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Principle of Digital Image Correlation

Digital Image Correlation (often referred to as “DIC”) is an easy to use proven optical method to measure deformation on an object surface. The method tracks the gray value pattern in small neighborhoods called subsets (indicated in red in the figure below) during deformation. Digital Image Correlation has been proven over and over to be accurate when compared to valid FEA models. The commercially available VIC-2D and VIC-3D systems from Correlated Solutions both utilize this advanced optical measurement technology.

 

Digital Image Correlation Overview

Below are the reasons why its versatility, robustness, and ease of use make it the only choice when it comes to digital image correlation.

DIC - Principle of Digital Image Correlation

Two-dimensional Example

In the two pictures below you can see a speckle pattern on an aluminum sample with two offset semi-circular cut-outs. The two pictures were taken from an animation with the left image taken from the beginning and the right picture taken from the end of the animation. Since the deformation is predominantly in-plane, a single camera can be used to measure the deformation.

VIC2D

Small Deformation

VIC2Da

Large Deformation

The pictures below show the horizontal strain measured by two-dimensional image correlation for the pictures shown above.

VIC2D-2 VIC2D-2a
VIC2D-2b

Three-dimensional Example

These two speckle images below were taken simultaneously with the left and right camera of a stereo-system. The sample itself is a piece of glass with the company logo sticker adhered to the surface.  The speckle pattern was applied using standard off-the-shelf flat white and black spray paint.  Can you make out the shape?

VIC3Dleft

Left View
 VIC3Dright

Right View

In the plot below the shape of the logo sticker measured with the VIC-3D System is shown. The thickness of the logo sticker is approximately 0.003″ or 0.070mm.

VIC3D-2
For more infomation have a look at our DIC products or contact us via info@correlatedsolutions.eu or +49 561 – 739798-0

Combination of Digital Image Correlation and Thermography

Thermography is based on the principle that induced heat in mechanical components causes a different temperature field in the area of defects or inhomogeneities. The digital image correlation is a powerful system for measuring and visualizing strain, deformation and 3D surface shape.

The VIC-3D IR System features an infrared (IR) camera that is integrated with the DIC cameras enabling temperature data to be accurately acquired and analyzed with the full-field strain and deformation data. The system works by first calibrating the intrinsic optical parameters of the IR camera and then calibrating the position of the IR camera relative to the stereo DIC system. This triangulation allows VIC-3D to place the thermal and strain (or deformation) as well as 3D surface data into a common coordinate system.

The calibration procedure has been streamlined by integrating the IR camera into our VIC-Snap image acquisition software, which allows the user to capture images from the IR and DIC cameras simultaneously. The result is an easy to use turn-key thermal imaging system that utilizes digital image correlation to accurately measure temperature and strain concurrently without any contact with the sample. Thermal and strain data can be viewed, analyzed, and extracted over the entire field or at precise locations.

The system is sold as a turn-key solution which includes all software, hardware, onsite installation, and one year of unlimited technical support and software upgrades giving you piece of  mind that your system functions as intended, so you can start acquiring data immediately. This unique thermal integration capability may also be added to any existing VIC-3D system for increased functionality.

System Configuration and Features

Both the IR camera for thermography and the two CCD cameras for 3D image correlation are mounted on a standard stereo bar.

  • Temperatures up to 2,000C
  • Synchronized IR and DIC images
  • User-friendly setup and calibration
  • Uniquely designed IR calibration targets
  • Analog data synchronization
  • Extract points, regions, or node locations for FEA validation
  • Accurately measure deformation and thermal data concurrently
  • Remotely view and acquire images using the Vic-Snap remote
  • Measure 3D full-field displacements and strains
  • All the features for the VIC-3D system included

Advantages of this multi-function device are all applications with component deformations caused by thermal energy like current flow. This product allows a simultaneously determination of the heat flows as well as strain and deformation.

