Abstract:
A method for structural component crack testing comprising: a) identifying a structural component hole and inserting a probe thereinto; b) for different emission directions, automatically performing the following: b1) controlling a probe ultrasound beam emission; b2) measuring a probe signal; b3) if the measured signal amplitude is above a predetermined threshold: determining a distance between the probe and a structural component discontinuity point; recording a data set comprising at least the distance between the probe and the discontinuity point, together with a data element corresponding to the probe emission angular direction, c) automatically searching for data sets corresponding to characteristic discontinuity points, and consequently establishing a correspondence between the probe emission angular directions and an angular reference frame linked to the component; d) based on the recorded data sets, automatically determining the discontinuity point positions; e) determining a dimensional characteristic of a crack based on the discontinuity point positions.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the French patent application No. 1653575 filed on Apr. 22, 2017, and the French patent application No. 1653576 filed on Apr. 22, 2017, the entire disclosures of which are incorporated herein by way of reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to the testing of a structural component of a vehicle, more particularly for the purpose of detecting a crack in such a component. Some structural components of a vehicle, such as an aircraft, are regularly subjected to stresses during the use of the vehicle, which may result in cracks in the components. In order to use the vehicle in adequate conditions of safety, these structural components must be tested in periodic test inspections. In these test inspections, operators check whether a structural component has a crack, and, if so, must estimate certain dimensional characteristics of the crack, particularly its length. For some components, the operators may detect a crack by visual inspection or by moving a sensor over the surface of the component. However, other components are more difficult to access, and such a procedure is not feasible for the detection of a possible crack. In particular, a component to be inspected may be assembled with another component, this other component preventing both visual inspection and the movement of a sensor over the component to be inspected. For example, a structural component  10  shown separately in  FIG. 1A  has a crack  15 . This structural component is assembled with another component  12 , shown separately in  FIG. 2A . These two components have a set of fastening holes such as the hole marked  21  in the figures. These fastening holes may be used to assemble the components by using fastening means such as bolts or rivets.  FIG. 3A  shows the structural component  10  and the component  12  assembled together. The component  12  prevents an operator from accessing the structural component  10  to inspect the crack  15 .  FIGS. 1B, 2B, and 3B  show the components  10  and/or  12 , in sections taken along the line A-A of  FIGS. 1A, 2A and 3A  respectively. Some structural components are also difficult for an operator to access: in some cases, the operator may touch a component, but finds it difficult to see it while manipulating the component. It would therefore be desirable to improve the methods of testing structural components, in order to facilitate the detection of a crack in a component which is difficult to access and/or is masked by another component. 
       SUMMARY OF THE INVENTION 
       [0003]    An object of the present invention is, notably, to provide a solution to these problems. It relates to a method for crack testing in a structural component of a vehicle. This method is remarkable in that it comprises the following steps:
       a) identifying a hole of circular cross section in the structural component and inserting into the hole a probe comprising at least one ultrasonic transducer, the probe being equipped with a rotation sensor and/or a motor;   b) moving the probe rotationally in the hole so as to move the direction of emission of an ultrasound beam by the probe, and, for each angular position of the probe among a set of different angular positions of the probe, performing the following sub-steps automatically, by means of a control system:   b1) controlling the emission of an ultrasound beam into the structural component;   b2) measuring a signal supplied by the probe, corresponding to an echo of the emitted ultrasound beam;   b3) if the amplitude of the measured signal is above a predetermined threshold:   determining a distance between the probe and a point of discontinuity in the structural component, on the basis of the measured signal;   determining the angular position of the probe;   recording a data set comprising at least the distance between the probe and the point of discontinuity, together with a data element corresponding to the angular position of the probe,   c) automatically searching, by means of the control system, among the data sets recorded for the different angular positions of the probe, for data sets corresponding to characteristic points of discontinuity of the structural component, and establishing a correspondence between the angular positions of the probe on the one hand, and an angular reference frame linked to the component on the other hand;   d) on the basis of the data sets recorded for the different angular positions of the probe, automatically determining, by means of the control system, the positions in the structural component of the points of discontinuity corresponding to the angular positions of the probe for which the amplitude of the measured signal is above the predetermined threshold;   e) determining at least one dimensional characteristic of a crack in the structural component on the basis of the positions of the points of discontinuity.       
