Abstract:
A method of evaluating the condition of a rolling burnishing element includes moving a burnishing element having an unknown condition against a surface in a preselected test pattern; while moving the burnishing element, recording at least one test force profile representative of a force acting on the burnishing element in at least one dimension; and comparing the at least one test force profile to at least one baseline force profile to determine a deviation of the condition of the second burnishing element from a baseline condition.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to methods for creating fatigue-resistant and damage-tolerant components, and more specifically, to a method of evaluating tools used to produce such components. 
   Various metallic, ceramic, and composite components, such as gas turbine engine fan and compressor blades, are susceptible to cracking from fatigue and damage (e.g. from foreign object impacts). This damage reduces the life of the part, requiring repair or replacement. It is known to protect components from crack propagation by inducing residual compressive stresses therein. Methods of imparting these stresses include shot peening, laser shock peening (LSP), pinch peening, and low plasticity burnishing (LPB). These methods are typically employed by applying a “patch” of residual compressive stresses over an area to be protected from crack propagation. 
   A typical burnishing apparatus includes rolling burnishing elements such as cylinders or spheres which are loaded with a burnishing force by mechanical or hydrostatic pressure. These burnishing processes require physical contact between the burnishing element and the workpiece. Even though lubrication is provided, wear of the burnishing element occurs during normal use and needs to be monitored. The quality of the burnishing relies on the condition of the burnishing element. Worn elements can cause material transfer between the element and the workpiece, which adversely affects the surface finish and residual stresses. 
   In the prior art, controlling degradation of the burnishing element condition relies on controlling its cumulative burnishing time. Indication of wear is determined with visual inspections of the burnishing element and the workpieces. Steps are also taken to prevent wear, for example by controlling the quality and the quantity of coolant/lubricant used in the burnishing process. However, there is no uniform, efficient test for burnishing element wear. 
   BRIEF SUMMARY OF THE INVENTION 
   The above-mentioned shortcomings in the prior art among others are addressed by the present invention, which according to one embodiment provides a method of evaluating the condition of a rolling burnishing element, including (a) moving a burnishing element having an unknown condition against a surface in a preselected test pattern;(b) while moving the burnishing element, recording at least one test force profile representative of a force acting on the burnishing element in at least one dimension; and (c) comparing the at least one test force profile to at least one baseline force profile to determine a deviation of the condition of the burnishing element from a preselected baseline condition to the unknown condition. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
       FIG. 1  is a perspective view of a prior art compressor blade having a burnishing process applied thereto; 
       FIG. 2  is a side view of a spherical burnishing element illustrating a first coordinate system; 
       FIG. 3  is a top view of a sensor pad illustrating a test pattern corresponding to the coordinate pattern of  FIG. 2 ; 
       FIG. 4  is a side view of a spherical burnishing element in a baseline condition; 
       FIG. 5  is a group of force profiles representative of the burnishing element of  FIG. 4 ; 
       FIG. 6  is a side view of a spherical burnishing element in a damaged or worn condition; 
       FIG. 7  is a group of force profiles representative of the burnishing element of  FIG. 6 ; 
       FIG. 8  is a side view of a spherical burnishing element illustrating a second coordinate system; and 
       FIG. 9  is a top view of a sensor pad illustrating a test pattern corresponding to the coordinate system of  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates an exemplary gas turbine engine compressor blade  10 . This component is used merely as an example of a part to which the method of the present invention may be applied. 
   The compressor blade  10  is shown undergoing treatment with a burnishing tool  12  of a known type including a rolling burnishing element  14  (a sphere is illustrated in this example). The burnishing element  14  is hydrostatically supported and lubricated by hydraulic fluid pumped through the burnishing tool in a known manner. The compressor blade  10  is treated by traversing the burnishing element through a preselected pattern “P”, using a multi-axis numerical- or-computer-controlled manipulator of a known type (not shown). 
   The burnishing element  14  will naturally wear during a burnishing process, and may also become damaged. In order to provide a basis for evaluating the condition of the burnishing element  14 , it is first tested using controlled parameters when it is in a “baseline” or unused condition.  FIG. 2  illustrates a spherical burnishing element  14  with an exemplary coordinate system having spaced-apart meridians  16  superimposed on its outer surface  18 , intersecting its equator  20 . 
