Metal deforming – By tool-couple embodying nonplanar tool-face
Reexamination Certificate
2001-05-15
2003-09-09
Crane, Daniel C. (Department: 3725)
Metal deforming
By tool-couple embodying nonplanar tool-face
C072S334000, C072S377000
Reexamination Certificate
active
06615636
ABSTRACT:
TECHNICAL FIELD
This invention is related to novel methods for manufacturing components and structures, such as metal components and structures, and more particularly for parts having apertures therein, or cutouts therein, and which parts are subject to repeated or prolonged stress, in order to improve structural integrity by providing improved resistance to metal fatigue. Such methods, and the apparatus made thereby, have widespread applications in components and structures for transportation systems, and for medical devices (particularly metal parts having apertures therein). More specifically, the invention is applicable to fabrication of structures and components having apertures designed for accommodating fasteners such as rivets and bolts, as well as for those for routing tubing, cable or wires (including, for example for fuel flow), or those apertures simply provided for weight reduction purposes, so that finished parts and the apparatus in which the components or structures are installed, have improved resistance to metal fatigue and consequently better structural integrity.
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 tracks 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. Also, 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 material adjacent to fastener holes or cutouts experience stress levels at much 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, 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. Basically, the presently known methods of cold working holes and other cutouts using tapered mandrel methods, coining, punching, and such are not adaptable to automated fastening systems and other automated environments because of their complexity and bulkiness of equipment. Also, presently known methods used by others do not treat the entire periphery of non-circular cutouts leading to potential fatigue life degradation. Finally, prior art countersink cold working methods require re-machining of the formed countersink, in order to achieve the desired fastener flushness.
Shortcomings of currently known methods for treating structures to provide enhanced fatigue life will be used as a basis for comparison with my novel, improved stress wave fabrication method. Heretofore known processes are not entirely satisfactory because:
they generally require that a starting hole be created in a workpiece, prior to initiating a stress fatigue life improving process;
they often require mandrels, split or solid, and disposable split sleeves, which demand precision dimensions, which make them costly;
mandrels and sleeves are an inventory and handling item that increases actual manufacturing costs when they are employed;
“mandrel” methods require a different mandrel for roughly each 0.003 to 0.005 inch change in hole diameter, since each sleeve is matched to a particular mandrel diameter, and consequently, the mandrel system does not have the flexibility to do a wide range of hole existing hole diameters;
each hole diameter processed with “mandrel” methods requires two sets of reamers to finish the hole, one for the starting dimension and another for the final dimension;
mandrel methods rely on tooling and hole dimensions to control the amount of residual stress in the part, and therefore the applied expansion can be varied only with a change of tooling;
mandrel methods require some sort of lubricant; such lubricants (and especially liquid lubricants), often require solvent clean up;
splits in a sleeve or splits in a mandrel can cause troublesome shear tears in certain 7000 series aluminum alloys;
the pulling action against mandrels, coupled with the aperture expansion achieved in the process, produces large surface marring and upsets around the periphery of the aperture;
split sleeve methods are not easily adapted to the requirements of automation, since the cycle time is rather long when compared with the currently employed automated riveting equipment;
mandrel methods are generally too expensive to be applied to many critical structures such as to aircraft fuselage joints, and to large non-circular cutouts;
mandrel methods have limited quality control/quality assurance process control, as usually inspections are limited to physical measurements by a trained operator.
OBJECTS, ADVANTAGES, AND NOVEL FEATURES
My novel stress wave manufacturing process can be advantageously applied to apertures for fasteners, to large holes in structures, to countersunk holes, to non-round cutouts from a workpiece, and to other structural configurations. Treating a workpiece structure for fatigue life improvement, prior to fabricating the aperture itself, has significant technical and manufacturing cost advantages. The method is simple, e
Crane Daniel C.
Goodloe, Jr. R. Reams
Stresswave, Inc.
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