Testing System for the Mechanical Properties of Small-Scale Specimens Based on 3D Microscopic Digital Image Correlation.

Xu Liu,Rongsheng Lu

doi:10.3390/s20123530

Abstract

The testing of the mechanical properties of materials on a small scale is difficult because of the small specimen size and the difficulty of measuring the full-field strain. To tackle this problem, a testing system for investigating the mechanical properties of small-scale specimens based on the three-dimensional (3D) microscopic digital image correlation (DIC) combined with a micro tensile machine is proposed. Firstly, the testing system is described in detail, including the design of the micro tensile machine and the 3D microscopic DIC method. Then, the effects of different shape functions on the matching accuracy obtained by the inverse compositional Gauss–Newton (IC-GN) algorithm are investigated and the numerical experiment results verify that the error due to under matched shape functions is far larger than that of overmatched shape functions. The reprojection error is shown to be smaller than before when employing the modified iteratively weighted radial alignment constraint method. Both displacement and uniaxial measurements were performed to demonstrate the 3D microscopic DIC method and the testing system built. The experimental results confirm that the testing system built can accurately measure the full-field strain and mechanical properties of small-scale specimens.

Highlights

With the rapid development of micro-electro-mechanical systems (MEMS), the mechanical properties of the materials used in MEMS are directly related to the design and reliability of MEMS products
zero-normalized cross-correlation is insensitive function cross-correlation (ZNCC)+inverse compositional Gauss–Newton (IC-GN) algorithms with zero-order, first-order, and second-order shape functions were interpolation method was used to calculate the gray value at the sub-pixel position
Ten uniformly deformed speckle images were generated by imposing a uniform strain of 0.01–0.1 compared to the set values measured by the IC-GN algorithm with three kinds of shape functions on Figure 6 in the horizontal direction by the inverse mapping method

Summary

Introduction

With the rapid development of micro-electro-mechanical systems (MEMS), the mechanical properties of the materials used in MEMS are directly related to the design and reliability of MEMS products. The deformation measurement of small-scale specimens is key to the direct tensile method, and directly affects the accuracy of the obtained mechanical performance parameters. The digital image correlation method (DIC) has the advantages of being a non-contact approach and having a high accuracy and low environmental and equipment requirements, and has become the most active method in the field of optical measurement It is widely used for measuring the shape, displacement, and full-field deformation of objects [12,13]. With the help of a stereo light microscope (SLM) vision system, digital image correlation methods can be applied to deformation measurement in small-scale areas [14,15,16,17].

System Specification

Micro Tensile Machine

Elements

Imaging System

Model of the Imaging Optical Path

Calibration Process

Figureand

Speckle

Calculated

Discussion

System Calibration

In-Plane Displacement MeasurementWRAC method