 
         [0015]    This method enables an ultrasound beam to be emitted in the thickness of the structural component from the hole in which the probe is inserted. By moving the probe rotationally in the hole, the direction of emission of the ultrasound beam by the probe may be oriented in a plurality of angular positions. It is thus possible to detect automatically, in the structural component, points of discontinuity corresponding to the crack which is being sought. For each detected point of discontinuity, the recording of the distance determined between the probe and this point, together with the corresponding angular position, enables the position of the point in the structural component to be determined automatically. By identifying characteristic points of discontinuity, a correspondence may be established automatically between the angular positions of the probe and an angular reference frame linked to the component: this makes it unnecessary to reference the position of the probe relative to the component, and thus facilitates the user&#39;s work. After determining the positions in the structural component of a set of points corresponding to the crack, it is thus possible to determine at least one dimensional characteristic of the crack. 
         [0016]    According to one embodiment, the probe being equipped with a motor, in step b) the rotational movement of the probe in the hole is controlled automatically by the control system. 
         [0017]    In a particular embodiment, the rotation sensor being a rotary encoder, in step a) the probe is inserted into the hole until the rotary encoder comes into contact with the structural component or with a component assembled onto the structural component. 
         [0018]    According to another particular embodiment, step b) is repeated after step c). 
         [0019]    Advantageously, step e) is executed automatically by the control system. 
         [0020]    Advantageously, step e) comprises the determination of a length of the crack in the structural component. 
         [0021]    According to a particular embodiment, in step a) the probe is inserted into the hole in the structural component through another component adjacent to the structural component. 
         [0022]    According to another particular embodiment, steps a), b), c) and d) are repeated for at least two holes in the structural component. 
         [0023]    In an advantageous embodiment, the probe being a multi-element ultrasonic probe, step b) is repeated with the ultrasound beam emitted toward a plurality of locations distributed within the thickness of the structural component. 
         [0024]    In another advantageous embodiment, the probe being a multi-element ultrasonic probe, steps b1) to b3) are repeated with the ultrasound beam emitted toward a plurality of locations distributed within the thickness of the structural component. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The invention will be more readily understood from a perusal of the following description and the accompanying figures. 
           [0026]      FIGS. 1A, 2A and 3A , already described, represent in a simplified manner a structural component of a vehicle and another component assembled onto this structural component. 
           [0027]      FIGS. 1B, 2B, and 3B , already described, show sections taken along the line A-A of  FIGS. 1A, 2A and 3A  respectively. 
           [0028]      FIGS. 4A and 4B , similar to  FIGS. 3A and 3B  respectively, represent a structural component into which a probe according to an embodiment of the invention has been inserted. 
           [0029]      FIGS. 5A and 5B  on the one hand, and  5 C and  5 D on the other hand, show detail views of  FIGS. 4A and 4B  respectively. 
           [0030]      FIGS. 6 and 7  show detail views, similar to that shown in  FIG. 5A , illustrating an embodiment of the invention. 
           [0031]      FIG. 8  shows an example of a probe equipped with a motor. 
           [0032]      FIG. 9  shows a display on a display screen according to an embodiment of the invention. 
           [0033]      FIG. 10  shows in a schematic manner an example of an automatic control system. 
           [0034]      FIG. 11A  shows in a simplified manner a multi-element ultrasonic probe. 
           [0035]      FIG. 11B  is a detail view illustrating the use of the probe shown in  FIG. 11A , according to an embodiment of the invention. 
           [0036]      FIG. 12A  is a sectional view, in a plane perpendicular to its longitudinal axis, of a multi-element ultrasonic probe. 
           [0037]      FIG. 12B  represents a two-dimensional multi-element ultrasonic probe. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    The structural component  10  and the component  12  assembled onto this structural component, shown in  FIGS. 4A and 4B , are similar to those described above and shown in  FIGS. 3A and 3B . A probe  35  is inserted into the hole  21  which is common to the structural component  10  and the component  12 . The probe  35  is equipped with a rotation sensor  38 . The probe  35  comprises at least one ultrasonic transducer  36 , as shown in  FIG. 5B . When its emission is controlled, this ultrasonic transducer emits an ultrasound beam  37  perpendicular to a longitudinal axis of the probe. The probe  35  is positioned so that the ultrasound beam is emitted in the thickness of the structural component  10 . Preferably, a commonly used gel is applied to the probe to ensure the transmission of the ultrasound between the probe and the structural component  10  in the hole  21 . If this ultrasound beam encounters a discontinuity in the component  10 , this beam is reflected in such a way that some of the emitted ultrasound is reflected toward the transducer  36  of the probe. The time interval between the emission of the ultrasound beam and the reception by the probe of an echo corresponding to the reflected ultrasound is characteristic of the distance between the probe and the discontinuity. This discontinuity may, notably, correspond to a crack  15  in the structural component  10 . In the example shown in  FIGS. 5A and 5B , the probe is oriented in the structural component  10  in such a way that the ultrasound beam is reflected at point B of the crack  15 . 
