Abstract:
The invention relates to a method of characterizing an adhesive bond, the method comprising a step of recording an ultrasound wave reflected by an adhesive bond interface between two fragments, a step of resolving said wave into its sinusoidal components in order to characterize a mode of breaking to be expected in the event of the adhesive bonding interface breaking in shear.

Description:
TECHNICAL FIELD AND PRIOR ART 
       [0001]    The invention lies in the field of methods of characterizing components in mechanical industry, and in particular in aviation industry. The components of interest are assemblies of adhesively bonded fragments, and the invention seeks to characterize the adhesive bonding interface between the fragments. Particularly, but not exclusively, the parts under study may be parts made of composite materials. 
         [0002]    The adhesive bonding under consideration is bonding using films of structural adhesive. There is a known method of characterizing adhesively bonded interfaces. That method consists in subjecting the interface to shear stresses until it breaks, and then in viewing the interface after it has broken, so as to determine the percentage of the area of the interface that has broken by cohesive breaking, and the percentage of the area of the interface that has broken by adhesive breaking. 
         [0003]    Such a method is destructive and has the drawback of needing to be performed on test-pieces and not on real parts. Test-pieces are standardized, and their dimensions do not correspond to those of a real part. Neither the bonded fragments, nor the adhesive joints of the adhesively bonded interfaces correspond to real parts. Furthermore, the nature of the material of the test-pieces and of the adhesive fluctuates from batch to batch, and the same applies to the working of the adhesive and the thickness of the final adhesive joint. Under such conditions, it is difficult to be sure that the characterization performed on a test-piece can be generalized to the real parts that it is desired to simulate. 
         [0004]    The invention seeks to solve this problem and to propose a non-destructive technique for characterizing an adhesively bonded interface between two fragments. 
       Definition of the Invention and the Associated Advantages 
       [0005]    In order to solve this problem, there is provided a method of characterizing an adhesive bond, the method comprising a step of recording an ultrasound wave reflected by an adhesive bond interface between two fragments, and a step of resolving said wave into its sinusoidal components in order to characterize a mode of breaking to be expected in the event of the adhesive bonding interface breaking in shear. By means of this method, a nondestructive technique is made available for determining the characteristics of an adhesive bond, specifically on an example of a component. 
         [0006]    It is specified that in one implementation, a wave is generated in emission/reflection mode in order to obtain the reflected wave, and a longitudinal wave is generated in order to obtain the reflected wave. Advantageously, the recording comprises recording at least seven successive echoes. 
         [0007]    For example, the determination is performed by comparing the amplitude of the maximum-amplitude sinusoidal component with predetermined ultrasound wave amplitude values, and/or by applying a linear relationship to the amplitude of the maximum-amplitude sinusoidal component. By way of example, the method is performed on a real aeroengine part, and it may be performed in order to develop an adhesive for adhesively bonding two fragments together. The braking mode is advantageously characterized by adhesive breaking and cohesive breaking percentages. 
         [0008]    In an implementation aspect, the electronic gate used for the recording has a field that includes the successive echoes in the material of the fragment through which the reflected wave passes, but not the echo from the interface between the water and the surface of said fragment. 
         [0009]    The invention is described below with reference to the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0010]      FIG. 1  shows a test-piece used during the calibration stage of the invention. 
           [0011]      FIG. 2  shows arrangements used during the calibration stage of the invention. 
           [0012]      FIG. 3  shows breaking patterns of the adhesive bonding interfaces of various test-pieces. 
           [0013]      FIG. 4  shows a later step of the calibration stage of the invention. 
           [0014]      FIG. 5  shows an analysis of the data obtained during the step of  FIG. 4 . 
           [0015]      FIG. 6  shows a chart established during the calibration stage. 
           [0016]      FIG. 7  shows the method of the invention, as performed on a component of made up of two fragments that are adhesively bonded together. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The prior art technique is shown in  FIGS. 1 to 3 . 
         [0018]      FIG. 1  shows a test-piece used for the calibration needed for performing the prior art method. It is constituted by two fragments  12  and  13 , each in the form of a rectangular parallelepiped, the fragments being assembled together with fragments of aluminum  11  and  14 , likewise in the form of rectangular parallelepipeds, all four fragments having sections of the same dimensions. 
         [0019]    The test-piece is obtained by cutting plates that have been adhesively bonded in a press. The fragments  11  and  12  are in alignment along with their long dimensions with a gap between them, and the fragments  13  and  14  are likewise in alignment along their long dimensions, likewise with a gap between them. A film of adhesive  15  connects together the fragments  11  and  12  and also the fragments  13  and  14 , the two gaps not being in register with each other, but being offset. Thus, the fragments  11  and  13  are adhesively bonded together over a long length, as are the fragments  12  and  14 . However the fragments  12  and  13  are adhesively bonded together along a short length, referred to as the “effective section”. This is an overlap zone between these two fragments. The fragments  11  and  14  are not adhesively bonded together. 