Example 1: Photovoltaic Module

Defective photovoltaic modules have been measured during operation over several minutes.  A preparation with speckle pattern is necessary for digital image correlation (here the backside of the panel). The marked areas are heated up due to electric/mechanical errors during operation.

Principal strain (epsilon 1) over time (index) of the points C0, C1, C2 shown in the image below. Following three dimensional presentation of the temperature values.

Three dimensional graph of the conture, superposed with the measured temperature (colour scale).

Three dimensional graph fo the conture, superposed with the strain measurement (colour scale).

Example 2: Analysis of simultaneous one-sided heating and compressive loading on an e-glass/vinyl ester/ balsa wood sandwich composite sample

Understanding thermo-mechanical behavior of material can be a vital component when designing vehicles  and structures that may become exposed to high temperatures. Virginia Tech’s Extreme Environments, Robotics, and Materials (ExtReMe) Laboratory focuses on the impact of extreme environments on materials. This includes research that is focused on understanding the thermo-mechanical behavior of materials both during and following fires. Experimental investigations are performed to understand the evolution of the material due to elevated temperature.

The senior research associates in the lab used the VIC-3D IR system to find the effects of a simultaneous one-sided heating and compressive loading test on an e-glass/vinyl ester/balsa wood sandwich composite sample. As one researcher stated, “The VIC-3D IR system identied several transient events during the compression tests which would not have otherwise been fully understood using either DIC or IRT independently. Through this testing, several features of sandwich composite thermomechanical behavior  were elucidated which would not have been possible with traditional point measurements (e.g. strain gages, defectometers, or thermocouples).”

Image above: Analysis of simultaneous one-sided heating and compressive loading on an e-glass/vinyl ester/ balsa wood sandwich composite sample, Identifying several transient events, which  would  not  have been  fully  understood  (stated by the enginners of Virgina Tech´s ExtReMe Laboratory) using  either  DIC  or  IRT independently or only applying traditional  point  measurements  (e.g.  strain gages, deectometers, or thermocouples).  Source:  Virgina Tech´s ExtReMe Laboratory

Vic-3D Stereo Microscope

The Vic-3D Micro system is a new addition to the Vic-3D product line of measurement solutions. Vic-3D Micro enables accurate displacement and strain measurements under high magnification.

BackgroundStereomikroskop2

Three-dimensional digital image correlation (DIC) has found widespread popularity for strain measurements due to its excellent accuracy, robustness and ease of use. However, 3D measurements have been difficult to obtain on specimens where high magnification is required. This is mainly due to the lack of optics with sufficient depth-of-field to acquire two high magnification images from different viewing angles.

Stereo microscopes overcome these depth-of-field limitations. However, the internal construction of stereo microscopes prevents proper correction of image distortions using traditional models, such as Seidel lens distortions. These uncorrected images will result in severely biased shape and strain measurements. In fact, it is not uncommon to observe bias levels of several thousand microstrain.

To overcome this problem, Correlated Solutions, Inc., has developed and patented an easy-to-use calibration method that does not suffer from the problems associated with traditional parametric distortion models. The calibration method computes the non-parametric distortion fields of the stereo microscope and has been shown to completely eliminate shape and strain bias from the measurements.

System Features

  • Field of view (zoom range): 0.8mm-7mm
  • Full-field measurements of 3D coordinates, displacements, velocities, and complete strain tensors
  • Automatic calibration
  • Image pairs can be automatically overlapped with a simple adjustment
  • Powerful tools for visualizing data
    • Contour displays which can be overlaid onto images of the test specimen
    • Data extraction from 3D plots based on user defined lines and circles
    • Post-processing tools for statistical analysis, stress-strain curves, and more
  • Convenient exporting of data with the FLEXPort data tool
    • Data can be exported in Tecplot/plain ASCII, Matlab, and STL formats
    • Node data can be easily extracted for FEA validation
  • One year of technical support and software upgrade
  • One-year replacement warranty for defects in materials and/or workmanship on all parts

Application examples about Vic-3D Micro:

Servered ceramic capacitor chip under bending load

Combination Stereomicroscope and Vic-3D digital image correlation

Combination of Fulcrum and FFT module

The combination of the new Vic-3D FFT-Module and the known synchronisation / trigger device with Fulcrum module of isi-sys GmbH / Correlated Solutions INC for Vibrocorrelation permits full modal and vibration analysis using conventional low speed cameras. It can be applied on objects with high frequency excitation, which can replace High-Speed (HS) cameras, when conventional shakers are used for excitation.

