Determination of the elastic properties of glasses and polymers exploiting the resonant characteristic of depth-sensing
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Depth-sensing indentation tests can be used to estimate the Young’s modulus, hardness, and other characteristics of material behavior. For many materials, the unloading segment of the load–depth curve contains only elastic recovery while the loading segment can contain elastic and plastic deformation. In this paper a new method is presented to determine the Young’s modulus of a material from the loading segment of an indentation test. A depth-sensitive hardness tester was used with a load cell integrated into the closed-loop system. Defined mechanical oscillations with constant frequency were generated by adding a piezoelectric stack to the closed loop of the hardness measurement system. Thus the resonance response of the system was obtained, which includes information regarding the stiffness of the tested material. This new method was tested on two polymers and two glasses, an optical and a conventional one. The results obtained for the Young’s modulus were in good agreement with other accepted methods.
I. INTRODUCTION
Recently, measuring the hardness, especially the depth-sensing microhardness, has preferentially been used in the microsystem technology and in microelectronics to characterize mechanical behavior on the micrometer to submicrometer scale. In depth-sensing indentation, an indenter of known geometry, like the Vickers’ or Berkovich’ pyramid, is forced into the surface of a specimen, and the load on the indenter and the penetration depth are continuously recorded. The mechanical parameters measured most frequently with this technique are the Martens hardness (HM) during loading,1,2 and the Young’s modulus (E) and contact hardness (H) both of which can be determined from the unloading data via the method used by Oliver and Pharr.3 If the shape of the unloading curve is strongly influenced by time-dependent deformation, such as creep behavior in polymers, the Young’s modulus and the contact hardness are determined inaccurately. The loading segment, which can contain both plastic and elastic deformations, can only be analyzed if the Young’s modulus or the contact hardness of the material is known.4
a) b)
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http://journals.cambridge.org
J. Mater. Res., Vol. 16, No. 6, Jun 2001 Downloaded: 11 Mar 2015
One way to reduce these problems is to determine the stiffness of the contact between the indenter and sample, S ⳱ dF/dhS, where dF is the incremental change of the applied load and dhS is the incremental elastic change of the penetration depth. The stiffness during loading and unloading is continuously measured using the continuous stiffness mode (CSM) of the Nanoindenter威II3,5 (MTS Systems Corporation, Nano Instruments Innovation Center, Oak Ridge, TN) or the equivalent method of the Triboscope6 (Hysitron Inc., Minneapolis, MN). With this system, an oscillation with nanometer-scale amplitude is applied to the loading force, and the stiffness is determined from the resulting phase shift and amplitude ratio between load and displaceme
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