         [0039]    When the probe  35  is inserted into the hole  21 , an operator positions the rotation sensor  38  in contact with the component  12 , as shown in  FIGS. 5A and 5B . For the sake of clarity, the component  12  is not shown in  FIG. 5A . 
         [0040]    Advantageously, the rotation sensor is a rotary encoder. It comprises two parts which are rotationally movable relative to one another, namely a first part fixed to the probe  35  and a second part designed to be fixed to the component  12  when the probe  35  is inserted into the hole  21 . In one alternative, the rotary encoder is an incremental encoder; in another alternative, it is an absolute encoder. When it is linked to a control system, if the first part and the second part move rotationally relative to one another, the rotation sensor  38  delivers a signal representative of the rotation. Thus the control system determines the current angular position of the probe  35 . According to a first variant, a surface of the second part of the rotation sensor  38 , designed to be in contact with the component  12  when the probe  35  is inserted into the hole  21 , is of a non-slip type. This surface is, for example, coated with a material such as rubber. Thus the operator may move the probe  35  rotationally in the hole  21 , while applying a light longitudinal pressure to the probe or to the rotation sensor in the direction of the component  12 , without causing the rotation of the second part of the rotation sensor  38 : thus this second part remains fixed to the component  12 . In a second variant, the rotation sensor  38  comprises a fastening flange  38   f  fixed to the second part, as shown in  FIG. 6 . This fastening flange has a hole designed to interact with another hole in the component  12 , for example one of the holes  20  or  22 . When he positions the rotation sensor  38  on the component  12 , the operator causes this hole in the fastening flange  38   f  to be superimposed on the other hole  20  or  22  in the component  12 . In the example shown in the figure, the hole in the fastening flange is superimposed on the hole  20  in the component  12 . If the other hole  20  is empty, the operator places an insert  24  into this at least one other hole, to fix the second part of the rotation sensor  38  to the component  12 : because of this insert  24 , the fastening flange  38   f , and consequently the second part of the rotation sensor  38 , cannot revolve around the probe  35 . The insert  24  is, for example, a clip made of plastic material. If the other hole  20  or  22  already contains an assembly screw or bolt, the insert  24  may be a screw head or a nut on the bolt. The above description relates to the insertion of the probe  35  into the hole  21  from the end of the hole  21  corresponding to the component  12 . This procedure is particularly useful if the other end of the hole, corresponding to the structural component  10 , is difficult or impossible to access. Without departure from the scope of the invention, if this other end of the hole is accessible, the operator could insert the probe into the hole  21  from this other end corresponding to the structural component  10 . He would then place the rotation sensor  38  in contact with the structural component  10 . 
         [0041]    An angular position of the probe  35  in the hole  21  corresponds to a direction of emission of the ultrasound beam  37  that may be emitted by the transducer  36  of the probe. The ultrasound beam shown in  FIG. 6  is reflected by the crack  15  at a point B. As mentioned above, the measurement of the reflected ultrasound may be used to determine the distance between the probe and the point B. The point B may thus be characterized by a pair of data elements corresponding, on the one hand, to the angular position of the probe determined by a control system on the basis of a signal received from the rotation sensor  38  and, on the other hand, to the distance measured between the probe and the point B. This pair of data elements forms polar coordinates in a reference frame centered on the point  21 . In an exemplary embodiment, the probe  35  is linked to a measuring instrument marketed by the company TESTIA® under the trade name “Smart U32.” This measuring instrument controls the emission of the ultrasound beam  37  by the probe, measures the reflected ultrasound, and automatically indicates the distance between the probe and the point B. Advantageously, the control system is incorporated in this measuring instrument. In fact, the measuring instrument has available communication ports, of the USB® type for example, and is based on a Windows® software environment in which special-purpose software can be installed, in addition to the basic measurement functions available in the measuring instrument. An example of a control system  55  based on this measuring instrument is shown in  FIG. 10 . The control system  55  comprises a processing unit  58 , for example a processor or a microprocessor. It also comprises a display screen  50 , designed, notably, to display measurements made by means of the measuring instrument. The probe  35  is linked to the control system  55  (the measuring instrument) as indicated above. The rotation sensor  38  is also linked to the control system, if necessary via an adapter which is not shown in the figure, and which is connected to a communication port of the control system. The processing unit comprises a software function for determining the angular position of the probe  35  on the basis of signals received from the rotation sensor  38 . 
