B3)+(B2-B4)<*>(B2-B4)). The calculation takes place at pixel level, so that each pixel in II is assigned a value which is a function of corresponding pixel values in II, B1, B2, B3 and B4, given by the equation above. 5. The lighting geometry is changed so that you get a new decorrelated speckle pattern in the image, and you go back to point 3 above and bring in 4 new phase-shifted images. A new size 12 is then calculated in a similar way to II described above. This procedure is repeated until you have a number of values (images) II, ...IN, and these images are summed and averaged to a single resulting image RI. N can typically be from 2 to 100, or higher. 6. The phase between the object and mirror vibration is changed by 90 degrees, and points 3-5 above are repeated so that a new resulting image R2 is obtained. This is done a total of four times, so that you get 4 sizes (resultant images) RI, R2, R3 and R4. 7. Steps 3-5 are repeated a further 2 or more times, but without object vibration, only with vibration on the mirror, so that further new resultant images are collected. For example, one can create one resultant image R5 without object or mirror vibration, and one resultant image R6 without object vibration but where the mirror is vibrated corresponding to the "first dark stripe" in the Bessel function 8. Based on the acquired data sets (images), the amplitude A(x, y) and phase F(x,y) of the object's vibration (or the gradient of the vibration in shearography) is calculated on a pixel basis using an algorithm that involves the use of inverse trigonometric functions.

When the phase and amplitude have been calculated, the output U(x,y) of the vibration gradient for a certain vibration phase can be calculated using the expression U(x,y) = A(x,y) -cos( F(x,y) + T), where T is a phase with a value in the interval (0 - 360) degrees. By selecting several (e.g. 20) different phases T distributed within (0-360) degrees, one can calculate the vibration output as it progresses through an entire vibration period, and plot or show this in a graphic representation as an animation. Figure 3 shows an example of a single such plot in three dimensions, where the gray scale shows the division of the result in relation to a selected unit.

The shearographic instrument can be designed differently from that shown in figure 2. One can have other shear mechanisms, for example based on birefringent materials or based on the use of two imaging systems or other designs. The essential thing is that you can perform phase stepping by stepwise movement of a mirror and that you can phase modulate (vibrate) the one light wave in a controlled manner. It is also essential that one can perform averaging over many speckle patterns by decorrelating the speckle pattern in the light entering the ccd array.

The vibration applied to the structure being inspected may be transmitted by direct contact with one or more shakers, by the use of non-contact shakers or transducers, by acoustic transmission or by other means. If the energy is transferred in one or more small points, a vibration can be achieved in the form of traveling waves or surface waves that spread beyond the structure. If these traveling waves pass an area with a material defect, for example a delamination or a debond, the traveling waves can initiate local forms of vibration in the defect area, where the vibration deviates from the vibration you would get if you did not have a defect present. In this way, the defect can be detected. Defects can also be recorded if the object is vibrated with a normal vibration mode (not traveling wave or surface wave) as long as the areas with defects vibrate with a vibration pattern that deviates from the vibration as it would be without defects present.

Since the invention is based on the acquisition and averaging of a number of images during the measurement, the measurement is relatively insensitive to external noise since such noise can be averaged away. Although there may be noise frequencies present that have amplitudes much higher than the imprinted vibration amplitudes, the invention will filter out and eliminate these, and vibration amplitudes (or gradient amplitudes with shearography) down into the nanometer range can be recorded. The good resolution of small amplitudes is crucial for using the invention for registering defects.

The measurements are made contactless and can be made at a distance of several metres, and areas from

less than one square centimeter and up to several square meters can be measured in each individual recording. The excitation can be done using a shaker or several shakers that are held against the surface of the object using, for example, a long rod or several rods, so that the operator and equipment can be located at a distance of several meters from the object.

Compared to many other methods, the method will be very time-saving, and it can be used even if the object has an irregular shape, and measurements can be made on

or near corners and flanges and so on on constructions such as boat hulls and the like.

As mentioned, the invention can also use an averaging algorithm for static measurements or for long periods, for example in connection with vacuum loading or thermal loading. This is done in the following way: i) Three or more phase-shifted images are used to calculate the optical phase difference ( ax ( x, y) - a2 ( x, y)) in equation (1) between the two interfering light waves on the image plane (on the ccd array)

ii) The object is subjected to a static deformation through the use of heat, vacuum or in some other way, so that a possible material defect can be registered with good or less good contrast

iii) Three or more phase-shifted images are again used to calculate the new optical phase difference between the two interfering light waves on the image plane (on the ccd array)

iv) One subtracts the optical phase difference calculated in point i) (before object loading) from the phase difference calculated in point iii) (after object loading) and obtains a resulting phase map as for example shown in Figure 4. This phase map contains direct information about the object's movement which a consequence of the object loading.

v) The spatial gradients of the phase map are calculated so that a

measurement image as shown in figure 4b.

vi) The lighting geometry described earlier is changed so that a

new decorrelated speckle pattern from the object surface

vii) The procedure at points i) to vi) is repeated several times so that you get more

measurement images (typically 5-50 images) corresponding to the image in Figure 4.

viii) All these measurement images are then summed (averaged) so that a significantly reduced noise level is achieved in the image, so that any defects can be registered more easily. This is illustrated in figure 5.

