BE1025565B1 - DEVICE AND METHOD FOR NON-DESTRUCTIVE CONTROL OF STRUCTURES - Google Patents

Abstract

The invention relates to methods that utilize a device for non-destructive control of an object by holographic interferometry, comprising a set of elements (2, 3, 4) for acquiring an interferometric image of an area of the object (10), said assembly comprising at least one coherent light source (2), an interferometer (4) and a camera (3) for sensing the interferometric image, characterized in that the device comprises in addition to a thermal camera (11) capable of capturing a thermographic image of at least a portion of said area of the object (10).

Description

DEVICE AND METHOD FOR NON-DESTRUCTIVE INSPECTION OF STRUCTURES

FIELD OF THE INVENTION The invention relates to devices and methods intended for non-destructive testing (NDT, Non-Destructive Testing) of structures by holographic interferometry technology, which includes methods such as shearography or ESPI (electronic speckle pattern interferometry).

STATE OF THE ART [0002] Shearography and ESPE are NDT methods well known in the industry. These are technologies based on the principles of holographic interferometry and in which an interferometer and a digital camera are used to detect a deformation of the surface of an object subjected to a non-destructive stress, such as a stimulus. mechanical or thermal. A structural defect, for example local delamination of a composite material, appears on the interferometric image in the form of a characteristic interference pattern. Shearography is a particularly advantageous technique because, unlike ESPE, it takes into account the spatial derivative of the deformation, thus making it possible to get rid of rigid body movements.

In this regard, it is known to apply a thermal stimulus as a constraint to the object. For example, an infrared lamp can be used for this purpose which will heat the object by radiation for a certain time. In this case, it is important to know when the object is sufficiently stimulated to

BE2017 / 5666 allow acquisition with optimal dynamics. It is known in this regard to use a pyrometer measuring the temperature of a local area of the surface of the object. For objects of complex shape or in the case of large areas to be inspected, this method is however not adequate, in the sense that the information is not complete but partial, since the temperature measurement obtained by the pyrometer is local information giving no information on the spatial distribution of the thermal impact.

It is advantageous to equip the shearography devices or

ESPI of a means of thermal stimulation (by radiation) without contact, integrated in the device. This arrangement makes it possible to apply a reproducible stimulus. The disadvantage is that this type of device offers little freedom in the positioning of the thermal source. The use of a source not integrated into the device offers the advantage of greater freedom of positioning of the stimulation, but at the cost of a loss in the reproducibility of the measurements.

Another drawback of existing devices and methods is related to the fact that certain defects are difficult to detect, since they are embedded in an overall deformation which is the result of the stimulus.

A measurement system which combines shearography and thermography is disclosed for example in the document 'Improved defect detection with combined shearography and thermography, Schmidt, Internation Conference on Applications for Image Based Measurements, 6 and 7 March 2012, Ulm, Germany.

Characteristic Elements of the Invention The present invention relates to methods as described in the claims. The invention relates more particularly to methods which use a device for non-destructive testing of an object by holographic interferometry, such as shearography, comprising a set of elements which make it possible to acquire an interferometric image of an area. defined object, said assembly comprising at least one coherent light source, an interferometer and a camera for capturing the interferometric image, characterized in that the device further comprises a

BE2017 / 5666 thermal camera capable of capturing a thermographic image of at least part of said defined area of the object.

The invention also relates to a method of non-destructive testing of an object by holographic interferometry, such as shearography, using a device as described above, comprising the following steps:

• Application of a thermal stimulus to at least one defined zone of the object, by the activation of a thermal source at an instant ti and the deactivation of said source at an instant t2, • Obtaining an interferometric acquisition of reference and an interferometric measurement acquisition, said acquisitions being relative to said area of the object, respectively at a reference instant and at a measurement instant, the reference and measurement instants corresponding to two distinct thermal states, an interferometric image of said zone expressing a difference between the reference and measurement acquisitions, characterized in that the instant t2 is chosen in real time on the basis of thermographic images of at least part of said zone, obtained during the application of the stimulus, the thermal source being deactivated as soon as at least part of the thermographic image corresponds to a th desired image. According to another embodiment, the method according to the invention, for non-destructive testing of an object by holographic interferometry, such as shearography, using a device as described above, comprises the following steps :

• Application of a thermal stimulus to at least one defined zone of the object, by the activation of a thermal source at an instant ti and by the deactivation of said source at an instant t2, • Obtaining an interferometric acquisition of reference and an interferometric acquisition of measurement, said acquisitions being relative to the laditezone of the object, respectively at a reference instant and at a measurement instant, the reference and measurement instants corresponding to two distinct thermal states,

BE2017 / 5666 • production of an interferometric image of the said zone which expresses a difference between the reference and measurement acquisitions, characterized in that:

• the method also includes taking a sequence of thermographic images of at least part of said zone, before, during and after the application of the stimulus, • the reference time is chosen on the basis of the correspondence d 'at least part of the thermographic image to a desired image.

