METHOD FOR CHARACTERIZING THE CONTENT OF AN OBJECT SUBJECTED TO MECHANICAL, THERMAL OR THERMOELASTIC STRESS THAT CAN GENERATE ELASTIC WAVES IN THE OBJECT

Description

[0001] The invention relates to a method for characterizing the integrity of an object, particularly for detecting defects in an object subjected to mechanical, thermal, or thermoelastic stress likely to generate elastic waves within the object. The invention also relates to a system capable of implementing the method, as well as to a method for the additive manufacturing of an object. The invention is particularly well-suited to characterizing objects with high roughness, especially objects produced by an additive manufacturing process. In the context of this invention, the term "object" is not limited in size; the invention also applies to structures such as pipelines or aeronautical structures.

[0002] During the manufacturing process of an object, or during its use, it is important to check the object's integrity. Integrity checking consists of verifying whether the object is defective, either from the manufacturing stage or due to its use (for example, a crack).

[0003] Among the methods of characterizing the integrity of an object, non-destructive testing (NDT) methods advantageously allow an object to be inspected without degrading it.

[0004] Non-destructive testing is known to be carried out using non-contact vibration measurement methods. For this, the object to be inspected is subjected to a thermal, thermoelastic (also called photothermal, optoacoustic) or mechanical stress, which generates elastic waves (ultrasonic waves).

[0005] Two types of non-contact vibration measurement methods are usually distinguished: the so-called passive mode, and the so-called active mode.

[0006] In the passive mode, we mean: "Thermal stresses" refers to stresses related to the solidification process, as well as residual stresses during an additive manufacturing process; "thermoelastic stresses" refers to elastic waves induced by a laser, also known as the optoacoustic or photothermal effect; "mechanical stresses" refers to stresses related to a crack opening or closing during cooling following an additive manufacturing process, as well as stresses related to a lack of fusion (e.g., residual stresses) during an additive manufacturing process.

[0007] In active mode, we mean: "Thermoelastic stresses" or "thermal stresses" involve applying a pulsed laser beam to the surface of the object. Thermal energy is absorbed at the surface, causing a rapid expansion and contraction of a small volume around the surface, thus generating elastic waves; "mechanical stresses" involve applying eddy currents (EMAT technology for ElectroMagnetic Acoustic Transducer) or generating vibrations using airborne or object-contact piezoelectric transducers, or capacitive transducers (cMUTs technology for Capacitive Micromachined Ultrasound Transducers).

[0008] The measurement of elastic waves is performed using an optical metrology device, such as a laser vibrometer, which emits a laser beam onto a point on the object's surface. The laser beam is reflected off the object's surface, and the optical metrology device receives the reflected beam. Due to the propagation of elastic waves within the object, the frequency of the reflected beam differs from that of the emitted beam (Doppler effect). The optical metrology device measures the temporal evolution of the vibration, which can be transient or monochromatic. This operation is then repeated at multiple points on the object's surface. This results in a map of the elastic waves propagating within the object. The mapping of potential defects on the object can be dynamic, in the form of a video sequence, or static.

[0009] The surface condition of the object, in particular its roughness, nevertheless has a strong influence on the quality of the measurement.

[0010] Objects produced by additive manufacturing have a high degree of roughness. This high roughness can be related to: to powder particle size, particularly in the context of selective laser melting (SLM) or deposition under energy (DED) methods, and / or to the intrinsic characteristics of the additive manufacturing process, particularly for wafer arc additive manufacturing (WAAM) processes.

[0011] The surface condition affects the reflection of the measurement laser on the sample. Under ideal conditions, i.e., with a probe laser perpendicular to a measurement surface, the entire light intensity of the laser signal will be reflected back to the metrology device, ensuring an accurate measurement. This condition is, of course, impossible to meet in practice. On an inclined surface, the majority of the reflected signal is lost. Generally, for this type of measurement, the aim is to have a surface as perpendicular as possible to the measurement laser. However, the laser has a spot diameter, i.e., a measurement area over which it performs the measurement, typically on the order of a few tens of microns.Moreover, the measurement surface is never flat, especially for rough objects, such as those from additive manufacturing, or corroded metals (corrosion not being seen here as a defect to be inspected), or composite materials.

