Method and Apparatus to Coldwork Holes

The stress concentration at the edge of holes shortens fatigue lives of structures and machinery. To avoid premature fatigue failures, the design stress can be decreased to an appropriate low value. However, such a decreased design stress value will result in an increase in weights of the structures and machinery. Metal fatigue is a peculiar design concern in the aircraft industry because of the stringent light weight design requirement and the abundance of holes in the airframe structures. The fatigue life of holes can be enhanced by hole coldworking which creates a beneficial compressive residual stress at hole edge. Several prior art methods are available to enhance fatigue lives of holes, for example, split sleeve method described in U.S. Pat. No. 3,566,662, seamless sleeve method described in U.S. Pat. No. 4,164,807, stress coining method described in U.S. Pat. No. 3,895,922, and split mandrel method described in U.S. Pat. No. 4,665,732. The split sleeve method is the common method currently used in the aircraft industry. The disadvantages of the prior art of split sleeve and split mandrel methods are:

i) The split in the sleeve or the split in the mandrel creates a ridge and shear discontinuity in the wall of coldworked hole. The ridge and shear discontinuity reduce the effectiveness of coldworking to enhance fatigue life.

ii) A final reaming step is often required to clean up the ridge and shear discontinuity, and to size the coldworked holes. The final reaming step requires extra labor and prolongs process time; therefore, it incurs significant manufacturing cost.

iii) The final reaming step removes material from a coldworked hole, thereby reduces the beneficial compressive residual stress at the hole edge, and thereby reduces the effectiveness of coldworking.

The prior art of split mandrel method does not use a sleeve to protect the hole in a workpiece; this would save time and cost. However, it has the propensity to create galling on the hole wall because there is no medium between the hole and split mandrel to protect the hole wall. It also suffers the aforementioned disadvantages because of the split in the mandrel.

The prior art of seamless sleeve method does not create ridge and shear discontinuity in the hole wall and does not require a final reaming step; it should result in better fatigue life improvement than the prior art of split sleeve and split mandrel methods. However, the disadvantage of this prior art method is that the plastically deformed seamless sleeve become stuck with the hole wall and is under the compressive stress exerted by the hole wall. Thus, it is difficult to remove the sleeve out of the hole after coldworking. To leave the seamless sleeve in the coldworked hole for isolating the fretting phenomenon has not been a desired practice because the sleeves made of steels increase the weight of airframe structures. Another disadvantage of this prior art method is that it requires the mandrel to be taken off the mandrel pulling means for assembling the seamless sleeve onto the small diameter portion of the mandrel. The prior art of various stress coining methods, for example, radius coining, pad coining, and ring pad coining, have limited application because of the relative complexity of the methods; they are primarily used for non-circular holes and very large circular openings where the prior art of split sleeve and split mandrel methods are not applicable. All of the aforementioned disadvantages of the prior art of coldworking methods can be eliminated by the present invention which utilizes a tubular seamless sleeve made of shape memory alloys.

The present invention described herein is an improvement of the prior art of split-sleeve, split mandrel, and seamless sleeve coldworking methods. The improvement is to eliminate the aforementioned disadvantages of the prior art methods and to increase the intensity of the fatigue life enhancement.

A tubular seamless sleeve coldworking method and apparatus utilizing shape memory alloys (abbreviated hereinafter as SMA in this document) has been invented and described hereinafter to enhance the fatigue life of holes. The shape memory alloys tubular seamless sleeve coldworking method will eliminate the final reaming step of the prior art of split sleeve and split mandrel coldworking methods described in U.S. Pat. Nos. 3,566,662 and 4,665,732. In contrast to the prior art of seamless sleeve coldworking method described in U.S. Pat. No. 4,164,807, the diameter of the SMA seamless sleeve will restore owing to the superelastic property of SMA, after hole expansion, to a size small than the diameter of the coldworked hole to ease the removal of the seamless sleeve. Hence the present invention not only reduces manufacturing time and cost but also creates higher beneficial compressive residual stress at hole edge than the split sleeve and split mandrel method.