The method described in the previous paragraph can also include the following characteristics:

• a sequence of interferometric acquisitions is obtained before, during and after the application of the stimulus, • the sequence of thermographic images is synchronized with the sequence of interferometric images, • the reference interferometric acquisition is the acquisition obtained at the instant when at least part of the thermographic image corresponds to a desired image.

According to another embodiment, the method according to the invention, for non-destructive testing of an object by holographic interferometry, such as shearography, using a device as described above, comprises the following steps :

• Application of a thermal stimulus to at least one zone of the object, by the activation of a thermal source at an instant ti and by the deactivation of said source at an instant t2, • Obtaining an interferometric acquisition of reference and an interferometric measurement acquisition, said acquisitions being relative to said area of the object, respectively at a reference instant and at a measurement instant, the reference and measurement instants corresponding to two distinct thermal states, an interferometric image of said zone which expresses a difference between the reference and measurement acquisitions, characterized in that:

BE2017 / 5666 • the method also comprises taking a sequence of thermographic images of at least part of said zone, before, during and after the application of the stimulus, said sequence comprising an image obtained at the time of measurement, • a mathematical operation is performed between the respective values of a matrix of digital values derived from said interferometric image and a matrix of digital values derived from the thermographic image obtained at the time of measurement, the matrices having the same size and relating to a common part between the interferometric and thermographic images, the operation making it possible to obtain a recombined image by said mathematical operation, • detection in the recombined image of one or more details (such as defects) of the object, said details having improved visibility compared to the visibility of these details in the image i nterférométrique.

Said mathematical operation can be an algebraic operation, such as a subtraction or a division of respective values of the data sets.

The method described in the previous two paragraphs can also include the following characteristics:

• a sequence of interferometric images is obtained before, during and after the application of the stimulus, • the sequence of thermographic images is synchronized with the sequence of interferometric images.

Brief description of the figures [0014] FIG. 1 represents a schematic view of a shearography system as known in the state of the art.

Figure 2 shows a shearography device according to a first embodiment of the invention.

FIG. 3 represents an example of a thermal cycle to which an object is subjected during a shearography test, with the temperature in the ordinate axis and the time in the abscissa axis.

BE2017 / 5666 [0017] Figure 4 shows an example of a thermographic image and a shearographic image of the same area of an object.

FIG. 5 represents the profiles detected on the basis of the images shown in FIG. 4.

Figure 6 shows the subtraction of the data shown in Figure 5.

Figure 7 shows a second embodiment of the device according to the invention.

Detailed description of the invention The present invention will be described below and in detail in preferred embodiments of the invention and with the aid of the appended figures which represent preferred and non-limiting variant embodiments of the invention.

The invention will be described on the basis of a shearography device, without the scope of the invention being limited to this type of device. A shearography device as known in the state of the art is shown diagrammatically in FIG. 1. The following elements are indicated:

- an infrared lamp 1 for heating an object to be inspected 10,

- a coherent light source, such as a laser 2 to illuminate the object and obtain the interference pattern,

- a digital camera 3, such as a CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) camera to capture the images to be analyzed. The camera operates in ‘global frame’ mode: all the pixels in the image are acquired at the same time.

- an interferometer 4 of the Michelson type, provided with an inclined mirror 5 and a mirror movable 6 by a piezoelectric actuator (PZT) 7, as well as a beam splitter 8. The inclination of the first mirror 5 is such that the reflected beams of two separate points of the object will coincide on a single point of the image plane of the CCD camera. The movable mirror 6 allows the calculation of the gradient of the deformation in a direction outside the plane of the surface of the object. This gradient is proportional to the

BE2017 / 5666 phase difference between two acquisitions, said phase difference being determined by a technique called "phase shifting" and known per se to those skilled in the art. Other types of interferometers having the same functionality are also possible in a shearography device (for example a Fabry-Perrot interferometer),

- a digital unit 9 which regulates and controls the operation of the laser 2, of the CCD camera 3, carrying out the processing of the measurements and the display of the results. Unit 9 is generally a PC (Personal Computer) type computer, configured or programmed to perform the functions described above.

