DIC systems are often used for large-deformation testing of rubber, plastics, flexible materials and composites.

Unlike contact measurement with a fixed gauge length, DIC computes displacement by continuously tracking the speckle features on the specimen surface, so it has a large deformation measurement range.

But the maximum strain DIC can measure is not determined by software parameters alone, nor can it be simply summarized by one universal number.

The actual measurement capability depends on:

▪ Whether the speckle remains recognizable after deformation;

▪ Whether the specimen stays within the measurement field of view;

▪ Whether the deformation between adjacent images is too large;

▪ Whether the specimen has obvious out-of-plane motion;

▪ Whether image resolution and imaging quality are sufficient;

▪ Whether cracks, occlusion or reflections appear in the calculation area;

▪ Whether specimen deformation remains continuous.

Therefore, judging whether a DIC system is suitable for large-deformation testing should consider the entire measurement process, not just the maximum strain range in the parameter table.

Why Is DIC Suitable for Large-Deformation Measurement?

DIC measurement does not rely on mechanical sensing structures mounted on the specimen, nor does it require the gauge length to always keep a fixed shape.

As long as the speckle on the specimen surface can be continuously recognized, the software can track position changes in different areas.

For rubber, films, flexible electronic materials and soft tissue simulants, the specimen may simultaneously undergo:

▪ Large elongation;

▪ Transverse contraction;

▪ Local necking;

▪ Strain concentration;

▪ Warping;

▪ Shape change.

DIC can obtain deformation results of different areas from the same set of images, and thus analyze whether the overall deformation is uniform, when local necking begins, and whether the final fracture location is consistent with the earlier strain concentration areas.

Which Factors Affect DIC Large-Deformation Measurement?

1. Whether the speckle can deform synchronously with the specimen

In large-deformation testing, the speckle needs to stretch together with the specimen surface.

If the speckle coating is too hard while the specimen itself is very soft, as the specimen stretches, the coating may crack, fall off or form texture changes unrelated to the material.

At this point, what DIC tracks may not be the true deformation of the material but the failure process of the coating.

For flexible materials such as rubber and films, the speckle needs sufficient adhesion and flexibility.

Clear speckle in the initial state does not mean it remains effective after several-fold elongation, so a full-range pre-test should be performed before the formal test.

2. Whether the Speckle Still Has Enough Features After Stretching

After the specimen is greatly stretched, the originally nearly circular speckles may gradually elongate, and speckle spacing increases.

If the initial speckle is too large or too sparse, large-area texture deficiency may appear after deformation, causing image correlation failure.

If the speckle is too small and occupies only a few pixels in the current field of view, it is also unfavorable for stable recognition.

Reasonable speckle design must ensure that from the beginning to the end of the test, the calculation area always contains a sufficient number of randomly distributed grayscale features.

3. Whether the Specimen Will Move Out of the Field of View

After the specimen elongates, the position and size of the target area both change.

If the initial field of view only barely covers the specimen gauge length, the specimen ends or key areas may gradually move out of the frame during stretching.

Field-of-view design cannot be based only on the initial specimen size; it also needs to estimate:

▪ Maximum elongation;

▪ Transverse contraction;

▪ The overall movement direction of the specimen;

▪ Additional fixture displacement;

▪ Possible fracture locations.

But the larger the reserved field of view, the fewer pixels per unit length. Therefore, large-deformation measurement requires a balance between covering the complete process and retaining spatial resolution.

4. Whether the Change Between Adjacent Images Is Too Large

The DIC algorithm needs to recognize the same area between consecutive images.

If the loading speed is fast while the camera sampling frequency is low, the speckle between adjacent frames may undergo excessive displacement or deformation, making it difficult for the software to establish a stable correspondence.

With the same 500% final strain:

Slow stretching to 500% may use a lower sampling frequency;

Stretching quickly to 500% in a short time places higher demands on camera frame rate, exposure time and lighting.

Therefore, the sampling frequency should be determined according to the deformation per unit time, not only by the final strain magnitude.

5. Whether There Is Obvious Out-of-Plane Motion

During large deformation, many flexible materials not only elongate but may also warp, bulge, twist or move toward the camera.

With 2D DIC, out-of-plane motion may cause image scale changes that are mistaken for in-plane deformation.

The following situations call for 3D DIC:

▪ Films wrinkling;

▪ Rubber specimen bending;

▪ Soft materials bulging outward under compression;

▪ The specimen surface itself is curved;

▪ Torsion occurring during the test;

▪ Z-direction displacement is needed.

Large deformation does not necessarily require 3D DIC, but you must first confirm whether the specimen can maintain an approximately planar state.

6. Whether the Calculation Area Remains Continuous

The DIC algorithm usually assumes that the same area can still be recognized in consecutive images.

After the specimen cracks, fractures, folds or is severely occluded, some areas lose continuity.