In general, the method of coldworking holes of this invention comprises the steps of assembling a prelubricated tubular seamless sleeve made of SMA and the associated parts to the coldworking mandrel pulling means, securing firmly the sleeve in the mandrel pulling means, passing the mandrel forwardly through the sleeve, inserting the mandrel and sleeve into a hole to be coldworked in an associated workpiece, holding the sleeve in the hole while retracting the mandrel backwardly from the workpiece, thereby expanding the sleeve to contact with hole wall and thereby compressing the materials of the sleeve and hole in a radial direction taken from the center axis of the hole, and removing the sleeve whose diameter restores to its original size prior to coldworking out of the coldworked hole. The apparatus uses a pre-lubricated tubular seamless sleeve made of SMA, a mandrel having a larger diameter end, a nose cap with rough conical surface, and a spacer with rough complementary conical surface.

It is therefore an object of the present invention to provide a hole coldworking method which eliminates the need for the final reaming of holes, thereby reduces manufacturing time and cost, and thereby better improves fatigue life of holes than the prior art methods of coldworking.

It is a further object of the present invention to provide a hole coldworking method and apparatus using tubular seamless sleeve made of shape memory alloys, which reduces the necessary steps and eliminates the disadvantages of the prior art methods of coldworking, and further provides high fatigue life improvement of holes.

Another object of the present invention is to provide a novel method and apparatus using tubular seamless sleeve made of SMA for coldworking holes wherein the fatigue life versus the coldworking interference is significantly improved.

The present invention has the following advantages when compared with the prior art of split sleeve, split mandrel, and seamless sleeve coldworking methods:

i). Eliminates the aforementioned disadvantages of the split sleeve, split mandrel, and seamless sleeve methods.

ii). Reduces the time to perform coldworking because there is no need to do final reaming of the hole after coldworking, and thereby reduces manufacturing cost.

iii). Preserves higher beneficial compressive residual stress at hole edge because the invention eliminates the final reaming step which relieves portion of the compressive stress. This will result in better fatigue life enhancement.

iv) Achieves higher compressive residual stress at hole edge with lower interference level of coldworking. This advantage will make it easier to coldwork those holes in the short transverse direction of the high strength aluminum thick plates wherein ductility is low and is susceptible to stress corrosion.

Further objects and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, which are illustrated in cross sectional and isometric views for explanatory purpose:

FIG. 1 is a cross sectional view of the parts of the invention, the forward end of the cylinder of a mandrel pulling means in which the invention is carried, and the workpiece. Also shown in FIG. 1 is an isometric view of the shape memory alloys seamless sleeve.

FIG. 2 is a cross sectional view of the parts of the invention, which are assembled together with a mandrel pulling means.

FIG. 3 is a cross sectional view which shows the expansion of the SMA tubular seamless sleeve while the mandrel passing forwardly through said seamless sleeve.

FIG. 4 is a cross sectional view to show that the SMA tubular seamless sleeve restores to its original diameter after the larger diameter portion of a mandrel passing forwardly through and out of said seamless sleeve.

FIG. 5 is a cross sectional view which shows the insertion of the SMA tubular seamless sleeve and a mandrel into the hole of the associated workpiece, and the relative diameters of said hole, seamless sleeve, and mandrel.

FIG. 6 is a cross sectional view which shows the expansion of the SMA tubular seamless sleeve and the hole of the associated workpiece while the mandrel retracting backwardly through said seamless sleeve.

FIG. 7 is a cross sectional view which shows the position of the mandrel after retracting backwardly, the diameter of the hole in an associated workpiece being enlarged, the diameter of the SMA seamless sleeve being restored to its original size and being sufficiently smaller than the diameter of said hole. The change of the diameter of the coldworked hole can be visualized by comparing with FIG. 5.

When considering the most essential steps of this invention to improve the prior art of coldworking methods, references should be made to FIGS. 1 through 7.

The present invention of coldworking utilizes a seamless sleeve made of shape memory alloys (abbreviated as SMA hereinafter in this document) which has the superelastic property. The superelastic property of SMA is described in C. M. Wayman's article entitled "Shape Memory Alloys" which was published in Materials Research Society Bulletin, Volume 18, Number 4 Apr. 1993. To demonstrate how can the SMA be used to overcome the difficulty of removing the seamless sleeve out of a coldworked hole in the prior art of seamless sleeve method, the underlying concept of present invention is explained with the following example to coldwork a 0.1875 inch diameter hole using a SMA tubular seamless sleeve. All dimensions below are in the unit of inch.