At least all of the elements 2, 3 and 4 are incorporated in a housing which forms the shearography device, also called a shearography camera.

According to a particular embodiment illustrated in Figure 2, the shearography device used in the present invention further comprises a thermal camera 11. This type of camera is known per se. Such a camera is able to take a succession of thermographic images of the object by representing the temperature distribution over a surface of the object. The device according to the invention therefore makes it possible to apply the principles of active thermography in combination with the shearography technique. Active thermography is a non-destructive testing technique that also uses heating as an excitation. A change in the thermal field, caused by heating, is detected by the thermal camera, and is capable of indicating a potential defect in the object under study.

According to the form illustrated in Figure 2, the operation of the thermal camera 11 is controlled by the configured digital unit 9, in addition to its mission of managing the shearography functions, to adjust the thermal camera 11 as well as the processing measurements and display of the results obtained by the thermal camera 11.

In general, the integration of a thermal camera 11 makes it possible advantageously to obtain, by a single thermal stimulus, an evaluation of the object by two different NDT techniques, and thus to increase the information obtained.

BE2017 / 5666 [0027] In addition, the device makes it possible to improve the measurement of shearography by using thermographic data. The improvements brought by the thermographic data are the following: the improvement of the reproducibility of the measurements, and the improvement of the detectability of certain defects. These uses will be described below using methods applying a device according to the invention.

A shearography measurement always requires two steps:

- a first shearographic acquisition of the object which serves as a reference,

- a second shearographic acquisition of the object which is used as a measurement, and which will be compared with the reference acquisition in the form of a shearographic image expressing a phase difference between the measurement acquisition and the reference acquisition.

A "shearographic acquisition" is defined in the present context as a phase image acquired by the CCD camera, ie an image which represents the phase of the intensity signal measured by the camera on an area of the object. The equation of the intensity of the optical wavefront returning from the object at a given time t can be described by the formula:

l (x, y) = A (x, y) cos (œt + φ),

In this equation:

• l (x, y) representing the intensity at a point of coordinates (x, y) what is called the shearogram, • A (x, y) represents the amplitude modulation of the point of coordinates (x, y ) due to the object, • ω represents the pulsation of the wavelength λ taken into consideration, ω corresponding to 2π / λ • φ represents the optical phase [0029] Any displacement d of the object outside the plane of the camera will induce a phase modification by a factor of 2πό. The information on the out-of-plane position of the skin of the object is therefore carried by the phase φ, which can be extracted from the shearogram for example by the known method of phase unwinding. Preferably, the shearographic acquisition (i.e. the phase) is

BE2017 / 5666 calculated on the basis of a series of shearograms acquired in sequence almost instantaneously according to the phase shifting technique. The almost instantaneous acquisition makes it possible to limit the influence of the temporal evolution of the phase on the acquisitions and thus to improve the quality of the individual acquisitions. Also in this context, a sh shearographic image ’represents the phase difference between two sh shearographic acquisitions’ (as defined above) taken at two separate times. The application of a stimulus between the time when the reference phase is taken and the time when the measurement phase is taken allows under certain conditions to highlight possible inhomogeneities / heterogeneities.

FIG. 3 shows an example of a typical thermal excitation cycle expressed as a function of the temperature T at a point of the object studied, as a function of time t. In the device shown in Figure 2, the infrared lamp 1 is turned on at an instant ti marking the start of the thermal cycle. The lamp is off at time t2. The cycle ends when the temperature has returned close to its initial level, at an instant t3. The device of the invention makes it possible to take a sequence of shearographic acquisitions for the duration of the cycle, as well as to take a sequence of thermographic images, the two sequences being synchronized over time. In other words, for each shearographic acquisition, a corresponding thermographic image is taken. The thermographic and shearographic fields of view overlap at least partially.

The thermal information available in a thermographic image is much greater than the measurement of a point temperature by a pyrometer. The thermographic image shows the distribution of temperature over the part of the surface of the object studied. This distribution can be quantified by studying the coefficient of determination R ² and the parameters of a trend curve applied to the distribution. To improve the reproducibility of the measurement, thermographic data can be used in several ways.

