A testing method for detecting at least one surface-breaking defect on the surface of an aircraft mechanical part.

Abstract

A non-destructive testing method is described for detecting at least one surface-breaking defect (12) forming at least one surface discontinuity present on the surface of a mechanical part (10) for an aircraft. The method comprises the following steps: application (E1) of a liquid-phase product to the mechanical part (10); optical inspection (E2) by an imaging device, during an inspection time (Δt) in which all or part of the previously applied product evaporates, of at least one inspection zone (Z) of the mechanical part (10); the imaging device providing data representative of the evolution, during the inspection time (Δt), of the optical appearance of the monitored inspection zone (Z); and analysis (E3) of the data previously provided by the imaging device. Figure 1

Description

Title of the invention: Control method for detecting at least one emerging defect present on the surface of a mechanical part for an aircraft Technical field of the invention

[0001] The present invention relates to a non-destructive testing method for detecting at least one emerging defect forming at least one surface discontinuity present on the surface of a mechanical part for an aircraft. State of the art

[0002] Today, industrial products must meet demanding quality criteria while remaining as economically competitive as possible. This optimum aims for the best balance between the use of advanced manufacturing technologies to produce a product of good quality and usable directly and the downstream use of means of evaluation and control of the products and the materials constituting them, detecting any non-conformities which may remain present.

[0003] The metal processing industries follow these rules, particularly for products applied to land, maritime and aeronautical transport, in particular to control and minimise the risks of anomalies, imposing even more severity given their impact on mechanical strength and final safety.

[0004] The evaluation and control methods available are numerous and are divided into two categories: destructive evaluation methods and non-destructive evaluation methods. The first category is generally used in the research, development or even industrialization phases, which are not ultimately intended to obtain a deliverable product for a user client but to validate a stage of its development. On the contrary, the second category of methods has an extremely broad scope of application, being able to intervene in development and design stages, or in manufacturing and production stages of new deliverable products or even during any necessary phase during the life of the product, that is to say during monitoring of the product in service.

[0005] Non-destructive testing constitutes a broad spectrum of implementation and the industry uses it in all these phases in order to measure properties and detect possible anomalies while ensuring that these examinations do not in any way harm the final use of the product thus obtained.

[0006] More specifically in the field of parts used in the construction of turbo-machines for aircraft, the design, manufacture, sale and in-service support Such parts require the use of a wide range of these non-destructive testing techniques, often in combination, as no single technique is self-sufficient.

[0007] Among these techniques, penetrant testing occupies a prominent place, such testing having the aim of highlighting a through defect forming at least one surface discontinuity present in the part to be tested. Such a through defect may or may not be through the local thickness of the part, and may have the form of a simple hole, a porosity, a crack, a slot, a fissure, etc. Penetrant testing is a technique known for many years which may conventionally comprise the following steps: - degreasing and possible stripping of the surface of the part to be checked, - application of a colored penetrating product having suitable characteristics in terms of wettability to promote its diffusion and spreading, and in terms of color depending on the observation technique, - rinsing of excess penetrant on the surface - drying, - application of a revealing product to the surface of the part, allowing a residual quantity of penetrant to be restored, possibly located at the level of the defect to be revealed, and giving a visually detectable colored indication, - detailed observation, in real time or later, by adapting the light used (in particular depending on the penetrant used: either under white light or under ultraviolet light) and the tool possibly used (binocular, endoscope, digital or not, with recording or not), knowing that the observation can also be done with the naked eye.

[0008] Optionally, it is known to implement an additional step between the application of the developer product and the observation, allowing a removal of doubt, in order to confirm or deny a previous uncertain indication. This additional step consists of applying acetone allowing the penetrant to rise to the surface by a combined mechanism of capillarity and dilution. This doubt removal step is commonly referred to by the English term “wipe-off”.

[0009] Penetrant testing is mainly carried out on an unmounted or isolated part, allowing easy handling. For example, document FR3082941A1 describes an example of a handling technique used in penetrant testing. In addition, there are applications allowing penetrant testing to be carried out on a part mounted in an assembly, or even in situ and in conditions of non-accessibility.

[0010] To give reliable results, the penetrant testing technique is sensitive to implement, or even complex, because it involves certain very influential key parameters such as an operational range which must be precise with production times to be controlled, equipment used and rigorous calibration required, training and certification of control agents, etc.

