Abstract:
A detection wire cutter including a conducting wire mechanically secured to a structure that is to be monitored. This wire is electrically powered at one end and at another end feeds back an electrical voltage corresponding to the power supplied, provided that the structure that is to be monitored has not undergone unacceptable mechanical damage. As an improvement, two arrays of wires extending over flat supports in two directions in space are proposed. Each array is powered by a distinct network electrical supply. The two networks extend in layers, with parallel wire portions. The position of a crack does not impede the passage of current between certain terminals, but certain other terminals will no longer receive any current because the crack has broken a network supply general connection between the connection of one wire and that of another.

Description:
FIELD OF THE INVENTION 
     The invention relates to aircraft structure testing devices of the wire cutter type. It concerns non-destructive checking operations, for the maintenance, for example in the aircraft field, typically of aircraft structure junctions, for example between fuselage shrouds and longitudinal stiffeners. 
     BACKGROUND 
     Detection of fatigue fissures in aircraft structures is a very onerous maintenance operation when the part to be inspected is not easily accessible and requires dismounting operations. One current solution consists in manually checking, by means of eddy currents, a crack initiating surface, which requires to dismount some structural elements. The problem lies thus in detecting the apparition of fatigue cracks and in determining their length and position without requiring dismounting operations. 
     Sensors, previously installed such as wire cutter sensors or eddy current sensors or even acoustical detection sensors, contribute to reduce the costs involved as they prevent the environment of the piece to be monitored from being dismounted during the checking operations. Any technology has however its own advantages and drawbacks. 
     A detector of the wire cutter type uses an electrically conducting wire stuck on the fissure initiating surface. This detector enables to detect the possible apparition of a crack by testing the electrical continuity of the conducting wire. The wire is cut when the crack extends at the point where it is stuck. This solution is disadvantageous in that it gives a binary indication and does not enable either to estimate the length of the crack or to locate it for the breaking is likely to appear anywhere along the length of the conducting wire. 
     The invention uses the wire cutter principle, the innovation lying in its pattern which notably enables to estimate both the length and position of the fissure. According to this new pattern, instead of having one wire in the wire cutter, a lot of wires are arranged and connected in two arrays. Both arrays can be moreover interconnected for simplification reasons. The arrays form then a mesh network, preferably in a perpendicular arrangement, in order to precisely locate the damage. 
     SUMMARY OF THE INVENTION 
     Thus, the invention aims at an aircraft structure testing device of the wire cutter type, comprising
         a conducting wire, mechanically secured to a structure to be monitored, and   connected at one end to an electrical supply and outputting at another end an electrical current and/or voltage corresponding to the supply, if no unacceptable mechanical damage has been done to the structure to be monitored,       

     characterized in that it comprises
         two wire arrays extending on flat supports in two spatial directions,   the supports being secured to the structure,   each array being connected to a distinct electrical array supply,   each wire of a wire array being connected to the electrical array supply of the array it belongs to,   both arrays extending in layers, with portions of wires being disposed in a parallel arrangement,   parallel portions of wires of one array being oriented in a direction perpendicular to that of the portions of the other array.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood from the following description and the accompanying figures. These figures are given only as an indicative, and not at all as a limitative, representation of the invention. In the figures: 
         FIG. 1  shows a device according to the invention having a matrix geometry of a wire cutter; 
         FIG. 2  shows a variant of the wire cutter having a comb-shaped geometry. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a device according to the invention. This device comprises a conducting wire, for example the wire  1 , secured to a structure to be monitored. The structure, not represented, is for example flat. But it could have a step or corner edge configuration, even a boss configuration or other. In practice, the conducting wire  1  is fixed to a support, for example a layer of polytetrafluorethylen, metallized on one face, and cut for uncovering the conducting wire  1 . This layer is then firmly stuck on the structure and thus follows the outline thereof. Particularly, a fissure  2  extending into the structure, for example from an edge  3  of this structure, results in the breaking of the wire  1 . 