A first way concerns the control of the duration of the thermal stimulus. According to this application, the instant t2 at which the

BE2017 / 5666 heater is off is based on a real-time evaluation of the thermographic image. For example, a comparison between the thermal distribution and a predefined distribution is the basis for the extinction of the stimulus and therefore for the instant t2. In general, the choice of time t2 is based on a comparison between at least part of the thermographic image and a desired image. Said desired image can be defined by a thermal distribution, a temperature gradient, a maximum heating temperature or previously specified heating temperatures. This way of controlling the thermal stimulus will ensure that the excitation cycle is performed in a reproducible manner.

A second improvement in the reproducibility of the measurements relates to the choice of the reference time. Knowing that it is common to carry out several measurements of the same object, and therefore to subject the object to several warm-ups, sometimes with a long period between two measurements, it is important to know when the object reaches a state particular thermal that can serve as a reference state. The analysis of the thermographic images makes it possible to choose this instant and to make sure that the object has reached the desired state. Thus, we arrive at a better reproducibility of the results. The choice of the reference time is based on a comparison of at least part of the thermographic image with a predefined thermographic image. This comparison can take several forms, for example: direct comparison of the images (pixel by pixel) or comparison of the temperature distribution of the respective images. Preferably, the choice of the reference time is not made in real time, but this choice is made on the basis of an entire sequence of thermographic images acquired during a thermal cycle.

Referring to Figure 3, a first possibility for the choice of the reference time is the time t _r before the start of the cycle. A thermographic image taken before the first application of the stimulus can be used to quantify the reference thermal distribution. For each subsequent measurement, the reference instant will be determined by choosing the instant at which the thermographic image corresponds with the reference thermographic image (for example based on a comparison of the images or of the distributions).

BE2017 / 5666 However, in certain cases, it is useful to choose the reference instant at an instant in the thermal cycle when the temperature of the object is high. For example, the times t _r 'or t _r ”shown in Figure 3 can be used as reference points. t _r 'slightly precedes the instant t2 during which the heat source is switched off, and therefore after it has been observed that the temperature T stabilizes towards a maximum level. t _r ”is found after time t2. After an abrupt fall 12, the temperature T continues to decrease at a slower speed. The measurement acquisition can be taken at another instant of the cycle or even outside of the cycle ti-t3 ,. The choice of the measurement instant may depend on a number of factors including the type of material of the object tested, the type of defect sought, their depth in the object or certain geometric parameters of the object.

The choice of the reference time during high temperature regimes, such as the points V or t _r ”of the cycle, facilitates the detection of certain types of faults, for example in the case of a workpiece. significant thickness. Since these reference points are chosen during less stable regimes than outside the cycle, means for verifying the thermal state of the object are necessary. As such, we will advantageously use the thermal camera which then demonstrates all its effectiveness. Without the control of the temperature stability on the basis of thermographic images, it would not be possible to choose reference times in a reproducible manner.

The use of thermographic images to define the instant t2 and / or the reference instant t _r , t _r 'or t _r ”makes it possible to produce a device whose source of the thermal stimulus is external, and therefore to have great freedom in the positioning of the source relative to the object studied, but without losing the advantage of the reproducibility of the results.

A third avenue for improving reproducibility consists in verifying the spatial distribution of the thermal excitation (centering, orientation, homogeneity, etc.). This verification, carried out on the basis of thermographic images obtained by the camera according to the invention, makes it possible to optimize the thermal stimulus in terms of intensity, position of the lamp, etc.

The shearographic camera according to the invention also makes it possible to compare the thermal distribution measured by the camera with a distribution

BE2017 / 5666 simulated thermal. This simulated distribution can be obtained on the basis of a numerical model of the object, for example a model calculated by the finite elements model (Finite Elements Model), based on the knowledge of a number of physical parameters, such as the material , the shape and dimensions of the object. These parameters also make it possible to model certain types of defects, as well as to simulate the shearographic and thermographic response to different thermal excitations. The simulation optimizes the thermal stimulus so as to maximize the detectability of certain types of fault. The simulated thermal distribution will serve as a reference during a shearographic measurement carried out with a camera according to the invention.

