The Complete Guide to Sample Marking for Video Extensometers

Best Practices for Accurate Strain Measurement in Tensile Testing

Introduction

As non-contact measurement technologies become more common in modern materials testing laboratories, video extensometers have emerged as one of the most versatile and accurate tools for strain measurement. Unlike traditional clip-on extensometers, video extensometers rely on optical tracking rather than physical contact with the specimen.

However, the accuracy of any video extensometer depends heavily on one often-overlooked factor: proper sample marking.

Whether performing routine tensile testing, measuring Poisson’s ratio, or conducting full-field strain analysis, correct sample marking is essential for reliable, repeatable strain data. This article provides a comprehensive guide to sample marking for video extensometers, explains how digital image correlation (DIC) works, and outlines best practices for achieving high-quality strain measurements across a wide range of materials.

Video Extensometer with High Contract Sample Markings

What Is a Video Extensometer?

A video extensometer is a non-contact extensometer that measures strain by tracking visual features on a test specimen using high-resolution cameras and advanced image-processing software. As the specimen is loaded in a universal testing machine, the video extensometer continuously captures images and calculates displacement and strain in real time.

Key Advantages of Video Extensometers

• No physical contact with the specimen

• Ideal for delicate, brittle, soft, or high-elongation materials

• Flexible gauge lengths without mechanical reconfiguration

• Capable of axial, transverse, and full-field strain measurements

Because video extensometers do not physically grip the specimen, optical contrast and pattern tracking replace mechanical contact as the primary source of measurement accuracy.

How Video Extensometers Measure Strain

At a fundamental level, strain is calculated using:

Strain = Change in Length ÷ Original Gauge Length

A video extensometer determines this change in length by tracking the movement of visible features or markings applied to the specimen surface. These features act as reference points, allowing the software to measure displacement as the material deforms during testing.

This is where sample marking becomes critical.

Digital Image Correlation (DIC): The Foundation of Optical Strain Measurement

Strain Measurement with a Video Extensometer Using Digital Image Correlation

What Is Digital Image Correlation?

Digital Image Correlation (DIC) is an optical measurement technique that calculates deformation, displacement, and strain by analyzing changes in digital images taken during a test. DIC compares a reference image (before loading) with subsequent images captured as the specimen deforms.

By tracking how specific pixels or patterns move between images, the software reconstructs strain with extremely high precision.

2D vs. 3D Digital Image Correlation

• 2D DIC: Uses a single camera and assumes deformation occurs in a single plane

• 3D DIC: Uses stereo cameras to capture out-of-plane motion and complex deformation

Modern video extensometers often use DIC-based algorithms, even for simple axial strain measurements.

Why Sample Marking Is So Important

Video extensometers and DIC systems rely on pattern recognition algorithms. These algorithms require:

• Clear, high-contrast visual features

• Unique patterns that can be reliably tracked

• Stable markings that remain visible throughout the test

Poor marking can lead to:

• Lost tracking

• Noisy strain data

• Incorrect gauge length definition

• Invalid test results

In many cases, sample marking quality directly determines measurement accuracy.

Types of Sample Marking Techniques

1. Simple Gauge Length Marking

For standard tensile strain measurements, simple marking techniques are often sufficient.

Common methods include:

• Two dots defining the gauge length

• Circles applied using a stencil

• Short lines or markers placed at gauge endpoints

These markings are typically applied using high-contrast pigment ink paint pens and gauge-length stencils. Common gauge lengths include 25 mm, 50 mm, 100 mm, 1 inch, and 2 inches, depending on the test standard.

This method is ideal for:

• Routine tensile testing

• Quality control environments

• Elongation and modulus measurements

2. Advanced Dot Patterns for DIC and 3D Strain

For full-field strain measurement and advanced DIC applications, a more complex marking pattern is required.

Rather than uniform grids, DIC performs best with non-uniform dot patterns, where:

• Dots vary slightly in spacing and size

• No repeating geometric pattern exists

This randomness helps the software uniquely identify each region of the specimen and prevents tracking ambiguity.

These patterns are typically applied using:

• Dot-pattern stamps

• Speckle pattern sprays

• Ink pads and stamping tools

3. Marking for Transverse Strain and Poisson’s Ratio

When measuring Poisson’s ratio, strain must be measured in both the axial and transverse directions.

Typical marking layouts include:

• Two axial gauge length marks

• Additional dots placed horizontally across the specimen width

This allows the video extensometer to simultaneously measure longitudinal elongation and lateral contraction.

Choosing the Right Marking Color and Contrast

High Contrast Sample Markings for Video Extensometer

High contrast between the specimen surface and the marking is essential.

General guidelines:

• Black ink on white or light samples

• White ink on dark samples

• Blue ink on gray or metallic surfaces

Paint-based pigment markers are preferred because they:

• Provide consistent opacity

• Resist smearing during deformation

• Remain visible throughout the test

Best Practices for Sample Marking

• Ensure markings are fully dry before testing

• Avoid glossy or reflective finishes near the measurement area

• Keep markings within the camera’s field of view

• Do not use perfectly repeating or symmetrical patterns for DIC

• Apply markings only in the region of interest

• Verify visibility during system setup and focusing

Proper lighting and camera alignment should always be verified after marking.

Common Sample Marking Mistakes

• Insufficient contrast between marks and specimen

• Marks applied outside the gauge region

• Uniform grids that confuse pattern recognition

• Ink that flakes or smears during testing

• Poor lighting that reduces image clarity

Addressing these issues early can dramatically improve strain measurement reliability.

Video Extensometers vs. Traditional Clip-On Extensometers

While clip-on extensometers remain effective for many standardized tests, video extensometers offer unmatched flexibility.

Feature	Video Extensometer	Clip-On Extensometer
Contact with specimen	None	Physical
Gauge length flexibility	Very high	Limited
Delicate materials	Excellent	Often unsuitable
Full-field strain	Yes (with DIC)	No

In many labs, both technologies are used depending on the application.

Implementing Sample Marking in the Lab

Establishing a standardized sample marking procedure improves:

• Test repeatability

• Operator consistency

• Data quality

• Compliance with internal SOPs and test standards

Documenting marking methods as part of your test procedure is strongly recommended.

Conclusion

Sample marking is not a minor detail—it is a critical component of accurate strain measurement when using video extensometers. From simple gauge length dots to advanced DIC speckle patterns, the right marking technique ensures reliable tracking, clean strain curves, and confidence in your test results.

Understanding how digital image correlation works—and how it depends on optical contrast and unique patterns—allows engineers and lab technicians to fully leverage the capabilities of modern non-contact extensometers.

Have Questions About Video Extensometers or Sample Marking?

Universal Grip provides complete solutions for video extensometers, non-contact strain measurement, tensile testing systems, and accessories, including marking tools, stencils, and software options.

If you have questions about selecting the right video extensometer, optimizing sample marking, or upgrading your strain measurement capabilities, contact Universal Grip today.

👉 Visit: www.UniversalGripCo.com📩 Reach out to speak with an applications specialist