 

Excitation signal

A periodic vibration signal with a wide frequency spectrum (e. g. chirp, fast sweep) is applies to a structure via an electrodynamic, hydraulic or piezo shaker. The cameras of a stereoscopic system are set to a short exposure duration (we use 200μs here) and they are triggered though the Fulcrum module of Vic-Snap.

The excitation signal is shown in the images below. A pulse is generated by the function generator for each cycle of the excitation (chirp) signal and connected to the synchronisation device below.

Anregungssignal

Image 1 – Frequency spectrum

Anregungssignal 2

Image 2 – Excitation signal

 

SynchronisationTrigger device

 

The cameras are triggered through the standard synchronisation/trigger device  DAQ-STD-8D controlled by the Fulcrum Module for Vic-Snap/ Vic-3D.

 

 

FFT Module Evaluation Examples

The following results show the workspace of the FFT Module in Vic-3D with amplitude (left) and phase (right). Shifting the frequency value in the graphs below (amplitude or phase vs frequency) permits to select the corresponding mode shape for any analysed frequency. The amplitudes (left) are displayed here as 3D plot, other options are e.g. generation of animated videos of the mode shape vibration.

The results on a sqare plate 140 x 140mm of different resonance frequences are shown in the images below.

263Hz

Image 3 – Resonance at 263 Hz

 

707Hz

Image 4 – Resonance at 707 Hz

 

875Hz

Image 5 – Resonance at 875 Hz

 

 

Application example of the FFT module:

Operation mode analysis on a mobile phone during vibration alert

 

 

Different Loading Methods

Flexible and Efficient: One sensor system for dynamic, thermal and vacuum loading.

 

Dynamic Loading

For the dynamic loading the piezoshaker excitation systems are designed by isi-sys. It consists of a Piezoshaker, amplifier and function generator. For a detailled description please refer to the report „Products / Piezoshaker Systems „

 

Piezoshaker frontneu

 

The Piezoshaker can be attached by vacuum to the object surface.
The HVDA-0-180 amplifier is especially design for isi-sys Piezoshakers and mobile application, where high power at low weight and compact size is a big advantage for mobility and flexible handling. High frequencies up to 100kHz and more as well as large forces and acceleration have been generated in combination with the different isi-sys Piezoshaker modules.

 

 

 

Thermal LoadingThermal loading

isi-sys offers timed thermal loading modules to to heat-up the objects of measurement.

A automated precise control of heat loading and a corresponding timed shearographic measurement is required to achieve a reproducible test procedure for non destructive testing experiments and automated systems.

Any conventional halogen flood lights with 220V can be connected. isi-sys recommends some flood lights used for professional photography. Thus they can be mounted on standard light stand of e. g. Manfrotto etc. and easily directed to the area of interest independent of the sensor head. Also this lights offer some choices of reflector combinations.

The start and duration of the heat loading and the measurements can be set per computer controlled timer, which permits reproducible sequences of the test process.

 

 

 

 

 

 

Vakuum Loading

The vacuum is another loading method for non destructive testing. The modular system concept allows to combine the SE2 sensor with different vacuum loading devices, which reduces system expense and keeps the equipment smart and mobile compared to other solutions .

The vacuum methods can be categorized depending on the loading principle into vacuum chambers/vacuum cabins and vacuum hood/vacuum windows.