Another way to improve the detectability of certain faults is to use a combination of shearographic acquisitions and thermographic images taken in a synchronized manner. According to a method of using the device of the invention, a shearographic image, representing a phase difference between two shearographic acquisitions taken respectively at a measurement instant and at a reference instant, is mathematically combined with a thermographic image which represents the thermal state of the object at the time of measurement. More precisely, a mathematical operation is performed on the basis of corresponding pixel values of the two images. This calculation may be preceded by a recalculation of the pixel matrix of at least one of the two images. The thermographic image may have a lower resolution than the shearographic image or vice versa. The recalculation is then advantageously based on an interpolation algorithm between adjacent pixels. This type of algorithm being known in the state of the art, we do not go into more detail on this subject. The object of these preliminary manipulations is to obtain two matrices of values: a first matrix which represents the phase values obtained on the basis of the shearographic image, relating to a number of zones of the object (pixels or recalculated pixels ) and a second matrix which represents the thermal values, obtained on the basis of the thermographic image, relating to the same zones of the object. The mathematical operation is then carried out on the corresponding values of the two matrices, for example a subtraction or a division of the respective values of the two matrices is carried out, leading to

BE2017 / 5666 a resulting matrix. This resulting matrix will be presented in the form of a recombined image of the part of the object concerned.

FIG. 4 represents a thermographic image 15 and a shearographic image 16 of the same object. The thermographic image comprises a region 17 at high temperature, which is reproduced in the form of a region 17 'in the shearographic image due to the presence of deformation gradients which are directly linked to the increase in temperature in said image. region 17 according to the thermal expansion phenomenon. In addition, the shearographic image demonstrates local deformation 18 which is caused by a structural defect in the object. FIG. 5 shows a representation of the values detected and possibly recalculated as described above in order to correspond the number of pixels of the two images, at the level of the horizontal lines x and x 'indicated in FIG. 4. The vertical axes y and y' represent a standardized unit for evaluating the two curves with respect to a common reference. The two curves represent a similar profile, although the local deformation 18 appears only on the curve obtained on the basis of the shearographic image. In this representation, the local deformation 18 is still visible on the curve. In reality, the curve may be less regular or it may be affected by phenomena related to signal acquisition, so that the faults are drowned out in the available data.

In Figure 6 is shown the subtraction of the two curves shown in Figure 5. The corresponding portions of the curves are reduced to constant values, which shows the local fault 18 in a more visible manner. Instead of subtraction, other mathematical operations can be performed between the two matrices, such as dividing values, or more complex operations.

The mathematical operation performed on the shearographic and thermographic data therefore makes it possible to improve the signal / noise ratio of the defects, so as to facilitate the detection of details difficult to detect using the existing techniques. These details include faults but also other types of structural details detectable by interferometry.

According to a preferred embodiment, illustrated in FIG. 7,

BE2017 / 5666 a processor 25 is integrated in the device of the invention, said processor being configured to command and control the operation of the shearography elements 2, 3, 4 as well as of the thermal camera 11. In addition, the integrated processor 25 is configured to collect in real time the digital data delivered by the CCD camera 3 and the thermal camera 11, which makes it possible to constitute the shearographic acquisitions and the thermographic images respectively. This configuration makes it possible to acquire two synchronized sequences as described above. The acquisition of these two sequences makes it possible to choose any pair of shearographic acquisitions as reference acquisition and measurement acquisition, based on images of the thermographic sequence. The shearographic acquisitions and the thermographic images are recorded in a database which is part of an external computer 9 or which is connected to the computer 9. This data is then processed separately and combined (concept of "data fusion" ) using advanced mathematical image processing methods.

According to one embodiment of the invention, the device further comprises a sensor for measuring the distance between the device of the invention and the object inspected. The measured distance is saved in the database that collects the images, so that each recorded image is linked to a measured distance. This distance measurement introduces an autofocus function for the camera. It also makes it possible to reproduce the exact conditions of a particular test.

As already indicated, the invention is applicable to other holographic interferometry devices, such as an ESPI device. The thermal imager is integrated in the ESPI device or equivalent, in the same way as illustrated in FIG. 2 or 7. The description of the methods and uses of a shearography device are literally applicable to similar uses of non-shearographic devices. but which remain under the scope of the invention. It suffices to replace "shearographic" by "interferometric" throughout the description while also taking into account certain specific aspects which are known to those skilled in the art. For example, the interferometric image obtained by an ESPI device expresses a relative displacement between points of the object studied instead of a gradient of this displacement.

