Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
1999-03-31
2001-07-31
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Reexamination Certificate
active
06267011
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the measurement of true stress and true strain behavior of ductile polymeric materials. More particularly, this invention relates to methods and apparatus for obtaining data regarding such behavior as the material goes through deformation and thereafter using such data to evaluate the material for use in human joint implants (e.g. artificial knees, hips, etc.).
BACKGROUND OF THE INVENTION
Before placing an artificial implant into a human or animal body the implant must be sterilized. The material, furthermore, must not only be biocompatible, but have as long a life as is feasible after sterilization. The importance of this latter characteristic, i.e. useful life expectancy, is accentuated when the implant is a load-bearing one, such as a total joint arthroplasty (e.g. a hip or knee).
For many years, polyethylene, and in particular, ultra high molecular weight polyethylene (UHMWPE), has been used for this purpose. Sterilization is accomplished in at least one of several ways. One known way was to irradiate the implant (e.g. joint replacement) with a gamma ray dose of 2.5-4.0 Mrads in the presence of air. While highly effective as a sterilization technique, it was known to result, at times, in the formation of free radicals in the polyethylene which combined with oxygen to eventually degrade the polymer and thus reduce its effective useful life. Such degradation, it has been found, not only occurs during shelf storage but, unfortunately, can also continue to occur after implantation in the body.
As a result of this undesirable degradation, gamma sterilization in air is generally no longer used in most UHMWPE implant situations, and other alternatives have been devised. For example, low oxygen environment gamma irradiation, ethylene oxide, and gas plasma sterilization are currently more often used. Unfortunately, it is yet to be clearly ascertained to what extent these new sterilization methods either inhibit or cause mechanical degradation of the polyethylene during shelf storage and/or after implantation in the body.
There are two particular times at which the testing of an implant to determine its “degradation” characteristic (or susceptibility) normally occurs. The first, of course, is before implantation. The next is if the implant is removed for failure or is suspected of being near failure. Ascertaining the true cause of failure aids future improvements. Moreover, being able to predict in vivo wear behavior in advance would obviously materially aid the technology and patients, alike.
It was known prior to this invention that meaningful comparative mechanical behavior data could be obtained for analyzing polyethylene implants by deriving and comparing load vs. displacement curves using small sample punch test techniques on material of a known polyethylene “standard” (i.e. nondegraded) implant and on material of an implant suspected of having experienced degradation or being tested for use, failure, or potential degradation. While such techniques are very useful, they do not measure true stress and corresponding true strain which, if capable of measurement, would give a more accurate indication of levels of degradation due to the changes which a ductile polymeric material, such as UHMWPE, goes through during multiaxial (e.g. biaxial) deformation.
While various physical and chemical properties of polyethylene can be readily measured by well known techniques, it is only recently that techniques for performing measurements of mechanical behavior on localized sections of such a material have been developed. For example, uniaxial tensile testing of 200-400 &mgr;m thick sections prepared from acetabular components have been utilized to investigate changes in mechanical behavior of the material in heavily oxidized subsurface regions, including local oxidation in total knee replacements. Yet another study prepared miniature tensile specimens from tibial components to compare mechanical properties of implants sterilized with ethylene oxide and gamma radiation. Unfortunately, the highly curved surfaces of total joint replacement components makes the fabrication of numerous long, flat uniaxial tensile specimens from a single implant technically impractical, and sometimes unfeasible.
Miniature specimen small punch testing techniques have heretofore been developed for measuring mechanical behavior of metals. Such known techniques have, in fact, been used successfully to characterize the true stress—true strain behavior, as well as the ductility and fracture resistance of metals. This development of the small punch test for metallic materials was driven by the need to measure in-service degradation of mechanical properties of metals with a limited volume of available material. The small specimen sizes (e.g. 0.02 inches thick) required for the test also provided a useful method for characterizing the material at specific locations in a component or a structure.
Certain researchers have heretofore empirically correlated the results of small punch mechanical behavior with conventional, relatively large test specimen mechanical behavior in metals. A major disadvantage of this empirical approach is the need to accumulate a large volume of mechanical (e.g. tensile and fracture) data for a given material in order to make reliable engineering predictions from small punch test results.
A known nonempirical alternative interpretation of the results of the small punch test data accumulated during the testing of metals is disclosed in U.S. Pat. No. 4,567,774. The technique reported uses the finite element method, or FEM, to infer conventional tensile stress-strain properties. Another known nonempirical technique matches the observed small punch load-displacement curve of the metal under analysis with a database of experimental and analytically simulated small punch load-displacement curves. From such a comparison, tensile stress-strain behavior in that metal can be inferred (i.e. an inferred true stress vs. true strain curve can be obtained). Such a stress/strain curve has been used to compute the local strain energy density accumulated to initiate cracking (i.e. fracture property) in the small punch metal specimen. Tensile and fracture properties using this known approach have been reasonably accurate for a wide range of metals. However, due to limitations in the constitute theory in these various nonempirical alternatives, they do not provide satisfactory results when applied to polymers such as polyethylene.
In this respect, the von Mises yielding criterion, which has been incorporated into the finite element models when nonempirical techniques have been employed, has been validated for metals. However, the theory has significant limitations with polymers generally, and with polyethylene specifically. For example, when applied to large-scale deformation mechanical behavior under multiaxial loading conditions when polymers stretch significantly, the von Mises yielding criterion no longer applies. Thus, these methods do not produce reliable estimates of the large-scale mechanical behavior of polymers under multiaxial loading conditions during the drawing (i.e. stretching) phase, which may often be of particular interest for the particular polymer under investigation. “Large-scale mechanical behavior” is defined (and known) as the behavior of a body under conditions wherein strains experienced are plastic over much of the body's volume (i.e. the “stretching” or “drawing” phase). In short, the known finite element based methods have not been found useful in reliably measuring or predicting stress/strain behavior for ductile polymers during the “stretching” phase. This is particularly true for polyethylene during the multiaxial loading conditions produced during and by a small punch test.
Despite the above drawbacks, the load-displacement behavior obtained by the known small punch testing methods for polymers, in general, and for polyethylene, in particular, has provided some useful results. In such tests, the punch head is caused to interact with
Foulds Jude Reynold
Jewett Charles William
Kurtz Steven Michael
Davis Octavia
Exponent Inc.
Fuller Benjamin R.
Hall Priddy Myers & Vande Sande
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