Abstract:
A method for optimising a cycle and optionally high-cycle fatigue test rig, said test being intended to reproduce a support for turbomachine parts, wherein it comprises the steps consisting of determining variable geometric parameters of the support member and/or of the workpiece of the rig, in addition to ranges of variation of these parameters, determining at least one aim to be achieved, a variation in the values of at least a part of the abovementioned parameters having an influence on this aim, modifying one or a plurality of the values of the abovementioned parameters, in the respective ranges of same, and determining those that make it possible to achieve the aim, and producing or modifying a support member and/or a workpiece on the basis of the optimised parameters.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method for optimising a low-cycle, and optionally combined low-cycle and high-cycle, fatigue test rig, for reproducing a support of turbine engine parts, such as a support of at least one blade root on a recess projection of a rotor disc, and the corresponding support member thereof. 
       PRIOR ART 
       [0002]    A turbine-engine rotor disc comprises, on the periphery thereof, an annular array of recesses into which blade roots are fitted, which blade roots are for example of the dovetail type, to form a rotor wheel. In operation, the blades are subjected to centrifugal forces, and the roots thereof are supported by lateral projections of the recesses in the disc. The blades are further subjected to oscillations related to the aerodynamic forces which cause relative sliding between the blade roots and the disc. This loading affects the service life of the blade-disc attachments. 
         [0003]    The analysis of the service life of the blade-disc attachments is based on calculations which are made complex by the effect of contact on the calculated stresses and service lives. The calculation for predicting service lives is possible by means of a complete digital model. The difficulty of the model put in place lies in the input data required. The model requires a correlation between a stress field seen under the blade-disc contact and the number of cycles for initiating a corresponding crack. 
         [0004]    In view of this analysis, it is necessary to devise a test which is capable of reproducing, in laboratory conditions, blade-disc contact which is subjected to low-cycle fatigue (LCF) or low-cycle and high-cycle fatigue (HCF) loading. A test rig should make it possible to determine, by means of experiment, the service life of the blade-disc contact. These experimental data will subsequently be used to set the digital methodologies for determining service life on the actual parts for which it is impossible to determine a service life by means of experiment. 
         [0005]    In the current art, low-cycle, and optionally high-cycle, fatigue test rigs each comprise a support member which is fixed to a mount and defines at least one bearing surface, and a test piece which is connected to traction means for loading the test piece so that it bears against the or each bearing surface of the member. 
         [0006]    However, said test rigs are not completely satisfactory because they are designed without taking into account the features of the support member and the test piece, the quality of the contact between the support member and the test piece, the industrial application of the tested contact, the dynamic behaviour of the rig during a high-cycle fatigue test, etc. 
         [0007]    The aim of the present invention is in particular to provide a simple, effective and economical solution to at least some of the above-mentioned problems. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention proposes a method for optimising a low-cycle, and optionally combined low-cycle and high-cycle, fatigue test rig, said test rig being intended to reproduce a support of turbine engine parts, such as a support of at least one blade root on a recess projection of a rotor disc, and comprising a support member which is fixed to a mount and defines at least two bearing surfaces, and a test piece which is connected to traction means for loading the test piece so that it bears against the or each bearing surface of the member, the method comprising the steps consisting in: 
         [0009]    - determining variable, in particular geometrical, parameters of the support member and/or the test piece, as well as ranges of variation of said parameters, 
         [0010]    - determining at least one objective to be achieved or optimised, a variation of the values of at least some of the above-mentioned parameters affecting the objective, 
         [0011]    - modifying one or more of the values of the above-mentioned parameters, in the respective ranges thereof, and determining those which make it possible to achieve or optimise the objective so as to identify optimised parameters, and 
         [0012]    - producing a support member and/or a test piece based on fixed parameters and parameters optimised for equipping a new rig, or modifying the support member and/or the test piece of an existing rig based on the optimised parameters. 
