May/June 2003 EXPERIMENTAL TECHNIQUES 47

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FEATURE Technology Application Seriesby T. Schmidt, J. Tyson, and K. Galanulis

FULL-FIELD DYNAMIC DISPLACEMENT AND STRAINMEASUREMENT USING ADVANCED 3D IMAGECORRELATION PHOTOGRAMMETRY: PART 1

Full-field optical techniques are increasingly appreci-ated as deformation, stress and strain measuringtools. This article discusses an advanced deformationand strain analysis method based on 3D image cor-

relation photogrammetry that is substantially more robustand has greater dynamic range than other full-field imagingtechnologies such as ESPI and moire.

Underlying principles are reviewed, and several applicationscombining image correlation photogrammetry with strobo-scopic illumination and/or high-speed video cameras arepresented. Operational strains in ionic polymeric and dielec-tric elastomer artificial muscles are determined. Part 2 ofthis series will describe the use of short-duration white lightpulses to study automobile tires on road wheels at speeds upto 150 miles per hour. Initial work with a pulsed laser tostudy a flywheel in a spin pit at up to 35,000 rpm is alsodescribed. And a notched rubber dogbone sample is pulledto failure at 125% strain in 38 milliseconds, with hundredsof full-field strain maps captured.

Benefits of Full-Field Measurements • Visualize Strain Gradients and Hot Spots

• Verify and Iterate Finite Element Models

• Test Non-Homogeneous and Anisotropic Materials

• Reduce the Number of Required Prototypes ($$$)

• Quicker Time to Market

• Improved Product Quality at Lower Cost

• Make “Impossible” Measurements

Fig. 1: The benefits of full-field systems

INTRODUCTIONFull-field optical measuringtechniques are increasinglybeing used in industry asdevelopment and designtools and for production in-spection. Techniques suchas moire and holographicinterferometry, for example,have been thoroughly de-scribed from the theoreticalperspective1 as well as theapplication engineeringviewpoint.2,3 A variety ofcommercially available optical inspection systems combineexponentially increasing computer power, high-resolutiondigital cameras and compact mechanical design to provideturnkey operation for industrial applications.

At the risk of preaching to the converted, it is worthwhile tosummarize the advantages of full-field measurements. Whatjustifies the capital investment and other costs of change?

ET occasionally features short Industry/Application articles under the title, ‘‘Tech-nology Applications.’’ The short articles demonstrate real world application of bothmeasurement techniques and apparatus to be used primarily in industry and, insome cases, the classroom. This month we are featuring two Technology Applica-tions articles: 1) Full-field Dynamic Displacement and Strain Measurement UsingAdvanced 3D Photogrammetry; and 2) Residual Stress Measurement Using theHole Drilling Method and Laser Speckle Interferometry. Please contact Series Ed-itor, Dr. Kristin B. Zimmerman, at kristin.b.zimmerman@gm.com, if you are in-terested in submitting a Technology Applications article.

T. Schmidt (SEM Member) and John Tyson are affiliated with Trilion QualitySystems, W. Conshohocken, PA. K. Galanulis is affiliated with GOM mbH, Ger-many.

As shown in Figure 1, there are many good reasons to seri-ously consider. A single-point gauge cannot show strain gra-dients, and could miss hot spots. This is particularly the casewith non-homogeneous and anisotropic materials. Final re-sults of optical measurements are compatible with finite el-ement analysis software, and facilitate verification and it-eration of models. Reduction of the number of requiredprototypes, quicker time to market, and improved productquality at lower cost provide a compelling return on invest-ment justification. Perhaps most importantly, measurementsthat would otherwise be impossible become feasible, openingnew avenues of investigation.

This paper describes the use of the 3D image correlationtechnique in operational situations that take full advantageof its robust measuring principles. The power of image cor-relation provides superior dynamic range and ease of use formany test situations. Combined with pulsed illumination orhigh-speed video, it provides unparalleled capability.