While the seamless sleeve and mandrel are in the hole during coldworking, the sleeve inside diameter, 0. 1720, is expanded by the mandrel to the same size as the mandrel diameter, 0. 1830. The hoop strain of the sleeve is calculated below: ##EQU1##

This 6.4% of hoop strain is substantially larger than the yield strain of most materials except the SMA. To ease the removal of the seamless sleeve out of a hole, the strain of 6.4% must be smaller than the critical recoverable strain of a SMA so that the sleeve outside diameter can restore, via the superelasticity property of a SMA, to a size which is smaller than the plastically expanded hole diameter after coldworking. The critical recoverable strain is a characteristic and is specific to each SMA. For the Nitinol family of SMA, this critical strain is approximately 7% to 10%. Except the SMA, the recoverable elastic strains of most metallic materials are smaller than 1.0% which is well below the current practice of coldworking interference level of about 4%. This is the reason why the sleeve in the prior art of seamless sleeve is difficult to be removed out of the expanded holes after coldworking. Present invention utilizes the SMA tubular seamless sleeve to overcome this difficulty.

One embodiment of the present invention is described hereinafter to illustrate the essential characteristics of the invention.

In FIG. 1, a portion of the cylinder of a mandrel pulling means 4 having a mandrel 1 is coaxially lined up with a workpiece 20 having a hole 21. Placed between the workpiece 20 and the cylinder of a mandrel pulling means 4 are a spacer 7, a prelubricated tubular SMA seamless sleeve 11, and a nose cap 15. The forward end of the cylinder of a mandrel pulling means 4 has a screw-threaded outside surface 5 and a flat smooth end surface 6. The mandrel 1 has a larger diameter portion 3 and a smaller diameter portion 2. The spacer 7 is made of hard material, has a flat smooth surface 8 in the backward end, has an conical rough surface in the forward end 10, and has a central circular passageway 9. The tubular SMA seamless sleeve 11 is made of shape memory alloys having the superelastic property, does not have a seam or slot in the sleeve wall, and comprises a conical shoulder portion 12, a tubular portion 13, and a fillet radius 14 joining the conical shoulder portion 12 and tubular portion 13. The required recoverable elastic strain for a SMA is determined by the desired coldworking interference level as explained hereinbefore. Any kind of SMA which has a critical recoverable elastic strain in excess of the required value can be used to make the seamless sleeve of the present invention. Currently, the Nitinol family of SMA is a preferred material because they have sufficient critical recoverable strain and are technologically well developed. When technology advances in the future, new family of SMA may be available for making the seamless sleeve of this invention. The nose cap 15 is made of hard material, has inside screw threads 16 in the backward end, a conical rough surface 17 for said seamless sleeve to rest on, a central circular passageway 19, and a fillet radius 18 joining said central circular passageway 19 and conical rough surface 17.

The first step of coldworking operation, illustrated in FIG. 2, is to assemble coaxially the cylinder of a mandrel pulling means 4, spacer 7, prelubricated tubular SMA seamless sleeve 11, and nose cap 15. The conical contact surfaces in said spacer 10 and nose cap 17, and the conical shoulder 12 of said seamless sleeve are complementary to each other to ensure the coaxiality of the affected parts being automatically maintained during assembling. The fillet radii in said seamless sleeve 14 and nose cap 18 are complementary to each other to maintain substantially equal space between the central circular passageway of said nose cap 19 and the tubular portion of said seamless sleeve 13.

The second step of coldworking operation, also illustrated in FIG. 2, is to secure firmly the conical shoulder of the SMA seamless sleeve 12 between the spacer 7 and nose cap 15 with force by tightening the screw-threaded nose cap onto the screw-threaded cylinder of a mandrel pulling means. The step of tightening is facilitated by the smooth surfaces of the backward end 8 of the spacer 7 and the forward end 6 of the cylinder of the mandrel pulling means 4. The conical rough contact surfaces of said spacer 10 and nose cap 17 increase the friction, thereby enhancing the firm position of said seamless sleeve 11.

