The Tensile Stress-Strain Properties of Silicone Elastomers Are Not Affected by Specimen Type

ASTM breast implant standards F703 and F2051 specify that Type C specimens, as described in ASTM D412, must be used to determine the tensile properties of implant shells, whereas ISO breast implant standard 14607 specifies the much smaller Type 2 specimens described in ISO 37. There is ongoing interest in harmonizing these requirements.

The published literature on this topic is inconclusive regarding whether specimen size and dimensions affect the stress–strain behavior of polymers and elastomers. Some authors report large effects, requiring careful control of specimen shape and the application of corrections. Others report that the stress–strain behavior is largely independent of specimen geometry.
We conducted comparative tensile tests on specimens cut from three 1 mm thick silicone sheets, a smooth breast implant shell, and a textured breast implant shell. Our results show conclusively that, when all other test conditions are the same, ISO 37 Type 2 specimens and ASTM D412 Type C specimens yield equivalent stress–strain behavior of silicone elastomers. However, care must be taken to determine the true engineering strain by measuring the separation of the gauge markers in the center of the narrow portion of the specimens using an extensometer. Failure to do so will lead to erroneous results.

We discuss the advantages of using one specimen type over the other.