         [0042]    In a first embodiment, in order to inspect the crack  15  in the structural component  10 , the operator moves the probe  35  rotationally in the hole  21 , the probe being linked to the aforementioned measuring instrument  55 . This rotational movement of the probe enables the direction of emission of the ultrasound beam by the probe to be varied. If the ultrasound beam is reflected by a discontinuity in the structural component  10 , such as the crack  15 , the amplitude of the signal supplied by the probe (corresponding to the reflected ultrasound) and measured by the measuring instrument is above a predetermined threshold.  FIG. 9  shows an example of a display on the display screen  50  of the measuring instrument. A vertical scale A corresponds to the amplitude of the measured signal, and a horizontal scale d corresponds to the time interval between the emission of the ultrasound beam and the reception of reflected ultrasound. This horizontal scale d therefore corresponds to the distance between the probe and an ultrasound reflection point. A measured signal  54  is displayed on the screen. For distance at which the structural component has no discontinuity, the amplitude of the signal  54  is below a predetermined threshold S. However, for a distance D at which the ultrasound beam is reflected, the signal  54  has a peak amplitude 52 above this predetermined threshold. The display of the peak  52  on the screen enables the operator to read the distance D on the horizontal scale d. If no discontinuity is present in the component in the direction of emission of the ultrasound beam, the signal  54  does not have a peak  52  of this type, and its amplitude is below the predetermined threshold S over the whole of the horizontal scale. When the operator moves the probe rotationally  35  in the hole  21 , the direction of emission of the ultrasound beam by the probe varies as mentioned above, and therefore the distance D indicated by the measuring instrument on the screen  50  also varies. In practice, the operator moves the probe  35  rotationally, preferably through a rotation of at least 360°, while monitoring the display on the screen  50 . While the operator is moving the probe  35 , the control system  55  determines the current angular position of the probe  35  at different instants, by means of the software function of the processing unit  58 . Additionally, if the ultrasound beam is reflected at point of discontinuity of the structural component  10 , the distance D between the probe and this point of discontinuity is known by the control system  55  and is also displayed on the screen  50 . The processing unit  58  of the control system executes software configured to record in a memory pairs of data elements corresponding to the angular position of the probe and the distance D, for a set of points of discontinuity of the structural component  10 . In a first variant, the recording of the pairs of data elements corresponds to temporal sampling, coupled to a filtering procedure, so that the pairs of data elements are recorded only if the signal  54  measured and displayed by the measuring instrument has a peak amplitude 52 above the predetermined threshold S, this peak corresponding to a reflection of the ultrasound beam at a point of discontinuity of the structural component  10 . Because of this filtering, the pairs of data elements are recorded only if they correspond to points of discontinuity of the structural component  10 . In another variant, the recording of the pairs of data elements corresponds to angular sampling (for example, at every 1° of rotation of the probe  35 , based on its angular position determined by the control system  55 ), coupled to a filtering procedure similar to that described for the first variant. 
         [0043]    In a second embodiment, the probe  35  is equipped with a motor  40  as shown in  FIG. 8 . The motor comprises a rotor  40   r , fixed to the probe, and a stator  40   s . According to a first variant, a surface  41  of the stator  40   s , designed to be in contact with the component  12  (or with the structural component  10 ) when the probe  35  is inserted into the hole  21 , is of a non-slip type. This surface is, for example, coated with a material such as rubber. Thus, when the motor is controlled so as to move the probe  35  rotationally in the hole  21 , the operator simply has to apply a light longitudinal pressure to the stator in the direction of the component  12  in order to keep the stator fixed to the component  12 . In a second variant, the stator  40   s  comprises a fastening flange, similar to the fastening flange  38   f  described above for the rotation sensor. This fastening flange enables the stator  40   s  to be kept fixed to the component  12 . The motor  40  is linked to the control system  55  as shown in  FIG. 10 , if necessary via an adapter (not shown) which is connected to a communication port of the control system. According to a first alternative, the probe  35  is equipped with both the motor  40  and the rotation sensor  38 . According to a second alternative, the probe  35  is equipped solely with the motor  40 , and the control system  55  determines the angular position of the probe  35  on the basis of the control signals that it sends to the motor. In this second embodiment, after the probe has been placed on the component  12  (or on the structural component  10 ), the operator does not need to manipulate the probe  35 : the rotational movements of the probe are provided by the motor  40  controlled by the control system  55 . Thus the control system  55  records pairs of data elements (corresponding to the angular position of the probe and the distance D determined by the control system  55 ) as in the first embodiment, and its software is also configured to control the rotation of the probe  35  by means of the motor  40 . 