The third step of coldworking operation is to pass, illustrated in FIG. 3, the larger diameter portion 3 of the mandrel 1 through the central circular passageway 9 of the spacer 7 and the tubular portion 13 of the SMA seamless sleeve 11, and to position, illustrated in FIG. 4, said mandrel 1 at a location such that the smaller diameter portion 2 of said mandrel 1 is adequately inside of said SMA seamless sleeve 11. Referring to FIG. 3, the fillet radius 14 joining the tubular portion 13 and shoulder portion 12 of said seamless sleeve 11 provides a larger diameter than the inside diameter of said tubular portion 13 of said seamless sleeve 11 to ease the entrance of the larger diameter portion 3 of said mandrel 1. Because the larger mandrel diameter 3 is substantially larger than the inside diameter of said SMA seamless sleeve 13, a substantial axial force is applied to said SMA seamless sleeve while the larger diameter portion 3 of said mandrel 1 passing through said SMA seamless sleeve. The lubricant provided on the inside surface of the tubular portion 13 of said SMA seamless sleeve decreases the friction between said mandrel and sleeve, thereby reduces the required axial force. However, to prevent the slippage of said SMA seamless sleeve, the conical shoulder 12 of said SMA seamless sleeve 11 is held firmly between the conical contact surfaces of said spacer 10 and nose cap 17. This is achieved by using the rough contact surfaces of said spacer 10 and nose cap 17 to increase friction, and by making the conical contact surfaces inclined in a direction in favor of preventing slippage while the larger diameter portion 3 of said mandrel 1 passing forwardly through said SMA seamless sleeve 11. The diameter of the central circular passageway 19 of said nose cap is sufficiently large so that the outside surface of said SMA seamless sleeve 11 will not interfere with said nose cap 15 while said mandrel 1 passing forwardly through said SMA seamless sleeve 11. While the larger diameter portion 3 of said mandrel passes through said SMA seamless sleeve 11, it exerts a radial force on and thereby elastically expands the diameters of said SMA seamless sleeve 11. Referring to FIG. 4, after the larger diameter portion 3 of said mandrel 1 passes forwardly through and out of said SMA seamless sleeve 11, the diameter of said SMA seamless sleeve restores to its original size owing to the superelastic property of the shape memory alloys. This third step of operation eliminates the need to take a mandrel off the mandrel pulling means for assembling seamless sleeve onto the smaller diameter portion 2 of said mandrel 1 as required by the prior art of seamless sleeve method.

The fourth step of coldworking operation, illustrated in FIG. 5, is to insert the mandrel 1 and the SMA seamless sleeve 11 into the hole 21 of an associated workpiece 20. The position of the hole 21 in said workpiece 20 is coincident with the smaller diameter portion 2 of said mandrel 1 surrounded by said SMA seamless sleeve 11 whose conical shoulder 12 remains firmly held between the spacer 7 and nose cap 15.

The fifth step of coldworking operation, illustrated in FIG. 6, is to retract the mandrel 1 backwardly, therefore the larger diameter portion 3 of said mandrel 1 exerts a radial force onto the tubular portion 13 of the SMA seamless sleeve because the diameter of the larger diameter portion 3 of said mandrel 1 is substantially larger than the diameter of the tubular portion 13 of said seamless sleeve. The deforming of the tubular portion 13 of said sleeve is shown exaggerated in FIG. 6 for explanatory purpose. The movement of the tubular portion 13 of said sleeve in the axial or circulatory direction is insignificant because the shoulder portion 12 of said sleeve is held firmly between the spacer 7 and nose cap 15, and because the lubricant on the inside surface of the tubular portion of said sleeve decreases the friction between said mandrel 3 and sleeve 13, and because of the high friction of the contact surfaces between the hole 21 and tubular portion 13 of said sleeve. The diameter of the tubular portion 13 of said sleeve elastically expands beyond the diameter of the hole 21 in the associated workpiece 20, therefore the material of the hole 21 is compressed by the tubular portion 13 of said sleeve and the hole diameter expands beyond its yield strength. Thus, a compressive residual stress is created around the hole 21 to improve fatigue life.

The final step of coldworking operation is to remove the SMA seamless sleeve 11 together with the mandrel 1 and the mandrel puling means out of the coldworked hole 21 of the associated workpiece. As illustrated in FIG. 7, after the larger diameter portion 3 of said mandrel 1 retracts backwardly through and out of the tubular portion 13 of said sleeve, the diameter of the sleeve restores, because of the superelastic property of shape memory alloys, to its original size which is sufficiently smaller than the diameter of the coldworked hole 21 to ease the removal of said sleeve 1. The change in hole diameter 21 before and after coldworking, relative to the diameter of said sleeve 11, can be visualized by comparing FIG. 5 and FIG. 7. It should be understood that the dimensions are shown exaggerated for explanatory purpose.

The present invention and its attendant advantages will be understood from the foregoing description and it will be apparent that the form, construction, and arrangements of the tubular seamless sleeve made of shape memory alloys and the associated parts of the invention can be varied without changing the concept of the invention, the specific embodiment of the invention described hereinbefore is considered in all respects as illustrative and not restrictive, and furthermore that variation may be made without departing from the scope of the invention as defined in the accompanying claims.