Abstract:
The invention relates to the detection of liquid in a cavity of a structure, e.g. a sandwich-structure cell with honeycomb core and carbon fiber composite coating. The structure is subjected to a mechanical vibration, at a frequency f m  in the audible range, by an excitation wave, the area being explored is subjected to an incident electromagnetic wave and the reflected electromagnetic wave is analyzed to deduce therefrom the presence or the absence of liquid in cavities. The Faraday wave induced on the surface of the liquid in the cavity comprises subharmonic frequencies of f m  which modulate the reflected electromagnetic wave and make it possible to identify the presence of liquid. A detection apparatus comprises an excitation wave generation device for generating the vibration excitation wave, an electromagnetic wave generation device for generating the incident electromagnetic wave, a measuring device for detecting and measuring the reflected electromagnetic wave, and a signal processor.

Description:
RELATED APPLICATIONS 
     This application is a §371 application from PCT/EP2008/056796 filed Jun. 2, 2008 which claims priority from French Patent Application No. 07 55459 filed Jun. 5, 2007, each of which is herein incorporated by reference in its entirety. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention falls within the field of nondestructive testing of structures. In particular, the invention relates to the detection and location of liquids in internal cavities of structures. 
     BACKGROUND OF THE INVENTION 
     In the field of structures in mechanical constructions, the objective of producing structures as light as possible, while still ensuring that they are strong and rigid, often results in relatively highly loaded structural parts that include cavities. 
     The form and composition of such structural parts are very varied, one of the most common forms corresponding to what are called “sandwich” structures having a honeycomb cellular core. 
     A cellular-core sandwich structure  1 , as shown in cross section in  FIG. 1 , generally comprises a core  2  formed from hollow cells  12 , a priori containing air, having on each of its faces, namely the bottom face and the top face, a solid and strong skin panel, respectively  4 ,  3 . 
     This type of structure, because of its very favorable rigidity-strength/weight ratio, is used particularly in aeronautical constructions. 
     In one particular embodiment, which is also well known, the skin panels  3 ,  4  of the sandwich structure are made of composites comprising fibers—glass fiber, aramid fiber, carbon fiber, etc.—held in place in a cured resin, and the hollow cells  12  of the cellular core  2  are formed by means of walls produced in a sheet material shaped so as to define the cells in the form of a usually regular lattice. When this lattice consists of cells of hexagonal cross section, the expression “honeycomb” is generally used. 
     In most cases, the materials used for the skin panels and for the walls of the cells are relatively impervious to common fluids, resulting in a situation in which each cell constitutes a substantially closed and sealed cavity. 
     One drawback of this type of structure stems from the fact that the hollow cells are liable to fill up to a greater or lesser extent with water, without this water being able to be removed naturally. This has the effect, on the one hand, of unnecessarily increasing the weight of the structure, intended to be a lightweight structure, and, on the other hand, of reducing, through various physicochemical actions, the strength of the structure by impairing the specific mechanical properties of the materials used in producing the structure or by impairing the quality of the bonding between the various assembled elements, in particular between the cellular material and the skin panels. 
     The presence of water, or in general of liquid, in hollow cells which is generally considered to be a major defect of the structure, usually cannot be predicted and, whatever the causes of liquid being present—water formation process during manufacture of the structure or subsequent liquid penetration into cells—it is essential to detect the presence of the liquid in cells, to locate where the liquid is and to quantify the amount thereof so as to carry out the necessary actions for removing the largest possible amount of this liquid. 
     The detection of liquids in such cavitied structures is a long-standing problem. However, the conventional approaches suffer from major drawbacks. 
     Apart from visual inspections, limited to cases in which the materials used are sufficiently transparent for the liquid in a cell to be observed by optical means, the methods that have been used for the longest time and are well known to those skilled in the art are based on the principle of acoustic detection. 
     Echographic methods based on this principle are particularly well suited for continuous compact media. In the case of cellular—low-density cavitied—structures, the transmission of an acoustic wave is difficult and is unable to discriminate with the necessary precision the presence of liquid. 
     A purely acoustic, derived method, suitable for detecting the level of a liquid in a closed space—in this case a bottle—is disclosed in the patent published under the number EP 0 938 653. According to that patent, vibrations are induced in the bottle by means of a magnetic field and an acoustic signal in response to the vibrations is received by a microphone. 
     The presence and level of liquid in the bottle can be deduced from the characteristics of this received acoustic signal. However, in the case of highly cavitied materials, such as those considered here, this approach does not allow the presence of water in the cavities to be effectively detected because of the weakness of the echo. 
     Other methods aimed particularly at detecting the presence of water in cellular-core sandwich structures using other physical principles have also been imagined. 
     According to one of these other methods, since water has thermodynamic characteristics that are very different from the materials in which the presence of water is sought, it has been proposed to produce a thermographic image of a part subjected to temperature variation. In this case, the zones corresponding to water being present vary in temperature less rapidly than the zones where there is no water, because of the differences in thermal inertia between the water and the materials of the part, a thermographic image being capable of detecting the differences in surface temperatures and therefore zones containing water. 