         [0044]    In both the first and the second embodiment, after the recording of the pairs of data elements in the memory, the software of the processing unit  58  executes a function of searching for characteristic points of discontinuity of the structural component  10 . These characteristic points of discontinuity correspond to discontinuities of the structural component which are present even in the absence of a crack or defect in the component. They correspond, notably, to edges of the component, to holes pierced in the component, etc. By way of non-limiting example, the points C 1 , C 2 , C 3  and C 4  shown in  FIG. 7  correspond to characteristic points of discontinuity of the structural component  10 . The coordinates of the characteristic points of discontinuity are known from a plan of the component, for example a digital model of the component. According to a first variant, these coordinates are recorded in a memory of the control system. According to another variant, these coordinates are recorded in a database, and the control system  55  interrogates the database via a data link. If these coordinates are not already expressed in an angular reference frame centered on the hole  21 , the software executes a conversion function to find the coordinates of the characteristic points of discontinuity in an angular reference frame of this type centered on the hole  21  and having its orientation determined relative to the structural component  10 , for example the angular reference frame Ra shown in  FIG. 7 , the orientation of which is represented by an arrow corresponding to a direction 0. These coordinates then correspond to polar coordinates, such as the pairs of data elements recorded in the memory by the control system during the rotation of the probe. Regarding these pairs of data elements, the reference frame used is also centered on the hole  21 , but is oriented in any direction, since the rotation sensor  38  (or the motor  40 ) has not undergone an initialization procedure to fix a reference direction relative to the structural component  10 . By comparing the coordinates of the characteristic points of discontinuity (expressed in the angular reference frame Ra whose orientation is determined relative to the structural component  10 ) with the pairs of data elements recorded in the memory, the software searches for the pairs of data elements corresponding to these characteristic points of discontinuity. As a result of this, it calculates a correspondence, in the form of an angular offset, between the angular positions recorded in the pairs of data elements and the angular reference frame Ra linked to the component. 
         [0045]    Having determined this correspondence between the recorded angular positions and the angular reference frame linked to the component, the software of the processing unit  58  executes a transformation function based on this correspondence, to transform the pairs of data elements recorded in the memory into polar coordinates expressed in the angular reference frame Ra linked to the component. These polar coordinates define the positions, in the structural component  10 , of the points of discontinuity detected during the rotation of the probe  35  in the hole  21 . 
         [0046]    In a non-limiting specific embodiment of the invention, a new step of measurement acquisition is executed before the execution of the aforementioned transformation function. For this purpose, another rotation of the probe in the hole  21  is performed by the operator, or is controlled automatically by means of the motor  40 , and the control system records new pairs of data elements in the memory. The transformation function is then applied to these new pairs of data elements. 
         [0047]    Preferably, after the transformation of the pairs of data elements corresponding to the points of discontinuity into polar coordinates expressed in the reference frame linked to the structural component  10 , the software executes a filtering function which eliminates all the stored points of discontinuity which correspondent to characteristic points of discontinuity. This makes it possible to retain in memory only the useful data elements corresponding to anomalies detected in the structural component  10 , for example the crack  15 . In the example shown in  FIG. 7 , the useful data elements correspond to the coordinates of points B 1 , B 2  . . . B 10  located on the crack  15 . The software displays these useful data elements on the screen  50  so that the operator can become aware of the anomalies detected in the structural component  10 . In one embodiment, the software produces a report containing these useful data elements, which can be exported to a computer to make use of the data elements. 
         [0048]    The operator may then, for example, represent the points B 1 , B 2 , . . . B 10  on a plan of the component, regardless of whether this is a paper plan or a computer plan. The measurement of the distance between one edge of the structural component  10 , near the point B 1 , and the point B 10  enables the operator to determine a length of the crack  15  in the structural component  10 . 
         [0049]    Advantageously, the software of the processing unit  58  further comprises a calculation function configured for automatically calculating the length of the crack  15  on the basis of the useful data elements corresponding to the points B 1 , B 2  . . . B 10 , in order to display this length on the screen  50  and/or to include it in the report. 