[0012] There Figure 1 This illustrates what happens at the micrometer level when a laser beam is reflected off the surface of a rough object. The laser beam is scattered in all directions by the roughness of the reflecting surface. This phenomenon is called diffuse reflection (speckle), as opposed to specular reflection produced by a perfectly smooth surface. The arrows represent some of the laser's reflection paths. The light intensity returning to the metrology device, coinciding with the laser source, is therefore lower. The ultrasonic measurements, performed to create the map, are thus of highly variable quality depending on the local roughness.

[0013] In the paper "Influence of surface roughness from additive manufacturing on laser ultrasonic measurements" (Bakre, Chaitanya, Mostafa Hassanian, and Cliff Lissenden, AIP Conference Proceedings. 2102(1). AIP Publishing, 2019), the authors investigate the influence of surface roughness in additive manufacturing samples during ultrasonic measurements. The paper concludes that surface roughness is indeed problematic. However, no concrete solutions are proposed, but rather an understanding of how to choose operating parameters to limit the effects of surface roughness.

[0014] The document "Laser Doppler Vibrometry: Development of advanced solutions answering to technology's needs," Castellini et al., Mechanical Systems and Signal Processing, ELSEVIER, vol. 20, no. 6, August 1, 2006, describes various applications of laser Doppler vibrometry. The document "Imaging Material Texture of As-Deposited Selective Laser Melted Parts Using Spatially Resolved Acoustic Spectroscopy," Rikesh Patel et al., Applied Sciences, vol. 8, no. 10, October 19, 2018, describes the use of spatially resolved acoustic spectroscopy (SRAS) in additive manufacturing processes. The document "Influence of surface roughness from additive manufacturing on laser ultrasonics measurements," Chaitany Bakre et al., AIP Conference Proceedings, vol. 2102, January 1, 2019, presents the problem of surface roughness of objects obtained by additive manufacturing, to perform measurements by laser vibrometry.

[0015] The invention therefore aims to characterize, in a non-destructive, i.e. non-invasive way, the integrity of an object having a high roughness.

[0016] An object of the invention is therefore a method for characterizing the integrity of an object subjected to a mechanical, thermal or thermoelastic stress capable of generating elastic waves in the object as defined by claims 1 to 4.

[0017] The invention also relates to a system as defined by claims 5 to 8, configured for characterizing the integrity of an object subjected to mechanical, thermal or thermoelastic stress capable of generating elastic waves in the object.

[0018] The invention also relates to an additive manufacturing process as defined by claims 9 and 10.

[0019] Other features, details and advantages of the invention will become apparent from the description provided with reference to the accompanying drawings given by way of example, which represent, respectively: [ Fig. 1 ] There figure 1 , already described, illustrates the phenomenon of diffuse reflection; Fig. 2 ] There figure 2 illustrates the different stages of the characterization process according to the invention; [ Fig. 3 ] There figure 3 illustrates a system capable of implementing the characterization process according to the invention; [ Fig. 4 ] There figure 4 illustrates an example of a displacement device; [ Fig. 5 ] There figure 5 illustrates an example of reflectivity mapping; [ Fig. 6 ] There figure 6 illustrates the mapping of elastic waves, obtained with the characterization process according to the invention; [ Fig. 7 ] There figure 7 illustrates a detailed view of the figure 6 .

[0020] The characterization method according to the invention is described with reference to the figure 2 .

[0021] In the first step (a), a reflectivity map is established. This reflectivity map is created on the surface of the object, for which the elastic wave map will then be created. The reflectivity map indicates the roughness of the object's surface.

[0022] Reflectivity mapping is established by an optical metrology device, which is advantageously the same as that used for the subsequent stage of establishing elastic wave mapping.

[0023] Indeed, some optical metrology devices have information relating to the amplitude of backscattered light intensity, independently of any Doppler effect measurement.

[0024] However, it could be considered to use two different devices, one to establish the reflectivity map, and one to establish the elastic wave map.