     Such methods, one example of which is disclosed in the patent published under the number JP 62024134, have however the drawback, on the one hand, of usually requiring the part to be removed, in order for it to be investigated, and, on the other hand, of requiring substantial means for cooling or heating the part under conditions suitable for the measurements to be carried out. 
     According to another method, the presence of water in the cells is detected by means of electromagnetic microwaves, the propagation of which is modified in the presence of water. 
     However, the use of electromagnetic microwaves is constricting in terms of the precautions to be taken to protect personnel when the energy level used is significant. 
     Furthermore, difficulties arise in the case of sandwich structures using skin panels made of a carbon-fiber-based material or having a metallized surface, for example one metallized by means of a bronze mesh, which is the situation for many parts. 
     In the case of a structure based on carbon fibers, the carbon of the skin panels induces high losses and the signal-to-noise ratio is extremely unfavorable for detection. 
     When a bronze metallization mesh covers a surface, which is frequently the case for modern aeronautical materials, the skin panels are electrically conducting and prevent any measurements of the dielectric type. 
     To solve this problem, the solution described in the patent published under the number FR 2 880 424 describes a detection system using electromagnetic microwaves that includes antennas placed in the cellular material of a sandwich panel between the two skin panels. This solution, which is applicable in the case of carbon-fiber-based skin panels, does however require the parts to be modified in order to install the antennas. As a result, the parts are more complex, weaker and heavier, and the solution cannot be easily used for many existing parts or on parts that would not have been provided with these antennas during their manufacture, for cost reasons or for other reasons. 
     In general, the presence of a bronze mesh prevents any approach of the type in which the level of fluid in the cavities is measured by a radar device (in the radio, microwave or millimeter frequency ranges), since the bronze mesh reflects most of a radio wave in these frequency ranges. To alleviate this drawback, it is usually necessary to reduce the working frequency, but this has the effect of dramatically degrading the precision of the measurements and prevents a measurement of the liquid level from being carried out with sufficient precision. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The objective of the present invention is specifically to alleviate the difficulties encountered by the techniques of the prior art for detecting a liquid in cavities of a structure by coupling a vibratory excitation of the structure in the acoustic range with electromagnetic detection of the vibratory response of the structure. 
     Thus, according to the method of detecting the presence of a liquid in a cavity of a structure, in which the structure is subjected to a mechanical vibration by means of an excitation wave and is subjected to an incident electromagnetic wave in a zone of said structure in which the presence of the liquid in a cavity is sought. 
     Finally, a reflected electromagnetic wave, reflection of the incident wave on elements of the structure, is analyzed so as to deduce therefrom the presence or absence of liquid in cavities, the reflected wave having characteristics that are modified according to whether or not liquid is present in the cavities. 
     To create a Faraday wave on the surface of a liquid present in a cavity of the structure, which is perfectly distinct from the frequencies at which the structure responds in the absence of liquid, one component of the excitation wave is generated by an approximately sinusoidal vibratory excitation of frequency f m , preferably in the acoustic range, capable of initiating an induced oscillation of the surface of the liquid in a cavity at a subharmonic frequency of the excitation frequency f m . 
     To improve the sensitivity of the method, one component of the excitation wave is also generated by a vibratory excitation of frequency f m2  shifted relative to the frequency f m , thereby enabling a standing wave pattern to be created on the surface of the liquid and increasing the vibratory response of the liquid relative to the response of the structure. 
     To improve the signal-to-noise ratio, the frequency of one of the components of the excitation wave is advantageously modulated and more particularly is modulated by a white or psuedo-white noise. 
     Because of the very different frequencies of the vibratory responses of the liquid present in cavities from that of the structure, the method makes it possible, with a high signal-to-noise ratio, to deduce the presence of liquid from the detection of predefined spectral lines in the spectrum of the reflected wave. 
     In particular, the presence of liquid is deduced from the detection of at least one predefined spectral line in the spectrum of the reflected wave corresponding to a subharmonic of frequency f m /2 and/or f m2 /2 of the excitation frequency f m  and/or f m2  respectively. 
     According to the invention, a device suitable for implementing the method of detecting the presence of liquid in a cavity of a structure comprises:
         first means for generating at least one vibratory mechanical excitation wave in the structure;   second means for generating an incident electromagnetic wave in a zone of the structure in which the presence of a liquid is sought;   third means for measuring an electromagnetic wave reflected by the structure receiving the incident wave; and   fourth, processing and/or display means capable of demonstrating the presence in a spectrum of the reflected wave of at least one spectral line characteristic of an induced wave on a surface of a liquid contained in a cavity of the structure by the mechanical excitation wave.       

