纳米压痕过程的三维有限元数值试验研究

采用有限元方法模拟了纳米压痕仪的加、卸载过程，三维有限元模型考虑了纳米压痕仪的标准Berkovich压头．介绍了有限元模型的几何参数、边界条件、材料特性与加载方式，讨论了摩擦、滑动机制、试件模型的大小对计算结果的影响，进行了计算结果与标准试样实验结果的比较，证实了模拟的可靠性．在此基础上，重点研究了压头尖端曲率半径对纳米压痕实验数据的影响．对比分析了尖端曲率半径r＝0与r＝100nm两种压头的材料压痕载荷—位移曲线．结果表明，当压头尖端曲率半径r≠0时，基于经典的均匀连续介质力学本构理论、传统的实验手段与数据处理方法，压痕硬度值会随着压痕深度的减小而升高．

3D FINITE ELEMENT SIMULATION OF THE NANOINDENTATION PROCESS

In recent years, micro-scale indentation, especially nanoindentation has become a standard test to measure mechanical properties of various new type surface materials. However, numerical simulation by finite element method (FEM) to simulate the micro-scale indentation process plays an important role as well in explaining experimental phenomena and obtaining more accurate parameters of material surface mechanical properties. Bhattacharya, Nix, Laursen and Sino developed the researd in this field earlier. In order to save computation cost, Vickers indenter and Berkovich indenter as a standard indenter of microhardness and nanoindentation equipment with geometrical shapes of regular four-sided pyramid and triangular pyramid were substituted by conic indenter and thus the 2D axial symmetrical element model was adopted to simulate the micro-indentation process is their study. As a mather of fact, the micro-scale indenters are not axisymmetric due to unevenness of all material in micro-scale. These properties could not be presented in 2D cone model. In this paper 3D finite element method is used to explore the mechanics of the nanoindentation process for Berkovich indenter. The mesh and boundary condition of this model and the material property of sample in FE simulation are introduced. The tests of standard sample are comducted for FEA. The influence of friction, slide -and sample size on the results is discussed. Based on the 3D finite element simulation, the effects of tip radius on nano-indenter are investigated. The nanoindentation results obtained by ideal Berkovich indenter and blunt tip indenter are comparatively analyzed, indicating that the measured hardness by using of blunt tip indenter decrease with the depth of indentation, even for the materials complying with traditional homogeneous continuum constitutive relation under regular testing and evaluating condition. The effect of tip radius on nanoindentation is an explanation other than the strain graduate effect.