         [0013]    The method is notable in that the support member further comprising two middle portions respectively supporting the two bearing surfaces, each middle portion being connected, on the side opposite the traction means, by a first arm to a base for fixing to the mount and, on the side of the traction means, by a pair of second arms to ends of two parallel crossbars which are at a distance from one another, the opposite ends of the bars being connected by another pair of second arms to the other middle portion, the method includes among the variable parameters at least one dimension of the second arms of each pair, and/or the angle of inclination of said second arms with respect to the corresponding crossbar or with respect to the bearing surface of the corresponding middle portion. 
         [0014]    The invention thus proposes a method making it possible to optimise specific parameters of the support member and the test piece in order to improve in particular the representativeness of the test with respect to the industrial application, as well as the reliability of the objectives of said test. The variable parameters are preferably geometrical parameters but can be other types of parameters or not only geometrical parameters. The invention further makes it possible to modify a test rig so that it adapts to any type of support of turbine engine parts, and further has the advantage of being able to be applied to an existing rig from the prior art. 
         [0015]    The method according to the invention can be implemented by a computer system comprising an optimisation software such as the software DesignXplorer marketed by the company Ansys. 
         [0016]    The variable parameters can include at least one dimension of the first arms, such as the length and/or thickness thereof, and/or the angle of inclination of said arms with respect to the base or with respect to the bearing surface, and/or the rigidity of said arms, and/or the height or length between the base and the bearing surface. 
         [0017]    The objective to be achieved can be the parallelism and the contact of the bearing surfaces between the test piece and the support member, and/or a maximum amplitude of sliding between said surfaces, and/or a substantially homogeneous contact pressure between said surfaces. 
         [0018]    When the surfaces of the test piece and the support member bear against a substantially rectangular region, the contact pressure can be considered to be substantially homogeneous when the ratio between the contact pressure in the region of a lower edge of the region and that in the region of an upper edge of the region is equal to approximately one. 
         [0019]    In the case in which the rig is used for low-cycle and high-cycle fatigue tests and comprises two I-shaped parts having a flexible middle portion, one of which connects the support member to the mount and the other of which connects one end of a vibrating blade to the traction means, the other end of the blade being connected to the test piece, the rig further comprising excitation means cooperating with the I-shaped part connected to the blade for making said blade vibrate during the tests, the objective to be achieved can be a target vibration frequency of the blade. In the case in which at least two objectives are determined, at least some of said objectives are ranked in order of importance. Thus, in the case in which several values of parameters make it possible to optimise said objectives, the parameters chosen may be those which allow the best optimisation of the most important objective. 
         [0020]    In the case in which the method is used to optimise a test rig reproducing the support of at least one blade root against a recess projection of a rotor disc, the bearing surfaces of the test piece can represent recess projection bearing surfaces of a rotor disc, and the bearing surfaces of the support member represent bearing surfaces of a blade root. 
         [0021]    The invention also relates to a support member comprising at least two bearing surfaces which are intended to cooperate with bearing surfaces of a test piece in a test rig to reproduce a support of turbine engine parts, such as a support of at least one blade root against a recess projection of a rotor disc, and carry out low-cycle, and optionally high-cycle, fatigue tests. Since the support member is intended to be fixed to a mount and the test piece has to be connected to traction means for loading the test piece so that it bears against each bearing surface of the support member, the support member is characterised in that it further comprises two middle portions respectively supporting the two bearing surfaces, each middle portion being connected, on the side opposite the traction means, by a first arm to a base for fixing to the mount and, on the side of the traction means, by a pair of second arms to ends of two parallel crossbars which are at a distance from one another, the opposite ends of the bars being connected by another pair of second arms to the other middle portion. 