     For detecting the breaking, the wire  1  is supplied at one end  4  with an electrical voltage and/or current. Another end  5  outputs the voltage and/or current by electrical conduction. The drawback of such a device lies in the fact that the localization of the fissure at a particular place of the wire  1  is not possible. The voltage, output at  5 , enables to know, only in a binary manner, whether the fissure  2  is present or not. 
     According to the invention, the device comprises two arrays of wires: the arrays  6  and  7 , represented separately in a magnification of a detail in  FIG. 1 . Both arrays are in fact stuck to one another, and together firmly stuck on the structure to be monitored. Both arrays form then a mesh network. These both arrays extend on this structure, each on its own flat support, in a spatial direction different from that in which the other array extends. Thus, the array  6  extends in a direction, represented here in  FIG. 1 , perpendicular to the direction of its conducting wires such as  1 . The conducting wires of one array are parallel to one another. For the array  1 , they all are parallel to the wire  1 . The array  7  extends in a direction, here vertical, perpendicular to the direction of its conducting wires, or at least perpendicular to the portions  8  to  11  of its conducting wires which are parallel to one another. The portions are parallel to one another and are perpendicular to the extension direction of the array  7 . 
     The supports are secured to the structure supporting them and are subjected together with it to deformations and breakings this structure has suffered. 
     While the array  6  is connected to an electrical array supply through a terminal  12 , the array  7  is supplied through an array terminal  13 , distinct from the terminal  12 . Both arrays are connected to distinct supplies. “Distinct supplies” does not necessarily mean a different electric voltage or waveform. But, possibly and preferably, it also means one time-shared supply. Notably, in this preferred last case, both arrays can be connected to one another through a common terminal  14 . 
     The arrays are formed in such a manner that the wires of each wire array are connected to the electrical array supply of the array they belong to. Thus, all the wires  8  to  11  of the array  7  are connected to the terminal  13 . In the figures representing the arrays, when electrical connections are established, points at wire intersections represent these connections. Thus, each array can comprise a general connection oriented in the extension direction of the array and to which all the conducting wires of the array will progressively be connected. 
     While both arrays extend in layers, with portions of wires having a parallel configuration, the portions of wires of one array are oriented in a direction perpendicular to that of the portions of the other array. 
     In one embodiment, the electric continuity of the injection point,  12  or  13 , at each of the reading terminals such as  5  for the array  6 , or respectively  15  for the array  7 , can be sequentially tested for determining the position of the crack. For this purpose, the device comprises, or is associated to, an electronic circuit  16  provided with a microprocessor  17  connected, through an address, data and control bus  18 , to a program memory  19 , to a data memory  20  (for recording measure results) and to an input/output interface  21 . The interface  21  is in communication with a transport bus  22  (typically an aircraft bus). The bus  22  is connected to a decoder  23  located in the vicinity of or on the support of the arrays  6  and  7 . The decoder  23  is connected on one hand to the bus  22  and on the other hand to the injection terminals  12  and  13 , and to the reading terminals such as  5  and  15 . Under the control of the processor  23 , the decoder is used for successively, or even simultaneously but separately, acting upon the output connections. 
     When an array is supplied, the method enables to determine an interval between two wires where the crack  2  is located. For example for the array  7 , the position of the crack  2  does not indeed disturb the passage of the current between the terminal  13  and the terminal  15 , any more than between the terminal  13  and the terminal  24  or the terminal  25  of the conducting wires  8  and  9  which are the closest to the wire  8 . On the other hand, the terminal  26  supplied by the wire  11  does not receive current anymore for the fissure extending from the edge  3  has broken the general connection supplying the array  7  between the connection of the wire  10  and that of the wire  11 . 
     In the case of the array  7 , the most important thing is that the general connection should be carried by the structure it is supposed to monitor. It is this general connection that accounts for the breaking since it is broken too. It is also possible that the wires  8  to  11  are not themselves submitted to the constraint. For example, where they are located, the support is not stuck on the structure. 