[0025] In the first step a), a first "quick" scan of the surface of the object to be characterized is performed. This scan is called quick because, unlike the scan performed to establish the map of elastic waves, it does not require measuring the temporal evolution of the vibrations of each point.

[0026] Reflectivity mapping represents a value of the backscattered light intensity return of the optical metrology device, namely an internal metric of the laser linked to the optical energy received on the receiver (photodiode or low-noise broadband photodetector).

[0027] In the following description, the optical metrology device will be considered a single-point laser Doppler vibrometer, hereafter referred to as a vibrometer. The vibrometer comprises a laser source with a wavelength of 1550 nm and a Mach-Zehnder interferometer. The interferometer measures the Doppler effect (or Doppler-Fizeau effect) due to the modulation between the emitted and reflected laser beams. From this, the frequency and phase of a vibration are deduced. A processing unit within the vibrometer then determines the velocity, or by integration, the displacement of the point caused by mechanical, thermal, or thermoelastic stress.

[0028] The optical metrology device could be more simply described as follows: a Mach-Zehnder heterodyne interferometer, which can directly measure a phase shift between the reflected and emitted beams; a Fabry-Perot interferometer; a two-wave mixing interferometer employing a photorefractive crystal; a homodyne interferometer employing a multi-detector; an interferometer implementing a holographic or speckle interferometry method; a Doppler vibrometer using an IR wavelength at 1550 nm.

[0029] Then, a computing unit, connected to the interferometer, would in the same way be able to measure a displacement of the point.

[0030] Step a) can be implemented in two ways.

[0031] According to a first embodiment, step a) comprises the following substeps: In a first substep a1), the optical metrology device is placed facing the surface of the object. The distance between the metrology device and the surface is not critical, but is dictated by application constraints. However, the distance between the metrology device and the surface depends on the dimensions of the surface: the larger the surface, the greater the distance between the metrology device and the object must be, in order to limit the geometric distortion of the measurement grid and to compensate for distortion upstream of the measurement.

[0032] In a second substep (a2), the surface is illuminated with the beam. A point on the surface is illuminated, which will correspond to a pixel in the reflectivity map. For this point, a portion of the backscattered light amplitude / intensity, also called a speckle grain, is measured.

[0033] The higher this value, the less rough the dot. The pixel reflectivity is thus quantified by the metrology device. This value is then acquired and digitized by an analog-to-digital conversion device, such as a DAQ data acquisition card or an oscilloscope, before being recorded on a computer.

[0034] The conversion device preferably has a sufficient dynamic range, i.e. at least with sixteen bits of resolution.

[0035] The second substep a2) is repeated for different points on the surface. To do this, the orientation of the beam emitted by the metrology device is modified. With reference to the figure 3 , the scanning of the surface of the object OBJ can be operated by a DDE displacement device of the measurement point.

[0036] The DDE displacement device is advantageously a galvanometric head, illustrated by the figure 4 The galvanometer head includes a set of motorized mirrors driven by servomotors, which allow the metrology device to measure specular reflectivity at different points on the surface, without needing to move the metrology device.

[0037] The galvanometer head consists of two mirrors, M1 and M2, which can be rotated by several degrees using two servomotors, S1 and S2. Each mirror is controlled with very high repeatability, specifically less than 15 µrad, and very high angular accuracy, specifically less than 0.0008° or 14 µrad. The angle of rotation of these mirrors depends on the applied voltage. For each point on the surface where roughness is to be measured, the incident laser beam, emitted by the DMO metrology device, is reflected by the first mirror, M1. After this reflection, the laser beam strikes the second mirror, M2, before being projected onto the measurement point on the surface of the object (OBJ) or sample. The control of a galvanometer head is precise because the rotational displacement of a mirror is proportional to the applied voltage, for example, 0.5 V / °.The galvanometer head must also have a travel speed between 100 Hz and 10 kHz, preferably equal to 1 kHz, and with an angular opening approximately equal to 20°.