         [0022]    Advantageously, the first arms of the support member are substantially collinear to the shearing forces applied to the bearing surfaces and the second arms are substantially collinear to the normal forces applied to said surfaces. This allows decoupling of the normal and shearing stresses. To be more precise, the geometry of the member (which is collinear both with the normal forces and with the shearing forces) makes it possible, by means of the deformations of such a structure, to maintain perfect plane-to-plane contact regardless of the loading. 
         [0023]    The present invention also relates to a low-cycle, and optionally high-cycle, fatigue test rig, characterised in that it comprises a support member as described above and in that it is optimised by the method as described above. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0024]    The invention will be better understood, and other details, features and advantages of the invention will become apparent upon reading the following description, given by way of non-limiting example and with reference to the accompanying drawings, in which: 
           [0025]      FIG. 1  is a very schematic view of the attachment of a blade root in a recess of a rotor disc of a turbine engine, 
           [0026]      FIG. 2  is a partial schematic view of a low-cycle fatigue test rig and shows a test piece and a support member of said rig, 
           [0027]      FIG. 3  is a larger-scale view of part of  FIG. 2  and shows the region of contact between the test piece and the support member, 
           [0028]      FIG. 4  is a schematic view of the support member from  FIG. 2  during a fatigue test, 
           [0029]      FIG. 5  is a graph showing the change in the opening angle in the contact region as a function of the position of said region, 
           [0030]      FIG. 6  is a partial schematic perspective view of another low-cycle fatigue test rig, 
           [0031]      FIG. 7  is a half view of the support member and the test piece of the test rig from  FIG. 6 , 
           [0032]      FIG. 8  is a schematic perspective view of a low-cycle and high-cycle fatigue test rig, 
           [0033]      FIG. 9  is a larger-scale view of part of  FIG. 8  and shows the region of contact between the test piece and the support member, 
           [0034]      FIG. 10  is a graph showing the change in the contact pressure between the test piece and the support member, between an upper edge and a lower edge of the contact region, 
           [0035]      FIGS. 11 and 12  are graphs showing the effect of the variation of geometrical parameters of the support member on the homogeneity of the contact pressure between the test piece and the support member and on the amplitude of sliding therebetween, and 
           [0036]      FIG. 13  is a graph showing the change in the amplitude of displacement of the contact region as a function of the vibration frequency of a blade of the test rig. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    Reference is first made to  FIG. 1 , which schematically shows a blade-disc attachment of a turbine engine, the blade  10  comprising a root  12  which is engaged in a recess  14  in the periphery of a rotor disc  16 , said disc comprising an annular array of recesses  14  of this type for receiving blade roots. The assembly formed by the disc  16  and the blades  10  form a rotor wheel of the turbine engine. In this case, the root  12  is of the dovetail type. Two adjacent recesses  14  in the disc  16  are separated from one another by a tooth  15 , the teeth  15  located on either side of the root  12  of the blade from  FIG. 1  being shown in part. 
         [0038]    In operation, the blade  10  is subjected to centrifugal forces (arrow  18 ) and the vane thereof has a tendency to oscillate (arrow  20 ), causing the lateral portions of the blade root  12  to bear and slide against lateral projections  22  of the recess  14  in the disc. The arrows  24  show normal forces which are applied to the surfaces opposing the blade root  12  and the recess  14 , and the arrows  26  denote shearing forces which are applied to said surfaces. 
         [0039]      FIGS. 2 to 4  show a test rig which is designed to reproduce two blade-disc contacts which are subjected to low-cycle fatigue (LCF) loading, in order to determine, by means of experiment, the service life of said contacts. 
         [0040]    The test rig  100  basically comprises two portions, a first portion  102  which is connected to traction means  104  and which is intended to reproduce a tooth of a rotor disc, and a second portion  106  which is connected to a fixed mount  108  and which is intended to reproduce portions of two blade roots cooperating with said tooth. 
         [0041]    The first portion  102  comprises a test piece  110  which is fixed at the end of a blade  112 , the other end of which is connected to the traction means  104 . Said traction means  104  comprise for example an actuator, the free end of the rod of which is connected to the blade  112 , and the cylinder of which is supported by a fixed portion of the test rig. Said actuator is preferably oriented in parallel with the blade  112  in such a way that the traction force is parallel to the longitudinal axis of the blade  112 . 