     While the position of the fissure  2  (between the wires  10  and  11 ) can thus be determined with the array  7 , its extension, its length, can be determined with the array  6 . The conducting wire  1 , located between the terminal  4  and the terminal  5 , is cut first by the fissure  2 . It is then sufficient to know which wire among the wires  1  and  27  to  29  of the array  6  is the first one to give an answer for measuring the length of the fissure. In the same way, the continuity of the injection point  12  at each of the reading points  30  to  32  is sequentially tested in order to determine the crack length (in fact a length interval). In the case of the array  6 , the general connection leading to the particular connections is not cut (preferably shall not be cut). Only the particular connections l a  27  to  29  are likely to be cut. 
     When applied to the structure at the place to be monitored, such a device requires then that the leading edge of the fissure, the edge  3 , should be parallel to the general connection of the array  7  and to the particular connections of the array  6 . If the fissure extends in another direction, it is suitable that these general and particular connections should be perpendicular to it. 
     Otherwise, if the extension direction of the fissure is unknown, or if all the conducting wires are carried by the fissure, it is all the same possible, thanks to the last supplied wire  10  of the array  7  and to the last supplied wire  33  of the array  6 , to known the limits of this fissure  2 . 
     In another embodiment, the reading terminals such as  5  and  30  to  32 , or such as  15  and  24  to  26 , can be connected to one another for example by means of an electrical conductor or of the decoder  23 . Such connections thus form two independent electrical circuits, one used for determining the position with the array  7 , the other for determining the crack length with the array  6 . The resistance of the so-formed electrical circuits is then modified when a part of the wires is cut. Each conducting wire has indeed an electrical resistance in parallel with that of the other conducting wires. 
     If the resistances of the conducting wires are named R 1 , R 2 , RN, the equivalent resistance R of such a circuit with N wires is 1/R=1/R 1 +1/R 2 + . . . +1/RN. If the wires  1  to k are cut by the fissure, the resistance R of the circuit becomes 1/R=1/Rk+1+1/Rk+2+ . . . +1/RN. The measure of the total resistance of the circuit thus enables to know the number of wires that have been cut and in consequence the length, or respectively the position, of the fissure. 
     According to the method used, the microprocessor  17  will implement a program of sequential measures or of the measure of an equivalent resistance. 
     The electrical tracks or wires can be supported by a flexible substrate such as a layer of polytetrafluorethylen metallized or directly deposited on the surface of the piece. Preferably, the flexible substrate and the tracks have a maximal deformation which is superior to that of the material of the monitored structure, in order to prevent a track from being broken before the extension of the fissure under it. 
     In the case of a flexible substrate solution it is also possible that the different tracks should be deposited on their own independent support, or on a unique support cut into thin strips in order to prevent the wires from being cut because of the movements of the flexible substrate, which could result in an erroneous measure of the length or position of the fissure. 
     For example, in the represented detail, we can see between the particular connections  1 ,  27 ,  28 ,  29 , and the general connection long notches  34  to  37 . These notches  34 - 37  are long and parallel to the connections  1  and  27  to  29 . Also, the notches  38  to  40  are located between the particular connections of the second array  7 . These notches do not extend up to the general or particular connection intended to detect the fissure. 
     The wire cutter solution is interesting if the device can be remotely interrogated, even without any wire for example without the bus  22 , which prevents the piece from being dismounted. However, the invention substantially relates to the device pattern, whatever the technique used for the interrogation. 
     In both geometric configurations in  FIGS. 1 and 2 , the horizontal and vertical distances between the wires can be adapted to the contemplated application (dimensions of the piece, critical fissure length, . . . ). 
     The  FIG. 2  shows another embodiment of the device of the invention. According to this other comb-shaped embodiment, both arrays are juxtaposed on the surface to be monitored instead of being superposed as in  FIG. 1 . The connection point  14  can be maintained. The array  7  is placed immediately close to the expected place  3  for the fissure. In that case, the useful connections of the array  7  comprise parallel portions  8  to  11  connected, through perpendicular back bends and connections, to an assembly  38  of terminals aligned with the terminals of the reading connections of the array  6 . In this variant, as in the first variant, the connections can be sequentially or simultaneously read, by measuring then the equivalent resistance. In the case of  FIG. 2 , the back connections are preferably not involved. They are supported for example by a part of the support which is not stuck on the structure to be monitored.