[0038] The repetition of the second substep (a2) for different points on the surface is performed by controlling the rotation of one or the other of the mirrors at specific points: before emitting the laser beam, one or the other mirror is rotated. The mirror rotation requires a stabilization period. Then, once the mirror is stabilized, or after a predetermined time corresponding to the mirror stabilization, the measurement takes place. The laser beam is emitted continuously and provides irrelevant information when the mirrors are not stabilized. The data acquisition, for example by the oscilloscope, is only synchronized to perform measurements when the mirrors are stabilized. The laser beam reflected by the surface also passes through the galvanometer to measure the reflectivity. Thus, with the first embodiment, the DDE displacement device is stopped for each measurement point.

[0039] The measurement operation is repeated for all points on the surface, with the finest possible resolution, along a predetermined path. The resolution is that of the DDE displacement device. For a commercially available galvanometer head, a resolution of 0.0008° can be achieved, which corresponds to pixels spaced a few microns apart if the metrology device is placed a few centimeters / decimeters from the object to be manufactured, within the context of an additive manufacturing process for a mechanical part, with the DDE displacement device located within the additive manufacturing chamber. Apart from this case of integration within the additive manufacturing chamber, it is possible to work at a distance of more than one meter from the object, preferably one to three meters, in order to limit the effects of geometric distortion, and also to scan surfaces with complex geometries or strong curvatures.

[0040] If the reflectivity mapping resolution is coarse, most points with high reflectivity may not be scanned, and very few measurement points will be extracted. This may be acceptable, however, if only a few spot measurements are required.

[0041] An alternative to the galvanometer head would be to mechanically move the metrology device using a Cartesian bench (motorized translation stage or robotic arm), but this configuration is less advantageous in terms of the time it takes to move the measurement point, functionality and transportability of the device.

[0042] The galvanometer head offers the advantage, compared to the other solutions mentioned above, of a compact size, making it suitable for integrity control applications in additive manufacturing processes. For example, only the galvanometer head could be housed inside the additive manufacturing machine, while the vibrometer could be placed outside and measured through an optical window. Additive manufacturing machines, such as SLM powder bed fusion machines, have relatively limited internal space, which allows for the addition of compact instrumentation to better control the process. Therefore, only the mechanical movement device (galvanometer head) would be subjected to the manufacturing conditions and / or the inerting process used to prevent oxidation effects during production.

[0043] In the first embodiment, the reflectivity map is established as follows: each position of the DDE displacement device is recorded on a computer, along with its reflectivity, which represents the backscattered intensity return. A reflection value can therefore be associated with each scanned point. The positions of the DDE displacement device correspond, for example, to the rotation angles of the galvanometer head mirrors, and thus to the measured XYZ coordinates. A scale, for example from 0 to 100, allows the roughness of each point to be quantified in a way that is usable by the user. From a user interface perspective, the reflectivity map can be presented as a two-dimensional map, with color levels corresponding to the aforementioned scale.Mapping can also be presented in the form of a table, which correlates the position of the point on the surface with the reflectivity, for example on a scale of 0 to 100.

[0044] The metrology device can be equipped with autofocus, which maximizes the signal return at each spatial position. This option is particularly useful when the distance between the object and the measuring instrument is not constant, ensuring that the signal return value measured at a point is obtained under optimal conditions.

[0045] According to a second embodiment, the first step a) comprises a first substep a1') of arranging the metrology device facing the surface of the object. The positioning of the metrology device is identical to that of the first embodiment.

[0046] In a second substep (a2'), the orientation of the incident beam is continuously modified, namely: the metrology device emits a continuous laser beam, and the DDE displacement device continuously modifies the orientation of the emitted laser beam. The DDE displacement device orients the laser beam along a predefined scanning path, enabling uninterrupted scanning. Advantageously, a meandering or zigzag path allows the entire surface to be scanned without interruption.

[0047] This method continuously collects the return signal information, resulting in a spatial resolution far superior to the first embodiment. The surface can be scanned at a virtually infinite number of points. The number of points thus collected therefore depends on the acquisition frequency of the return signal and the distance traveled by the laser beam.

[0048] Given the current performance of galvanometer heads, in point-by-point mode, only a few dozen points per second can be measured, representing a distance ranging from centimeters to decimeters (depending on the desired resolution). Between two points spaced one millimeter apart, the roughness can vary significantly locally, and therefore even if both points have a low return signal value, it is possible that a highly reflective point lies between them and has been missed. However, by continuously moving the laser probe, it becomes possible to measure thousands of points per second with a much finer spatial resolution, and thus a much greater probability of detecting highly reflective points, resulting in a low-noise local measurement of normal displacement.

