Metal deforming – With means to effect compound motion of tool
Reexamination Certificate
2001-02-09
2004-06-01
Crane, Daniel C. (Department: 3725)
Metal deforming
With means to effect compound motion of tool
C072S334000, C072S377000
Reexamination Certificate
active
06742376
ABSTRACT:
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The inventor 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.
TECHNICAL FIELD
This invention is related to novel methods for the manufacture of fatigue prone structures, and their components, and particularly metal parts having apertures therein, including, but not limited to, apertures utilized (a) for accommodating connecting elements, such as rivets, bolts, pins, screws or other fasteners, or (b) for accommodating tubing, cable, wires, rods, or other actuators, (c) for weight reduction purposes. Additionally it can be applied to guns, pressure vessels or other structures carrying pressurized fluid loads. Individual components, sub-structures, and overall finished structures can be manufactured utilizing the method and apparatus disclosed herein in order to achieve improved resistance to metal fatigue, and thus to have improved structural integrity.
BACKGROUND
Metal fatigue is a problem common to just about any component or structure that experiences cyclic stresses or repetitive loading. Such problems are especially important in the metal structures utilized in various components of transportation systems as they experience a varying amount of repetitive loads during normal operation. Structures or components that are prone to fatigue damage include, but are not limited to, commercial and private transport aircraft, general aviation, military aircraft, helicopters, jet engines, turbines, passenger cars, trucks, off-road equipment, construction vehicles, heavy construction equipment, boats, ships, trains, rolling stock, railroad track, stationary and moving bridges, medical implants, pressurized pipes and vessels, guns, cannons and the like.
Metal fatigue can generally be defined as the progressive damage, usually evidenced in the form of cracks, that occurs to structures as a result of cyclic or repetitive loading. 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, maneuvering, taxi and take-off loads, etc., which over the service life of the aircraft can produce fatigue damage.
Fatigue damage is generally observed, at time of initiation, in the form of growth of small cracks from areas of highly concentrated stress. Typical stress concentrators include holes, fillet radii, abrupt changes in section, notches, and the like. Fatigue damage can often be hidden to the untrained because it generally occurs under loads that do not generally cause yielding or deformation of the structure. In fact, failure usually occurs under loads typically experienced in the operation of the structure. Undetected, a fatigue crack can grow until it reaches a critical and catastrophic size or length. At the critical length, the unstable crack races through the metal, causing sudden failure of the component. Catastrophic failure of the entire structure, such as a wing or fuselage, can occur when other members of the structure can not carry the additional load from the failed member.
Even stationary objects such as railroad track, pressurized vessels and artillery equipment may fail in fatigue because of cyclic stresses. Cyclic loads caused by repeated loading due to rail car wheels running over an unsupported span of railroad track are the cause of many track failures. In fact, some of the earliest examples of fatigue failures were in the railroad and bridge building industry. Sudden pressure vessel failures can also be caused by repeated pressurization cycles acting on initially small cracks. It is not surprising that U.S. governmental studies report that fatigue damage is a significant economic factor in the U.S. economy.
While many methods have been developed and utilized for the manufacture of structures having improved fatigue life at fasteners, it would nevertheless still be desirable to reduce the amount of handling involved in producing such structures. That is because such a development would facilitate reduced manufacturing costs of enhanced fatigue life structures, thus reducing the cost of end products utilizing such structures, and/or enabling more widespread use of improved fatigue life components in industrial applications.
SUMMARY
An novel tool for working a structure to improve the fatigue strength at a selected location in the structure has been developed. Specifically, the tooling involves the provision of a compound indenter, of either a solid one-piece integral construction, or of adjustable multi-part construction, which includes a primary indenter with a contacting end for engagement with and deformation of a pre-selected portion of a first surface of the structure being worked, to impart a desirable residual stress profile in said body of the structure. The primary indenter has a first shaped surface with a preselected profile designed to impart the desired stress profile, and a sloping peripheral wall to facilitate removal of the indenter from the workpiece. The compound indenter, whether of the solid, integral one-piece design or of the adjustable design, also includes a secondary indenter having a second shaped surface having a preselected surface profile. The primary indenter and the secondary indenter are configured for engagement with the structure being worked. For the creation of the usual round holes in a workpiece (such as for rivets or other fasteners), the primary indenter and the secondary indenter are arranged concentrically on the working end of the compound indenter. In this manner, the secondary indenter is preferably situated, longitudinally, so as to form an annular shoulder having an inner ring edge around the primary indenter. In some cases, a very narrow, annular secondary indenter is followed by, radially outwardly, a sloping blend radius, and then a tertiary indenter surface. Next, another blend radius is located radially outward of the tertiary indenter surface. Ideally, a concave foot portion is located radially outward from the final indenter (as described, the tertiary indenter), and finally, a flat foot portion extends radially outward in the same plane as the top surface of the work piece being indented. When a circular hole is being formed, and a circular indenter is being utilized, the foot is annular in shape and confiningly structurally surrounds the outermost (normally secondary or tertiary indenter) to protect said first surface of the structure being worked against surface upset when the compound indenter acts on the first surface of the structure.
Importantly, in thick stacks of workpieces, a second compound indenter, of similar construction to that just described for the first compound indenter, can be utilized in the same fashion against a second side of the lowermost workpiece. In this fashion, desirable residual compressive stresses can be created at a preselected location throughout the body of each workpiece in the thick stack.
Use of the novel tooling described herein enables the practice of an improved method for the manufacture of a joint that includes overlapping at least first and second structural members. The method involves contacting a preselected portion of the first structural member with a first compound indenter at a pressure greater than the yield point of the composition of the structural member to deform a portion of the first structural member in a manner so as to impart a pre-selected residual stress at a location at or near a selected location for a first fastener aperture through the first structural member. Preferably, the indenter shape and the amount of indentation are selected in order to impart a residual compressive force that is substantially uniform along the entire length through the body of the first structural members along sidewall portions of a first fastener aperture. A second struc
Easterbrook Eric T.
Juhlin Nils
Crane Daniel C.
Goodloe, Jr. R. Reams
Stresswave, Inc.
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