Indentation fracture of silicone gels
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Joseph L. Lenhart and Phillip J. Cole Sandia National Laboratories, Albuquerque, New Mexico 87185
Kenneth R. Shulla) Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (Received 31 July 2008; accepted 28 October 2008)
Indentation tests were performed, using a flat punch probe, on silicone gels to induce failure under compression. The silicone gels were formed from networks of vinyl-terminated polydimethylsiloxane (PDMS) with molecular weights of 800 and 28,000 g/mol and a sol fraction of trimethylsiloxy-terminated PDMS with molecular weights ranging from 1250 to 139,000 g/mol. Cone cracks were observed in samples that fractured from defects at the sample surface, but failure more commonly originated from the corners of the indenter. Ring cracks were observed for the most highly compliant samples that fractured at indentation depths approaching the overall thickness of the sample. In these cases we generally observed a delayed fracture response, with a time delay that increased with increasing sol fraction and decreased with increasing indentation load.
I. INTRODUCTION
There is an increased need for simple yet quantitative methods for determining the fracture behavior of polymer gels and other soft solids. Indentation methods are a convenient method for probing fracture processes,1 but the use of indentation techniques has been confined primarily to hard, brittle materials with fracture strengths much lower than the elastic modulus. When highly compliant polymeric materials are tested in this geometry, they usually act as a soft substrate and are coated with a thin brittle layer,2,3 although more brittle bulk polymers have been studied as well.4 Fracture studies of polymer gels that apply indentation techniques have focused on the puncture of biological tissues and silicone rubbers to understand the wounding of skin and the breaking of latex gloves.5–8 These experiments are often performed in geometries in which the material is not fully supported by a rigid substrate,5–7 allowing the punch to penetrate all the way through the soft material, emerging through the opposing surface. The detailed geometry of the supporting structure must be appropriately accounted for in the analysis of the results. An additional complication arises when highly deformable materials such as polymer gels are used, since these may sag under their own weight during the experiment. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0128 J. Mater. Res., Vol. 24, No. 3, Mar 2009
These complicating factors can be eliminated by supporting the indented material on a flat, rigid substrate. An additional advantage of this geometry is that multiple indentation tests can be applied to the same sample in different locations on the gel. Data collected in this way improve the reliability of the results, since the process of fracture is statistical in nature. We illustrate the use of this indentation method with a set of well-characterized silicone gel
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