Method and apparatus for determining the true stress and true strain behaviour of a ductile polymer

摘要

Measurement of true stress and true strain of a ductile polymeric material during deformation involves: (a) mounting the polymeric material in an engageable alignment with a punch head of a punch system; (b) deforming the specimen with the punch head; and (c) determining the true stress and true strain of the specimen using predefined formulae. True stress and true strain of the polymeric material can be measured during deformation. The polymeric material has a planer shape defining parallel first and second spaced planar surfaces having an initial, uniform thickness. A punch system for deformation includes a movable punch head comprising a first hemispherical end having a predetermined radius and a device for mounting the material in an engageable alignment with the first end of the punch head. Determination of true stress and true strain involves: (a) mounting the ductile polymeric material in an engageable alignment with the punch head; (b) engaging the planar surface of the polymeric material with the first end of the punch head; (c) equibiaxially deforming the polymer with the punch head; (d) determining the thickness of the polymer during deformation; (e) determining the load applied by the punch head during deformation of the polymer; and (f) determining the true stress and true strain of the polymer at at least one point in time of the deformation by the formulae (I) and (II). true stress = P / 2pi Rt + pi t2 (I) true strain = In(t0/t) (II) P : the load applied; t0initial thickness of the polymer; t : thickness of the polymer at the point in time of the determination during deformation; R : radius of the punch head; and ln : natural logarithm. Independent claims are also included for the following: (A) deformation of the acceptability of a first ductile polymeric material for use in an artificial body implant involving: (i) selecting a quantity of a material and forming the material into the specimen; (ii) repeating steps (a) - (f); (iii) comparing the true stress and true strain so calculated with that of a second acceptable ductile polymeric material comprising the same polymer; and (iv) using the comparison to determine the acceptability of the first material; and (B) an improvement in a small punch test apparatus comprising: (a) a device for calculating the true strain of the polymer using formula (II); and (b) a controller for controlling the relative movement between punch head and the mounting device to achieve a constant strain rate, in response to the calculation of true strain.