Development of a methodology for non-contacting strain measurements in fluid environments using computer vision

doi:10.1016/S0143-8166(99)00066-4

Optics and Lasers in Engineering

Volume 32, Issue 4, October 1999, Pages 367-377

https://doi.org/10.1016/S0143-8166(99)00066-4 Get rights and content

Abstract

Using first order, ray optics theory, a method has been developed and experimentally verified for measurement of surface strains on structures immersed in fluid environments. The methodology utilizes an imaging system that is contained within an air environment and separated from the fluid media by an optical quality, transparent window. Theoretical considerations demonstrate that non-contacting strain measurements in fluid systems are somewhat sensitive to the relative orientation of the transparent window, providing insight into the system setup required to make accurate strain measurements in fluid environments. Underwater experimental measurements using a laboratory-based system are in excellent agreement with theoretical predictions, demonstrating that the proposed methodology is both viable and capable of measuring strains accurately.

Introduction

The measurement of full-field, surface displacements and strains in structural components submerged in a fluid is important in several applications. For example, recent work [1], [2] measuring surface crack opening displacement (COD) in air environments has demonstrated clearly that crack closure [3], [4], [5] is a strong function of position behind the crack tip. Since the regime where no crack closure occurs is oftentimes considered the “intrinsic” material response, determining the presence of crack closure throughout the wake region as a function of environment and crack growth is of primary concern.

The crack closure effect during fatigue loading is complicated by the presence of saltwater or other fluid media, which have the potential to increase crack growth due to corrosive effects. Thus, the ability to determine “intrinsic” material response (i.e., the regime without crack closure) is essential for proper understanding of the physical processes resulting in crack growth. In these cases, careful measurements of COD on specimens that are submerged and dry while undergoing fatigue loading is essential to separate the various effects contributing to crack growth.

In this work, we will describe and verify a non-contacting, optical method for measuring the surface displacements and strains on a structural component submerged in a fluid that is undergoing mechanical loading. The measurement method utilizes well-known, two-dimensional computer vision principles to make all measurements. It is shown both theoretically and experimentally that the effects of the variations in indices of refraction for the optical media can be minimized so that accurate surface displacement and strain measurements for submerged specimens can be made.

In Section 2 and the Appendix, the theoretical development is presented and theoretical predictions of the apparent strain as a function of window orientation are presented graphically. In Section 3, the experimental setup is described. A comparison of theoretical predictions and experimental measurements is presented. Section 4 presents conclusions from this work.

Section snippets

Theoretical considerations

Fig. 1 shows a schematic of a laboratory-scale optical setup with a submerged specimen. As shown in Fig. 1, the imaging system (camera–lens–computer combination) views the submerged specimen via an optical quality window. Conceptually, this arrangement is identical to underwater cameras that are housed within an air environment, viewing objects through a transparent window.

For the purposes of analysis, Fig. 2 presents a simplified optical arrangement that has the key features required for

Experimental considerations

To assess the accuracy of the predictions based on Eq. (5), a series of laboratory experiments were performed using specimens immersed in distilled water. Fig. 3, Fig. 4 present two photographs of the experimental setup. As shown in Fig. 3, Fig. 4, the setup consisted of (a) CCD camera with photographic quality lens, (b) fiber-optic white light source, (c) Plexiglas immersion container, (d) circular, optical quality window, (d) adjustable specimen mount for immersion of specimen and (e)

Discussion of results

As shown in Table 1 and Fig. 6, the theoretical predictions are in excellent agreement with the average strain values from five separate sets of experimental measurements throughout the range of rotation angles.

Most significantly, the results in Fig. 6 indicate that for $θ⩽1°$ , the measured and predicted strains are ⩽76 microstrain throughout the field of view. Thus, careful control of the rotation angle between the specimen and the viewing system during underwater experiments is sufficient to

Acknowledgments

The authors wish to thank Dr. Jeffrey D. Helm and Correlated Solutions, Incorporated for their donation of the VIC-2D program. In addition, the authors wish to thank Mr. Hubert Schreier and Mr. Daniel Wilhelm for their technical advice and assistance in performing the experiments. Finally, the financial support of NASA through grant NCC5-174 is deeply appreciated.