         [0050]    In particular, the hole  21  is a hole corresponding to a fastening which is common to the structural component  10  and the component  12 . To enable the probe to be inserted into this hole, the operator first removes this fastening, and then puts it back into position when he has finished the inspection of the crack in the structural component. 
         [0051]    In an advantageous embodiment, the crack testing method is repeated with the probe  35  inserted into at least two holes in the structural component  10 , for example the aforementioned hole  21  and the hole  22  and/or  20 . This makes it possible to detect points of reflection of the ultrasound beam  37  from the crack which would not be accessible from the first hole  21 , for example points which would be masked by another hole in the component. This embodiment also enables a plurality of measurements to be made of the position of the same point, thereby improving the precision of the position of the crack  15 . 
         [0052]    In an advantageous embodiment, the probe  35  is a multi-element ultrasonic probe (also known as a “phased array” in English), as shown in  FIG. 11A . The probe then comprises a sensor  36  which has a plurality of transducers  36   a ,  36   b  . . .  36   k  placed parallel to a longitudinal axis of the probe. Each of the transducers  36   a ,  36   b  . . .  36   k  may emit an ultrasound beam  37   a ,  37   b  . . .  37   k  respectively, as shown in  FIG. 11B . Thus, the use of this multi-element probe makes it possible to emit ultrasound beams in various locations distributed through the thickness of the structural component  10 , as shown in  FIG. 11B . The probe is controlled by the measuring instrument so as to emit the various ultrasound beams successively in time, so that the echoes of the beams do not interfere with one another. The use of a plurality of ultrasound beams enables a finer analysis of the crack  15  to be made. This is, notably, useful if the crack  15  affects only a limited part of the thickness of the component  10 , as in the case shown in  FIG. 11B : the crack is detected by the ultrasound beam  37   k , while it is not detected by the other beams. In particular, instead of controlling the emission of a plurality of beams  37   a ,  37   b  . . .  37   k  successively, the measuring instrument may be configured to control the probe in a mode called the angular scanning mode, making it possible to choose the trajectory of an ultrasound beam emitted into the structural component. 
         [0053]    In a variant shown in  FIG. 5C , the probe used is a multi-element probe comprising a sensor  36  as shown in  FIG. 5D . Using this multi-element probe enables the structural component to be tested without moving the probe. This sensor  36  comprises a plurality of ultrasonic transducers  36   s ,  36   t  . . .  36   z , as shown in  FIG. 12A . As the sensor  36  comprises a plurality of ultrasonic transducers ( 8 , for example), this sensor is placed inside the probe  35  rather than on its surface, for reasons of integration. The space between the sensor  36  and the surface of the probe is then filled with a material which conducts ultrasound. When it is controlled by a control system, the sensor  36  emits an ultrasound beam  37  perpendicular to a longitudinal axis of the probe. The ultrasound beam is emitted in a direction of emission which is controlled by the control system, according to what is known as an angular scanning method. The probe  35  is positioned so that the ultrasound beam is emitted in the thickness of the structural component  10 . Preferably, a commonly used gel is applied to the probe to ensure the transmission of the ultrasound between the probe and the structural component  10  in the hole  21 . If this ultrasound beam encounters a discontinuity in the component  10 , this beam is reflected in such a way that some of the emitted ultrasound is reflected toward the transducer  36  of the probe. The time interval between the emission of the ultrasound beam and the reception by the probe of an echo corresponding to the reflected ultrasound is characteristic of the distance between the probe and the discontinuity. This discontinuity may, notably, correspond to a crack  15  in the structural component  10 . In the example shown in  FIG. 5C , the ultrasound beam is emitted in a direction of emission controlled by the control system, in such a way that the ultrasound beam is reflected at point B of the crack  15 . 
         [0054]    When the probe  35  is inserted into the hole  21 , an operator positions the stop  38  against the component  12 , as shown in  FIGS. 5C and 5D . For the sake of clarity, the component  12  is not shown in  FIG. 5C . The stop  38  enables the longitudinal position of the probe  35  relative to the structural component  10  to be secured, so that the ultrasound beam is emitted in the thickness of the structural component  10 . The hole  21  may be a hole with a circular cross section, but this is not essential. If the hole does not have a circular cross section (for example, if it is a hole with a rectangular, square or oblong cross section), the cross section of the probe is preferably chosen to be complementary to that of the hole, as a result of which the orientation of the probe relative to the structural component  10  is fixed because of its insertion into the hole. During the manufacture of the probe, the position of the sensor  36  in the probe is then chosen in such a way that the sensor can emit an ultrasound beam in a set of directions that enables the whole of an area of interest  14  to be tested, when the probe is controlled by the control system. If the hole  21  has a circular cross section, the operator orientates the probe  35  in the hole in such a way that the probe can emit an ultrasound beam in a set of directions that enables the whole of an area of interest  14  to be tested, when the probe is controlled by the control system, without any movement of the probe. 