         [0042]    The test piece  110  comprises a portion which is shaped into a disc tooth, said portion reproducing portions of two adjacent recesses in the disc. Said portion has a dovetail general shape and comprises two lateral faces which are shaped to reproduce the projections  120  of two adjacent recesses in the disc. Each of said projections  120  comprises a relatively planar bearing surface  124  ( FIG. 3 ). 
         [0043]    The second portion  106  of the test rig  100  comprises a support member  126  comprising a base  128  which is fixed to the mount  108  and two crossbars  130  which are parallel to one another and to the base and are at a distance from one another, said bars  130  being connected to the base by arms  132 ,  134  which support bearing middle portions  138  of the test piece  110 . 
         [0044]    The base  128  has a parallelepiped shape and is preferably fixed in a flat manner in a horizontal position on the mount  108 . Said base is connected by two opposite ends to lower ends of first arms  132 , the upper ends of which are connected to middle portions  138  supporting bearing surfaces  148 , said middle portions  138  being connected to the lower ends of second arms  134 , the upper ends of which are connected to the ends of the crossbars  130 . The bearing surfaces  148  are intended to cooperate with the bearing surfaces  124 . 
         [0045]    There are two first arms  132  or lower arms, each arm  132  connecting an end of the base  128  to a lower end of the middle portion  138 . In the resting position, said arms  132  are substantially perpendicular to the base  126 . 
         [0046]    There are four second arms  134  or upper arms, each middle portion  138  being connected by a pair of second arms  134  to first ends of the crossbars  130 , the opposite ends of which are connected by the other pair of second arms  134  to the other middle portion  138 . The second arms  134  of each pair are parallel and at a distance from one another, each crossbar  130  and the second arms  134  which are connected to said bar being located substantially in the same plane. In the resting position, said arms  134  are substantially perpendicular to the bars  130 . 
         [0047]    In the assembled position shown in  FIG. 2 , the blade  112  passes between the bars  130  and the test piece  110  extends between the middle portions  138 , in such a way that the surfaces  124  of the test piece  110  bear against the surfaces  148  of the middle portions  138 . 
         [0048]    As can be seen in  FIG. 3 , during a fatigue test, even if the bearing surfaces  124 ,  148  of the test piece  110  and of the support member  126  are perfectly parallel and bear against one another at the start of the test, it is possible, as a result of the deformations of the parts, for said surfaces to become misaligned and move away from one another, which leads to the appearance of an opening angle a in the contact region. 
         [0049]    This drawback is eliminated as a result of the optimisation method according to the invention which makes it possible to modify the support member and/or the test piece so as to ensure that the opening angle a remains zero for the entire duration of the test. 
         [0050]    As can be seen in  FIG. 4 , the parts of the test rig  100  and in particular the support member are subjected to forces and undergo deformations which can be seen here by deformations of the arms  132 ,  134  of the support member  126  which lead to the middle portions  138  and the bearing surfaces  148  moving away from one another and risk leading to the appearance of an opening angle a between said surfaces. 
         [0051]    As explained above, the method according to the invention makes it possible to optimise one or more variable, in particular geometrical, parameters of the test rig in order to best achieve an objective. The desired objective in this case is to prevent the appearance of the opening angle α during a test; said angle must therefore remain zero. The variable geometrical parameter in this case is the position of the contact region which corresponds in  FIG. 4  to the dimension L extending between the upper edges of the bars  130  and the lower ends of the arms  134  (and is substantially equal to the sum of the length of the arms  134  and the thickness of the bars  130 , said dimensions being measured in a direction which is substantially parallel to the longitudinal axis of the above-mentioned blade  112 ). 
         [0052]    The position of the contact region is expressed as a percentage of the total length L′ of the support member  126 . The position P of the contact region is thus equal to the ratio (L:L′)*100. 
         [0053]    The method according to the invention consists in particular in determining by calculation the effect of the variation in P on the opening angle α and in determining for which value of P the objective is achieved (α=0). The method can consist in creating a graph as shown in  FIG. 5  in which the range of variation in P is [20%-40%]. It is noted that the angle α is zero for P=approximately 27%. The region of contact between the test piece  110  and the support member  126  of the test rig  110  from  FIGS. 2 and 4  must thus be approximately 27% of the total length of the support member, measured from the crossbars  130 . 
         [0054]      FIGS. 6 and 7  show another low-cycle fatigue (LCF) test rig  100 ′ which has been designed and optimised by means of the method according to the invention. 
