Metal deforming – With cutting – By shearing tool-couple
Reexamination Certificate
1999-03-16
2002-05-21
Crane, Daniel C. (Department: 3725)
Metal deforming
With cutting
By shearing tool-couple
C072S412000, C072S377000
Reexamination Certificate
active
06389865
ABSTRACT:
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
FIELD OF THE INVENTION
This invention is related to novel methods and tools for use in manufacturing parts with improved fatigue life, particularly for parts having fastener apertures therein, or cutouts therein, and which parts are subject to repeated or prolonged stress. More specifically, this invention relates to improved techniques for utilizing the stress wave method for working parts, to improved tools for utilizing the stress wave method for working parts, and to finished parts made thereby, which parts have improved stress fatigue resistance characteristics.
BACKGROUND
Metal fatigue is a problem common to just about everything that experiences cyclic stresses. Such problems are especially important in transportation equipment, such as aircraft, ships, trains, cars, and the like. Metal fatigue can be defined as the progressive damage, usually evidenced in the form of cracks, that occurs to structures as a result of cyclic loading. This failure mode is not to be confused with a failure due to overload. The lower surface of an aircraft wing is a classical example of the type of loading that produces fatigue. The wing is subjected to various cyclic stresses resulting from gust, maneuver, taxi and take-off loads, which over the lifetime of a particular part eventually produces fatigue damage. Similarly, the pressurized envelope of an aircraft, including the fuselage skin and rear pressure bulkhead, are subject to a stress cycle on each flight where the aircraft interior is pressurized.
One problem inherent in fatigue damage is that it can be hidden since it generally occurs under loads that do not result in yielding of the structure. Fatigue damage is most often observed as the initiation and growth of small cracks from areas of highly concentrated stress. Undetected, a crack can grow until it reaches a critical size. At that point, the individual structural member can suddenly fail. Catastrophic failure of an entire structure can also occur when other members of the adjacent portions of the overall structure can not carry the additional load that is not being carried by the failed structural member.
Even stationary objects, such as railroad track or pressure vessels, may fail in fatigue because of cyclic stresses. Cyclic loads for railroad track are caused by repeated loading from the wheels running over an unsupported span of track. In fact, some of the earliest examples of fatigue failures were in the railroad industry and in the bridge building industry. Sudden pressure vessel failures can be caused by fatigue damage that has resulted from repeated pressurization cycles. Importantly, government studies report that fatigue damage is a significant economic factor in the U.S. economy.
Fatigue can be defined as the progressive damage, generally in the form of cracks, that occur in structures due to cyclic loads. Cracks typically occur at apertures (holes), fillets, radii and other changes in structural cross-section, as at such points, stress is concentrated. Additionally, such points often are found to contain small defects from which cracks initiate. Moreover, the simple fact that the discontinuity in a structural member such as a fuselage or wing skin from a hole or cutout forces the load to be carried around the periphery of such hole or cutout. Because of this phenomenon, it is typically found that stress levels in the structure adjacent to fastener holes or cutouts experience stress levels at least three times greater than the nominal stress which would be experienced at such location, absent the hole or cutout.
It is generally recognized in the art that the fatigue life in a structure at the location of a through aperture or cutout can be significantly improved by imparting beneficial residual stresses around such aperture or cutout. Various methods have been heretofore employed to impart beneficial residual stress at such holes or cutouts. Previously known or used methods include roller burnishing, ballizing, and split sleeve cold expansion, split mandrel cold working, shot peening, and pad coining. Generally, the compressive stresses imparted by the just mentioned processes improve fatigue life by reducing the maximum stresses of the applied cyclic loads at the edge of the hole. Collectively, these processes have been generically referred to as cold working. The term cold working is associated with metal forming processes where the process temperature is lower than the recrystallization temperature of the metal. A similar term, “cold expansion”, as used by Fatigue Technology Inc., of Tukwila, Wash., is often used interchangeably with cold working, but as applied specifically to their split sleeve cold expansion process. However, of all the methods used to cold work holes, presently the most widely used processes are the split sleeve process and split mandrel process. Together, these processes are referred to as mandrel cold working processes.
Historically, mandrel cold working was accomplished through strictly manual means. As an example, split sleeve cold expansion of holes is still done using hand-held hydraulic tools attached to air-actuated hydraulic power units. The variables involved in tool selection, implementation, and control of the cold expansion process requires skilled operators to reliably produce properly treated holes. Unfortunately, the requirement of having a skilled operator to perform the task is a disadvantage in that it continuously presents the risk of improper or inaccurate processing. Also, such labor-intensive techniques effectively preclude automated feedback necessary for statistical process control. Although development of that process continues, the complexity of the split sleeve processes and the apparatus utilized presently precludes the widespread adoption of the process for automated fastening environments. The split mandrel process it at a similar stage of development; manually performed, but with some minor automation.
The mandrel cold working processes have a particular disadvantage in that they require precision in the size of the starting holes, usually in the range of from about 0.002 inch to about 0.003 inch in diametric tolerance, in order to achieve uniform expansion. Also, an undersize starting hole is required in that process, in order to account for the permanent expansion of the hole and the subsequent final ream that is necessary to remove both the localized surface upset around the periphery of the hole, as well as the axial ridge(s) left behind by the edges of the sleeve split or mandrel splits at their working location within the aperture, and of course, to size the holes. Moreover, treatment requires the use of two reamers; one that is undersized, for the starting hole diameter, and one which is provided at the larger, final hole diameter.
Another undesirable limitation of mandrel cold working processes is the requirement for, presence of, and residual effect of lubricants. For the split sleeve cold expansion process the starting hole must be free of residual lubricants (used for drilling) to prevent sleeve collapse during processing. A collapsed sleeve can be very difficult to remove and necessitates increasing the hole diameter beyond the nominal size, to remove the subsequent damage. The split mandrel process uses a liquid cetyl alcohol lubricant that must be cleaned from the hole after cold working, in order to ensure proper paint adhesion. In either case, the cold worked hole must be cleaned with solvents, in order to remove lubricants. Such chemical solvents are costly, require additional man-hours for handling and disposal, and if not effectively controlled during use or disposal, can have a deleterious effect on operators and/or the environment.
Still ano
Crane Daniel C.
Goodloe, Jr. R. Reams
Stresswave, Inc.
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