[0049] The second embodiment is therefore much more precise than the first embodiment.

[0050] Furthermore, it is faster because it is not necessary to wait for the DDE displacement device to stabilize for each measurement point, particularly mirror stabilization if the DDE displacement device is a galvanometer head. However, the second embodiment does not allow for autofocus. It should be noted that it would be possible to preprogram the laser's depth of field with the predicted laser-object focal length, for example, using Computer-Aided Design (CAD) data for the object.

[0051] To then establish the surface reflectivity map, the position of each point is associated with the value of the return signal. If the DDE displacement device is a galvanometer head, the position of each point is determined by the angle values ​​of the mirrors after calibration of the device.

[0052] As in the first embodiment, reflectivity is recorded along with the position of each point. This data can be presented to the user as a two-dimensional diagram, representing reflectivity according to a predetermined color code, with the value calibrated between 0 and 100, for example. Alternatively, the data can be presented in tabular form.

[0053] As illustrated by the figure 2 , the method for characterizing the integrity of an object conforming to the invention includes a second step b) of selecting a set of measurement points, determined according to the reflectivity mapping established during step a).

[0054] By "set of measurement points", we mean a portion of the measurement points that were scanned during the previous step.

[0055] The selection of all measurement points can be done in several ways.

[0056] First, the set of measurement points can correspond to points on the reflectivity map whose reflectivity exceeds a predefined threshold. When reflectivity values ​​are on a scale of 0 to 100, for example, all measurement points with a reflectivity greater than 80 can be included in the set. Naturally, the user can modify this threshold via a human-machine interface, for example, through the computer to which all the elements of the DMO metrology device and the DDE displacement device are connected.

[0057] Secondly, the point set comprises the N points determined in step a) with the highest proportion of backscattered light intensity. The set thus includes N favorable points. The number of points N in the set can also be parameterized by the user. The number N can also be determined so that the set comprises a given proportion of the total number of points in the backscattered intensity map.

[0058] Third, the two methods described above can be combined. This might be the case if a large majority of points on the surface have very high reflectivity (above the threshold). To avoid mapping the elastic waves on all these points, particularly to save time, we can select the N most reflective points from among them—that is, the brightest points at that wavelength and angle of incidence. Conversely, if the selected points exceed a threshold, but that threshold is low, we can choose the N most reflective points from among those above the predefined threshold.

[0059] These three ways of selecting the set of points are compatible with the two embodiments of step a), described previously (point-by-point scanning, or continuous scanning).

[0060] There figure 5 illustrates an example of a set of measurement points obtained following step b) of the characterization process according to the invention.

[0061] In addition, step b) involves applying spatial interpolation. Indeed, following the selection of the set, a random and / or non-uniform spatial distribution of the backscattered intensity map can be expected. Applying interpolation thus allows us to establish the elastic wave map (step c) at points that are not initially favorable, in order to characterize the object uniformly.

[0062] To achieve this, the object's surface is divided into several zones. The number of zones, as well as their distribution across the object's surface, can be configured via a user interface. A uniform surface mesh, such as a grid, or any other type of segmentation (triangular, mixed, etc.) can be used.

[0063] Thus, in reflectivity or sensitivity mapping, each area has a predetermined number of points or a predetermined proportion of points relative to the total number of points in the reflectivity map. For example, if the user knows that, in the context of additive manufacturing of an object, defects are often found in a region of interest, they can configure the point selection to ensure that this area is sufficiently represented (refined) within the selected set of measurement points.

[0064] The application of the point interpolation function can be carried out on a set of points determined according to one or the other of the aforementioned variants (points above a threshold, N most favorable points, or the combination of both).

[0065] Such an interpolation process is described in particular in patent application PCT / EP2019 / 072136.

[0066] Step c) of the process includes, for the measurement points of the set determined in step b), the emission of a laser beam, the reception of the laser beam following its reflection on each of the measurement points, and the measurement of the Doppler effect due to the difference in frequency between the emitted beam and the reflected beam caused by the propagation of elastic waves in the object OBJ, i.e. the measurement of the normal displacement.