[0011] Furthermore, given the numerous operational phases which are specific to this technique and which have been described above, its implementation presents substantial constraints and requires a significant amount of time to complete. The penetrant testing technique has the disadvantage of being long to implement. Indeed, it requires on the one hand the implementation of several successive steps, which makes the solution complex and requires the use of several products. On the other hand, each of the steps is quite slow, for example in the step of applying the developer product the absorption of the penetrant by the developer requires a certain time depending on the movement of the liquid to the surface of the part.The observation step is also deliberately quite slow because it requires a high level of attention from the operator in order to maximize the detection of any defects (whether the analysis is done directly or later) and is not dynamic in nature. On the contrary, the observation is interpreted once the bleeding phenomenon has been established and completed, similar to a static observation.

[0012] Despite these constraints, the evaluation capability that is derived from the penetrant testing technique can still justify its regular industrial practice. Moreover, in certain even more delicate cases where avoiding the disassembly of the part from its complex assembly makes it possible to avoid delays, costs and the associated cumbersomeness, the technique of localized in-situ penetrant testing remains used even if this requires an even more specific implementation. Indeed, to do this, suitable means are used allowing not only the numerous operational phases of penetrant testing to be carried out but also the final evaluation carried out thanks to a dedicated endoscopic observation.

[0013] In this context and faced with these challenges, it became imperative to provide a non-destructive testing process for aircraft parts that could be simple and quick to implement, while being very reliable. Subject of the invention

[0014] The aim of the present invention is to propose a non-destructive testing method making it possible to detect at least one emerging defect forming at least one surface discontinuity present on the surface of a mechanical part for an aircraft which responds to the problems mentioned above in connection with the known solutions.

[0015] In particular, the aim of the invention is to propose such a control method which meets at least one of the following objectives: - be economical, - be simple to implement, - provide high reliability of fault detection.

[0016] This aim can be achieved by implementing a non-destructive testing method making it possible to detect at least one emerging defect forming at least one surface discontinuity present on the surface of a mechanical part for an aircraft, the method comprising the following steps:

[0017] - application of a product in liquid phase on the mechanical part,

[0018] - optical control by an image acquisition device, for a period of control during which all or part of the previously applied product evaporates, from at least one control zone of the mechanical part, the image acquisition device delivering data representative of the evolution, during the control period, of the optical appearance of the monitored control zone,

[0019] - analysis of the data previously delivered by the image acquisition device.

[0020] This control method has the advantage of being economical, simple to implement, and providing high reliability in fault detection.

[0021] Alternatively, the optical inspection can be carried out with the naked eye.

[0022] Some preferred but non-limiting aspects of this control method are as follows. These aspects may be implemented separately or in combination.

[0023] According to one embodiment, the control method comprises a step of generating an alert, triggered if the result of the analysis step delivers information representative of the suspicion of the presence of said at least one emerging defect present on the surface of the mechanical part.

[0024] According to one embodiment, during the application step, the product in liquid phase is applied to at least a portion of a first face of at least one part to be checked of the mechanical part, and during the optical control step, said control zone belongs to a second face of the part to be checked of the mechanical part.

[0025] According to another embodiment, the first face may be identical to the second face. In other words, the liquid phase product is applied to the same face as the face where the optical control is carried out. This can promote ease of implementation, and feasibility for parts where only one face is accessible.

[0026] Alternatively, the first face may be distinct from the second face, the first face and the second face being two opposite faces of the mechanical part at the level of the part to be checked, said two opposite faces delimiting the thickness of the mechanical part in the part to be checked. This may favor the implementation of an application of the product in liquid phase prolonged over time, possibly repeated over time, at least in part at the same time as all or part of the optical control step, favoring reliability of the detection. In addition, this technique applicable for the case of through defects, has the advantage of allowing visualization of the defect immediately, without having to wait for the parts to provided with defects do not see the previously applied liquid phase product evaporate.

[0027] According to one embodiment, the applied liquid phase product comprises at least one organic solvent. This has the advantage of making the solution simple and economical.

[0028] According to one embodiment, the liquid phase product applied is characterized by a surface tension of less than 40 mN / m, preferably less than 30 mN / m, and in particular less than 25 mN / m. This has the advantage that the liquid phase product used has very high wettability and promotes its natural and rapid spreading over the controlled surface of the part.

[0029] According to one embodiment, the liquid phase product applied in the application step has an evaporation index Ie relative to ethyl ether according to standard DIN 53170 of less than 3. A technical effect is that it allows rapid drying, typically less than 1 minute under the conditions of said standard. This has the advantage that the liquid phase product used has a very high capacity to evaporate naturally and extremely quickly under normal ambient conditions of pressure and temperature.

