https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A30834/datastream/PDF/Polatidis-2020-Deformation_mechanisms_in_a_superelastic-%28published_version%29.pdf

Deformation mechanisms in a superelastic NiTi alloy: An in-situ high resolution digital image correlation study

Metallic materials experience irreversible deformation with increasing applied stress, manifested in localized slip events that result in fatigue failure upon repeated cycling. We discerned the physical origins of fatigue strength in a large set of face-centered cubic, hexagonal close-packed, and body-centered cubic metallic materials by considering cyclic deformation processes at nanometer resolution over large volumes of individual materials at the earliest stages of cycling. We identified quantitative relations between the yield strength and the ultimate tensile strength, fatigue strength, and physical characteristics of early slip localization events. The fatigue strength of metallic alloys that deform by slip could be predicted by the amplitude of slip localization during the first cycle of loading. Our observations provide a physical basis for well-known empirical fatigue laws and enable a rapid method of predicting fatigue strength as reflected by measurement of slip localization amplitude. Description Slipping into fatigue Materials that are cyclically deformed become easier to break due to fatigue. However, tying fatigue strength to microstructure has been challenging. Stinville et al. used nanometer-resolution digital image correlation to observe the slip localization on the surface of a wide range of alloys (see the Perspective by Omar and El-Awady). They found that after one deformation cycle, the amplitude of the early slip localization events determines fatigue strength. This observation helps to provide a physical basis for well-known fatigue laws and paves the way to easily predicting fatigue strength. —BG Observations of slip amplitude on the surface of a metal after deformation predict fatigue strength.

On the origins of fatigue strength in crystalline metallic materials

Developments in digital image correlation (DIC) in the last decade have made it a practical and effective optical technique for displacement and strain measurement at high temperatures. This overview aims to review the research progress, summarize the experience and provide valuable references for the high-temperature deformation measurement using DIC. We comprehensively summarize challenges and recent advances in high-temperature DIC techniques. Fundamental principles of high-temperature DIC and various approaches to generate thermal environment or apply thermal loading are briefly introduced first. Then, the three primary challenges presented in performing high-temperature DIC measurements, i.e., 1). image saturation caused by intensified thermal radiation of heated sample and surrounding heating elements, 2) image contrast reduction due to surface oxidation of the heated sample and speckle pattern debonding, and 3) image distortion due to heat haze between the sample and the heating source, and corresponding countermeasures (i.e., the suppression of thermal radiation, fabrication of high-temperature speckle pattern and mitigation of heat haze) are discussed in detail. Next, typical applications of high-temperature DIC at various spatial scales are briefly described. Finally, remaining unsolved problems and future goals in high-temperature deformation measurements using DIC are also provided.  We expect this review can guide to build a suitable DIC system for kinematic field measurements at high temperatures and solve the challenging problems that may be encountered during real tests.

Overview of High-temperature Deformation Measurement Using Digital Image Correlation

The effect of temperature on slip in microtextured Ti-6Al-2Sn-4Zr-2Mo under dwell fatigue

Background: Advancements in the Digitial Image Correlation (DIC) technique over the past decade have greatly improved spatial resolution. However, many processes, such as plastic deformation, have a temporal component spanning from fractions of a second to minutes that has not yet been addressed in detail, particularly for DIC conducted in-situ in the scanning electron microscope (SEM). Objective: To develop a methodology for conducting time-resolved digital image correlation in the SEM for analysis of time-dependent mechanical deformation phenomena. Methods: Microscope and electron beam scanning parameters that influence the rate at which time-resolved DIC information is mapped are experimentally investigated, providing a guide for use over a range of timescales and resolutions. Results: Time-resolved DIC imaging is demonstrated on a Ti-7Al alloy, where slip band propagation is resolved with imaging dwell times of seconds. The limits of strain resolution and strain collection speeds are analyzed. Conclusions: The new developed methodology can be applied to a wide range of materials loaded in-situ to quantify time-dependent plastic deformation phenomena.

Time-Resolved Digital Image Correlation in the Scanning Electron Microscope for Analysis of Time-Dependent Mechanisms

Digital image correlation of scanning electron microscope images is a powerful technique for measuring full-field deformation at microstructural length scales. A major challenge in applying this technique is the fabrication of speckle patterns small enough to facilitate full-field measurements with high spatial resolutions and at high magnifications. Current approaches are inconsistent, damaging to the substrate, or highly substrate dependent, which requires researchers to recalibrate or develop new patterning approaches when changing materials systems. Here, multi-layered Au, Ti, and Ag sputtered coatings are reconfigured in a NaCl solution to quickly form DIC-appropriate speckle patterns. Our proposed technique is shown to be substrate independent, as demonstrated on neat epoxy, Ti-6Al-4V titanium alloy, and tetragonal zirconia polycrystal samples, and allows for controllable particle distributions by varying the sputtered Ag layer thickness. Patterns produced by the proposed technique enable the use of correlation window (subset) sizes smaller than 1 μm, small enough to capture highly localized deformation gradients at material discontinuities areas. Capabilities of this method in characterizing highly heterogeneous deformation conditions at sub-micron scales are demonstrated by measuring localized deformations in a single fiber model composite system.

C. B. Montgomery

Papers

A Robust Patterning Technique for Electron Microscopy-Based Digital Image Correlation at Sub-Micron Resolutions