Video Extensometers
Video Extensometers for Non-Contact Strain Measurement
Video extensometers based on Digital Image Correlation (DIC) have become the preferred solution for non-contact strain measurement in modern materials testing laboratories. By combining high-resolution cameras with advanced image processing algorithms, these systems measure deformation with exceptional accuracy across a wide range of mechanical tests including tensile, compression, shear, and flexural testing.
Strain represents the deformation a material undergoes when subjected to an applied force over time and is typically expressed in units of displacement such as millimeters. Because video extensometers do not physically contact the specimen, they eliminate many limitations associated with traditional measurement devices while improving repeatability, safety, and overall data quality.
Universal Test Machine Co. is proud to offer the Vector Video Extensometer from Imetrum, an advanced optical strain measurement system that reflects the rapid evolution of Digital Image Correlation and non-contact testing technologies across the materials testing industry. As laboratories continue to prioritize higher accuracy, reduced operator influence, and improved testing efficiency, DIC-based extensometry has emerged as a leading solution for modern test environments.
The Vector system features a modular hardware and software architecture capable of supporting demanding strain-based applications in both production laboratories and research settings. Designed for compatibility with virtually any universal testing machine, the Vector integrates easily with leading platforms such as Instron, Galdabini, MTS, Tinius Olsen, Shimadzu, and ZwickRoell. Any machine equipped with an extensometer input can typically be configured with the appropriate electrical signal and connector for seamless integration. In addition to load frame operation, the Vector can also be used as a standalone measurement platform for R&D applications requiring true strain analysis beyond traditional tensile testing.
Why Use a Video Extensometer?
Laboratories that rely on multiple clip-on extensometers often find that transitioning to a video extensometer improves efficiency while significantly expanding testing capability. A single optical system can accommodate a wide range of specimen sizes and test configurations without physical adjustment or risk of instrument damage.
Application Flexibility
Compared to clip-on extensometers, strain gauges, and deflectometers, video extensometers offer exceptional flexibility. Because the system never contacts the specimen, it can be used across numerous test methods without influencing results or risking damage during sudden sample failure.
Common applications include tensile, compression, shear, and bend or flexural testing. Multi-camera configurations further extend capability into advanced measurement techniques such as 3D vibrography, full-field strain mapping, and real-time structural monitoring. These systems are increasingly used in construction materials testing and structural integrity evaluations where detailed deformation data is required.
Ease of Use and Reliability
Video extensometer systems are straightforward to set up and operate. Since no device is attached to the specimen, there is no risk of extensometer damage when a sample fractures. Measurements can be captured continuously throughout the entire test, providing a complete strain profile from preload through failure.
The Vector Video Extensometer is designed to integrate mechanically and electrically with nearly any load frame, and Universal Test Machine Co. provides full application support to assist with proper configuration and system performance.
2D vs. 3D Video Extensometers
2D Video Extensometers
Two-dimensional video extensometers provide an economical entry point into non-contact strain measurement and are well suited for basic point-to-point elongation testing. However, these systems require carefully controlled imaging conditions including precise focus, planar alignment, lighting, and working distance. Calibration can also be more sensitive to operator technique, particularly in less controlled environments.
3D Video Extensometers
The 3D Vector Video Extensometer incorporates advanced processing capabilities that automatically compensate for lighting variations, focus challenges, and out-of-plane specimen movement. The operator positions the specimen within a defined measurement volume, where tracking lasers establish reference points and immediately begin generating true strain data.
This level of automation significantly reduces user-induced error while improving laboratory throughput by minimizing configuration and setup time.
Choosing the Right Configuration
While both 2D and 3D systems are competitively priced, the optimal choice depends on laboratory priorities. A 2D system is ideal for controlled environments performing straightforward measurements, whereas a 3D extensometer is better suited for facilities seeking maximum consistency, faster setup, and advanced analytical capability.
System Overview for Tensile Testing
Camera Module
Camera options are selected based on the required field of view and working distance. Shorter working distances generally provide higher resolution and improved measurement accuracy. For high-elongation materials such as elastomers, dual-camera configurations can be used to stitch images together and maintain continuous tracking throughout large extensions.
Illumination System
High-lumen LED lighting is used to illuminate the specimen and enhance contrast between the surface and applied markings. Proper illumination is critical for achieving accurate and repeatable video-based strain measurements.
Power and Control Unit
The centralized power and control unit manages the camera and illumination modules. Multiple cameras and light sources can be controlled simultaneously or switched as needed for complex testing configurations.
Digital Image Correlation Fundamentals
Digital Image Correlation is a software-based technique that tracks the movement of visual features across sequential video frames. Each pixel within the camera’s field of view is mapped to a known distance, allowing precise displacement calculations.
For example, a 5.4-megapixel camera effectively creates a grid containing millions of measurable data points. When combined with multi-camera configurations, the system can capture X, Y, and Z displacement to generate highly accurate strain measurements.
Two primary algorithms drive DIC analysis. Pattern recognition algorithms identify and track surface features as they move, while pixel interpolation algorithms detect sub-pixel transitions — such as the boundary between contrasting colors — to achieve measurement accuracy beyond the limits of human visual perception.
Sample Marking Techniques
Accurate DIC measurement depends heavily on high-contrast, non-uniform specimen markings. Speckled patterns provide the most reliable tracking performance, though dots, lines, and cross patterns are sufficient for many static tensile tests.
Markings can be applied using markers, stencils, spray techniques, or adhesive targets depending on test requirements. Advanced strain mapping applications typically require randomized speckle patterns to maximize tracking fidelity across the full measurement area.
Advantages of Non-Contact Video Extensometers
Non-contact video extensometers offer several advantages over traditional measurement methods. Adjustable fields of view allow a single system to accommodate a wide range of specimen sizes without maintaining multiple clip-on devices. Calibration is fast and repeatable using a calibration plate, allowing the software to automatically scale the pixel grid.
Video extensometers can measure both elongation and lateral strain, enabling direct calculation of Poisson’s ratio without additional sensors. With no mechanical wear components, maintenance requirements are minimal, and the absence of physical contact eliminates slippage and external influence on the specimen.
Sample Marking Systems
Video extensometers rely on clearly visible targets to establish reference points. While basic marking can be performed using markers and calibration plates, many laboratories use dedicated sample marking devices to improve repeatability and throughput.
A sample marking machine applies retro-reflective or high-contrast adhesive targets directly onto the specimen surface, creating an ideal reference for optical tracking. Universal Test Machine Co. offers both manual and automated marking solutions designed to support consistent, repeatable video extensometer workflows.