After a crack opens, if the calculation area spans both sides of the crack, the software may fail to establish a reliable correspondence.

Therefore, fracture tests need to distinguish two objectives:

First, measuring continuous deformation before fracture;

Second, analyzing the displacement discontinuity after cracks appear.

Different objectives require different calculation areas and analysis methods.

Is a Larger Maximum Strain Parameter Always Better?

Not necessarily.

The maximum strain range is only part of a DIC system's capability. For actual testing, what matters more is:

▪ Whether computation can remain stably continuous within the target strain range;

▪ Whether the speckle remains clear after deformation;

▪ Whether local strain concentration can be resolved;

▪ Whether transverse contraction can be measured simultaneously;

▪ Whether the specimen will move out of the field of view;

▪ Whether fast deformation produces motion blur;

▪ Whether results are repeatable.

If the system's nominal strain range is large but the measurement field of view, speckle scheme and sampling frequency do not match the test, the actual calculation may still fail midway.

How Should Large-Deformation DIC Be Designed?

▪ Determine the field of view according to the maximum deformation state

The field of view should be based on the specimen's maximum size after deformation, while reserving space for overall movement, fixture displacement and fracture position.

▪ Configure resolution according to the target area

If only the overall stress-strain relationship is needed, the field of view can be appropriately enlarged.

If local necking, strain around holes and notch damage also need to be analyzed, these areas must have sufficient pixels.

When necessary, different fields of view or multi-camera combinations can be used to balance overall deformation and local details.

▪ Choose the sampling frequency according to deformation speed

Slow large deformation and high-speed large deformation are two different problems.

At high speed, besides increasing the frame rate, the exposure time must be controlled to prevent speckle smearing in a single frame.

▪ Choose the speckle method according to the material

A speckle scheme suitable for rigid materials is not necessarily suitable for flexible specimens.

Large-deformation speckle needs to consider adhesion, flexibility, contrast and post-deformation texture state simultaneously.

▪ Run a full-range pre-test

Problems in large-deformation testing often appear in the second half of the test.

The pre-test should cover the process approaching the maximum strain, checking whether the field of view, speckle, exposure and correlation remain stable.

How Does Haytham ConfigureLarge-deformation DIC solution?

The Haytham Technology DIC visual strain measurement system can be used for material testing from small strain to large deformation, with an applicable strain range covering 0.005% to 2000%. Actual configuration still needs to be determined by specimen size, material type, deformation speed and measurement field of view.

This range does not mean that all specimens can be measured with the same set of camera and lens.

For example:

▪ Large rubber stretching needs to focus on speckle following, field-of-view reservation and transverse contraction;

▪ Films and flexible materials may show wrinkling and out-of-plane motion simultaneously;

▪ High-speed tension needs to balance frame rate, exposure time and lighting intensity;

▪ Local necking research requires ensuring sufficient spatial resolution for the target area.

In large-deformation projects, Haytham usually determines the measurement field of view based on the initial specimen size and the maximum deformation state, then configures the camera, lens, sampling frequency and speckle scheme.

If the specimen remains approximately planar, 2D DIC can be used for in-plane deformation analysis; if the specimen bulges, warps, twists or changes curvature, single-camera 3D or stereo 3D DIC is preferred.

For fast tension, impact and dynamic large-deformation processes, a single-camera high-speed 3D visual strain measurement scheme can also be configured, acquiring spatial deformation data of the specimen surface while increasing the sampling frequency.

The system can set multiple analysis areas, paths and gauge lengths on the specimen surface, and output axial, transverse and local deformation results simultaneously, helping users analyze the relationship between overall elongation, transverse contraction, local necking and final failure.

Factors Affecting DIC Large-Deformation MeasurementWhich Materials Does It Suit?

Common applications include:

▪ Rubber and elastomers;

▪ Plastic films;

▪ Flexible composite materials;

▪ Textile materials;

▪ Polymer materials;

▪ Foam materials;

▪ Flexible electronic devices;

▪ Soft tissue simulant materials;

▪ Seals and damping parts;

▪ Large-deformation structural parts.

Different materials differ greatly in surface condition and deformation mode, requiring separately configured optical systems and speckle schemes.

Frequently Asked Questions

▪ Does DIC large-deformation measurement require a fixed gauge length?

No dependence on mechanically fixed gauge lengths. Different measurement areas, paths and gauge lengths can be set on the specimen surface according to analysis needs.

▪ At what strain does DIC fail?

There is no universal value applicable to all tests. Whether it fails depends on speckle, field of view, sampling frequency, image quality and deformation continuity.

▪ Why do blank areas appear in the contour map after large deformation?

Possible causes include insufficient speckle features, local reflections, the specimen moving out of the field of view, excessive changes between adjacent frames, or cracks and occlusion in the area.

▪ Does large-deformation testing always require high-speed cameras?

Not necessarily. Whether a high-speed camera is needed depends on the deformation speed, not the final total deformation.