Strain rate effect on semi-crystalline PLLA mechanical properties measured by instrumented indentation tests

Poly-L-lactic acid, PLLA, is a widely studied biomaterial, currently approved for use in a range of medical devices. Its mechanical properties can be tailored giving the material different crystallinity degrees. PLLA presents a complex non-linear behaviour that depends not only on structural parameters such as crystallinity degree but also on external parameters such as strain rate and temperature. Failure of polymeric implants is attributed to their intrinsic time-dependent performance under static loading conditions. This work explores the potential of instrumented indentation tests as a suitable technique to measure the complex non-linear time-dependent mechanical properties of PLLA. The applicability of the Oliver-Pharr method, typically used in ceramic and metallic materials, is discussed through the sensitivity of elastic modulus, hardness and material creep response to different strain rates. Likewise, the strain rate influence depending on the crystallinity degree is considered, providing important information about viscoelastic behaviour of PLLA. Whereas possible, results are compared with those obtained from tensile and Dynamic Mechanical Analysis tests extracted from the literature. (C) 2014 Elsevier Ltd. All rights reserved.