References (8)

Sutton MA, Zhao W, McNeill SR, Helm JD, Riddell WT, Piascik RS. Local crack closure measurements: development and...
Riddell WT, Piascik RS, Sutton MA, Zhao W, McNeill SR, Helm JD. Determining fatigue crack opening loads from...
W. Elber
Fatigue crack closure under cyclic tension
Engng Fract Mech
(1970)
W. Elber
The significance of fatigue crack closure
ASTM STP
(1971)

There are more references available in the full text version of this article.

Cited by (19)

Mechanical characterization and identification of material parameters of porcine aortic valve leaflets
2020, Journal of the Mechanical Behavior of Biomedical Materials
Citation Excerpt :
As the samples are immersed during the tests, 3D-DIC measurements can be affected by optical refraction. Sutton and McFadden (1999) have shown that errors are introduced when the angle between the optical axis and the interface changes. But their conclusion was that careful control of the rotation angle between the specimen and the observation system is sufficient to minimize the effect on strain measurements during underwater experiments.
The ideal artificial heart valve does not exist yet. Understanding of mechanical and structural properties of natural tissues is necessary to improve the design of biomimetic aortic valve. Besides these properties are needed for the finite element modeling as input parameters. In this study we propose a new method combining biaxial tests and digital image correlation. These tests are carried out on porcine aortic valves. In this work, we use a modified version of the HGO (Holzapfel-Gasser-Ogden) model which is classically used for hyper-elastic and anisotropic soft tissues. This model can include fiber orientation. The identification of HGO model parameters can be determined using experimental data and two different protocols. One protocol is based on the identification of collagen fibers orientation as well as the mechanical parameters. The second one, is based on a complementary experiment to determine orientation (confocal laser scanning microscope). Both lead to determine different sets of material parameters. We show that the model is more likely to reproduce the actual mechanical behavior of the heart valves in the second case and that a minimum of three different loading conditions for the biaxial tensile tests is required to obtain a relevant set of parameters.
Crack propagation in a PVA dual-crosslink hydrogel: Crack tip fields measured using digital image correlation
2019, Mechanics of Materials
As a non-contact, full field deformation measurement technique, 2D digital image correlation (DIC) is a good candidate for the measurement of the crack tip fields of gels. We apply 2D DIC to study the crack propagation of a Poly(vinyl) alcohol (PVA) hydrogel specimen immersed in mineral oil. Practical issues such as the quality of the speckle pattern and the possible optical distortion are discussed. The accuracy of the setup in measuring the high-gradient crack tip strain field is carefully validated. A hydrogel specimen with an existing crack is loaded to fracture and the strain field near the tip of the propagating crack is measured using DIC. We find that the crack propagates under steady state conditions. The moving crack induces high strain rates at material points ahead of the crack.
Experimental set up for the mechanical characterization of plane ITM membrane at high temperature
2015, Journal of the European Ceramic Society
Citation Excerpt :
The others plights are linked to the optical acquisition for Digital Image Correlation. In addition to usual precaution for such measurement [20,27], the high temperatures add some specific optical artifact: convective air movement, heat haze and radiation fluctuation induce by thermal resistance of furnace. Two of these artifacts are herein be by-pass thanks to set-up design and the last one is corrected by embedded routines in the Digital Images Correlation.
Mixed ionic and electronic conductors (MIEC) are promising materials for ion transport membrane (ITM) based technologies. A reliable design of these membranes requires a better knowledge of their macroscopic mechanical properties. These properties are linked to the composition of membrane materials, but also in a large part to the manufacturing process. Then, the mechanical characterization set-up must allow using specimen very similar in term of manufacturing process as the final parts in. The use of the diametric compression test on thin plates is investigated here, combined with full-field measurements. Guideline for sample size, solution for classical artifact and a dedicated I-DIC algorithm are proposed. Then, Young modulus and tensile strength of seven MIEC compositions of the lanthanum-ferrite perovskite series (La_(1−x)Sr_xFe_(1−y)Ga_yO_3−δ, La_0.5Ba_0.5Fe_0.7Co_0.3O_3−δ and La_0.5Ca_0.5Fe_0.7Co_0.3O_3−δ) were determined at room temperature and at 900 °C.
The in vitro inflation response of mouse sclera