     To produce a compact device, at least for a probe part, the probe comprises a torus using piezoelectric technology capable of mechanically vibrating so as to create the mechanical excitation wave at a frequency f m , or at a frequency f m  and at a frequency f m2  shifted relative to the frequency f m . 
     Advantageously, the hollow part of the torus is exploited so as to provide: 
     
         
         
           
             an antenna for transmitting and receiving the electromagnetic waves; and/or 
             all or part of the supply electronics for the piezoelectric component; and/or 
             all or part of the supply electronics for the antenna for transmitting the incident electromagnetic wave; and/or 
             all or part of the detection electronics for detecting the reflected electromagnetic wave; and/or 
             all or part of the signal processing electronics. 
           
         
       
    
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One method of implementation and one embodiment of the invention will be described in detail with reference to the figures, which show: 
         FIG. 1   a : a simplified diagram of the method of the invention and a device according to the invention; 
         FIG. 1   b : a simplified diagram illustrating an improvement of the method and the device shown in  FIG. 1   a;    
         FIG. 2   a : an illustration of an example of a reflected wave spectrum observed in a structure in the absence of liquid with a set-up of the type shown in  FIG. 1   a , the frequencies being plotted on the x-axis and the power on the y-axis; 
         FIG. 2   b : an illustration under the same conditions as in  FIG. 2   a  of an example of a reflected wave spectrum observed in a structure in the presence of liquid in a cavity, the frequencies being plotted on the x-axis and the power on the y-axis; 
         FIG. 3 : an illustration of the spectrum of the reflected wave in the case of a dual-frequency set-up corresponding to the diagram shown in  FIG. 1   b;    
         FIG. 4 : an illustration of the spectrum of the reflected wave in the case of a dual-frequency set-up corresponding to the diagram shown in  FIG. 1   b  in which one of the frequencies is modulated; and 
         FIG. 5 : a schematic illustration of a probe combining acoustic and radar functions for a device according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention, the principle of which is illustrated in  FIG. 1   a , is used for detecting the presence of a liquid  11 , for example water, in a structure  1  having at least one cavity  12 . 
     For the requirements of the detailed description of one way of implementing the method according to the invention and that of one embodiment of a device according to the invention, said structure having at least one cavity is represented by a structure  1  of a panel, which is substantially flat on the scale of a zone in question, comprising a cellular core  2  and skin panels—a top skin panel  3  and a bottom skin panel  4 —on the faces of the cellular core  2 . 
     Each cell  12  of the core, closed at its ends by the top and bottom skin panels, constitutes a cavity. 
     The panel  1  comprises in practice a multitude of cavities  12 , which makes the problem of detecting a liquid in some of the cavities more complicated in practice since it is then necessary to locate the cavity or cavities in which the liquid is present. 
     A panel having such a structure, in which the cells have regular hexagonal cross sections, is generally referred to as a honeycomb core sandwich panel. 
     However, the choice of such a frequently used structure to describe the invention is not limiting. 
     As is known, the skin panels  3 ,  4  of the panel in question are made of a composite formed from fibers held in place in a cured resin, for example glass fibers or carbon fibers held in place by a cured aramid resin, and the walls forming the cells of the honeycomb are made of a composite, in principle a less expensive one than the carbon-fiber-based materials, for example based on glass fibers or based on thin sheets of an organic material. 
     According to the method of the invention, the structure  1  to be inspected, comprising numerous cavities  12 , some of which are liable to contain a liquid  11 , is mechanically excited so as to vibrate. 
     The mechanical excitation is preferably produced by means of a sinusoidal excitation of frequency f m  falling within the acoustic frequency range. 
     The structure  1  to be inspected is mechanically excited in such a way that an acoustic wave  14 , called excitation wave, propagates in the structure  1 . 