[0030] According to one embodiment, the liquid phase product applied in the application step has an evaporation index Iab relative to butyl acetate according to the ASTM D 3539 standard greater than 1. A technical effect is that it allows rapid drying, typically less than 1 minute under the conditions of said standard. This has the advantage that the liquid phase product used has a very high capacity to evaporate naturally and extremely quickly under normal ambient conditions of pressure and temperature.

[0031] According to one embodiment, the liquid phase product applied in the application step is transparent in the visible spectrum. This greatly facilitates the implementation of the control method since there is no need to provide a subsequent cleaning step, and furthermore, this has the advantage of avoiding any residue after drying.

[0032] According to one embodiment, the liquid phase product applied in the application step comprises a ketone. This has the advantage of facilitating cleaning, dust removal and degreasing. Cleaning the surface is a step which ultimately facilitates the detection of defects because any residues, after evaporation of the ketone, are extremely low.

[0033] According to one embodiment, the liquid phase product applied in the application step comprises acetone. This characteristic has the same advantages as mentioned above, all with a very economical product.

[0034] According to one embodiment, the control duration is between 0 and 15 seconds, preferably less than 10 seconds, in particular of the order of 7 seconds. This advantageously promotes the speed and ease of implementation of the control process, and makes it easily repeatable.

[0035] According to one embodiment, the application step is implemented by means of a device applying the product in liquid phase according to one of the following methods: by projection, by spraying, by deposition, by dipping.

[0036] According to one embodiment, the image acquisition device is chosen from a binocular magnifier, an endoscope, a digital camera, a camera, a telephone. Summary description of the drawings

[0037] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which:

[0038] [Fig-1] is a flowchart representing the successive stages of a process of control according to the invention.

[0039] [Fig.2] is a schematic curve representing the evolution over time noted t (in s), taken by the visual contrast marked K, in the control zone (which includes a through defect), during the control duration At.

[0040] [Fig.3] schematizes the optical appearance of the control zone between times t0 and tl visible in [Fig.2].

[0041] [Fig.4] schematizes the optical appearance of the control zone between times tl and t2 visible in [Fig.2].

[0042] [Fig.5] schematizes the optical appearance of the control zone between times t2 and t3 visible in [Fig.2]. Detailed description

[0043] In the figures, the various elements are not shown to scale so as to enhance the clarity of the figures. Furthermore, the various embodiments and variants are not mutually exclusive and can be combined with each other.

[0044] The non-destructive testing method detailed below makes it possible to detect at least one emerging defect forming at least one surface discontinuity present on the surface of a mechanical part for an aircraft.

[0045] Such an emerging defect forming at least one surface discontinuity may or may not pass through the local thickness of the part, and may have the form of a simple localized hole, a crack, a porosity, a split, a fissure, or any other type of anfractuosity originating from a possible manufacturing defect or wear after a period of operation of the mechanical part.

[0046] The non-destructive testing method comprises a first application step E1 consisting of applying a liquid phase product to the mechanical part. This liquid phase product will be described later and can advantageously have precise properties in terms of wettability and in terms of evaporation capacity.

[0047] The non-destructive testing method also comprises an optical testing step E2 consisting of optically testing, in particular by an image acquisition device, during a testing period At during which all or part of the previously applied product evaporates, at least one testing zone Z of the mechanical part 10, the image acquisition device delivering data representative of the evolution, during the testing period At, of the optical appearance of the monitored testing zone Z.

[0048] Figures 3 to 5 show for example three successive phases of the optical appearance taken by the monitored control zone Z of the mechanical part 10 during the control duration At. For a good understanding, it is assumed that the mechanical part 10 has, in the control zone Z, at least one emerging defect 12 forming at least one surface discontinuity. The local thickness of the mechanical part is identified by the reference 14.

[0049] According to a complementary alternative, the optical control can be carried out with the naked eye.

[0050] Such data may, for example, correspond to each pixel of each acquired image. It may also be data from the operator's perception.

[0051] The non-destructive testing method also comprises an analysis step E3 consisting of analyzing the data previously obtained in step E2, in particular delivered by the image acquisition device.

[0052] Such an analysis can be done for example by image processing algorithms, consisting for example essentially of breaking down each acquired image into gray levels, these algorithms being able to be implemented by a processing unit configured and programmed for this purpose. This can however be done additionally by the perception of the operator if necessary.

[0053] This analysis can be done at least partially simultaneously with the optical control step E2, or possibly a posteriori.