PRINCIPLES OFOPERATIONThe object under test isviewed by a pair of high res-olution, digital CCD camerasfor the 3D deformation mea-surements. 3D image corre-lation technology is a combi-nation of two-camera imagecorrelation and photogram-metry. A random or regularpattern with good contrast isapplied to the surface of thetest object, which deformsalong with the object. The

deformation of this structure under different load conditionsis recorded by the CCD cameras and evaluated. The initialimage processing defines unique correlation areas known asmacro-image facets, typically 5–20 pixels square, across theentire imaging area. The center of each facet is a measure-ment point that can be thought of as an extensometer andstrain rosette. These facets are tracked in each successiveimage with sub-pixel accuracy. Then, using photogrammetricprinciples,4 the 3D coordinates of the entire surface of thespecimen are precisely calculated. The results are the 3Dshape of the component, the 3D displacements, and theplane strain tensor. An example of a commercial image cor-relation system configured for 3D measurement during atension test is shown in Fig. 3 below. The camera bar ismounted to a tripod that can simply be placed in front of thetest sample at the correct working distance. Because rigidbody motion has no effect on the measurements, this type ofsetup is perfectly adequate for use with servo-hydraulic ma-chines as well as electric screw models.

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DYNAMIC DISPLACEMENTAND STRAIN MEASUREMENT

3D ESPI• Interferometry

• Laser Speckle Pattern

• Highly Sensitive to Ambient Vibrations

• Sequential Measurements X Y Z

• Measuring Sensitivity Is Fixed;0.03 to 0.35 Microns

• Low Dynamic Range: 1–10 Microns Measuring Step

• Real-Time Display (1D ESPI)

3D Image Correlation• Photogrammetry

• Applied Stochastic Pattern

• Insensitive to Ambient Vibrations and Rigid Body Motion

• Simultaneous 3D Measure

• Measuring Sensitivity is 1/30,000 Field of View:0.3 to 30 Microns

• High Dynamic Range: Microns-Millimeters/Measuring Step

• Post-Processing

Comparison of 3D ESPI and 3D Image Correlation

Fig. 2: Comparison of 3D ESPI and 3D image correlation characteristics

Fig. 3: 3D image correlation camera bar on a tripod, readyfor measuring at a load frame. Since rigid body motion is notan issue, the same setup can be used with servo-hydraulic testmachines

Table 1—3D image correlation system sensitivity forvarious fields of view

FIELD OF VIEW SENSITIVITY

10 � 8 mm 0.3 microns15 � 12 mm 0.5 microns25 � 20 mm 0.8 microns35 � 28 mm 1.2 microns50 � 40 mm 1.7 microns

100 � 80 mm 3.3 microns200 � 160 mm 6.7 microns800 � 600 mm 27 microns1.2 � 0.9 m 40 microns

COMPARISON OF 3D IMAGE CORRELATION ANDESPI

In order to understand the applicability of image correlation,it is useful to compare it to the more well-known techniqueof electronic speckle pattern interferometry (ESPI). Fig. 2summarizes the key attributes of each technique. In ESPI,the sensor head projects a coherent speckle pattern onto thetest part, and then tracks lateral movements of the speckles.The resultant fringes are displacement isolines.

This gives ESPI high sensitivity, but also leaves it vulnerableto ambient vibrations. Measurement ceases once a specklehas shifted its own diameter, so dynamic range is limited.The X, Y and Z displacements are determined sequentiallyby varying the illumination geometry, thus limiting dynamicmeasurement capability.

The key to image correlation photogrammetry is that ittracks changes in an applied micro-pattern, rather than aprojected pattern, with sub-pixel accuracy, and thus does notsuffer from the speckle decorrelation limitations of ESPI.Even a pulsed ESPI system is limited to rotational speedson the order of 10 s of rpm. Theoretically, an optical dero-tator enables higher-speeds, but is cumbersome, and veryexpensive, to implement in practice.

