Can a DIC System Measure High-Temperature Specimens?

DIC systems can be used for displacement and strain measurement in high-temperature environments.

Since the camera does not need to touch the specimen directly, DIC suits high-temperature tests where contact sensors are hard to install, easily affected by temperature, or where full-field deformation observation is needed.

But non-contact does not mean that simply pointing a camera at a hot specimen yields reliable results.

After the temperature rises, the specimen surface condition, light propagation path, speckle adhesion and camera imaging conditions all change.

high-temperature DIC is often not whether the camera can withstand the temperature, but whether stable, clear and computable images can be continuously obtained throughout heating, holding and loading.

Why Does High Temperature Affect DIC Measurement?

1. Thermal Radiation Reduces Speckle Contrast

As the specimen temperature rises, the thermal radiation generated by the specimen itself gradually increases.

In visible light imaging, it may appear as:

▪ The whole image brightens;

▪ Local highlights;

▪ Speckle color changes;

▪ Reduced black-white grayscale difference;

▪ Image overexposure.

The DIC algorithm relies on the grayscale features of speckles to recognize the same area. If the specimen surface gradually brightens, the originally clear black-and-white speckle may become blurred, degrading image correlation.

The higher the temperature, the less effective simply increasing ordinary lighting brightness becomes. Usually, specific light sources, filtering schemes, exposure control and lens configurations are needed to reduce thermal radiation interference.

2. Hot Air Disturbance Produces False Displacement

After the specimen heats up, the density of the surrounding air changes with temperature.

Air of different temperatures has different refractive indices; as hot air flows, the light propagation path between the specimen and the camera keeps changing.

This phenomenon is similar to watching sun-heated pavement in summer, where distant scenery shimmers slightly.

Even if the specimen does not actually deform, hot air disturbance may cause the speckle positions in the image to fluctuate, forming false displacement and strain.

Therefore, high-temperature tests also require attention to:

▪ Heat flow direction;

▪ Air convection at the furnace mouth;

▪ Start-stop of heating devices;

▪ Whether the optical path passes through high-temperature gradient areas;

▪ Whether the camera mount is heated;

▪ Whether the protective window is heated unevenly.

3. Speckle may discolor, crack or fall off

Ordinary coatings used in room-temperature tests are not necessarily suitable for high-temperature environments.

After the temperature rises, the speckle may show:

▪ Softening;

▪ Discoloration;

▪ Carbonization;

▪ Cracking;

▪ Falling off;

▪ Deformation not synchronized with the specimen.

If the speckle is not firmly attached to the specimen surface, what DIC measures may be the change of the coating itself rather than the true deformation of the material.

High-temperature speckle must be chosen according to specimen material, target temperature, heating rate, holding time and atmosphere conditions.

Being able to withstand a certain temperature for a short time does not mean remaining stable at that temperature for a long time.

For high-temperature creep and high-temperature fatigue, the adhesion and grayscale stability of the speckle during continuous heating also need key verification.

4. The specimen surface may oxidize or reflect

Metal specimens may oxidize, discolor or form oxide layers at high temperature.

After some materials heat up, their surface reflection characteristics change obviously, and originally uniform lighting may form local highlight areas.

Heating elements, observation windows and metal inner walls inside the furnace may also produce reflections.

Once local overexposure occurs, speckle features are covered. Even without complete overexposure, surface brightness changing over time affects image grayscale consistency.

Therefore,high-temperature DIC requires a complete heating imaging test before the experiment, rather than only confirming speckle clarity at room temperature.

5. Observation Windows May Introduce Additional Errors

Many high-temperature DIC tests need to shoot the specimen through the furnace observation window.

The observation window can isolate heat and protect the camera, but may also cause:

▪ Glass deformation under heat;

▪ Refraction changes caused by inner-outer temperature difference;

▪ Soot attached to the surface;

▪ Distortion caused by window tilt;

▪ Multiple glass layers producing ghosting;

▪ The optical paths of the two cameras differ.