 

Vacuum hood and Vacuum window

Vakuum hoodThe vacuum hood is a modular extension accessories for the SE2 sensor. The sensor will be mounted inside of the hood and therfore the measurement area is covered from sun light influences.

The system has small dimensions and is for this reason very mobile and flexible, additionally the low required vacuum pump power is an advantage.

 

 

 

 

 

Vakuum window

If you use the vacuum window, the sensor is outside the vacuum volume. The system is monitoring the object through the window. The bending forces are only across the field of view.

Due to the small volume of the vacuum window hood it can be used in combination with the small isi-vacuum unit (also used for isi-piezoshakers with vacuum cup adapter). The unit includes two separate vacuum pumps, where the second can be used for the vacuum suction cup tripod.

 

 

Vacuum chamber and Vacuum cabin

Generally vacuum chambers or cabins have the advantage against vacuum hoods and windows, that the forces appear uniform from all sides on the object. This avoids superposed fringes by global deformation such as by hood, where a bending force on the object surface is superposed, as usually the pressure forces are applied only in one direction on one side of the surface, where the hood is applied.

In both systems the sensor is inside the vacuum volume, which causes pressure loading also on the sensor. The camber and the cabin are bigger than the object and therefore depending on object size the mobility is restricted.

 

isi-sys offers the newly designed testing vacuum cabin based on aluminium-foam sandwich elements, which is recommended e. g. for frequently manual spot tests or laboratory use.  Samples with a size up to 450 x 750 mm can be measured in this cabin.

Furthermore isi-sys GmbH cooperates with different manufacturer for large vacuum chambers.

 

 

 

 

 

SE-Sensors Types

No safety requirements: Laser class 1 with power from 0,1W up to 12W per module

High resolution and sensitivity: 5 MPixel sensor, 10nm light phase reconstruction

High quality and variety of lenses: Designed for professional Nikon-F-mount lenses

 

SE1 Sensor

SE1 shearography

 

The SE1 sensor modular system

for automated applications such

as tire testing.

 

 

 

 

 

 

SE2 Sensor

isi-sys_se2_2arrayneu2

 

SE2 Sensor with 1W light power by

10 adjustable laser diodes for flexible

laboratory use. The SE2 is the most

flexible and recently new upgraded

design in the SE-sensor family.

 

 

 

 

SE2 neu array

 

 

SE2 Sensor with 12 W laser diode module

for outdoor and large field of view

applications.

 

 

 

 

 

 

SE3 Sensor

SE3

 

SE3 Sensor

 

 

 

Setup. Control. Acquire.

x

VICSnap RemoteThe new Vic-Snap Remote app will change

the way you set up your image correlation tests.

 

Conveniently view live images on your mobile device or tablet while setting up the cameras. Zoom in with a pinch to adjust your focus. The crosshairs help align the cameras just perfectly. Control exposure to get your images well lit (the histogram helps, too). Acquire calibration images with a tap.  Compatible with most iOS and Android devices. Now available on the App Store and the Google Play Store.

 

 

AppStore1

AppStore2

___________________________________________________________________________________________

 

VICSnap Remote2

Features

  • Remotely view and acquire images from Vic-Snap image acquisition systems
  • Detect and control multiple Vic-Snap systems with one device
  • Pinch-to-zoom live images
  • Double-tap images for full-screen viewing
  • Adjust the cameras’ exposure time
  • Acquire calibration and test images with automatic file naming
  • Toggle cross-hairs for alignment
  • View gray scale histogram for adjusting exposure
  • Over and under-exposure indicated using red and blue

 

 

 

 

You will find further information about VIC Snap Remote on the following web page:
http://www.correlatedsolutions.com/vic-snap-remote/

Vic-3D High-Speed Vibration Analysis System

Vibration1               Vibration2

 

The Vic-3D™ Vibration Analysis System by Correlated Solutions, Inc. is a new addition to the Vic-3D product line of measurement solutions. Vic-3D Vibration enables full-field 3D viewing, measurement, and analysis of transient events. Full-field operational deflection shapes in the frequency domain can easily be seen and compared with levels of accuracy in the nanometer range.

