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Jin Haeng Leea) Division for Reactor Mechanical Engineering, Korea Atomic Energy Research Institute, Daejeon 305-353, Republic of Korea

Minsoo Kim and Hyungyil Lee Department of Mechanical Engineering, Sogang University, Seoul 121-742, Republic of Korea (Received 21 February 2012; accepted 20 June 2012)

The numerical approach of Lee et al. [Trans. Korean Soc. Mech. Eng., A 28, 816–825 (2004)] to spherical indentation technique for property evaluation of hyperelastic rubber is enhanced. The Yeoh model is adopted as the constitutive form of rubber material because it can express well large deformation and cover various deformation modes with a simple form. We first determine the friction coefficient between a rubber specimen and a spherical indenter in a practical viewpoint and perform finite element simulations for a deeper indentation depth than that selected by Lee et al. [Trans. Korean Soc. Mech. Eng., A 28, 816–825 (2004)]. An optimal data acquisition spot is selected, which features sufficiently large strain energy density and negligible frictional effect. We improve then two normalized functions mapping an indentation load–displacement curve onto a strain energy density–invariant curve, the latter of which gives the Yeoh model constants. The enhanced spherical indentation approach successfully produces the rubber material properties with an average error of less than 5%. The validity of our developed approach is verified by experimental evaluation of material properties with three kinds of rubber materials.

I. INTRODUCTION

Rubbers and rubber-like materials have been widely used in many industries because of their excellent properties, especially flexibility and durability. Considering the importance of rubber, however, the research outcome and its practical use on rubber have a long way to go. Hyperelastic rubber generally shows a nonlinear elastic behavior for large deformation. Viscoelasticity and hysteresis are also important characteristics of rubber. In addition, mechanical properties of rubber can easily change due to environmental effects such as heat, light, and aging. Therefore, it is hard to standardize property evaluation methods for rubber and even its constitutive equation and material properties. Kim et al.1 recommended using an equibiaxial tensile test method rather than a simple compression test method, which is sensitive to friction coefficient; however, because of the difficulty in making a rubber specimen and complicated testing process, the equibiaxial tensile test method has an impractical aspect. Lee et al.2 therefore proposed a new method to evaluate material properties of hyperelastic rubber by a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.241 J. Mater. Res., Vol. 27, No. 20, Oct 28, 2012

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using a nondestructive and in situ spherical indentation test. This method, however, considered only the oillubricated condition; thus, it has a drawback in practical use. In addition, the relativel