[0054] According to the non-limiting embodiment of [Fig. 1], the non-destructive testing method triggers a generation step E4 consisting of generating an alert if the following condition, marked C in the figure, is verified: the result of the analysis step E3 delivers information representative of the suspicion of the presence of at least one emerging defect forming at least one surface discontinuity present on the surface of the mechanical part. If condition C is not verified, then the non-destructive testing method does not report any emerging defect.

[0055] Such an alert can be of any nature, such as for example an audible signal, the publication of a report, or any other form adapted to the circumstances of implementation of the process.

[0056] Such information may, for example, correspond to the local presence of at least one processed image, at a given moment during the monitoring duration At, of a contrast level having a value greater than a predetermined threshold. A person skilled in the art, through his knowledge, knows how to adapt algorithms or programs to achieve such processing. Otherwise, the operator can also process and determine this information in addition.

[0057] By the terms “on the surface of the mechanical part”, it is understood that said at least one defect 12 opening onto the second face, which is defined later, of the part to be checked of the mechanical part 10.

[0058] This control method has the advantage of being economical, simple to implement, and providing high reliability in detecting emerging defects. Indeed, the number of steps is low, as is the number or nature of the product used. In addition, each step is very quick to implement and economical, which facilitates possible repeatability and large-scale implementation on complex mechanical parts, possibly inaccessible, and for in-situ applications if necessary.Unlike known techniques where the presence of localized colored products had to be identified, the present technique is based on monitoring the evolution over time t of the optical appearance (in particular on contrast criteria) within the control zone Z, under the sole effect of the evaporation of the product in liquid phase previously applied, and this over a very short time control period At which can in particular be less than one minute. It is therefore a simple, rapid, economical and reliable solution.

[0059] According to one embodiment, during the application step, the liquid-phase product is applied to at least a portion of a first face of at least one part to be checked of the mechanical part 10, and during the optical control step E2, said control zone Z belongs to a second face of the part to be checked of the mechanical part 10. This first face mentioned above corresponds to the face of the mechanical part 10 where the liquid-phase product is applied in step E1.

[0060] According to one embodiment, the first face may be identical to the second face. In other words, the liquid phase product is applied in step E1 to the same face as the face where the optical inspection is carried out in step E2. This can promote ease of implementation, and feasibility for parts where only one face is accessible. In Figures 3 to 5, the face where the optical inspection is carried out is marked 16. In this case, the liquid phase product is also applied in step E1 to face 16.

[0061] Alternatively, the aforementioned first face may be distinct from the second face, the first face and the second face being two opposite faces of the part. mechanical part 10 at the level of the part to be checked, said two opposite faces delimiting the thickness 14 of the mechanical part 10 in the part to be checked. In this case, the product in liquid phase is applied in step E1 on the face opposite the face where the optical check is carried out in step E2. In this embodiment, the product in liquid phase is applied in step E1 on the face 18 and the optical check is carried out in step E2 on the face 16, the faces 16 and 18 being separated locally by the thickness 14 already mentioned. This can promote the implementation of an application of the product in liquid phase prolonged over time, possibly repeated over time, at least in part at the same time as all or part of the optical check step E2, promoting reliability of the detection.Furthermore, this technique applicable to the case of through defects 12, has the advantage of allowing immediate visualization of the defect 12, without having to wait for the previously applied liquid phase product present on the parts without defects 12 to evaporate. These arrangements allow dynamic application over time of the liquid phase product on the first face, making it possible to dynamically and more easily reveal a through defect 12 opening onto both faces 16, 18 of the mechanical part 10.

[0062] The liquid phase product may be a solvent, for example water, pure or mixed with other solvents. According to one embodiment, the applied liquid phase product comprises at least one organic solvent. This has the advantage of making the solution simple and economical. There may be the presence of a single organic solvent or a mixture of a plurality of organic solvents, all potentially mixed with water and possible additives.

[0063] According to one embodiment, the applied liquid-phase product is characterized by a surface tension of less than 40 mN / m, preferably less than 30 mN / m, and in particular less than 25 mN / m. The surface tension determines the behavior of the liquid-phase product used, particularly with regard to wettability: the lower the surface tension, the greater the wetting of a solid (here the mechanical part) by the liquid-phase product. This has the advantage that the liquid-phase product used has very high wettability and promotes its natural and rapid spreading over the controlled surface of the mechanical part 10.

