Patent Application
20080160891
This invention relates generally to methods for creating fatigue-resistant and damage-tolerant components more specifically to a method of setting process parameters for a burnishing treatment.
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. The main objective of burnishing is to impart residual stress onto a surface to obtain material benefits, like fatigue and corrosion resistance and preventing crack formation and propagation. Of these benefits the aerospace industry is most interested in increasing fatigue life stress resistance. 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 against a workpiece at a selected burnishing pressure by mechanical or hydrostatic means, and traversed across the part surface in a series of strokes or segments. The magnitude of the residual stress is a function of a number of parameters, of which the most influential are the burnishing pressure and the degree of overlap of burnishing strokes. With the high costs of fatigue testing, the initial selection of these parameters can prove expensive given the broad range of burnishing pressures and degrees of overlap.
In the prior art, initial pressure and overlap selection is performed either arbitrarily or through trial and error. A trial and error approach is not only expensive but time consuming.
Furthermore, using parameters derived for a particular application may not have the same results for another application. For example, burnishing two thin plates of the same material under the same conditions but with different cross-sectional thickness will result in different degrees of overlap up to a critical thickness, and therefore will behave differently in fatigue testing. The critical thickness is the thickness for a given material at which the degree of overlap will remain constant at or above this value, if all other parameters are held constant.
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 determining parameters for a burnishing operation, including: using a rolling burnishing element to burnish at least two segments on a selected surface area of a material sample, the segments having a common width and overlapping each other by a preselected overlap value; measuring the resulting hardness of the surface in the selected area; and selecting a working overlap value for a subsequent burnishing operation on a workpiece, based on the measured hardness.
The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:
Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,
As illustrated, the pattern includes a plurality of segments 14 arranged in a series of S-turns along a path “P” defining the segment centerlines, and connected by lateral segments 16. The segments 14 are separated by a feed distance “F” (also referred to as a “step-over distance” or “offset”), which is the distance between adjacent legs of the centerline path P. Various paths may be used to suit a particular application. For convenience in set-up, programming, and measurement, the path P would most commonly comprise some combination of linear segments or strokes.
The width “W” of the segments 14 (also referred to as a “footprint”) is a function of the workpiece material and thickness, as well as the applied burnishing pressure and dimensions and properties of the burnishing element used. The relationship between the feed distance F and the footprint W determines the degree of overlap between the segments 14.
If the segments 14 are burnished side-by-side using a feed value F equal to the footprint value W, they will not overlap each other (
Initial parameters for a burnishing process as follows. First a material sample with a known material composition and thickness is selected. Test segments 14 are burnished on the sample workpiece and measurements made of the widths of these segments 14 to determine the burnish footprint at the selected burnishing pressure. This footprint value defines the 0% overlap setting as described above.
Next, using the defined overlap values, patched are burnished in selected areas on the sample workpiece at different overlaps between 0% and 100% overlap, and are measured for hardness. The hardness measurements are then analyzed to determine the desired overlap value. The overlap values used may be determined arbitrarily, for example by using even increments of overlap, or by using design of experiments (DOE) or other statistical methods. Generally, higher hardness values correspond to greater fatigue resistance and are desired. Once the hardness measurements are made, the overlap value corresponding to the desired hardness value (e.g. the highest hardness) is then used as a working overlap value to process subsequent workpieces.
The parameter setting process described above was applied to flat plates of Ti-6-4 alloy to find the initial process parameters for fatigue testing of gas turbine engine compressor blades. The following general results were observed for Titanium samples with a footprint of about 0.4178 mm (16.45 mils): Hardness results at about 90% to 100% overlap range (high overlap range) were generally lower than at lower overlap settings. High overlap settings also produce greater deformation on the samples This suggests that at high overlap settings the material sample may plastically deform in a macroscopic scale. On the other hand, hardness results at about 50% overlap or lower (low overlap range) generally decline as the overlap setting is reduced. By analyzing the burnishing footprints and hardness results, the initial pressure and incremental feed were selected for subsequent burnishing of compressor blades. Testing of the burnished blades showed that fatigue stress resistance of the blades was improved by about 200% of its original value at the test conditions.
This process described above is quick and inexpensive. It allows the use of inexpensive material samples instead of expensive finished products. It also uses inexpensive and quick tests (length measurements and hardness measurements) to narrow down parameter selection before any fatigue testing is performed.
The foregoing has described a method for setting parameters for a burnishing process. 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.