2010, Experimental Eye Research
Citation Excerpt :
DIC is an optical method that identifies a contrasting grayness pattern in a chosen subset of pixels in comparison to a reference image. The grayness pattern is matched between the deformed images and the reference image to calculate the displacement field (Sutton and McFadden, 2000). Multiple DIC tracking points were placed on the scleral edge of the reference image from points near the ONH to the holder.
The purpose of this research was to develop a reliable and repeatable inflation protocol to measure the scleral inflation response of mouse eyes to elevations in intraocular pressure (IOP), comparing the inflation response exhibited by the sclera of younger and older C57BL/6 mice. Whole, enucleated eyes from younger (2 month) and older (11 month) C57BL/6 mice were mounted by the cornea on a custom fixture and inflated according to a load-unload, ramp-hold pressurization regimen via a cannula connected to a saline-filled programmable syringe pump. First, the tissue was submitted to three load-unload cycles from 6 mmHg to 15 mmHg at a rate of 0.25 mmHg/s with ten minutes of recovery between cycles. Next the tissue was submitted to a series of ramp-hold tests to measure the creep behavior at different pressure levels. For each ramp-hold test, the tissue was loaded from 6 mmHg to the set pressure at a rate of 0.25 mmHg/s and held for 30 min, and then the specimens were unloaded to 6 mmHg for 10 min. This sequence was repeated for set pressures of: 10.5, 15, 22.5, 30, 37.5, and 45 mmHg. Scleral displacement was measured using digital image correlation (DIC), and fresh scleral thickness was measured optically for each specimen after testing. For comparison, scleral thickness was measured on untested fresh tissue and epoxy-fixed tissue from age-matched animals. Comparing the apex displacement of the different aged specimens, the sclera of older animals had a statistically significant stiffer response to pressurization than the sclera of younger animals. The stiffness of the pressure-displacement response of the apex measured in the small-strain (6–15 mmHg) and the large-strain (37.5–45 mmHg) regime, respectively, were 287 ± 100 mmHg/mm and 2381 ± 191 mmHg/mm for the older tissue and 193 ± 40 mmHg/mm and 1454 ± 93 mmHg/mm for the younger tissue (Student t-test, p < 0.05). The scleral thickness varied regionally, being thickest in the peripapillary region and thinnest at the equator. Fresh scleral thickness did not differ significantly by age in this group of animals. This study presents a reliable inflation test protocol to measure the mechanical properties of mouse sclera. The inflation methodology was sensitive enough to measure scleral response to changes in IOP elevations between younger and older C57BL/6 mice. Further, the specimen-specific scleral displacement profile and thickness measurements will enable future development of specimen-specific finite element models to analyze the inflation data for material properties.
Fixed membrane wings for micro air vehicles: Experimental characterization, numerical modeling, and tailoring
2008, Progress in Aerospace Sciences
Fixed wing micro air vehicles (wingspan between 10 and 15 cm) are aerodynamically challenging due to the low Reynolds number regime (10⁴–10⁵) they operate in. The low aspect ratio wings (typically used to maximize area under a size constraint) promote strong tip vortices, and are susceptible to rolling instabilities. Wind gusts can be of the same order of magnitude as the flight speed (10–15 m/s). Standard control surfaces on an empennage must be eliminated for size considerations and drag reduction, and the range of stable center of gravity locations is only a few millimeters long. Membrane aeroelasticity has been identified as a tenable method to alleviate these issues: flexible wing structures with geometric twist (adaptive washout for gust rejection, delayed stall) and aerodynamic twist (adaptive inflation for high lift, larger stability margins) are both considered here. Recent investigations in static aeroelastic characterization, including flight loads, wing deformation, flow structures, aeroelastic-tailoring studies through laminate orientation, as well as unconventional techniques based on membrane pre-tension, are reviewed. Multi-objective optimization aimed at improving lift, drag, and pitching moment considerations is also discussed.
Calibration-free single camera stereo-digital image correlation for small-scale underwater deformation measurement
2019, Optics Express

View all citing articles on Scopus

View full text

Development of a methodology for non-contacting strain measurements in fluid environments using computer vision

Abstract

Introduction

Section snippets

Theoretical considerations

Experimental considerations

Discussion of results

Acknowledgments

Fatigue crack closure under cyclic tension

Engng Fract Mech

The significance of fatigue crack closure

ASTM STP