[0067] Step c) is advantageously implemented using the same DMO optical metrology device and the same DDE displacement device, thus avoiding a registration step between the reflectivity mapping data and the elastic wave mapping. Furthermore, using the same components saves space in the overall system.

[0068] We can thus establish a map of the elastic waves propagating in the object, as illustrated in the figures 6 et 7 . There figure 6 represents an object OBJ, in this case a boat propeller, and a surface SURF of the object OBJ. The SURF is first scanned to establish the reflectivity map. Then, some points on this SURF are analyzed to establish the elastic wave map. To generate elastic wave propagation within the object OBJ, one or more ultrasonic wave generators (UGGs), for example, piezoelectric transducers, can be placed on the object OBJ. The signal emitted by the ultrasonic wave generator (UGG) can be, for example, a sinusoidal (monochromatic) signal with applied amplitude modulation, or a pulse or wave train.

[0069] As can be seen on the figure 6 and on the figure 7 , which represents a detailed view of the SURF surface of the figure 6When an ultrasonic wave (or mode) encounters a defect, and that defect has dimensions close to or greater than the equivalent acoustic wavelength of the mode, then its propagation is affected. This results in diffraction, attenuation, and resonance phenomena (due to circumferential or cavity waves), and also generates mode conversions (L to T and vice versa, evanescent waves, circumferential in some cases). Other analytical methods allow information about the defect to be deduced from the waves reflected or diffracted by the defect; for example, acoustic tomography methods used in Structural Health Monitoring (SHM).

[0070] Thanks to the method according to the invention, the quality of the elastic wave mapping (also called "wavefield" in Anglo-Saxon terminology) is improved, because the measurement is only performed on points with the best reflectivity. Furthermore, the mapping process is faster, as it is carried out on fewer points.

[0071] It is also possible, during step c), to carry out measurements on points that are not part of the set of measurement points determined during step b).

[0072] Indeed, step a) allows the identification of a number of points with high roughness. If the elastic wave map were to be established on these points as well, it would be necessary to limit their impact on the elastic wave map by performing an amplitude compensation of the map (CSCAN) based on the reflectivity map obtained in step a).

[0073] The invention also relates to a method for the additive manufacturing of an object, comprising the following steps: deposition of successive layers of material; following the deposition of a predetermined number of layers, characterization of the object in accordance with the aforementioned process, in which an external mechanical, thermal or thermoelastic stress is applied to the object in step c).

[0074] This is therefore an active mode operation because the constraint is external to the object, as previously indicated.

[0075] This allows for non-contact inspection of a part during manufacturing, even if its surface is rough. The most reflective points are then selected. Since the characterization method is rapid (there are fewer measurement points than in known characterization processes), it can be implemented in real time, during the object's manufacturing process. If a defect is detected, the manufacturing process can then be adjusted or stopped to repair the object.

[0076] The compactness of the system composed of the vibrometer and the galvanometer head makes it possible to integrate it alongside additive manufacturing systems.

[0077] The process can also be implemented after the object has been polished.

[0078] The invention is also particularly well-suited to acoustic emission measurements, especially during additive manufacturing. The invention relates to an additive manufacturing process for an object, comprising the following steps: successive layers of material are deposited; following the deposition of a predetermined number of layers, the object is characterized according to the aforementioned process, in which an internal thermal, thermoelastic or mechanical stress within the object is detected.

[0079] This is therefore a passive mode operation because the constraint is internal to the object, as previously indicated.

[0080] Acoustic emission signals are generated during cracking, release of residual stresses, poor melting, during the solidification process, etc. These effects will locally release energy and generate transient elastic waves, which can be guided if we are dealing with thin-walled interfaces, and of more or less high amplitude depending on the nature and source of the event.

[0081] The quality of measurements is all the more important in acoustic emission, since the measurement of this vibration, due for example to the propagation of a crack, only occurs at a specific point.

