Micro/Nano-scale Strain Distribution Measurement from Sampling Moiré Fringes

Summary

A sampling moiré technique featuring 2-pixel and multi-pixel sampling methods for high-accuracy strain distribution measurements at the micro/nano-scale is presented here.

Abstract

This work describes the measurement procedure and principles of a sampling moiré technique for full-field micro/nano-scale deformation measurements. The developed technique can be performed in two ways: using the reconstructed multiplication moiré method or the spatial phase-shifting sampling moiré method. When the specimen grid pitch is around 2 pixels, 2-pixel sampling moiré fringes are generated to reconstruct a multiplication moiré pattern for a deformation measurement. Both the displacement and strain sensitivities are twice as high as in the traditional scanning moiré method in the same wide field of view. When the specimen grid pitch is around or greater than 3 pixels, multi-pixel sampling moiré fringes are generated, and a spatial phase-shifting technique is combined for a full-field deformation measurement. The strain measurement accuracy is significantly improved, and automatic batch measurement is easily achievable. Both methods can measure the two-dimensional (2D) strain distributions from a single-shot grid image without rotating the specimen or scanning lines, as in traditional moiré techniques. As examples, the 2D displacement and strain distributions, including the shear strains of two carbon fiber-reinforced plastic specimens, were measured in three-point bending tests. The proposed technique is expected to play an important role in the non-destructive quantitative evaluations of mechanical properties, crack occurrences, and residual stresses of a variety of materials.

Introduction

Micro/nano-scale deformation measurements are vitally essential for evaluating the mechanical properties, instability behaviors, residual stresses, and crack occurrences of advanced materials. Since optical techniques are non-contact, full-field, and non-destructive, various optical methods have been developed for deformation measurement during the last few decades. In recent years, the micro/nano-scale deformation measurement techniques mainly include the moiré methods^1,2,3,4, geometric phase analysis (GPA)⁵

Protocol

1. Confirmation of the Micro/Nano-scale Grid on the Specimen

1. Machining of the Specimen
   1. Cut the specimen to the size required by the specific loading device used under a microscope (e.g., 1 x 5 x 30 mm³), making the surface to be observed 1.5x greater than the region of interest.
   2. Polish the specimen surface to be observed (e.g., 1 x 30 mm²), successively using coarse and fine sand paper on an automatic polishing machine (e.g., use SiC foil #320 for 3 min and then #800 for 1 min at 150 rpm and 30 N). Clean the specimen using water after each polishing step.
   3. Po....

Results

The 2D displacement and strain distributions of two carbon fiber-reinforced plastic (CFRP) specimens (#1 and #2) were measured according to the moiré formation principle²³ and the measurement process (Figure 1). The CFRP specimens were made up of 10-11 µm-diameter K13D carbon fibers and epoxy resins. The deformation of CFRP #1 was determined using the reconstructed multiplication moiré method from two-step sampling moiré fringes, and t.......

Discussion

In the described technique, one challenging step is the micro/nano-scale grid or grating (abbreviated as grid) fabrication²⁶ if no periodic pattern exists on the specimen. The grid pitch should be uniform before deformation because it is an important parameter for the deformation measurement. If the material is a metal, a metal alloy, or a ceramic, UV or heating nanoimprint lithography (NIL)²⁷, electron beam lithography (EBL)², focused ion beam (FIB).......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Automatic Polishing Machine	Marumoto Struers K.K.	LaboPol-30, Labor Force-100
Carbon Fiber Reinforced Plastic	Mitsubishi Plastics, Inc.	HYEJ16M95DHX1
Computer	DELL Japan	VOSTRO	Can be replaced with another computer with C++ programming language
Image Recording Software	Lasertec Corporation	LMEYE7	Installed in a laser scanning microscope
Ion Coater	Japan Electron Optics Laboratory Ltd.	JEC3000F
Laser Scanning Microscope	Lasertec Corporation	OPTELICS HYBRID
Nanoimprint Device	Japan Laser Corporation	EUN-4200	Can be replaced with a electron beam lithography device or a focused ion beam milling device
Nanoimprint Mold	SCIVAX Corporation	3.0μm pitch	Customized
Nanoimprint Resist	Toyo Gosei Co., Ltd	PAK01
Polishing Solution	Marumoto Struers K.K.	DP-Spray P 15μm, 1μm, 0.25μm	Use from coarse to fine
Pipet	AS ONE Corporation	10mL
Sand Paper	Marumoto Struers K.K.	SiC Foil #320, #800	Use from coarse to fine
Spin Coater	MIKASA Corporation	MS-A100