     When the excitation wave  14  reaches a cell  12  containing a liquid  11 , for example water in liquid form, said wave creates on the free surface of the water, by a nonlinear effect, a new wave  16 , called induced wave, also known as a Faraday wave. 
     The induced wave  16  is predominantly a subharmonic at half that of the excitation wave, i.e. said induced wave is a wave of frequency f m /2 that only fluid phenomena can generate. 
     Furthermore, the structure  1 , in a zone in which the presence of liquid is sought, is subjected to an electromagnetic wave  17 . 
     The electromagnetic wave  17  is preferably a sustained radar wave of the type of waves generated by means of a CW (continuous wave) radar. 
     The electromagnetic wave  17 , called incident wave, is chosen with a frequency suitable for the materials making up the composition of the cellular structure and in particular according to the material of the skin panel lying between the source of the electromagnetic wave and the possible liquid that has to be detected, in such a way that said skin panel introduces only a reasonable attenuation, for example &lt;120 dB. 
     In particular, when the cellular structure  1  is covered, on a face through which the incident wave  17  passes and/or through which a reflected wave that has to be observed passes, with electromagnetic shielding, for example a conducting mesh, care will be taken to chose the frequency of the incident wave  17  so as to prevent said incident frequency, and also the expected frequencies of a reflected wave, from corresponding to a forbidden frequency band for which the attenuation would be maximal. 
     As is known, the choice of frequency of the incident wave also results from a compromise between, on the one hand, keeping the losses in the materials of the structure  1  at an acceptable level for the measurement means employed—the losses, which are dependent on the material, increasing when the frequency increases—and, on the other hand, the desired spatial sensitivity, the resolution being better when the frequency is increased. 
     By taking these parameters for choosing a frequency of the incident wave  17  into consideration, it is possible, according to the method of the invention, to limit the power used for said incident wave to relatively low levels, limiting the physiological risks associated with electromagnetic radiations. 
     The incident wave  17  is reflected by the various elements that are present along its path and especially by the free surface of the liquid possibly present in a cell in order to form a reflected wave  18  modulated by the mechanical vibrations of said various elements. 
     When a liquid  11  is present in a cell, the wave  18  reflected by the surface of said liquid, subjected to the mechanical excitation frequency f m , is modulated not only by the vibrations of the structure due to the excitation wave  14 , at the frequency f m , i.e. vibrations at the frequency f m  and multiple harmonics of f, but also those due to the induced wave  16  at said surface of said liquid with subharmonic frequencies and in particular the frequency f m /2. 
     By processing a measurement of the reflected wave it is possible to extract a useful signal for identifying the presence or absence of liquid in the cell or cells subjected to the incident wave  17 . 
     This processing comprises an analysis of the baseband spectrum of the reflected electromagnetic wave in order to detect a possible subharmonic of the excitation frequency. 
     One advantage of the method stems from the fact that, on the one hand, the electromagnetic wave, unlike acoustic waves, is insensitive to the presence of acoustic vacancies characteristic of cellular materials and, on the other hand, the consequences of the original excitation at the frequency f m  and of the higher-order (second, third, fourth, etc.) harmonics due to the nonlinearities within the solids constituting the structure, may be effectively filtered out, since they are well separated in the spectrum of the sought frequencies of the induced wave. 
     The direct consequences are a sufficient power of the useful signal, so that it can be measured without any particular difficulty, and a very favorable signal-to-noise ratio. 
     The following equations illustrate how the reflected wave  18  is influenced by the presence of a liquid in a cell. 
     If the incident wave  17  is characterized by an emitted electromagnetic signal e(t), for example a sinusoidal signal of frequency f e  and amplitude E e , the signal is expressed as:
 
 e ( t )= E   e   ×∈xp ( j× 2×π× f   e   ×t )
 
where j is the imaginary unit and using the so called exponential notations, t being the time variable. The operator × corresponds to multiplication.
 
