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Advancement of Optical Methods in Experimental Mechanics, Volume 3 of the Proceedings of the 2017 SEM Annual Conference & Exposition on Experimental and Applied Mechanics, the third volume of nine from the Conference, brings together contributions to this important area of research and engineering. The collection presents early findings and case studies on a wide range of optical methods ranging from traditional photoelasticity and interferometry to more recent DIC and DVC techniques, and includes papers in the following general technical research areas.



Chapter 1. A New Method for Improving Measurement Accuracy of Digital Image Correlation

Digital image correlation (DIC) method has been applied in wide fields including experimental mechanics. The displacement measurement accuracy plays an important role in these situations. The direct method to improve the measurement accuracy is to reduce the measurement error in DIC. In the paper, a new method has been developed to improve the DIC accuracy using the feature of DIC system error. The feature of DIC system error is analyzed. And the reduction of the DIC system error has been verified by experiment.
Li Bang-Jian, Wang Quan-Bao, Duan Deng-Ping

Chapter 2. Fatigue Analysis of 7075 Aluminum Alloy by Optoacoustic Method

The influence of fatigue damage on the elastic response of AA7075 aluminum alloy was investigated through a combination of optical and acoustical experiments. Specimens were previously subjected to fatigue cyclic loads at various fatigue levels within the fatigue life. Macroscopic deformation process under a certain load below the yield point (elastic region) for the pre-fatigued specimen was visualized by electronic speckle pattern interferometry (ESPI). At the same time, the acoustic velocities of vertical and shear waves propagating in the fatigued specimen were measured using an ultrasonic probe. The acoustic analysis showed the following change in residual stress by the fatigue cyclic load; an increase in compressive residual stress with the number of pre-fatigue cycles (NP) below 103, and relaxation of the residual stress NP over 103. The visualization using ESPI demonstrated that the strain heterogeneity in the macroscopic elastic regime was enhanced with increase of the pre-fatigue cycle. The correlation between the optical and the acoustical measurement results is discussed based on the change in the residual stress, localized plastic deformation, and the crack initiation.
Tomohiro Sasaki, Hiroshi Ono, Sanichiro Yoshida, Shuich Sakamoto

Chapter 3. Early Strain Localization in Strong Work Hardening Aluminum Alloy (2198 T3): 3D Laminography and DVC Measurement

The effect of strain hardening on localization in front of a notch is assessed by following the interactions between strain concentrations, damage, initial microstructure and grain orientations. A CT-like specimen made of strong work hardening 2198 T3 aluminum alloy is subjected to an in situ synchrotron laminography experiment. Kinematic fields are measured via digital volume correlation. The final results are bulk displacement and strain fields including their corresponding resolutions. The reported results refer to the portion of the specimen around 1 mm away from the notch root. With the selected spatial resolution, damage nucleation and growth is evaluated in strained bands until the very end of the loading process.
Ante Buljac, Lukas Helfen, François Hild, Thilo F. Morgeneyer

Chapter 4. On the In-Plane Displacement Measurement by 3D Digital Image Correlation Method

In this study, a three-step object was moved in-plan to different positions and determined the displacement by 3D DIC method. To monitor the out-of-plane displacement of the object during movement, a laser triangulation method was performed together with a typical 3D DIC. The laser triangulation setup consists of a laser source with a cylindrical lens mounted on the center of the camera-pair to generate a line pattern and then the discontinuous line patterns were then recorded by left- and right- cameras for step-height calculation. The measurement results reveal that there is displacement deviation introduced by viewing angle between camera and the object as the object is positioned at different locations. The experiment also shows that the deviation of 3D DIC determined in-plane displacement is small but the determined displacement is higher than the nominal displacement determined by a precision stage; in addition, out-of-plane displacements have been reported and cannot be ignored. In the final, a single camera model based on geometrical parameters of 3D DIC is proposed to correct the 3D DIC determined displacements; according to current results, the in-plane displacement of the 3-steps object can be tremendously reduced from 9.7% to 1.7%.
Chi-Hung Hwang, Shou Hsueh Wang, Wei-Chung Wang

Chapter 5. Noise Reduction in Amplitude-Fluctuation Electronic Speckle-Pattern Interferometry