[0064] According to one embodiment, the liquid phase product applied in application step E1 has an evaporation index L relative to ethyl ether according to standard DIN 53170 of less than 3. A technical effect is that it allows rapid drying, typically less than 1 minute under the conditions of said standard. This has the advantage that the liquid phase product used has a very high capacity to evaporate naturally and extremely quickly under normal conditions. ambient pressure and temperature.

[0065] According to one embodiment, the liquid phase product applied in application step E1 has an evaporation index Iab relative to butyl acetate according to the ASTM D 3539 standard greater than 1. A technical effect is that it allows rapid drying, typically less than 1 minute under the conditions of said standard. This has the advantage that the liquid phase product used has a very high capacity to evaporate naturally and extremely quickly under normal ambient conditions of pressure and temperature.

[0066] According to one embodiment, the liquid-phase product applied in application step E1 is transparent in the visible spectrum. This greatly facilitates the implementation of the control method since it is not necessary to provide a subsequent cleaning step, and furthermore, this has the advantage of avoiding the presence of any residue after complete evaporation. It remains, however, possible to envisage a product applied in step E1 which would be non-transparent provided that it would have suitable wettability and evaporation capacity characteristics.

[0067] According to one embodiment, the liquid-phase product applied in the application step E1 comprises a ketone. This has the advantage of facilitating cleaning, dust removal and degreasing. Cleaning the second face is a step which ultimately facilitates the detection of defects 12 and the fact that any residues, after evaporation of the ketone, are extremely low. There may be the presence of a ketone or a mixture of a plurality of ketones, all potentially mixed with water and possible additives.

[0068] According to one embodiment, the liquid phase product applied in the application step comprises acetone. This characteristic has the same advantages as mentioned above, all with a very economical product.

[0069] According to one embodiment, the control duration is between 0 and 15 seconds, preferably less than 10 seconds, in particular of the order of 7 seconds. These arrangements are in particular obtained thanks to acetone according to the tests which have been carried out. This advantageously promotes the speed and ease of implementation of the control method, and makes it easily repeatable. This very rapid nature makes it possible to repeat the cycle of steps E1 to E4 as many times as necessary in the event of uncertainty about the determination of the presence or absence of a defect 12 and this in a very easy and rapid manner, and without any alteration of the mechanical part 10. This method has no limitation in terms of the number of repetitions, unlike that of traditional “wipe-off” in penetrant testing which is limited to 2 or 3 repetitions.

[0070] According to one embodiment, the application step E1 is implemented by means of a device applying the product in liquid phase according to one of the following methods: by projection, by spraying, by deposition, by dipping. For this purpose, it is so possible to use at least a spray nozzle, or a brush, or a soaked cloth or a pad or a brush etc...

[0071] According to one embodiment, the image acquisition device (it may be still images or a video stream) is chosen from a binocular magnifier, an endoscope, a digital camera, a camera, a telephone. In other words, the acquisition device is advantageously chosen from existing devices, implying that the control method does not require the development of a specific image acquisition device.

[0072] The general principle is now explained in connection with figures 3 to 5, assuming that the product in liquid phase is applied in step E1 to the same face as the face where the optical control is carried out in step E2, namely the face marked 16.

[0073] The instant t0 corresponds to the instant at which the control duration At begins, while the instant t3 corresponds to the instant at which the control duration At ends. The instant t0 may be simultaneous with the instant at which the step E1 of applying the product in liquid phase (for example acetone) is carried out, or else be very slightly later by the shortest possible duration, ideally less than one second.

[0074] In [Fig.2], the evolution of the contrast K, which is a characteristic of the optical appearance of the control zone Z, in time t has a Gaussian appearance. Between time t0 and time t1, the contrast K increases but remains quite low. Between time t1 and time t2, the contrast K increases to a maximum value and then decreases. Between time t2 and time t3, the contrast K decreases while remaining quite low.

[0075] Generally speaking, the contrast K taken between the times t0 and tl in [Fig. 2] corresponds to the situation shown diagrammatically in [Fig. 3]: in the control zone Z, the zone internal to the defect 12 is then at least partially filled with liquid-phase product (hatching in a first orientation) previously applied in step E1, and at the same time a certain quantity of liquid-phase product (hatching in a second orientation) previously applied in step E1 is also present in the zone located outside the defect 12. This uniform presence of liquid-phase product throughout the control zone Z implies that the contrast K is quite low, while increasing as the liquid-phase product evaporates, the contrast evaporating much more quickly in the zone outside the defect than in the zone internal to the defect 12 which, by its hollow nature, locally stores more liquid-phase product.