     The reflected wave  18  is characterized by a signal s(t). 
     The signal s 0 (t) reflected by a stationary object corresponds to the emitted signal e(t) phase-shifted according to the electrical distance from the object T, i.e.:
 
 s   0 ( t )= E   e   ×∈xp[jx (2× π×f   e   ×t+T )]= E   e   ×∈xp ( j× 2×π ×f   e   ×t )×∈ xp ( j×T )
 
     When the signal s(t) is reflected by an object mechanically vibrating at a frequency f m , the signal received is of the form:
 
 s ( t )= E   e   ×∈xp[j ×(2 ×π×f   e   ×t+a ×sin(2 ×π×f   m   ×t ))]×∈ xp ( j×T )
 
in which expression a is a coefficient proportional to the mechanical amplitude with which the object vibrates.
 
     This expression for the reflected signal is also expressed by means of k-order Bessel functions J k : 
     
       
         
           
             
               s 
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                 t 
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     This expression corresponds to a symmetrical spectrum of lines, centered on the frequency f e . 
     Each line is separated from a neighboring line by a frequency f m . 
     In the particular case of the panel  1  corresponding to the application of the method, the signal will a priori be reflected several times, in particular in the cavities. 
     In the absence of liquid in the cells, the signal therefore has the following form, noting the parameters associated with the various reflections by the index i:
 
 s ( t )=Σ i   └E   e     i     ×∈xp ( j× 2×π× f   e   ×t )×∈ xp ( j×T   i )×∈ xp ( j×a   i ×sin(2×π× f   m   ×t ))┘
 
     The spectrum of this signal again corresponds to a symmetrical spectrum of lines centered on the frequency f e , each line of which is separated from an adjacent line by a distance f m . 
     In the presence of a liquid in at least certain cells, the signal corresponding to the reflected wave contains a new term, due to the presence of the induced wave  16  on the surface of the liquid  11 , of the form:
 
 s ( t )= E   e     i     ×∈xp[j ×(2×π ×f   e   ×t+a ×sin(2 ×π×f   m /2× t ))]×∈ xp ( j×T )
 
assuming a subharmonic response of the liquid dominated by the half-frequency ½ subharmonic, the above expression also being expressed as:
 
     
       
         
           