Amplitude-Fluctuation Electronic Speckle Pattern Interferometry is used in a variety of vibration analyses. The technique utilizes the fact that when the vibration frequency of the object is significantly higher than the frame rate of the imaging device, the interference term can be approximated by the lowest (0th) order of the first kind Bessel function. Since the 0th order Bessel function takes the maximum value of unity when the vibration amplitude is null, the amplitude of a given vibration can be estimated from the reduction of the interference term relative to the case when the object is still. In reality, however, various environmental noises, such as temperature fluctuation of air in the interferometric paths and floor motion transferred through the optical table, cause low frequency fluctuations of the interference term, and thereby compromise the integrity of data. In this paper, we discuss typical environmental noise on standard optical interferometer settings, and propose to reduce the effect of the noise on the signal by introducing a carrier fringe system and analyzing the fringe pattern in the spatial frequency domain. The effectiveness of the proposed method is assessed for our recent Michelson interferometer experiment in which vibration of thin-film specimen is characterized.
Sanichiro Yoshida, David Didie, Jong-Sung Kim, Ik-Keun Park

Chapter 6. Evaluating Path of Stress Triaxiality to Fracture of Thin Steel Sheet Using Stereovision

A stereovision technique based on digital image correlation is applied to the evaluation of the stress triaxiality and fracture strain of thin steel sheet. A tensile testing specimen with notches made of high strength steel sheet is loaded and the surface displacements are measured from both sides of the specimen surface using two stereovision systems. Not only the in-plane strains but the through-thickness strains are evaluated from the measurement results of the displacements on the both surfaces of the specimen. The variation of the stress triaxiality at an evaluation point is evaluated from the measured strains. The fracture strain is also evaluated from the strain measurement results. Experimental results show that the stress triaxiality and the fracture strain of thin steel sheet can be evaluated by the surface strain measurement. The results can be utilized for simulating deformation and predicting fracture of a component made of thin steel sheet.
D. Kanazawa, S. Chinzei, Y. Zhang, K. Ushijima, J. Naito, S. Yoneyama

Chapter 7. Studying with a Full-Field Measurement Technique the Local Response of Asphalt Specimens Subjected to Freeze-Thaw Cycles

Asphalt is a strongly heterogeneous material, whose global thermo-mechanical behavior is generally studied at a large spatial scale. Observing the local strain fields which occur between aggregates and mastic (i.e., asphalt binder + filler) is however crucial to understand the phenomena that influence the global response of such materials, and then better design them. In this study, full-field measurements were used to investigate the difference in behavior between various types of asphalt containing Recycled Asphalt Pavement (RAP). We focused here on their response when they were subjected to freeze-thaw cycles. The local contraction/expansion of the specimens subjected to various cooling/heating phases was observed with the grid method. The strain distribution has been found to be very heterogeneous because of the difference in coefficient of thermal expansion between aggregates and mastic. The influence of the percentage of RAP on the global response of the specimen is observed and discussed.
M. C. Teguedi, B. Blaysat, E. Toussaint, S. Moreira, S. Liandrat, M. Grédiac

Chapter 8. Mechanical Shape Correlation: A Novel Integrated Digital Image Correlation Approach

Mechanical Shape Correlation (MSC) is a novel integrated digital image correlation technique, used to determine the optimal set of constitutive parameters to describe the experimentally observed mechanical behavior of a test specimen, based on digital images taken during the experiment. In contrast to regular digital image correlation techniques, where grayscale speckle patterns are correlated, the images used in MSC are projections of the sample contour. This enables the analysis of experiments for which this was previously not possible, because of restrictions due to the speckle pattern. For example, analysis becomes impossible if parts of the specimen move or rotate out of view as a result of complex and three-dimensional deformations and if the speckle pattern degrades due to large deformations. When correlating on the sample outline, these problems are overcome. However, it is necessary that the outline is large with respect to the structure volume and that its shape changes significantly upon deformation, to ensure sufficient sensitivity of the images to the model parameters. Virtual experiments concerning stretchable electronic interconnects, which because of their slender wire-like structure satisfy the conditions for MSC, are executed and yield accurate results in the objective model parameters. This is a promising result for the use of the MSC method for tests with stretchable electronics and other (micromechanical) experiments in general.
S. M. Kleinendorst, J. P. M. Hoefnagels, M. G. D. Geers

Chapter 9. On the Boundary Conditions and Optimization Methods in Integrated Digital Image Correlation