[0076] Then, the contrast K taken between the instants t1 and t2 in [Fig.2] corresponds to the situation shown diagrammatically in [Fig.4]: in the control zone Z, a situation very quickly arises where the product in the liquid phase is completely evaporated in the zone outside the defect 12 (this is the reason for the absence of hatching outside the defect 12) while the product in the liquid phase is still present in the zone internal to defect 12 (this is the reason for the presence of hatching inside defect 12). This differential presence of the product in liquid phase between the outside and inside of defect 12 implies a strong increase in the contrast K up to a maximum value, the contrast K then evolving in a decreasing manner as the product in liquid phase present in defect 12 tends to evaporate.

[0077] Finally, the contrast K taken between times t2 and t3 in [Fig.2] corresponds to the situation shown diagrammatically in [Fig.5]: in the control zone Z, the zone outside the defect 12 remains devoid of liquid phase product and at the same time the zone inside the defect 12 is less and less filled with liquid phase product as it evaporates, until complete evaporation (this is the reason why in [Fig.5] the zone inside the defect 12 is devoid of hatching). This reduction in the differential presence of liquid phase product throughout the control zone Z implies that the contrast K is quite low, while decreasing as the liquid phase product evaporates. At time t3, all of the liquid phase product previously applied in step E1 has been evaporated, outside and inside the defect 12.

[0078] Step E4 is for example implemented if the result of the implementation of the analysis step E3 shows that during the control duration, in particular between the times t1 and t2, the contrast K takes at a given time a value greater than a predetermined threshold, for example equal to 0.5.

Claims

Claims

1. Non-destructive testing method for detecting at least one emerging defect (12) forming at least one surface discontinuity present on the surface of a mechanical part (10) for an aircraft, the method comprising the following steps: - application (El) of a product in liquid phase on the mechanical part (10), - optical testing (E2) by an image acquisition device, during a testing period (At) during which all or part of the previously applied product evaporates, of at least one testing zone (Z) of the mechanical part (10), the image acquisition device delivering data representative of the evolution, during the testing period (At), of the optical appearance of the monitored testing zone (Z), - analysis (E3) of the data previously delivered by the image acquisition device.

2. Control method according to claim 1, comprising a step of generating (E4) an alert, triggered if the result of the analysis step (E3) delivers information representative of the suspicion of the presence of said at least one defect (12) emerging on the surface of the mechanical part (10).

3. Control method according to one of claims 1 or 2, in which during the application step (El), the product in liquid phase is applied to at least a portion of a first face of at least one part to be controlled of the mechanical part (10), and in which during the optical control step (E2), said control zone (Z) belongs to a second face of the part to be controlled of the mechanical part (10).

4. A control method according to claim 3, wherein the first face is identical to the second face.

5. Control method according to claim 3, in which the first face is distinct from the second face, the first face and the second face being two opposite faces of the mechanical part (10) at the level of the part to be controlled, said two opposite faces delimiting the thickness (14) of the mechanical part (10) in the part to be controlled.

6. Control method according to one of claims 1 to 5, in which the applied liquid phase product is characterized by a surface tension of less than 40 mN / m, preferably less than 30 mN / m, and in particular less than 25 mN / m.

7. Control method according to any one of claims 1 to 6, in which the liquid phase product applied in the application step (El) has an evaporation index Ie relating to ethyl ether according to standard DIN 53170 of less than 3.

8. Control method according to any one of claims 1 to 7, in which the liquid phase product applied in the application step (El) has an evaporation index Iab relating to butyl acetate according to standard ASTM D 3539 greater than 1.

9. A control method according to any one of claims 1 to 8, wherein the liquid phase product applied in the application step (El) is transparent in the visible spectrum.

10. A control method according to any one of claims 1 to 9, wherein the liquid phase product applied in application step (E1) comprises a ketone.

11. A control method according to claim 10, wherein the liquid phase product applied in the application step (El) comprises acetone.

12. Control method according to any one of claims 1 to 11, in which the control duration (At) is between 0 and 15 seconds, preferably less than 10 seconds, in particular of the order of 7 seconds.

13. Control method according to any one of claims 1 to 12, in which the application step (El) is carried out by means of a device applying the product in liquid phase according to one of the following methods: by projection, by spraying, by depositing, by dipping.

14. Control method according to any one of claims 1 to 13, in which the image acquisition device is chosen from a binocular magnifier, an endoscope, a digital camera, a camera, a telephone.

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