To summarize, the invention relates in particular to a method for measuring small vibrations in the surface of materials, where the preferred embodiment may include the following points: - use of an instrumentation where one or both of the light waves in the interferometer can be phase-shifted in steps (phase stepping) - use of an instrumentation where one or both of the light waves in the interferometer can be phase-shifted or phase-modulated by a vibration with a single frequency at a time - use of an instrumentation where the speckle pattern on the detector array can be changed or decorrelated so that one can perform averaging over several images to reduce speckle - the noise in the resulting image

- use of instrumentation for vibration excitation of the measurement object

- that one vibrates the object with a selected frequency

- that one vibrates (phase modulates) one of the light waves in the interferometer with the same or approximately the same frequency - that one controls the amplitude of both the object vibration and the phase modulation of the one light wave in the interferometer - that one controls and changes the vibration phase difference between the object vibration and the phase modulation of the one light wave in the interferometer during the recording - that one acquires and digitizes various images while the computer controls the vibration of the object, the vibration or the phase modulation

of the one light wave in the interferometer, the phase stepping in the one light wave in the interferometer as well as the speckle decorrelation unit 10. - that one uses the digitized images (pixel by pixel) in an algorithm to calculate the amplitude and vibration phase of the object over the entire imaged field - that one calculates the fluctuation of the vibration for several phases within (0-360) degrees and generates animations of the vibration - that one measures small vibrations (or gradients of the vibration since it is shearography) in the object to be tested in the manner described above, and that one identifies defects in the material in that the area of the defect vibrates in a different way than one would expect vibration to be without a defect. One can also study the amplitude map and/or the phase map directly to detect defects in the material

According to the invention, the method can also be used to measure small static deformations in the surface of the object, in the following steps: - use of instrumentation where one or both light waves in the interferometer can be phase-shifted in steps (phase stepping) - use of instrumentation where the speckle pattern on the detector - the array can be changed or decorrelated so that one can perform averaging over several images to reduce the speckle noise in the resulting image - use of so-called phase shift recordings to calculate the deformation or deformation phase of the object when the object is loaded by pressure/vacuum, by heating, by static loading or otherwise

- calculation of the spatial gradients of the deformation phase

- use of the speckle decorrelation unit 10 to change the speckle pattern on the detector array - repeated acquisitions as described, and then a summation or averaging of the images with the spatial gradients - use of the method above to detect defects in materials with increased resolution of defects

Claims (14)

1. Method for measuring properties in or under the surface of a material, which material is exposed to an essentially periodic movement, comprising the following steps: - projection of an expanded light beam onto the surface, where the light beam has a selected coherence length, - reception and dividing the light that is reflected from the surface into two light paths, which light paths have a length difference that is smaller than the coherence length of the light, and - combining the light beams and directing them towards a detector device to form an interference pattern on the surface of the detector device, where the detector device forms a imaging of the pattern on the surface characterized in that the method also includes - imposing a periodic fluctuation in optical path length in at least one of the light paths, where this period includes one or more frequency components corresponding to expected frequency components in the movement in the surface, 2. Method according to claim 1, regulation of the length of at least one of the light paths, preferably in stages. 3. Method according to claim 1, where the movement in the surface is impressed with a selected transducer, and where the periodic length fluctuation in at least one of the light paths is impressed with essentially the same frequency as the impressed movement in the surface. 4. Method according to claim 3, where the phase difference between the impressed movement in the surface and in the light path is controlled. 5. Method according to claim 1, where the light paths are defined by a Michelson interferometer where at least one of the mirrors has a variable position along the optical axis. 6. Method according to claim 1, comprising providing a variation in the light beam directed towards the surface, e.g. by periodically or stepwise displacement of this, to reduce speckle noise on the sensor, and formation of the image by recording that number of images or image series, where the lighting is changed between each image/image series. 7. System for measuring properties in or under the surface of a material, which material is exposed to an essentially periodic movement, comprising: - a light source with a selected coherence length, - projection means for projecting a light beam onto the surface, - beam splitting means for dividing the light that is reflected from the surface into two light paths, which light paths have a length difference that is smaller than the coherence length of the light, and - beam combining means for combining the light beams in the two beam paths and directing them towards a detector device to form an interference pattern on the detector device's surface, where the detector device is arranged to register the incident light and form an image of the interference pattern characterized in that the system also includes - activation means for imposing an essentially periodic length fluctuation in at least one of the light paths, where this period includes one or more frequency components corresponding to expected frequency components in the movement in the surface. 8. System according to claim 7, where at least one of the light paths has an adjustable, and preferably incrementally adjustable length. 9. System according to claim 7, where the movement in the surface is impressed with a selected transducer, and where the periodic length fluctuation in at least one of the light paths has essentially the same frequency as the impressed movement. 10. System according to claim 9, where the phase difference between the impressed movement in the surface and in the light path is controlled. 11. System according to claim 7, where the light paths are defined by a Michelson interferometer where at least one of the mirrors has a variable position along the optical axis. 12. System according to claim 7, comprising a device for varying the light beam directed towards the surface, e.g. by periodic or gradual displacement of this, to reduce speckle noise on the sensor, and where the system includes recording means to combine that number of images or image series where the lighting has changed between the images/image series 13. Application of the system according to claim 7 for shearography measurements. 14. Application of the system according to claim 7 for detection of discontinuity, such as delamination, breakage and the like, in or below the surface.