Study On Mechanics Problem In The Indentation Testing

In order to obtain mechanics parameter of materials, many methods and means of experiment were developed and the testing standards were established according to the material's working condition. But with the development of material producing, micro-electronics technique and micro-processing technique, the dimensions of new material and micro-mechanical device become smaller and smaller. It is difficult for a conventional testing apparatus to measure the mechanical properties of nano-materials and nano-parts. In fact, it is impossible. How to develop feasible testing method to measure material's mechanical properties is a challenge topic in modern experiment mechanics.Among various testing techniques to evaluate hardness and elastic modulus, indentation test has widely been adopted and used in determining the mechanical behavior of materials at small scales. Its attractiveness stems largely from the fact that the mechanical properties can be determined directly from indentation load and displacement measurements without the need to image the hardness impression. With high-resolution testing equipment, this method facilitates the measurement of properties at the micrometer and nanometer scales. For this reason, the method has become an effective technique to determine the mechanical properties of thin films and microstructure devices.However, indentation test has some problems and needs further studies to perfect. In fact, indentation experiment is a complicated process, which the materials beneath the imprint in different location experience different deformation process and are in various stress state. The past research concentrated on the deformation history for the materials beneath the imprint and neglected the deformation in the plane of the imprint. Secondly, indentation size effects have been reported. Many analyses on indentation size effects have been undertaken from various aspects and some explanation are presented, but many of them are contradicted. Thirdly, the accuracy of testing for nano indentation test is a crucial issue, because even the tiny differences may cause different results.This paper proposes an evaluation technique for indenter displacement and displacement field in the plane of indenter by applying digital speckle image correlation method to test hardness. The curves of indentation depth versus time in the case of loading, constant and unloading can be obtained from speckle image correlation calculation of the displacement of imprint. Based on this analysis, the possibility to measure material's creep properties by using traditional hardness instrument is investigated. The plastic deformation law in the surface of indenter is given by the calculation the displacement field in the surface of the indenter. Based on experiment and simulation, the influence of the imprint shape, the surface microstructure and contact point on testing accuracy are analyzed. Then we present factors which affect the accuracy of nano-indentation test and the limitation of this test method. By analyzing the results of testing hardness on different micro hardness testing instrument, we discuss the possible factors causing indentation size effect and present some new ideas. The relationship between hardness and indentation depth is obtained by using plastic strain gradient theory and FEM simulation. The validity of experiment result is verified analytically.