【作者】 刘美华；

【导师】 李鸿琦；

【作者基本信息】 天津大学 ， 固体力学， 2007， 博士

【摘要】 压痕深度测试法以它特有的优势在众多的力学测试法中脱颖而出,并越来越为人们所关注。该方法的优势主要表现在能够精确地连续测量得到载荷-位移数据,据此可建立与其他力学参数之间的关系。目前,压痕深度测试法在实际中已得到广泛的应用。本文首次将数字散斑相关技术应用于硬度测试中,通过散斑相关计算得到被测表面压痕周围的塑性变形场,同时结合不同显微硬度设备上的硬度测试结果及有限元模拟,系统地讨论压痕尺寸效应问题,认为压痕尺寸效应的产生与测试方法有关,与压痕的几何相似性有关,与测试过程压头尖端不规则形状有关,与压头顶部表面与被测材料之间的摩擦有关等。数字散斑相关技术计算及有限元的模拟计算发现了压痕周围的形变特征,这为压痕法的计算和理论研究提供了技术支持。比较不同实验法得到的弹性模量时发现,纳米压痕法测量的弹性模量高于常规的拉伸实验法的弹性模量,由此讨论了纳米压痕法的精度问题,根据推导的压痕接触面积与深度之间的关系可半定量化地确定压痕尖端曲率半径,提出使用纳米压痕仪测试材料力学性能的局限性。探讨在洛氏硬度计上,采用球形压头测试高分子材料蠕变的方法可行性,压头在保持载荷阶段的压痕深度与时间之间的关系可作为研究材料蠕变的基础,由此得到的蠕变曲线符合Kelvin模型,使用该方法可以确定材料的蠕变指数。试验表明,微米压痕硬度相对值的平方与压痕对角线的倒数之间有近似的线性关系。更多还原

【Abstract】 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.更多还原