Method and apparatus for determining the true stress and true strain behavior of a ductile polymer
AssigneeExponent, Inc.
InventorsSteven Michael Kurtz, Charles William Jewett, Jude Reynold Foulds
Life of this patent
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Abstract
A small punch test apparatus is used to determine true stress and true strain in a ductile polymeric material as it undergoes deformation by the punch head. The information gathered can be used comparatively to assess the acceptability of a body implant subjected to sterilization against similar information obtained on another implant of the same polymer already known to be acceptable.
Claims24
A method for measuring true stress and true strain of a ductile polymeric material during deformation, the steps comprising:
- a) providing a ductile polymeric material specimen having a generally planar shape defining substantially parallel first and second spaced planar surfaces having an initial, substantially uniform thickness to therebetween;
- b) providing a punch system which includes a movable punch head comprised of a substantially hemispherical end having a predetermined radius (R) and means for mounting the specimen in engagable alignment with said end of the punch head;
- c) mounting said specimen in engagable alignment with said punch head;
- d) engaging a planar surface of said specimen with said first end of said punch head;
- e) substantially equibiaxially deforming said specimen by applying a load thereto with said punch head;
- f) determining a thickness of said specimen during said deformation of said specimen by said punch head;
- g) determining said load applied by said punch head during said deformation of said specimen;
- and
- h) determining said true stress and true strain of said specimen at at least one point in time of said deformation, according to the following formulae:
- true stress = P 2 π Rt + π t 2 ( 1 ) true strain = ln ( t o / t ) ( 2 )
- P is the load applied;
- to is the initial thickness of the specimen;
- t is the thickness of the specimen at the point in time of said determination of said thickness during deformation;
- R is the radius of said punch head;
- and ln is the natural logarithm of the quantity (to/t).
A method for determining acceptability of a first ductile polymeric material, said method comprising the steps of:
- a) selecting for use a quantity of a ductile polymeric material;
- b) forming said material into a specimen having a generally planar shape defining substantially parallel first and second spaced planar surfaces having an initial substantially uniform thickness (to) therebetween;
- c) providing a punch system which includes a movable punch head substantially hemispherical in shape and having a predetermined hemispherical radius R and means for mounting said specimen in engagable relationship with said movable punch head;
- d) mounting said specimen in said mounting means;
- e) moving said punch head into engagement with said specimen;
- f) continuing said movement of said punch head with respect to said specimen to apply a load to said specimen and to cause substantial deformation of said specimen;
- g) determining said load applied by said punch head during said deformation;
- h) determining a thickness of said specimen during said deformation;
- i) calculating from said determinations acquired from steps g) and h) true stress and true strain of said specimen during said deformation according to the following formulae:
- true stress = P 2 π Rt + π t 2 ( 1 ) true strain = ln ( t o / t ) ( 2 )
- P is the said load applied;
- to is the said initial thickness of the specimen;
- t is a said thickness of the specimen at a point in time of determination of said thickness during deformation;
- R is the said radius of said punch head;
- and ln is the natural logarithm of the quantity (to/t);
- thereafter
- j) comparing said true stress and true strain so calculated with said true stress and true strain of that of a second, acceptable ductile polymeric material comprised of substantially said same polymer providing said same method steps a)-i);
- and
- k) using said comparison to determine acceptability of said first ductile polymer material from which said specimen is formed.
In a small punch test apparatus including a punch head, means for mounting a substantially planar specimen in a position so as to be capable of being deformed by relative movement between said punch head and said mounting means, means for causing said punch head to deform said specimen and means for measuring a plurality of thicknesses of said specimen during deformation, the improvement comprising:
- a) means for calculating true strain of said specimen during deformation using said thicknesses as measured according to the formula:
- true strain=ln (to/t)
- wherein to is an initial thickness of said specimen;
- t is a said measured thickness of the specimen;
- and ln is the natural logarithm of the quantity (to/t);
- and
- b) means for controlling relative movement between said punch head and said mounting means to achieve a constant strain rate during deformation in response to said calculation of true strain.
Dependencies are inferred from the claim text.