In integrated digital image correlation (IDIC) methods attention must be paid to the influence of using a correct geometric and material model, but also to make the boundary conditions in the FE simulation match the real experiment. Another issue is the robustness and convergence of the IDIC algorithm itself, especially in cases when (FEM) simulations are slow. These two issues have been explored in this proceeding. The basis of the algorithm is the minimization of the residual. Different approaches for this minimization exist, of which a Gauss-Newton method is used most often. In this paper several other methods are presented as well and their performance is compared in terms of number of FE simulations needed, since this is the most time-consuming step in the iterative procedure. Beside method-specific recommendations, the main finding of this work is that, in practical use of IDIC, it is recommended to start using a very robust, but slow, derivative-free optimization method (e.g. Nelder-Mead) to determine the search direction and increasing the initial guess accuracy, while after some iterations, it is recommended to switch to a faster gradient-based method, e.g. (update-limited) Gauss-Newton.
S. M. Kleinendorst, B. J. Verhaegh, J. P. M. Hoefnagels, A. Ruybalid, O. van der Sluis, M. G. D. Geers

Chapter 10. Extension of the Monogenic Phasor Method to Extract Displacements and Their Derivatives from 3-D Fringe Patterns

Determination of displacement field and its derivatives from fringe patterns entails four steps: (1) information inscription; (2) data recovery; (3) data processing; (4) data analysis. Phase retrieval and processing are very important in fringe analysis. In [1], these steps were discussed for 1D signals, introducing a 2D abstract space as phase evaluation requires a vectorial function. The Hilbert transform allows to obtain signal in-quadrature defining local phase. A 3D abstract space must be generated to handle the analysis of 2D signals and simultaneously extend Hilbert transform to 2D. The theory of a monogenic function introduced in [2] is elaborated here: the 3D vector in a Cartesian complex space is graphically represented by a Poincare sphere. This provides a generalization of the Hilbert transform to a 2D version of the Riesz transform, a modified version of that described in [2]. Theoretical derivations are supported by actual application of theory and corresponding algorithms to 2D fringe patterns and by comparing obtained results with literature.
C. A. Sciammarella, L. Lamberti

Chapter 11. Deformation Measurement within a Volume of Translucent Yield Stress Material Using Digital Image Correlation

This paper introduces a method of determining in-plane deformation of a translucent yield stress material (YSM) at any depth using digital image correlation. A 2D plane of uniquely shaped speckles are introduced to a volume of the YSM using a 3D printing technique. A cylindrical object, is dragged through the 2D plane at four different speeds each with four different diameters. The displacements caused by the cylinder were found and analyzed.
A. McGhee, P. Ifju

Chapter 12. Surface Deformation with Simultaneous Contact Area Measurement for Soft Transparent Media due to Spherical Contact

We present a method to measure surface deformations between a steel sphere and a flat PDMS surface. A sphere was chosen as the specimen to ensure the resulting deformation measurement can be compared to known theoretical models. A 36 mm diameter steel sphere was pressed into contact against flat, transparent polydimethylsiloxane (PDMS) sheets with a constant load rate controlled by an Instron testing machine. The modulus of the PDMS samples range from 241 kPa to 2.1 MPa. A digital image correlation technique was used to measure the surface deformation of the PDMS with increasing applied load.
A. McGhee, D. Nguyen, P. Ifju

Chapter 13. Towards Measuring Intergranular Force Transmission Using Confocal Microscopy and Digital Volume Correlation

We aim to show the feasibility of using confocal microscopy imaging techniques for Digital Volume Correlation (DVC) and analysis of granular mechanics experiments. The first part of this study validates the DVC and confocal microscopy imaging methods for a general problem of uniaxial compression of continuous media. The second part investigates the specific problem of granular mechanics. Intergranular force transmission will be analyzed from 3D image stacks captured using confocal microscopy. DVC methods will be used to analyze the full 3D grain motions and deformations. The intergranular forces will be determined inversely using the Granular Element Method (GEM). Preliminarily results show that confocal microscopy is a useful volumetric imaging method for DVC analyses and shows promise in furthering the study of intergranular force chains and shear bands.
Kimberley Mac Donald, Guruswami Ravichandran

Chapter 14. Using Anti-aliasing Camera Filters for DIC: Does It Make a Difference?

Aliased speckle patterns are a known problem for digital image correlation (DIC). By definition, aliased speckles are smaller than the resolution limit of the camera and add “noise” to images via the spatially-aliased frequency content. Aliased speckles occur quite frequently in practical DIC applications, especially when using spray paint to speckle a surface, where control of the speckle size is difficult at best. This paper compares DIC results from aliased speckle patterns imaged with typical machine cameras with and without physical anti-aliasing filters applied to the camera detectors. Additionally, physical anti-aliasing filters are compared with post-processing, digital low-pass filters of aliased images to quantify the influence of the two types of filters on the quality of DIC results. A key result from this work is that the loss of contrast associated with the addition of physical anti-aliasing filters is generally more detrimental to DIC results than the noise resulting from aliased speckles.
PL. Reu

