Effects of angular misalignment on material property characterization by nanoindentation with a cylindrical flat-tip ind
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Nanoindentation techniques are commonly used to characterize nanomechanical properties of microscaled and nanoscaled materials. Nanoindentation using a cylindrical flat-tip indenter has a constant contact area which makes it a reliable source to find material’s yield strength as well as other mechanical properties. However, an angular misalignment of the indenter with the specimen results in experimental error. In this work, the effects of angular misalignment on the nanoindentation testing with a cylindrical flat-tip indenter were numerically analyzed. A three-dimensional nanoindentation solid model was generated, computer modeling based on finite element analysis was conducted. The angle of misalignment ranged from 0° to 1°. Young’s modulus and hardness were evaluated. Based on the hemispherical stress–strain distribution assumption of an elastic plastic indentation, corrected depths and modifiers were proposed for adjusting material’s 0.1% offset and 0.2% offset yield strengths. Low carbon steel AISI 1018 was selected as sample material for indentation testing and modeling validation.
I. INTRODUCTION
Nanoindentation measurement involves continuously monitoring the load and depth of an indenter during an indentation process at the microscale and nanoscale. This technique is often used to assess materials’ nanomechanical properties at high spatial resolution. The material systems being measured include thin-films, coatings, nanostructured material, surface, interface/interphase, microelectromechanical systems, and functionally graded materials, with which the conventional mechanical tests become ineffective or inaccessible based on the size of specimen and the testing equipment.1–4 Material’s various mechanical properties can be determined by nanoindentation, such as Young’s modulus, hardness, yield strength, and stress–strain relationship, in a micro/nano scale level which undoubtedly facilitated the reliable design of devices and structural applications of materials with microsized or nanosized features. With the advancement of technology, different indenter tips have been made with reduced size, different shapes, and improved accuracy. The accuracy and reliability of the load versus depth measurements have also been improved. However, since the pyramidal sharp-tip, such as Berkovich, conical, Vickers, or spherical tip, constantly changes its contact area as it is undergoing an indentation, the true projected
Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.478
contact area of the indenter on the specimen is unknown which makes the material property extracting from the testing data more difficult or less accurate. A cylindrical flat-tip indenter has a constant contact area allowing for the indenter to press on the specimen. A mechanical model proposed for directly extracting the yield strength of the tested materials, based on the hemispherical stress–strain distribution assumption, was analytically derived and numerically validated.5
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