         [0020]    For calibration, traction tests in shear are performed in a tensile test machine fitted with a 100 kilo newton (kN) measurement cell. The extensometer used is shown in  FIG. 2 , together with the enclosure of the tensile test machine. 
         [0021]      FIG. 3  shows various breaking patterns of test-pieces that have been subjected to a shear test. In each example, the left and right portions are photographs of the surfaces of the fragments  12  and  13  in contact with the film of adhesive  15 . From top to bottom, there can be seen patterns with 70% cohesive breaking and 30% adhesive breaking, 100% adhesive breaking, and 50% cohesive breaking with 50% adhesive breaking. 
         [0022]      FIG. 4  shows how the calibration stage continues. 
         [0023]    Use is made of the elements obtained by breaking the structure shown in  FIG. 1 . Thus, there is a fragment  101  of the material under study, corresponding to the fragment  12  or  13  of  FIG. 1 , which is adhesively bonded by the film  15  to a fragment  102  that corresponds to the fragment  11  or  14 . An ultrasound transducer  103  is directed towards the surface of the fragment  101  that is remote from the film  15  (face  1 ), which transducer is a plane Panametrics V313 15/0.25″ sensor, for example. It emits a longitudinal ultrasound wave propagating in a direction perpendicular to the interface constituted by the film of adhesive  15 . The pulse vibrates at a frequency of 16.7 megahertz (MHz). A portion A 0  of this wave is reflected on the outside surface of the fragment  101 , and therefore does not penetrate into the material. It therefore performs no more than a go and return trip through the water between the transducer  103  and the surface of the fragment  101 . 
         [0024]    A second portion A 1  penetrates into the fragment  101  and is reflected by the interface constituted by the film of adhesive  15 . It passes back through the fragment  101 , leaves the fragment, and travels the distance between the fragment and the transducer  103 . 
         [0025]    A third portion A 2  passes through the interface constituted by the film of adhesive  15  and is reflected by the opposite face of the fragment  102  (face  2 ). It thus passes in the opposite direction through the fragment  102  and the fragment  101 , and likewise travels the distance between the fragment  101  and the transducer  103 . 
         [0026]    Using the transducer  103  in transceiver mode, a series of echoes are recorded that come from the thickness of the fragment  101 , which in this example is about 10 millimeters (mm), with measurement being to within 1 micrometer (μm). The echoes are shown in the left portion of  FIG. 5 , as a function of time, and numbered  201  to  209 . They are of decreasing amplitude. Only those that originate from within the thickness of the fragment  101  are concerned, i.e. in this example the echoes  202  to  208 , and the echo  201  originating from the outside surface of the fragment  101  (face  1 ) is excluded, this echo corresponding to the above-mentioned portion A 0  of the wave. 
         [0027]    A Fourier transform is performed on the accumulated echoes  202  to  208 , thereby revealing multiple resonances. Resonance is associated with the quality of the adhesive and cohesive bond. There can be seen a set of resonance peaks  301 ,  302 , . . . ,  30   n,  having an envelope  320  forming a Gaussian curve. The amplitude of the maximum  325  of this envelope is then measured accurately. 
         [0028]    This measurement work is performed on a plurality of test-pieces using different adhesives, and it is found that a good quality linear relationship  410  exists between the percentage of adhesive breaking as observed visually in the images of  FIG. 3 , and the amplitude of the maximum  325  of the envelope as measured in  FIG. 5 . This relationship is shown by the graph of  FIG. 6 , which plots adhesive breaking percentage along the abscissa axis and the above-mentioned amplitude up the ordinate axis (a succession of points  401 ,  402 , . . . ,  40   n , obtained for n test-pieces during the calibration stage). 
         [0029]    Once this chart has been established, the method of the invention is performed on a real part made up of two fragments  501  and  502  that are adhesively bonded by a film  515 , as shown in  FIG. 7 , and the invention is performed optionally on each real part produced, or else on parts selected at random for quality control purposes. 
         [0030]    The fragment  501  has the same thickness as the fragment  101  that was used for preparing the chart. 
         [0031]    The method consists in pointing the ultrasound transducer  103  towards the surface of a fragment  501  that is remote from the film  515 . The transducer once more emits a longitudinal ultrasound wave propagating in a direction perpendicular to the interface constituted by the film of adhesive  515 . A series of echoes B 1  coming from within the thickness of the fragment  501  is recorded using the transducer  103  in transceiver mode, with the echo BO that is associated with the wave traveling the distance between the fragment  501  and the transducer  103  being excluded. Echoes are selected in the same manner as described above ( FIG. 5 ), after which the Fourier transform (FT) of the resulting signal is obtained, and then the amplitude of the maximum of the envelope is read. This is referred to the chart of  FIG. 6  in order to determine the expected adhesive rupture percentage for the assembly constituted by the part in question, and by taking the difference, the expected cohesive rupture percentage. 
         [0032]    The invention is not limited to the implementations described, that extends to any variants within the ambit of the scope of the claims.