Electricity: measuring and testing – Magnetic – Stress in material measurement
Reexamination Certificate
2002-08-20
2004-04-27
Le, N. (Department: 2862)
Electricity: measuring and testing
Magnetic
Stress in material measurement
C324S242000, C073S779000
Reexamination Certificate
active
06727690
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of testing a metal structural element, using precise measurements of eddy currents generated on the surface of the structural element being tested, to determine the useful life of the structural element without destroying it. Such eddy currents are induced in a mass of conducting material by a varying magnetic field.
BACKGROUND OF THE INVENTION
Metal structural elements of a vast array of devices are routinely subjected to severe stresses under which they are designed to operate over the endurance life of the devices. Such stresses are caused by forces producing, or tending to produce deformation in a device or a portion of it; the stresses are measured by the force applied per unit area, for example as dynes per square centimeter (or pounds per square inch); the forces are typically axial torsional or bending. To increase their service life, metal structural elements of ferrous metals, aluminum, titanium and other metals which are susceptible to an increase in residual compressive stress at the surface when peened or shot-peened, are routinely shot-peened, which delays fatigue failure.
Fatigue refers to the failure of materials under the action of repeated stresses; it is responsible for a large proportion of the failures occurring in any one of a myriad structural parts of an aircraft, wheels of heavy duty trucks and rail cars, and a wide array of machine parts. But the expected service life of any of the structural parts is purely conjectural; the expected life is typically estimated from prior experiences with actual failures, or by destructive testing of an essentially identical device, such testing being carried out under what is believed to be the same spectrum of stresses to which such devices are expected to be subjected.
Typically, a series of fatigue tests are carried out on a number of specimens of a particular structural element at different stress levels, until each specimen fails; the stress endured by each specimen is then plotted against the number of cycles sustained. For steel structural elements, choosing lower and lower stresses, a value for stress may be found which will not produce failure even after a very large number of cycles. This stress value is termed the “endurance limit” and the diagram is referred to as a stress-cycle diagram or S-N diagram. In structural elements made of aluminum alloys, the build up of residual stress is more cumulative than in steel and less predictable. In the design and construction of devices where weight and cost are critical, designing a device to operate at stress values low enough to produce the endurance limit is not an option; the goal is the opposite, namely to design a device to operate at as high a stress value as will fall just short of the endurance limit.
For example, the landing gear of an aircraft is designed to operate for some predetermined period of time under preselected operating conditions. Failure of a critical strut in the landing gear under a chosen cyclical load, can be observed when the strut breaks. For the chosen strut, and every other structural element, there is a combination of peak load and number of cycles which provides a 50% failure point, that is, the point at which 50% of all the parts tested will have failed under those test conditions. In the field, an engineer does not know what peak load a particular part has endured, nor, typically, the number of cycles. Therefore the time when the part will fail during its normal operation is unpredictable.
Assuming one was to test a single strut, from a batch of many essentially identical struts which had been in similar service, until that strut failed, the test information might be used to predict the useful life of the remaining struts. Unfortunately, one cannot predict with reasonable certainty, the period after which a strut on the landing gear of an aircraft will fail after the aircraft is placed in service. Landing gear, typically of aluminum or titanium, is designed to withstand the forces generated by that aircraft not only while it is at rest, or while it is hurtling down a runway prior to take-off, but also when it lands. As is well-known, each landing is different from another, some, for example those on a pitching deck of an aircraft carrier, generating stresses an order of magnitude (ten times), or more, greater than those on a deck of the carrier on a calm sea. It therefore is imperative that the aircraft and its landing gear be removed from service well before its imminent fatigue failure. Knowing when to do so, until the discovery disclosed herein, has not been possible.
It should be recognized that, were it possible to identify precisely, the 50% failure point for a structural element which was still in service, the identification, in reality, would have been too late, because by definition, there was a 50% probability that the structural element would have already failed.
A structural element such as a strut of a ferrous metal or any other structural element of a device or machine may be checked by magnafluxing the element, which requires a large enough disturbance of the magnetic flux to allow the magnetic powder to gather in the vicinity of a non-uniformity, such as a crack. When this occurs the accumulation of residual stresses in the part has already reached or exceeded a “safe-operation” point where failure of the part is imminent, that is, the accumulation has progressed too far to allow the device to be operated safely. An accumulation of stresses past the “safe operation” point may also be observed in steel and aluminum devices with dye penetrants commercially available in “spot check” kits.
Checking the structural element by X-ray provides information relating to a change in strain as evidenced by changes in a diffraction pattern from surface atoms, where there is displacement of atoms or distortion of grain structure, to a depth limited to less than 50 &mgr;m (microns), typically less than 20 &mgr;m, and often as little as 10 &mgr;m. Seeing such variations provides no information as to how many stress cycles the structural element has endured, nor the magnitude of the strain. If, just before the X-ray measurements are made, the sample has relaxed its accumulated internal strains sufficiently so as “to report” normal atomic spacing, the conclusion derived from such information, though an excellent method for determining the condition of the element at that particular time, would be misleading. Moreover, X-ray measurements are too costly and time consuming, therefore generally impractical.
The Problem: In the example of the landing gear just provided, over the course of several years, it is presently not possible to make an educated, economical appraisal of the condition of any of its structural elements at any time after it has been in service, and no method of determining how close to failure that element might be. If one could predict that a stressed component of any device would fail within a specified window of time, assuming operation of the device was continued, then, without otherwise interrupting operation of the device, that component could be taken out of service no later than, and preferably before the prediction indicated that the component had reached a “safe-operation” point, despite the component appearing to be in good condition. The problem is to find a method which allows one to make that prediction with reasonable accuracy.
U.S. Pat. No. 5,610,515 teaches a method of measuring eddy currents modified by residual stress in non-ferromagnetic metal objects, using certain circuit elements in an alternating current circuit the values of near-surface residual stress can be inferred. Generated eddy currents measured are modified by near-surface compression or tension resulting from working the surface for example, by shot-peening it, or by rolling material from which a structural element is fabricated. The term “near-surface” refers to a superficial zone having a depth in the range from about 0.025 mm (0.001″) to 0.5 mm (0.020″) for struc
Lobo Alfred D.
Zaveri Subhash
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