         [0055]    As mentioned above, if the probe  35  is connected to a control system, this control system can control the emission of an ultrasound beam by the probe in a direction of emission controlled by the control system, using an angular scanning method. The ultrasound beam  37  shown in  FIG. 6  is reflected by the crack  15  at a point B. As mentioned above, the measurement of the reflected ultrasound may be used to determine the distance between the probe and the point B. The point B may thus be characterized by a pair of data elements corresponding, on the one hand, to the direction of emission of the ultrasound beam by the probe  35  (controlled by the control system) and, on the other hand, to the distance determined between the probe and the point B. This pair of data elements forms polar coordinates in a reference frame centered on the point  21 . In an exemplary embodiment, the control system to which the probe  35  is linked is a measuring instrument marketed by the company TESTIA® under the trade name “Smart U32.” This measuring instrument controls the emission of the ultrasound beam  37  by the probe, measures the reflected ultrasound, and automatically indicates the distance between the probe and the point B. 
         [0056]    To test for the crack  15  in the structural component  10 , the control system  55  comprises special-purpose software. This special-purpose software, executed by the processing unit  58 , is configured to control the emission of an ultrasound beam by the probe  35  in successive directions of emission of a set of directions of emission specified to test the whole of the area of interest  14 , without any movement of the probe between the measurements corresponding to the different directions of emission. In an exemplary embodiment, successive directions of emission are spaced apart angularly by an angle of 1°. If the ultrasound beam is reflected by a discontinuity in the structural component  10 , such as the crack  15 , the amplitude of the signal supplied by the probe (corresponding to the reflected ultrasound) and measured by the measuring instrument  55  is above a predetermined threshold. The software of the control system  55  is configured in such a way that, if this ultrasound beam is reflected by a discontinuity of the structural component  10  in one direction of emission, the software controls the recording in the memory of a pair of data elements corresponding to the direction of emission of the ultrasound beam by the probe and to the distance D determined between the probe and the discontinuity. Thus the software successively records in the memory a set of pairs of data elements, each corresponding to the direction of emission of an ultrasound beam by the probe and to the distance D between the probe and a point of discontinuity corresponding to this direction of emission. These pairs of data elements correspond to the set of points of discontinuity of the structural component  10  detected by the control system  55 . The recording of the pairs of data elements by the control system  55  requires no action by operator on the probe after it has been positioned in the hole  21 . 
         [0057]    After the recording of the pairs of data elements in the memory, the software of the processing unit  58  executes a function of searching for characteristic points of discontinuity of the structural component  10 . These characteristic points of discontinuity correspond to discontinuities of the structural component which are present even in the absence of a crack or defect in the component. They correspond, notably, to edges of the component, to holes pierced in the component, etc. By way of non-limiting example, the points C 1 , C 2 , C 3  and C 4  shown in  FIG. 7  correspond to characteristic points of discontinuity of the structural component  10 . The coordinates of the characteristic points of discontinuity are known from a plan of the component, for example a digital model of the component. According to a first variant, these coordinates are recorded in a memory of the control system. According to another variant, these coordinates are recorded in a database, and the control system  55  interrogates the database via a data link. If these coordinates are not already expressed in an angular reference frame centered on the hole  21 , the software executes a conversion function to find the coordinates of the characteristic points of discontinuity in an angular reference frame of this type centered on the hole  21  and having its orientation determined relative to the structural component  10 . This reference frame is called the first reference frame in the remainder of the description. The coordinates of the characteristic points of discontinuity then correspond to polar coordinates, such as the pairs of data elements recorded in the memory by the control system. Regarding these pairs of data elements, the reference frame used is also centered on the hole  21 . It is called the second reference frame in the remainder of the description. If the position of the probe  35  is fixed relative to the structural component  10  as a result of its insertion into a hole  21  having a non-circular cross section, then, unless there is an error in manipulation or measurement, this second reference frame relating to the pairs of data elements must substantially correspond to the first reference frame. If the hole  21  has a circular cross section, the operator orientates the probe in the hole in such a way that it can emit an ultrasound beam throughout the whole of the area of interest  14 . However, in this case the orientation of the