Claims (7)

1. Method for non-destructive testing of an object (10) by holographic interferometry, using a device comprising a set of elements (2, 3, 4) which make it possible to acquire an interferometric image of a defined area of l object (10), said assembly comprising at least one coherent light source (2), an interferometer (4) and a camera (3) for capturing the interferometric image, characterized in that the device also comprises a thermal camera (11) able to capture a thermographic image of at least part of said defined area of the object (10), the method comprising the following steps: • Application of a thermal stimulus to at least one defined zone of the object, by the activation of a thermal source (1) at an instant ti and the deactivation of said source at an instant t2, • Obtaining a interferometric reference acquisition and an interferometric measurement acquisition, said acquisitions being relative to said area of the object, respectively at a reference instant and a measurement instant, the reference and measurement instants corresponding to two distinct thermal states, • Production of an interferometric image of said zone expressing a difference between the reference and measurement acquisitions, characterized in that the instant t2 is chosen in real time on the basis of thermographic images of at least part of said zone , obtained during the application of the stimulus, the thermal source (1) being deactivated as soon as at least part of the thermographic image corresponds nd to a desired image. 2. Method for non-destructive testing of an object (10) by holographic interferometry, using a device comprising a set of elements (2, 3, 4) which make it possible to acquire an interferometric image of a defined area of the object (10), said assembly comprising at least one source of BE2017 / 5666 coherent light (2), an interferometer (4) and a camera (3) for capturing the interferometric image, characterized in that the device also comprises a thermal camera (11) capable of capturing a thermographic image of at least part of said defined area of the object (10), the method comprising the following steps: • Application of a thermal stimulus to at least one defined zone of the object, by the activation of a thermal source at an instant ti and by the deactivation of said source at an instant t2, • Obtaining an interferometric acquisition of reference and an interferometric measurement acquisition, said acquisitions being relative to said area of the object, respectively at a reference instant and at a measurement instant, the reference and measurement instants corresponding to two distinct thermal states, • production an interferometric image of said zone which expresses a difference between the reference and measurement acquisitions, characterized in that: • the method also includes taking a sequence of thermographic images of at least part of said zone, before, during and after the application of the stimulus, • the reference time is chosen on the basis of the correspondence d 'at least part of the thermographic image to a desired image. 3. The method according to claim 2, in which: • a sequence of interferometric acquisitions is obtained before, during and after the application of the stimulus, • the sequence of thermographic images is synchronized with the sequence of interferometric images, • the reference interferometric acquisition is the acquisition obtained at the instant when at least part of the thermographic image corresponds to a desired image. 4. Method of non-destructive testing of an object by holographic interferometry, using a device comprising a set BE2017 / 5666 of elements (2, 3, 4) which make it possible to acquire an interferometric image of a defined zone of the object (10), said assembly comprising at least one coherent light source (2), an interferometer (4) and a camera (3) for capturing the interferometric image, characterized in that the device further comprises a thermal camera (11) capable of capturing a thermographic image of at least part of said defined zone of the object (10), the method comprising the following steps: • Application of a thermal stimulus to at least one zone of the object, by the activation of a thermal source at an instant ti and by the deactivation of said source at an instant t2, • Obtaining an interferometric acquisition of reference and an interferometric measurement acquisition, said acquisitions being relative to said area of the object, respectively at a reference instant and at a measurement instant, the reference and measurement instants corresponding to two distinct thermal states, an interferometric image of said zone which expresses a difference between the reference and measurement acquisitions, characterized in that: The method also comprises taking a sequence of thermographic images of at least part of said zone, before, during and after the application of the stimulus, said sequence comprising an image obtained at the instant of measurement, a mathematical operation is performed between the respective values of a matrix of digital values derived from said interferometric image and a matrix of digital values derived from the thermographic image obtained at the time of measurement, the matrices having the same size and being relative to a common part between the interferometric and thermographic images, the operation making it possible to obtain a recombined image by said mathematical operation, • detection in the recombined image of one or more details of the object, said details having improved visibility by relation to the visibility of these details in the interferometric image. BE2017 / 5666 5. The method according to claim 4, wherein said mathematical operation is an algebraic operation, such as a subtraction or a division of respective values from the data sets. 5 6. The method according to claim 4 or 5, in which: • a sequence of interferometric images is obtained before, during and after the application of the stimulus, • the sequence of thermographic images is synchronized with the sequence of interferometric images. o 7. A shearography method according to any one of claims 1 to 6.