A 3D image correlation system, based on photogrammetry,uses ordinary white light, rather than coherent laser light.The system tracks the stochastic pattern applied to the mea-surement surface with sub-pixel accuracy. This means thatas long as the object remains within the field of view of thecameras, all of the local deformations can be tracked. Thus,large deformations can be analyzed in a single measure-ment. As previously mentioned, massive rigid body motionshave no influence, and can also be calculated from the orig-inal pixel registration. In fact, measurements can be contin-ued after a part has been removed, processed and replacedwithin the camera viewing zone.

Sensitivity with 3D image correlation is 1 /30,000 the fieldof view. For example, with a 30 mm field of view, sensitivityis 1 micron, and with a 300 mm field of view, it is 10 microns,as shown in Table 1. The system intrinsically measures 3D

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DYNAMIC DISPLACEMENTAND STRAIN MEASUREMENT

Fig. 4: Captured image of prepared ionic polymeric musclesample during a flexure cycle. The larger section flexedoutside the field of view in this setup, so only the smaller‘‘finger’’ was analyzed

Fig. 5: Strains were almost entirely compressive at the 19thcaptured position, except near the tip, which was out-of-phase

Fig. 6: A multi-stage diagram can be used to study the strainprogression for any subset of the captured deformation states.There are clearly repeating minima and maxima locations

shape, and therefore 3D deformations are measured simul-taneously, rather than sequentially. This significantly im-proves robustness and accuracy. Furthermore, as will beshown, the use of pulsed high intensity illumination facili-tates dynamic deformation analysis, with results far supe-rior to pulsed ESPI. Complete 3D deformation measure-ments occur at up to 20 Hz or 485 Hz with standardcameras, while higher-speed cameras are also available.

DYNAMIC DEFORMATION MEASUREMENTSThe robustness of 3D image correlation photogrammetry be-comes clearest when dynamic deformations are considered.Furthermore, by adding a precision triggering module andstroboscopic illumination, the system can capture high-speedevents, and can also be applied to impact studies, non-stationary responses, and rotating objects such as tires andjet engine components. The following examples demonstratethese capabilities.

IONIC POLYMERIC ARTIFICIAL MUSCLE STRAINANALYSISIonic polymeric materials, which are non-homogeneous andexhibit large dynamic deformations, are becoming increas-ingly important as soft robotic actuators, artificial musclesand dynamic sensors in the micro-to-macro size range. Thereis a need to further understand the local strain behavior ofsuch materials, which depends on granular physics and in-ternal water transport, among other effects.5 This is a per-fect application for 3D image correlation, which has beenpreviously demonstrated on several other challenging high-strain biomechanics applications, including aorta, tendonand ligament testing.6

For this test, the standard measurement cameras were setto run at 20 frames per second, and a sequence of 28 imageswas recorded for a single flexure cycle. Because of internalrecovery mechanisms, these materials function best slowly;the frequency was 0.5 Hz. Intense illumination from a hal-ogen lamp enabled a shutter time of 1 millisecond, whichprevented blurring during each exposure. For related cyclicfatigue tests, the cameras can be precisely triggered syn-chronous with displacement peaks, to ensure capture ofmaximum strains.

Figure 4 shows one of the captured images. The larger sec-tion tended to collapse beyond the field of view of the cam-eras, so only the smaller ‘‘finger’’ was analyzed. For subse-quent work, the fixturing was controlled more precisely toenable strain measurement on complete samples.

The next figures are color graphics representing the full-fieldstrain values, together with quantitative results extractedfrom a section line. In Fig. 5, strains were compressive ex-cept at the tip, which was moving out of phase. Fig. 6 ex-tends the analysis further using a multi-stage diagram, inwhich the profile line strains for numerous flexure stages areoverlaid. It can be seen that there are clearly repeating min-ima and maxima locations within a single flexure cycle. Thefinal results included a full animation of strains displayedon the 3D deforming shape.