             
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                                 f 
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     The spectrum of this signal corresponds to a symmetrical spectrum of lines centered on the frequency f e , each line of which is separated from an adjacent line by a distance f m /2. 
     The presence of liquid  11  in a cell  12  is therefore demonstrated by the presence in the spectrum of the reflected wave  18  of lines at frequencies f m /2, 3f m /2, 5f m /2, etc., which frequencies are absent in the case in which there is no liquid in the cells. 
       FIG. 2   a  illustrates an example of an experimentally observed spectrum in the absence of a liquid in the cells and  FIG. 2   b  illustrates an example of a spectrum in the presence of water in cells of a panel of a honeycomb sandwich structure. 
     In the spectrum of  FIG. 2   a , a spectral line is characteristic of the frequency f m , and no subharmonics can be distinguished, unlike in the spectrum of  FIG. 2   b  which shows subharmonics of the frequency f m . 
     In an advanced form of the method according to the invention corresponding to the illustration in  FIG. 1   b , to improve the sensitivity of liquid detection, the structure  1  is excited in mechanical vibration by means of two frequencies, the frequency f m  and a second frequency f m2 , said two frequencies being close together. 
     In fact it is known (see for example Thomas Besson and W. Stuart Edwards, “Two-frequency parametric excitation”, Physical Review, July 1996) that setting a surface of a liquid into vibration using two frequencies produces a subharmonic stationary wave. 
     The mechanical excitation at the frequency f m2  generates a wave  42  that propagates in the material  1  and interferes with the mechanical excitation wave  14  at the frequency f m  in order to create an interference pattern on the surface of the liquid  11  when a cell  12  contains such a liquid. 
     The mechanical excitation signal corresponding to the frequency f m2  may correspond to a pure sinusoidal signal or a modulated sinusoidal signal. 
       FIG. 3  shows schematically in this case a spectrum received in baseband by the electromagnetic sensor. 
     Advantageously, the excitation signal corresponding to the frequency f m2  is modulated by a white or psuedo-white (M-sequence) noise enabling signal processing methods of the autocorrelation type for improving the signal-to-noise ratio to be used. 
     In practice, the modulation by a M-sequence is capable of exciting nonlinear modes of the structure or of the fluid (parasitic mixing phenomenon). However, if the mechanical excitation frequencies are chosen in such a way that said frequencies are close together, i.e. |f m2 −f m |&lt;&lt;f m /2, the bands due to the frequencies mixing do not overlap the ½ harmonic, thereby making it possible to determine the acoustic transfer function of the material around the ½ harmonic. 
     In this case, the parasitic mixing properties will be advantageously used to determine the mechanical properties, i.e. the non-fluid properties of the part. 
       FIG. 4  illustrates these phenomena by a schematic representation of a spectrum of mechanical frequencies of a part. 
     A frequency  303  corresponds to the first mechanical vibration at the frequency f m  and another frequency  302  corresponds to the second mechanical vibration at the frequency f m2 , which is modulated by an M-sequence  301 . 
     Moreover, because of the presence of a cavity containing a liquid, mechanical vibrations of half frequencies  306 ,  307  are highlighted together with the white noise modulation multiplied by the transfer function of the fluid (a known result in the context of linear systems), denoted by  304  in the diagram shown in  FIG. 4 . 
     Furthermore, close to the zero frequency, a mixing product  305 , characterized by ±m×f m1 ±n×f m2  gives the mechanical transfer function around the zero frequency. This results from the fact that, by mixing, the white noise is transferred to the zero frequency and undergoes a temporal convolution by the mechanical transfer function of the structure  1 . 
     The measured signal may in certain circumstances exhibit spectral lines  308 ,  309  outside the spectrum directly associated with the mechanical excitation. 
     Such lines are due to the separation of small droplets from the free surface of the liquid in the cavity, the kinetics of which small droplets are specific to each droplet. Such lines  308 ,  309  are characteristic and are advantageously used to improve the way in which the presence of liquid is detected. 
     Finally, other lines  310  of purely parasitic type are sometimes observed, these corresponding predominantly to nonlinearities of the structure. 
     Thus, using the method, by analyzing the spectrum of the reflected wave  18  it is possible to detect, owing to the presence of certain lines  306 ,  307 ,  304 ,  305 ,  308  in said spectrum, the presence of a liquid  11  in cavities  12 . 
     A device according to the invention for detecting the presence of a liquid  11  in at least one cavity  12  of a structure  1 , such as a so called honeycomb core sandwich structure, comprises:
         first means  10 ,  13 ,  40 ,  41  for generating at least one vibratory mechanical excitation wave  14 ,  42  in the structure  1 ;   second means  21 ,  22  for generating an incident electromagnetic wave  17  in a zone of the structure  1  in which the presence of a liquid is sought;   third means  15 ,  19  for measuring an electromagnetic wave  18  reflected by the structure  1  receiving the incident wave  17 ; and   fourth, processing and/or display means  20  capable of demonstrating the presence in a spectrum of the reflected wave  18  of at least one spectral line characteristic of an induced wave  16  on a surface of a liquid  11  contained in a cavity  12  of the structure  1  by the mechanical excitation wave  14 ,  42 .       