   The burnishing element  14  is tested using a sensor pad  22 , shown in  FIG. 3 , which is capable of sensing a pressure and/or force applied thereto and generating a signal representative of that pressure or force, and optionally the location of the sensed pressure or force within the active area of the sensor pad  22 . In this example the sensor pad  22  uses a Cartesian frame of coordinates with X, Y, and Z (i.e. into-the-page) axes. The sensor pad  22  may be constructed of an array of a known type of sensor such as piezoelectric elements, load cells, etc. (not shown). Additional pressure or force sensors may be associated with the apparatus (not shown) used to move the burnishing element  14 , for example to sense X- and Y-axis forces while the sensor pad  22  records Z-axis forces. The output data of the sensor pad  22  is connected to a computer (not shown) operable to store, analyze, manipulate, display, and/or otherwise manipulate that data. 
     FIG. 3  illustrates a test pattern “T 1 ” selected to cover the outer surface  18  of the burnishing element. The test pattern T 1  includes a plurality of linear line segments  24  arranged in a series of S-turns and connected by transverse line segments  26 . The linear segments  24  have a length “L” and are separated by a step-over distance “S”. The length L is selected to be equal to a circumference of the burnishing element  14 , while the step-over distance S is equal to the distance between individual meridians  16  at the equator  20 . The exact step-over distance S is a trade-off between spatial resolution (i.e. ability to map very small features) and the time required to complete the test pattern T 1 . 
   The burnishing element  14  in baseline condition ( FIG. 4 ) is traversed through the test pattern T 1  while in contact with the sensor pad  22 . The output from the sensor pad  22  and other sensors results in a group of force profiles for X-, Y-, and Z-axes, labeled  28 A,  28 B, and  28 C, respectively in  FIG. 5 . The vertical axis in these profiles  28  is representative of force or pressure magnitude, and the horizontal axis is representative of time and/or total distance traversed. This procedure is carried out under conditions (burnishing pressure, etc.) identical to a subsequent burnishing operation. The selection of a “baseline” condition for the burnishing element  14  may be varied to suit a particular application. For example, the baseline condition could be a defined by a test standard which is finished to regular production standards, or to a more exacting standard. Alternatively, the baseline condition could be defined by the individual burnishing element  14  before it is used for any burnishing operations, or alternatively by an average measurement of several such elements. 
   Once the force profiles for the baseline condition are established, a burnishing element  14  can be tested at selected intervals, for example before every burnishing operation, to evaluate its condition. This is done by traversing the burnishing elemnt  14  through the test pattern T 1  under the sme parameters as the baseline condition test. Any defects or wear in the burnishing element  14  will result in test force profiles  30  which are different than the baseline condition force profiles  28 . For example,  FIG. 6  illustrates a burnishing element  14 ′ which has been used and which contains a defect  32  such as a groove or scratch.  FIG. 7  illustrates a set of X-, Y-, and Z-axis test force profiles labeled  30 A,  30 B, and  30 C, respectively, which correspond to the testing of the burnishing element  14 ′. The Z-axis force profile  30 C differs from the Z-axis force profile  28 C shown in  FIG. 5  as a result of the defect  32 . 
   The testing as described above can be used to develop a usage limit beyond which the burnishing element must be rejected, reconditioned, or replaced by correlation of the test force profiles  30  with physical observation and/or measurements of the burnishing element and/or the resulting workpiece quality. Once such a usage limit has been determined, burnishing elements can be accepted or rejected during regular testing solely by reference to the test force profiles  30 . This may be done by manual inspection of the test force profiles  30 . Alternatively, appropriate software may be used to compare the test force profiles  30  to the baseline force profiles  28 , determine a degree of deviation from baseline conditions, and then reject burnishing elements which exceed a pre-established degree of deviation. Similar software may be used for surface mapping, quantitative analysis, etc. of the burnishing element. 
   Various patterns can be used for testing of the burnishing elements  14  so long as the outer surface is adequately covered. For example,  FIG. 8  illustrates a burnishing element  14 ″ which has a three-dimensional spiral surface pattern  34  superimposed on its outer surface. Such a pattern may be developed using surface mapping software or other analytical methods, and has the possibility of covering the surface area of the burnishing element with a minimum amount of travel.  FIG. 9  illustrates a spiral test pattern “T 2 ” which is a two-dimensional development of the surface pattern  34  laid out on a sensor pad  22 ′. The test pattern T 2  is defined in terms of a polar coordinate system (see the exemplary vector with length “R” and angle “θ”), and would result in R-θ-, and Z-axis force profiles rather than X-, Y-, and Z-axis profiles. In other respects, both baseline establishment and testing would be the same as described above. 
   The foregoing has described a method for evaluating the condition of a burnishing element. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.