DIELECTRIC ELASTOMER ARTIFICIAL MUSCLEACTUATORSDielectric elastomers are currently being developed due totheir significant potential advantages over alternatives suchas shape memory alloys, piezoelectrics, Terfenol, etc.7 Im-portant criteria are high elastic strain, low power dissipa-

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DYNAMIC DISPLACEMENTAND STRAIN MEASUREMENT

Fig. 7: Full-field graphic showing range of tensile andcompressive strains in the x direction

Fig. 8: Section line plot shows strain profile across thediameter of the actuator and substrate

Fig. 9: Overlay of strain plot on corresponding live imageprovides further insight. Note that the left edge of theactuator was inactive, possibly due to misaligned electrodesduring fabrication.

tion, wide operating temperature range, and rapid actuationwith precise control. 3D image correlation photogrammetryis an excellent tool for evaluating the performance of theseactuators.

Figure 7 is a strain map from a circular actuator consistingof a pre-tensioned acrylic substrate with compliant carbon-based electrodes on each side. Under applied high voltage,the circle rapidly grows in diameter. The test method wassimilar to that used for the ionic polymer muscles. The sec-tion line result in Fig. 8 indicates the strain profile acrossthe diameter of the actuator and surrounding substrate. Atransparent overlay of the strain map onto the correspond-ing live camera image, as shown in Fig. 9, provides furtherinsight. It becomes evident that the left edge of the actuatorwas inactive, possibly due to misaligned electrodes.

3D image correlation enables other types of measurementsthat support actuator development. As an example, it is de-sirable to determine the hysteresis of linear actuators thatmay have 10 mm of travel. Here wide dynamic range is cru-cial. It is no problem to track and quantify rigid body mo-tions such as the end of an actuator. The camera shuttersync output can be combined with excitation voltages to de-termine hysteresis over numerous complete cycles, at veryhigh data acquisition rates. With a high-speed camera setupsuch as the one described in the next article, high frequencydevices such as loudspeaker membranes can be fully char-acterized.

ACKNOWLEDGMENTSWe are grateful to Dr. Mohsen Shahinpoor of the Universityof New Mexico for his application with ionic polymeric mus-cle.

Thanks to Roy Kornbluh and his colleagues at SRI Inter-national for allowing use of the electroactive polymer results.

All of the results in this paper were obtained using the GOMARAMISTM 3D image correlation photogrammetry system.

Further details, including animations (.avi files) of dynamicresults, are available at www.trilion.com.

REFERENCES1. Cloud, G., Optical Methods of Engineering Analysis, Cam-

bridge University Press, 1995.2. Williams, D.C, Ed., Optical Methods in Engineering Metrology,

Chapman and Hall, 1993.3. Tyson, J., ‘‘Noncontact Full-Field Strain Measurement With

3D ESPI,’’ Sensors, 17(5), pp 62–70, May 2000.4. Mikhail, E., Bethel, J., and McGlone, J., Introduction to Mod-

ern Photogrammetry, John Wiley and Sons, 2001.5. Shahinpoor, M. and Kim, K.J., ‘‘Ionic Polymer-Metal Compos-

ites: I.’’ Fundamentals, SMART Materials and Structures, Volume10, pp 819–833, 2001.

6. Tyson, Schmidt and Galanulis, ‘‘Biomechanics Deformationand Strain Measurements With 3D Image Correlation Photogram-metry,’’ EXPERIMENTAL TECHNIQUES, 26(5), pp 39–42, Sept /Oct 2002.

7. Kornbluh, R. Pelrine, R., Pei, Q., and Shastri, V. Applicationof Dielectric EAP Actuators, Electroactive Polymer (EAP) Actuatorsas Artificial Muscles: Reality, Potential and Challenges, ed. Y. Bar-Cohen, pp 457–495, SPIE Press, Bellingham, Washington, 2001. �