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The effect of specimen thickness on the mechanical behavior of UHMWPE characterized by the small punch test
Herr, M., Kurtz, S. M.
Abstract
The small punch test has been validated for conventional and highly crosslinked UHMWPE used in orthopedic implants and is currently being considered for standardization by ASTM. Because only a limited volume of material may be available for characterization in a retrieved component, however, reducing the specimen size may be useful to increase the number of possible test specimens from a particular implant. However, it is unclear how the reduction in specimen thickness could affect the properties of the small punch test. Therefore, the goal of this study was to examine the relationships between the small punch specimen thickness and the metrics of the small punch test.
Ram extruded GUR 1050 UHMWPE was used to make a total of 20 small punch specimens. Four thicknesses were chosen: 0.254mm, 0.330mm, 0.432mm, and 0.508mm. Five specimens of each thickness were manufactured and tested. Specimen diameters were all 6.35mm. Custom shim spacers were used in the small punch test apparatus to fill the void created by thinner specimens and to maintain a uniform contact area. Specimens were tested following an established protocol using indentation with a hemispherical-head punch at a constant displacement rate of 0.5 mm/min until specimen failure. Tests were performed in a servo-hydraulic test machine with an environmental chamber at 21±1 °C.
Linear relationships were observed between specimen thickness and peak load, ultimate load, and work-to-failure of the small punch test (R2 = 0.99, R2= 0.99, and R2=0.94, respectively). A cubic relationship was observed between specimen thickness and initial stiffness, which was proportional to the elastic modulus of the UHMWPE, and was predicted based on beam theory (R2 =0.98). The results of this study therefore provide a reliable basis for normalizing small punch test measurements to account for variations in specimen thickness.
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