The image above on the left is a mounted model jet plane which has undergone a transient vibration event.  To the right, the 3D vibration data obtained from the event is shown as a 2D contour overlay on this image.  The data overlay enables the user to see exactly how much deformation is occurring and where it is occurring.  This information is available at each frequency where deflection shapes occur.

The data below displays one operational deflection shape obtained with the VIC-3D™ Vibration Analysis System. Three-dimensional displacements, strains, velocities, and accelerations can all be computed in the analysis software.  The result is not only visual, but also enables the user to export quantitative data (ASCII, MatLab, CSV, etc.) from the sample’s behavior for FEA analysis and validation.  A 3D animation of the model jet’s operational deflection shape at 441 Hz is shown below, with only 12 microns of displacement amplitude.

 

Film Vibration3D_W_431Hz

 

What is transient vibration analysis? 

Transient vibration analysis is the process of monitoring, measuring, and analyzing the condition of samples during a transient event. Material properties can also be observed through the analysis of operational deflection shapes and mode shapes created by the event. These full-field 3D shapes can provide useful information which can be used to improve and correct a sample’s balance, displacement, flexure, rigidity, and overall product performance.  Measuring operational deflection shapes can help answer the question: “How much is this structure actually moving at a particular frequency?”

 

Why is it important to you?

Operational deflection shapes created from transient tests show how a sample can have a nonuniform thicknesses, surface irregularities, weak points, cracks and/or other imperfections & flaws.  This information can be useful at any point in the process of achieving your overall goal, from writing a project proposal to testing a functional or failing part in the field: The Vic-3D™ Vibration Analysis System

  • Has a justifiable price point when writing proposals
  • Can be used:
    • To create and validate FE models when designing parts & equipment
    • During the research and design process
    • In the product testing phase
    • To provide manufacturing quality assurance
    • To certify and assure equipment is operating as expected and required
    • To measure and analyze parts post-installation
    • To ensure product quality and performance over time and in the field
    • When reassessing product functionality over time

 

Examples of transient events which can be measured are the following:

▪ Door Slams ▪ Modal Hammer Strikes
▪ Engine Start-ups ▪ Drop Tests
▪ Explosive Testing ▪ Ballistic Testing

 

System Features 

  • View, compare, animate, graph, extract, and export data for easy FEA comparison
  • Measure 3D full-field, high frequency mode shapes with nanometer resolution
  • Measure extremely low amplitudes with extremely high accelerations
  • Full-field strain, deformation, and shape variables are still available
  • Easy procedures, accurate results
  • User friendly interface
  • Only a fraction of the price of a laser vibrometer system

 

Advantages over other measurement techniques

While traditional vibration measurement techniques can be useful, they also have many drawbacks.  For example, accelerometers can become unglued during testing, can mass-load a sample, and can only provide point to point measurements, often only in a single plane. Pre-testing and testing can also take days or even weeks to perform on large structures.  With the Vic-3D™ Vibration Analysis system, there are no adhesives, wires, signal analyzers, power amplifiers, or load cells necessary for detailed vibration results.  Obtaining thousands of data points for a tiny, complex structure or a large one is as easy as changing a pair of lenses. Similar to digital image correlation, laser vibrometers can provide a non-contact measurement solution, but similar to accelerometers, they are also only able provide point to point measurements.  A 3D measurement can be achieved with multiple scanning vibrometers, but these are usually mounted on large robot arms which can take up valuable laboratory space and cannot be moved easily once installed.  Additionally, these systems are unreasonably expensive for many applications.  The Vic-3D™ Vibration Analysis system can be taken into the field with any compatible laptop and together with the Vic-3D™ Workstation, the system can become mobile and secure inside your facility.  Vic-3D Vibration Analysis is only a fraction of the price of a 3D scanning laser vibrometer system, and since the module can be added onto any existing 3D system, it’s even more affordable.