Chapter 15. Investigation of Electronic Speckle Pattern Interferometry with Line Laser Scanning for Large Area Deformation Measurement

For measuring small deformations in large structures by electronic speckle pattern interferometry (ESPI), the increase of laser power is required for providing the sufficient laser power per area. However, high-power lasers lead increasing the risk of exposure to laser, the size of the equipment and the cost. In this study, ESPI with line laser scanning is investigated for large area deformation measurements without increasing the laser power. A dual-beam interferometer for a horizontal displacement measurement which can illuminate horizontal line lasers and can scanning for vertical direction is constructed. A static in-plane deformation of an aluminum sheet specimen is measured. Speckle images for each vertical position are captured at the initial and the deformed state. Analyzed phase maps for each line show mismatch of the phases. Additionally, a method for integrating the mismatched line phase maps are investigated.
Shuichi Arikawa, Yuta Ando

Chapter 16. Internal Heat Generation in Tension Tests of AISI 316 Using Full-Field Temperature and Strain Measurements

Full-field temperature and strain measurements were recorded during tension tests of AISI 316 on a hydraulic load frame at a strain rate of 1 s−1. The temperature increase was measured on one side of the specimen using a high speed IR camera while the deformation was measured on the opposite side with a visible camera, each at a frame rate of 500 FPS. Uniform deformation of the specimen was observed up to strains of 0.25 until necking occurred and localization strains reached up to 0.75 at failure. The maximum temperature as measured by the IR camera was 260 °C before failure. The fraction of plastic work converted to heat (ß) was calculated over the entire gage length of the specimen using the local measurements of stress, strain, and temperature and varied between 0.6 and 0.9 throughout the test.
Jarrod L. Smith, Veli-Tapani Kuokkala, Jeremy D. Seidt, Amos Gilat

Chapter 17. A Short Survey on Residual Stress Measurements by HDM and ESPI

In this paper a review of progresses occurred along the years in measuring residual stresses by optical methods is presented. These methods allow to implement the hole drilling procedure for residual stress profile measuring, without applying the strain gage rosette. This approach presents several advantages such as easier and cheaper preparation of the test beside the possibility to avoid eccentricity issues and to increase the amount of available data in view of the fact that each pixel can be considered as a measure point. In particular the evolution of the Electronic Speckle Pattern Interferometry (ESPI) technique will be presented showing how set up, methodologies and calculation approaches have developed along the last three decades.
C. Pappalettere

Chapter 18. Feasibility of Using Fringe Projection System for Corrosion Monitoring in Metals of Interest in Cultural Heritage

Cultural heritage is affected by degradation processes related to intrinsic factors, environmental or human activities. Corrosion is one the most deteriorative phenomena and it can introduce huge damages. Patina or thick corrosion crust formed on artifacts, may display complex products and structures. The identification of the corrosion products or characterization of the patina on artifacts is an essential task to acquire a better knowledge about the condition of ancient objects, corrosion processes and conservation treatment or preventive procedures for long-term, stable preservation. These considerations show that corrosion monitoring is a very important aspect for the evaluation of the degradation of cultural heritage. It is important to have a system that could be used for easy monitoring of the surface condition to put in evidence the beginning of dangerous localized corrosion phenomena. Nowadays optical methods allow detecting and representing, with remarkable accuracy, three-dimensional objects such as sculptures and archaeological finds, as well as large items such as architectural structures. One of the most powerful approaches is the Fringe Projection System (FP) based upon the projection of a complex pattern of light on the object to be reconstructed. In the following work, Fringe Projection is proposed for monitoring and to make quantitative evaluations of the corrosion process occurring on surfaces. The corrosion tests have been performed. Before starting the test, a fringe pattern is projected and the frequency spectrum of the recorded image is analyzed. Successively the samples are submerged in a solution of synthetic acid rain. At different time intervals the specimens were extracted from the solution and analyzed. Fringe pattern is projected again on the surface and new analysis of the frequency spectrum is run. Moreover surface analyses by microscopy and weight loss measurements were performed. At the end of the test, the specimens extracted from the solution were studied by surface analyses and weight loss measurement. The corrosion behavior of the bronze sample has been checked by fringe projection and optical microscopy observation during the exposure to synthetic rain. The average weight loss values were calculated using weight of sample measured before and after exposure to synthetic rain to quantify the corrosion rate of the sample in the corrosive solution.
C. Casavola, P. Pappalardi, G. Pappalettera, G. Renna
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