         [0055]    Said test rig  100 ′ differs from that  100  described above in particular in that the arms  132 ,  134  thereof are inclined with respect to the base  128  and to the crossbars  130 , the upper ends of the arms  132  each being connected to the lower end of a middle portion  138  formed as a portion of a cylinder, the upper end of which is connected to the lower ends of the arms  134 . The arms  132  are substantially collinear to the shearing forces applied to the surfaces  124 ,  148 , and the arms  134  are substantially collinear to the normal forces applied to said surfaces. 
         [0056]    The method according to the invention has been applied using the following parameters as variable geometrical parameters: the angle of inclination of each arm  132 ,  134  (with respect to the base  128  for example), the thickness of the different portions of the support member  126 , and the rigidity and the length of the arms  132 ,  134 . In  FIGS. 6 and 7 , the arrows denoted by reference numerals  150 - 159  show some of said parameters: arrows  150 ,  152  show the lengths of the arms  132 ,  134  respectively, arrows  154 ,  156  show the heights of said arms  132 ,  134  respectively, arrow  158  shows the height or position of the contact region, and arrow  159  shows the width of the base  128  or the support member  126 . 
         [0057]      FIG. 8  shows a test rig  200  which is designed to reproduce two blade-disc contacts which are subjected to low-cycle fatigue (LCF) and high-cycle fatigue (HCF) loading. 
         [0058]    The test rig  200  has all the above-mentioned features of the rig  100 ′, and additionally the following features. 
         [0059]    The member  126  is fixed to the mount by means of an I-shaped part  158 . Said part  158  comprises two parallel, substantially parallelepipedal, solid blocks  160  which are interconnected by a flexible wall  162  which is perpendicular to the blocks. The base  128  of the member  126  is applied and fixed to one of the blocks  160 , the second block being fixed to the mount  108 . 
         [0060]    The blade  112  is fixed to the traction means by means of another I-shaped part  164 , which is substantially identical to the first  160 . One of the blocks  166  of said part  164  is fixed to one end of the blade  112  (opposite the test piece  110 ) and the other block  166  is connected to the traction means. The flexible walls  162 ,  168  of the I-shaped parts are substantially coplanar. 
         [0061]    The test rig  200  comprises high-frequency excitation means, such as a shaker, which bear against the I-shaped part  164  which is connected to the blade  112 , for example in the region of the block  166  which is connected to said blade, for making the blade  112  vibrate. 
         [0062]      FIG. 9  is a larger-scale view of the region of contact between the test piece  110  and the support member  126  from  FIG. 8 . During the fatigue test, it is important for the contact pressure between the bearing surfaces  124 ,  148  of the test piece  110  and the support member  126  to be substantially homogeneous over the entire extent of said surfaces. The contact region can be equated to a substantially rectangular and planar surface. The contact pressure is considered to be homogeneous when the contact pressure P 1  located in the region of the lower edge (in C 1 ) of the contact region is substantially equal to the contact pressure P 2  located in the region of the upper edge (in C 2 ) of the contact region, that is to say that the ratio P 1 :P 2  is substantially equal to 1. The graph in  FIG. 10  shows the change in the contact pressure P(mPa) as a function of the position in the contact region, measured in millimetres from C 1 , in one embodiment. 
         [0063]      FIGS. 11 and 12  show one embodiment of the method according to the invention which is used in this case to achieve two objectives simultaneously: the first being the above-mentioned pressure ratio P 1 :P 2  which must be equal to approximately 1, and the second being the amplitude of sliding of the bearing surfaces  124 ,  148 , which must be the highest possible. 
         [0064]      FIGS. 11 and 12  show the response surfaces of a series of calculations, that is to say the value of the optimisation criteria as a function of the variable input parameters. Said response surfaces consequently make it possible to propose the best candidate with respect to input criteria. 
         [0065]      FIG. 11  shows the change in the pressure ratio (in X), as a function of the length of the arms  132  (arrow  154  in  FIG. 7 —in Y (mm)) and the length of the arms  134  (arrow  156  in  FIG. 7 —in Z (mm)).  FIG. 12  shows the amplitude of sliding of the bearing surfaces  124 ,  148  (in X (10 −4 m), as a function of the length of the arms  132 ,  134  (in Y (mm) and Z (mm) respectively). 
         [0066]    The table below includes the objectives of the optimisation steps of the method according to the invention. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Length of  
                 Length of  
                 Amplitude  
                 Pressure 
               
