Metal deforming – With cutting – By shearing tool-couple
Reexamination Certificate
2000-09-22
2004-03-30
Crane, Daniel C. (Department: 3725)
Metal deforming
With cutting
By shearing tool-couple
C072S377000, C029S525060, C403S408100
Reexamination Certificate
active
06711928
ABSTRACT:
TECHNICAL FIELD
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 novel manufacturing techniques for providing improved fatigue life in parts, to 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, helicopters, ships, trains, cars and the like. Fatigue can also be present in other less obvious applications such as pressurized vessels, space vehicles, farm equipment, internal combustion engines, turbine engines, medical implants, industrial equipment, sporting equipment. 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.
Fatigue can be a problem for holes and cutouts found in frames and bulkheads of fighter aircraft. Typically these structures have a variety of hole shapes and sizes (some non-round in shape) for the purpose of routing cables, wires, tubing and actuators through the aircraft. They can also serve as a means for allowing fuel flow from one bay to the next. In addition to serving as passageway holes they can also serve as lightening holes for reducing the weight of the structure. Lightening holes can also be found on bridges, trusses, construction equipment, semi-trailers and the like. Regardless of the function or purpose of the hole if they experience cyclic stresses they are subject to fatigue damage.
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 cannot carry the additional load that is not being carried by the failed structural member.
Automotive vehicles are also subjected to the damaging effects of cyclic stress. Vehicles driven on rough roads or off-road experience far more damaging loads on suspension, steering, wheels and the like than for those driven on smooth pavement. The firings of the pistons create cyclic loads on valves, valve guide holes, piston and connecting rod assembly, holes in and connecting both blocks and heads. Some fatigue is a result of high vibration of small stress. Metal covers surrounding and protecting mechanical assemblies may crack at holes due to vibratory loads. Holes created for the purpose of providing flow of lubricant or fluids are sometimes located in areas of high stress. These too, may experience fatigue damage.
Fortunately, failure due to the fatigue of an automotive component has generally less severe consequences than with an aircraft component failure. Even so, fatigue in automotive components has a large economic impact on the manufacturer because of the extent of the problem. Fatigue failures may show up only after the production of hundreds of thousands of units. Warranties work on that many vehicles can be very expensive and create a negative public image. Since fatigue damage usually occurs on highly stressed and typically more expensive parts these are the ones that are generally most costly to fix.
Large cylindrical and tubular rollers used in the manufacture of paper are perforated with thousands of holes allowing for the escape of liquids associated with the pulping process. The rollers used in paper production are basically rotating cylinders that are simply supported at both ends. The action of squeezing the pulp or pressing the paper under very high pressures creates bending stresses in the rollers. At the bottom of the roller tensile stresses are created and at the top of the roller compressive stresses are created. As the roller rotates through one complete turn the material experiences one cycle of alternating stresses; negative to positive. These applied cyclic stresses, coupled with the stress risers of many thousands of holes produce many potential fatigue damage sites on the rollers. Even non-perforated rollers experience fatigue because of the need for high speed, vibration free operation. Since the rollers typically rotate at a high velocity, any imbalance in the system can cause severe vibration. Typically, balance weights are attached to the roller through small bolt holes. The holes are subjected to the previously mentioned alternating stress cycle. Because the holes concentrate the stress they are a major source of fatigue cracks.
Orthopedic implants are subjected to repetitive cyclic loading from patient movements. Consequently, such implants are designed to resist fatigue. Orthopedic implants frequently include holes through which screws and other fasteners pass to attach the implant to the bone. The holes, while necessary for attachment to the bone, reduce the overall strength of the implant since they provide less cross-sectional area to accommodate the loads being transferred to the implant through the bone and also act as stress risers which reduce the ability of the implant to tolerate cyclic fatigue loading. The problem is particularly acute in trauma implants, such as bone plates, intramedullary nails and compression hip screws since these devices, in effect, stabilize broken bone fragments until healing occurs. Thus the loading imposed on the bone during the normal movements of the patient is immediately translated to the trauma implant which is then placed under greater stresses than a permanent prosthetic implant might be. The situation is aggravated if the bone does not heal as expected. In that case, the implant is required to accommodate not only greater stresses but also a longer cyclic loading period. Under such conditions, fatigue failure of the trauma implant is more likely.
Even stationary objects, such as railroad track or pressure vessels, may fail in fatigue because of cyclic stresses. The repeated loading from wheels running over an unsupported span of track causes fatigue loads for railroad 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, which occur in structures due to cyclic loads. Cracks typically occur at apertures (holes), notches, slots, 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
Crane Daniel C.
Stresswave, Inc.
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