     The first means advantageously comprise an actuator  10  in contact with said structure. 
     The actuator  10  is supplied by a power source  13  preferably delivering a sinusoidal signal of frequency f m , said frequency preferably lying within the acoustic frequency range. 
     Furthermore, when the first means generate an excitation wave at the frequency f m2  different from f m , which may be modulated, said first means advantageously comprise a second actuator  40  in contact with the structure  1 , said second actuator being supplied by a generator  41  delivering the signal at the frequency f m2 . 
     Such actuators  10 ,  40  are advantageously produced in a known manner using piezoelectric technology and are supplied by voltage sources corresponding to the excitation signals that have to be applied to the structure. 
     Preferably, the actuators  10 ,  40  are excited in a continuous regime in order to sustain the induced wave  16 , which are rapidly damped. 
     The second means, for generating the incident electromagnetic wave  17 , essentially comprise a radiofrequency generator  22  and an antenna  21  which is designed to radiate in the direction of the zone of the structure  1  that has to be analyzed, a priori close to one or both of the actuators  10 ,  40  producing the mechanical excitation wave  14 ,  42 . 
     The third means, for measuring the reflected electromagnetic wave  18 , essentially comprise an antenna  15  and means  19  for demodulating said reflected wave that are capable of establishing a frequency spectrum of said wave. 
     In a particularly inexpensive embodiment, the means  19  for demodulating the reflected electromagnetic wave  18  essentially consist of a simple self-oscillatormixer. 
     The emitting antenna  21  of the second means and/or the receiving antenna  15  of the third means are preferably directional antennas, it being possible for one and the same antenna to both emit the incident wave  17  and receive the reflected wave  18 . 
     The directivity of said second and/or third means makes it possible to define the zone of the structure  1  under investigation and therefore to locate the position in said structure of the cavities  12  containing a liquid  11  when the presence of liquid is detected. 
     The fourth means  20  consist of any means for processing the spectrum of the reflected electromagnetic wave  18 , making it possible to demonstrate one of the effects signifying with a high probability the presence of a liquid  11  in a cavity  12  of the structure  1 . 
     In an embodiment having minimal data processing, the spectrum is presented in a graphical form, thereby enabling an experienced operator to interpret the various lines of the spectrum within the context of an investigation in progress. 
     Preferably, the signal is processed, for example after sampling by a computer unit, in order to allow automatic detection of lines in the spectrum of the reflected wave  18  signifying the presence of liquid in a cavity. 
     In this case, a situation corresponding to a probable presence of liquid is indicated by a display, optionally together with an audible signal for drawing the attention of an operator, the display and/or the audible signal preferably being modulated so as to provide information about the amount of liquid that may be present in the zone undergoing measurement by the device, it being possible for such information to be deduced from the relative intensities of the various lines of the reflected wave spectrum. 
     In one particular embodiment, means (not shown), for example a system for depositing an ink by micronozzles, are associated with the device so as to deposit colored marks, for example with an ink having a color in contrast with the surface of the structure, and being able to be easily removed, on zones detected as containing liquid in cavities. Such a marking device thus makes it possible to scan, by means of the device, large areas of the structure without interrupting a search phase and, in a subsequent processing phase, to be able to instantly identify the zones potentially containing liquid in order to carry out a more detailed local examination or to carry out the draining operations that are generally necessary in order to remove the liquid from the structure. 
     One embodiment, which is particularly advantageous economically and from the operational standpoint, is shown in  FIG. 5  for producing a compact probe  401  suitable for the device and for implementing the method, capable of carrying out the mechanical excitation, electromagnetic radiation and electromagnetic measurement functions. 
     The compact probe  401  of the device comprises a torus  407 , which is not completely symmetrical, produced in piezoelectric technology, which is connected to frequency generators that generate frequencies corresponding to the mechanical excitation frequencies f m  and f m2  as seen above. 
     It is known that a torus made of piezoelectric materials supports two degenerate vibrational modes, i.e. modes having the same frequency. This property is only true if the symmetry is perfect. 
     When the symmetry is broken, the torus is capable of resonating at two frequencies f m  and f m2  that are close together but separate. A weight or a grove  408  is generally sufficient to break the symmetry of the torus  407 . 
     Remaining inside the torus is a space  409  sufficient to install, if this should prove to be advantageous, i.e. if because of their size or their thermal dissipation it is not desirable to install them elsewhere, the following:
         the antenna  402  used for transmitting and receiving the electromagnetic waves  17 ,  18 . The antenna  402  consists, for example, of an open coaxial cable, as illustrated in  FIG. 5 , which is capable of radiating in the frequency range in question, from a patch or a resonant cavity (whether iris-coupled or not) or any other suitable antenna;   all or part of the electronics for the frequency generator and of the electronics for amplifying the signal for supplying the piezoelectric component;   all or part of the supply electronics for the antenna for emitting the incident electromagnetic wave  17 ;   all or part of the electronics for the electromagnetic detection part  403 , for example a self-oscillator mixer or else an oscillator connected to a mixer; and   all or part of the signal processing electronics  404 .       

     A device may therefore be produced that uses a small lightweight probe that can be easily moved up to the surface of a structure in order to detect the presence of liquid, particularly water, in cavities of the structure. 
     Said device is capable of detecting water in liquid form in panels of the honeycomb core sandwich type having carbon-fiber-based skin panels, but not exclusively in such panels, including through a metallization mesh when such a mesh covers one of the skin panels, in general on the side of the more accessible face of the panel. 
     The device makes it possible to seek the presence of water not only in the factory but also, because it can be made to have a sufficiently small size, to be used without removing an installed part, for example on an aircraft.