Identifying the limitation of Oliver and Pharr method in characterizing the viscoelastic-plastic materials with respect
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1137-EE10-23
Identifying the limitation of Oliver and Pharr method in characterizing the viscoelasticplastic materials with respect to indenter geometry Keerthika Balasundaram 1, Yanping Cao 2, Dierk Raabe 1 1 Max-Planck-Institut fuer Eisenforschung, Max-Planck-Str. 1, Duesseldorf- 40237, Germany 2 Institute of Biomechancis and Medical Engineering Department of Engineering Mechanics Tsinghua University, 100084, Beijing, P. R. China ABSTRACT Nanoindentation tests are widely used in recent years to characterize the mechanical properties of viscoelastic-plastic materials like polymers and biomaterials at the micro or nano-scale using the analysis method proposed by Oliver & Pharr (OP). However, recent studies revealed that the mechanical properties of viscoelastic-plastic (polymeric) materials determined using the OP method does not lead to a correct evaluation of Young’s modulus. A systematic experimental study is performed with different indenter geometries like spherical and Berkovich geometries using various polymers in order to identify the limitations of the OP method. 1. INTRODUCTION Nanoindentation tests are widely used in recent years to characterize the mechanical properties of viscoelastic-plastic materials like polymers, biomaterials at the micro or nanoscales 1, 2. In most cases, the procedure proposed by Oliver and Pharr 3 (OP method) is adopted. However, recent studies 4-6 revealed that the mechanical properties of viscoelastic-plastic materials determined using the OP method leads to the incorrect evaluation of the reduced modulus. Usually pile-up, viscoelasticity, or adhesion energy is considered as the most important reasons for the failure of the OP method. By computational and experimental study it is understood from previous work an experimental procedure can avoid viscoelastic effects in modulus determination 4-6, 9. Pile up does not play a major role on determining the analysis of the force curves at relatively smaller depths 9. Further it is identified that adhesion energy plays an important role in overestimating the contact area for depths less than a critical depth beyond which the influence of adhesion energy is negligible 7. The results reported for spherical indenter suggest that pile-up, viscoelasticity, or the effect of surface forces (adhesion energy) may be avoided while determining the reduced modulus. We made efforts to extend our experience in spherical indenter to Berkovich indenter for same polymers which failed to obtain the correct reduced modulus. Thus the motivation is to understand the possible factors for the failure of the OP method for Berkovich indenter. A novel work performed by Tranchida et al. 6 provides the bottle neck information for the failure of OP method on polymers. According to Tranchida et al.6 the failure of the OP method is from the hypothetical area function used from the polymer. The area function representing the indenter geometry is different for different polymers, implying that every polymer shows its own unique contact behavior 2. Here a systematic n
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