probe may be imprecise, and there may be an angular offset between the first reference frame and the second reference frame. When it executes the function of searching for characteristic points of discontinuity, the software compares the coordinates of the characteristic points of discontinuity (expressed in the first reference frame) with the pairs of data elements recorded in the memory (the coordinates expressed in the second reference frame), in order to search for the pairs of data elements corresponding to these characteristic points of discontinuity. As a result of this, it calculates a correspondence, in the form of an angular offset, between the directions of emission recorded in the pairs of data elements (the coordinates expressed in the second reference frame) and the first reference frame. Having determined this correspondence, the software of the processing unit  58  executes a transformation function based on this correspondence, to transform the pairs of data elements recorded in the memory into polar coordinates expressed in the first reference frame linked to the component. These polar coordinates define the positions, in the structural component  10 , of the points of discontinuity detected by the control system  55 . The function of searching for characteristic points of discontinuity of the structural component  10  and the transformation function are especially useful when the hole  21  has a circular cross section and a template  18  is not used. However, the function of searching for characteristic points of discontinuity may also be useful outside this situation: if there is an error in manipulation or measurement, there is a risk that the function of searching for characteristic points of discontinuity may not find the characteristic points of discontinuity among the pairs of data elements recorded, and the software may then alert the operator to a problem. 
         [0058]    Preferably, the software executes a filtering function which eliminates all the points of discontinuity stored in the memory which correspond to characteristic points of discontinuity. This makes it possible to retain in memory only the useful data elements corresponding to anomalies detected in the structural component  10 , for example the crack  15 . In the example shown in  FIG. 7 , the useful data elements correspond to the coordinates of points B 1 , B 2  . . . B 10  located on the crack  15 . The software displays these useful data elements on the screen  50  so that the operator can become aware of the anomalies detected in the structural component  10 . In one embodiment, the software produces a report containing these useful data elements, which can be exported to a computer to make use of the data elements. 
         [0059]    The operator may then, for example, represent the points B 1 , B 2 , . . . B 10  on a plan of the component, regardless of whether this is a paper plan or a computer plan. The measurement of the distance between one edge of the structural component  10 , near the point B 1 , and the point B 10  enables the operator to determine a length of the crack  15  in the structural component  10 . 
         [0060]    Advantageously, the software of the processing unit  58  further comprises a calculation function configured for automatically calculating the length of the crack  15  on the basis of the useful data elements corresponding to the points B 1 , B 2  . . . B 10 , in order to display this length on the screen  50  and/or to include it in the report. 
         [0061]    In an advantageous embodiment, the probe  35  is a two-dimensional multi-element ultrasonic probe, that is to say, a probe using a sensor whose ultrasonic transducers are arranged in two dimensions. An example of a sensor  36  of this type of probe is shown in  FIG. 12B . The sensor comprises a set of transducers arranged in the form of a matrix in the rows  36   a ,  36   b  . . .  36   k  and columns  36   s ,  36   t  . . .  36   z . Thus the sensor shown in the figure comprises 64 transducers, arranged in 8 rows and 8 columns. Other arrangements of the transducers are possible without departure from the scope of the invention. The columns of transducers  36   s ,  36   t  . . .  36   z  are placed parallel to a longitudinal axis of the probe. Each row of transducers may be controlled by the control system to emit an ultrasound beam in a direction of emission controlled by the control system, as mentioned above. Each of the rows of transducers  36   a ,  36   b  . . .  36   k  can thus emit an ultrasound beam when controlled by the control system. The use of this multi-element probe makes it possible to emit ultrasound beams in various locations distributed through the thickness of the structural component  10 . The probe is controlled by the measuring instrument so as to emit the various ultrasound beams successively in time, so that the echoes of the beams do not interfere with one another. The use of a plurality of ultrasound beams enables a finer analysis of the crack  15  to be made. This is, notably, useful if the crack  15  affects only a limited part of the thickness of the component  10 . In particular, instead of controlling the emission of a plurality of beams successively, the measuring instrument may be configured to control the probe in a mode called the angular scanning mode, making it possible to choose the trajectory of an ultrasound beam emitted into the structural component. As the sensor  36  is of a matrix type, the angular scanning is then controlled by the control system in two dimensions. 
         [0062]    While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.