Universal Video Extensometers
for Strain Measurement
Digital Image Correlation (DIC) has quickly become the preferred method for non-contact strain measurement in the material testing industry. A camera and computer system combined with pixel recognition software algorithms can accurately measure strain in a variety of applications including tensile testing. Strain is the measure of deformation that a material experiences while under stress (force) over time (t) and is represented in distance units, usually (mm).
The Universal Grip Company has teamed up with Sobriety s.r.o. to offer the Universal Video Extensometer (UVE). The UVE is a modular and flexible software + hardware offering for solving a wide array of strain-based material testing applications. The UVE video extensometer can be integrated with most modern Universal Testing Machines including the I-brand, M-brand, T-brand, and also the Japanese and European UTM brands. The UVE can also be used as a stand-alone system or on a home-brew DAQ setup.
Why Use a Video Extensometer?
Video Extensometers are highly versatile and more flexible than clip-on extensometers, strain gauges, and deflectometers. Without the need to physically contact the sample, Universal Grip's UVE system can be used for tensile, compression, shear and flexural or bend testing. This flexibility to perform virtually any test adds advanced capability to any laboratory or testing environment. The UVE system can also be configured with multiple cameras for advanced 3D vibrography and other real-time video measurement applications away from the Universal Testing Machine.
Easy To Use
The UVE camera and software system is simple to set up and use. Since nothing touches the sample, there is no risk of damage to the video extensometer when the sample breaks. Measurements can also be taken during the entirety of the test.
System Overview for Tensile Testing
There are several different options for the camera module based on the Field of View (FoV) and working distance required. Generally a closer working distance will yield higher resolution and therefore more accuracy. For high elongation, such as with elastomers, the image from two camera modules can be stitched together.
A high lumens LED is used to illuminate the sample and provide good contrast on the markings. Good marking technique is critical for accurate and repeatable video based strain measurements. See below for tips and tools on sample marking.
Power and Control Box
The power and control box is used to control the light and camera modules. If necessary, multiple lights and camera modules can be controlled and switched on and off.
Sample Marking Technique
Digital Image Correlation (DIC) software uses algorithms to recognize and track pixels based on their patterns. Therefore, it is critical to prepare the sample with high contrast and non-uniform markings. The image below shows the difference in markings between a dot, horizontal line, X, or a speckled pattern (most ideal).
Dots, lines, and crosses or X's are suitable for most static tensile tests. Dots and lines can be applied with either a marker + stencil or a sample marking machine (see below). For more advanced testing, such as shear testing or tests which require a full strain map, a speckled pattern must be used on the sample. There are several ways to apply a high-contrast speckled marking including with spray paint, with hand applied dots, with a stencil + spray paint, or with a printed on paper or vinyl applique.
5 Advantages of Non-Contact Video Extensometers
- Adjustable Field of View allows the extensometer to test a wide range of samples. No need for 3-5 different clip-on extensometers, video extensometers can measure the strain of virtually any sample size. Ideal for 3rd party test labs.
- Easy to calibrate with the Calibration Plate. Just place the plate at the correct working distance within the FoV and allow the software the automatically calibrate your pixel grid frame.
- Measures both elongation and striction. Measure strain in both the X and Y axis without the use of multiple strain gauges. A perfect solution for measuring Poisson's Ratio.
- Low Maintenance with no mechanical parts to service.
- Non-contact means no external forces on the sample. The video extensometer also has no chance of slipping or breaking.
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Digital Image Correlation Basics
DIC is a software-based method of tracking the movement of features from frame to frame within a digital video. Small features such as sample markings are registered and measured as they move over time. Each pixel within the FOV is assigned a specific distance and features can be tracked as they move within the video image. The entire FOV or video scene of an example 5.4 Megapixel camera can be thought of as a 5.4 million grid of tiny boxes each with a set height and width. X, Y, and even Z distance measurements (with multi-camera 3D setup) can be gleaned from this grid and act as perfect strain measurements in material testing applications.
There are two key algorithms at work within most DIC software programs. The first is the pattern recognition algorithm which identifies, registers and tracks patterns as they move from image to image. The second key algorithm is Pixel Interpolation which essentially identifies features which are halfway in a pixel. For example, think of the fine edge of a black line against a white surface. As the line moves and bisects a pixel, the pixel image will show as a grey color instead of being either fully black or fully white. The interpolation algorithm will recognize the grey pixel as the edge of the line and maintain tracking on that particular feature marking.
In a 1.0 Megapixel array, the human eye can only discern and select a feature's edge within about 3-5 pixels. The human eye cannot discern an individual pixel with today's high resolution cameras. Rather than tracking individual pixels, the software identifies a group of pixels and interpolates for its center spot.
Video Extensometers look for a high contrast mark or sticker on the sample to use as a reference point. Markers and a calibration plate can be used for basic sample marking. To ensure accurate and repeatable sample marking, a small machine is usually used. The sample marking machine is a small punch tool which deposits a small bit of retro-reflective or high contrast tape directly onto the sample. The tape sticks to the sample and provides a very high contrast target for the camera system to identify. Universal Grip offers both manual and automatic sample marking punch tools for video extensometers.
- Newsletter discussing recent system upgrades and capabilities.
- System Configuration and Component Overview.
- Operator's Manual for the Universal Video Extensometer.