[0082] When an internal mechanical stress is detected within an object, its integrity can be characterized non-destructively, without the need for a piezoelectric transducer, which is particularly advantageous in additive manufacturing. Piezoelectric transducers are typically located beneath the substrate on which the object is being manufactured. During the initial layer stacking, the transducer is relatively close to the upper layers, making wave propagation easy to interpret. However, as the object grows larger, the propagation distance increases, the wave becomes more attenuated and diffracted, thus reducing the measurement's sensitivity for detecting defects. By implementing an acoustic emission method, defect detection is performed as close as possible to the top layer or near the melt pool.

Claims

1. Method for characterising the integrity of an object (OBJ) subjected to a mechanical, thermal or thermoelastic stress liable to generate elastic waves in the object (OBJ), comprising the following steps and in this order: a) establishing a map of the reflectivity of a surface of the object (OBJ), comprising the following substeps: a1') arranging an optical metrology device (DMO) opposite the surface of the object, the metrology device being configured to generate an incident laser beam and to measure a return of said incident laser beam after reflection and diffraction; a2') by continuously modifying, using a displacement device (DDE) comprising a set of motorised mirrors driven by servomotors, orienting the incident beam along a predefined scanning path, illuminating the surface with the beam and continuously measuring the reflectivity of the surface, the reflectivity map corresponding to the reflectivity diffuses from the surface over the scanning path; b) selecting a set of measurement points, which set is determined depending on the reflectivity map, the set of measurement points corresponding to points of the reflectivity map, some of the backscattered light intensity of which is greater than a predefined threshold, and / or corresponding to N points having some of the backscattered light intensity greater among the points of the reflectivity map, N being a predefined integer; c) transmitting a laser beam at least to the measurement points of said set, receiving the laser beam following its reflection from each of the measurement points, and measuring the Doppler effect due to the frequency difference between the transmitted beam and the reflected beam caused by the propagation of elastic waves in the object (OBJ), so as to establish a map of the elastic waves propagating through the object.

2. Method according to claim 1, wherein, the surface being meshed in a plurality of zones, step b) comprises the application of an interpolation function, such that, in the reflectivity map, each zone has a predetermined number of points or a predetermined share of points with respect to the total points of the reflectivity map.

3. Method according to any one of the preceding claims, wherein, in step c), the map of the elastic waves is completed by interpolation, the interpolation being configured to increase the spatial discretisation of the map of elastic waves.

4. Method according to any one of claims 1 or 2, wherein, in step c), a laser beam is transmitted on points of the surface which do not belong to the set of points selected during step b), the map of the elastic waves propagating through the object being corrected, for said points of the surface, by an amplitude compensation, inversely proportional to the reflectivity determined during step a).

5. System configured for the characterisation of the integrity of an object (OBJ) subjected to a mechanical, thermal or thermoelastic stress liable to generate elastic waves in the object (OBJ), comprising: - an optical metrology device (DMO), configured to measure a vibration in an object (OBJ) to be characterised, and further configured to generate an incident laser beam on a measurement point of an object to be characterised and to digitise and quantify a return of said incident laser beam after reflection on said measurement point; - a device (DDE) for displacing the measurement point on the surface of the object; - a computer suitable for executing the steps of the method according to any one of the preceding claims.

6. System according to claim 5, wherein the optical metrology device (DMO) comprises a Doppler effect laser vibrometer.

7. System according to any one of claims 5 or 6, wherein the device for displacing the measurement point (DDE) comprises a galvanometric head, disposed between the optical metrology device (DMO) and the object (OBJ).

8. System according to claim 7, wherein the galvanometric head has a repeatability less than 15µrad.

9. Method for the additive manufacture of an object, characterised in that it comprises the following steps: - depositing successive material layers; - following the deposition of a predetermined number of layers, characterising the object according to the method according to any one of claims 1 to 4, wherein a mechanical, thermal or thermoelastic stress external to the object is applied to the object in step c).

10. Method for the additive manufacture of an object, characterised in that it comprises the following steps: - depositing successive material layers; - following the deposition of a predetermined number of layers, characterising the object according to the method according to any one of claims 1 to 4, wherein an ultrasonic signal from a variation of mechanical, thermal or thermoelastic stress internal to the object is detected.