Vic-Volume – Software

VIC Volume1The new Vic-Volume software by Correlated Solutions is an exciting addition to the Vic image correlation product line. Vic-Volume utilizes volumetric images from X-Rays or CT-Scanners to measure internal deformation of a specimen under an applied load. Vic-Volume analyzes the acquired images to create three-dimensional volumetric displacement and strain data of the specimen’s internal behavior. The resulting data is a full-field contour plot of the deformation data that can be viewed, animated, and extracted for FEA validation. The image to the left displays the internal strain (Ezz) of a rubber puck undergoing compression.
 X

 

 X

Technology Background

VIC Volume2

Digital Image Correlation (DIC) has found widespread popularity among scientists, researchers and engineers across the globe due to its accuracy, robustness, versatility, flexibility and overall ease of use. DIC is commonly used to measure 2D and 3D surface deformation and strain utilizing white light machine vision digital cameras. Correlated Solutions has offered turn-key  2D and 3D DIC systems since 1998, and continues to develop and add new advanced DIC products to our growing product line. More recently, Correlated Solutions has developed new software that utilities images from X-Rays or CT scanners to measure volumetric deformation of an object under an applied load.

The diagram above displays a typical setup of how the images are acquired during a test. The scanner acquires images at specific depth coordinates, and then Vic-Volume analyzes the image slices to construct a 3D volume made up of voxels. The individual voxels are the building blocks for the sub-volume, which contain the volumetric image correlation data.

 

Example

VIC Volume4

A reinforced rubber matrix composite is mounted between two grips, and a set of reference images are acquired from a CT scanner at know increments.  Each ‘slice’ of data is then analyzed to compute a static volume measurement.  After the specimen undergoes a tensile load, images are acquired again by the CT scanner at the same locations.  Digital Image Correlation algorithms are used to calculate the volumetric change or deformation at each individual voxel, which make up the 3D volume.

The above animation displays the internal strain (Ezz) of a reinforced rubber matrix composite undergoing tension. The volumetric strain data can be viewed, analyzed, or extracted as a volume or as individual data slices. The internal tension strain can clearly be seen.

Vic-Volume Software Features
  • Convenient AOI selection method through “Tweening’’
  • Semi-automatic initial guess computation
  • Optimized for accuracy reduce non-linear optimization to reduce bias and interpolation artifacts
  • Highly Advanced memory management permits analysis of huge volumetric data sets
  • Volumetric 3D displacements & strains


Vic-Gauge 2D/3D – Technology Overview

Vic-Gauge uses our optimized 2-D and 3-D correlation algorithms to provide real-time displacement and strain data for mechanical testing. Think of this tool as a set of virtual strain gauges: Data can be retrieved for multiple points and plotted live against analog load inputs. Results are saved for every analyzed point, and full images may be saved for full-field analysis in Vic-2D or Vic-3D.

VIC Gauge

Measurement System Feature Overview

The Vic-Gauge 3D system is offered as a turnkey strain measurement solution that doubles as a video extensometer and virtual strain gauge. The system performs a three-dimensional digital image correlation (DIC) analysis on a pair of images in real-time, processes the data, an then outputs a control signal. The measurements are displayed graphically, but also as virtual gauges values. The system utilizes the same robust and precise algorithms found in the Vic-3D system, and the same ease of use.

 

General Features:

  • Real-time measurement of strain and displacement at one or many discrete points
  • Measure strain data at points, or use virtual extensometers to connect locations
  • Analog value inputs for real-time load vs. strain reporting
  • Dual BNC analog outputs for non-contacting strain or displacement control of test
    frames, forming machines, etc.
  • Each gauge measures
    • X, Y, & Z coordinates
    • X, Y, & Z displacement
    • Full strain and shear tensor
    • First and second principal strains
  • Full test setup can be saved as a project for fast, consistent repeated tests