               
                   
                 the arms  
                 the arms  
                 of sliding 
                 ratio 
               
               
                   
                 132 (mm) 
                 134 (mm) 
                 (m) 
                 P1:P2 
               
               
                   
               
             
             
               
                 Objective 
                   
                   
                 To be 
                 Target value 
               
               
                   
                   
                   
                 maximised 
                 sought 
               
               
                 Target value 
                   
                   
                   
                 1 
               
               
                 Importance 
                   
                   
                   
                 High 
               
               
                 Candidate A 
                 99.951 
                 79.953 
                 −0.00027375 
                 0.83284 
               
               
                 Candidate B 
                 96.879 
                 79.944 
                 −0.00025343 
                 0.83521 
               
               
                 Candidate C 
                 81.903 
                 79.918 
                 −0.00016705 
                 0.99952 
               
               
                   
               
             
          
         
       
     
         [0067]    The range of variation in the length of the arms  132  is [80, 100] mm and that in the length of the arms  134  is [79, 80] mm. Each candidate A, B and C corresponds to a set of values of the parameters considered. As explained above, the objective in this case is both to maximise the amplitude of sliding of the bearing surfaces and to ensure that the pressure ratio P 1 :P 2  is as close to 1 as possible. In the criteria of importance, it is noted that the pressure ratio objective takes precedence over the sliding amplitude objective. 
         [0068]    It is noted that the candidates A and B make it possible to obtain a relatively high amplitude of sliding, and that the candidate C makes it possible to obtain a pressure ratio which is close to 1 together with a relatively good amplitude of sliding. Given the importance of the pressure ratio objective, the candidate C is chosen, that is to say that the corresponding values of the lengths of the arms  132 ,  134  are considered to be values which are optimised for achieving the above-mentioned double objective. 
         [0069]    In the case in which the objective to be achieved is a vibration frequency, it is necessary to adapt the rig so as to be able to apply said frequency because the excitation or HCF loading frequency is not constant. The HCF loading is primarily controlled by the geometry and the rigidity of the support member  126  and above all by the geometry and the rigidity of the blade  112 . The vibration frequency is not controlled and depends for example on the geometry and the rigidity of the support member  126  and the blade  112 . 
         [0070]    The optimisation method is applied with the same procedure as previously, thus proposing modifications of parameters of the test rig. The entire frequency behaviour of the new rig is then calculated and superimposed on the target response.  FIG. 13  is a graph showing the change in the amplitude of displacement (Amp (m)) of the bearing surfaces  124 ,  148  of the test piece  110  and the bearing member  126  as a function of the vibration frequency of the blade  112 . The curve  202  shows the target frequency response and the curve  204  shows the frequency response calculated by the method according to the invention. It is noted that the method for optimising the rig is effective and that the technology of the test rig makes it possible, by varying the parameters, to adapt the frequency response depending on the application. 
         [0071]    The method according to the invention can be implemented by a computer system which is intended in particular to carry out the optimisation calculations. 
         [0072]    The last step of the method according to the invention consists in producing a support member and/or a test piece based on fixed parameters and parameters optimised for equipping a new rig, or in modifying the support member and/or the test piece of an existing rig based on the optimised parameters.