For stereo 3D DIC, refraction in different directions caused by the window may also affect 3D reconstruction.

If the system needs to calibrate through a window, the calibration state should be as close as possible to the actual test state. After the window position, material or angle changes, the original calibration relationship may no longer apply.

Is Everything Measured at High Temperature Mechanical Strain?

Not necessarily.

When the specimen heats up, it deforms due to thermal expansion even without mechanical load.

DIC measures the total displacement and total deformation of the specimen surface, which may simultaneously include:

▪ Thermal expansion;

▪ Mechanical loading deformation;

▪ Warping caused by uneven temperature;

▪ Material phase change;

▪ Fixture thermal deformation;

▪ Displacement of the heating equipment itself.

If the test goal is to obtain mechanical strain, thermal deformation must be analyzed in combination with the temperature field, the material's thermal expansion coefficient and the test procedure.

For specimens with uneven temperature distribution, a single average temperature cannot be used for unified correction. In this case, synchronized measurement of the temperature field and strain field is more helpful for analyzing thermal-mechanical coupling processes.

How Should High-Temperature DIC Be Arranged?

▪ Reasonably select the measurement field of view

The larger the field of view, the fewer pixels per unit length.

If local strain concentration needs to be analyzed, ensure the target area has enough effective pixels in the image, rather than only ensuring the whole specimen fits in the frame.

▪ Control the distance between camera and heat source

The camera needs to be kept away from the high-temperature radiation area, with necessary thermal insulation, heat dissipation and protective structures.

But as the distance increases, the lens focal length, field of view and lighting also need to be re-matched; heat protection cannot be solved only by moving the camera farther away.

▪ Avoid areas of intense heat flow

The measurement optical path should avoid passing through obvious heat flows as much as possible.

The impact of air disturbance on imaging can be reduced by adjusting the camera angle, furnace mouth structure, air isolation method or heating device layout.

▪ Verify speckle in advance

High-temperature speckle should not be used for the first time directly on the formal specimen.

It is recommended to use test pieces of the same material and verify them following the real test's heating rate, target temperature and holding time, observing whether the speckle discolors, falls off or loses contrast.

▪ Synchronously record temperature, load and images

Material properties in high-temperature tests usually change with temperature.

Only by synchronizing DIC images with load, temperature and time information can the deformations caused by thermal expansion, mechanical loading and material damage be distinguished.

How Does Haytham Configure High-Temperature DIC Solutions?

High-temperature DIC is not accomplished simply by adding a furnace in front of a room-temperature system.

Different temperature ranges, heating methods and specimen materials impose different requirements on speckle, lighting, filtering, lenses and equipment protection.

According to actual high/low-temperature material testing needs, Haytham Technology can configure corresponding 2D, single-camera 3D and stereo 3D visual strain measurement schemes, combined with:

▪ Target temperature;

▪ Heating method;

▪ Holding time;

▪ Specimen dimensions;

▪ Observation window;

▪ Measurement distance;

▪ Deformation speed.

Design the optical imaging, filtering and lighting, speckle and equipment protection as a whole.

The Haytham visual strain measurement solution can also combine temperature field data to synchronously analyze temperature changes and the deformation process on the specimen surface, providing experimental data for thermal expansion, thermal stress and thermal-mechanical coupling research.

For large structures or specimens with complex curved surfaces, multi-camera measurement and point cloud stitching schemes can also be configured according to the measurement range, to analyze temperature and deformation differences between regions.

Which Tests Suit High-Temperature DIC?

Common applications include:

▪ Metal tension at high temperature;

▪ High-temperature creep;

▪ High-temperature fatigue;

▪ Welding thermal deformation;

▪ Thermal barrier coating testing;

▪ Ceramic and refractory material testing;

▪ High-temperature deformation of composite materials;

▪ Thermal expansion analysis;

▪ Local strain testing of high-temperature structural parts;

▪ Coupled analysis of temperature field and strain field.

Different tests differ greatly in requirements for temperature range, sampling frequency, measurement field of view and spatial resolution, and need to be configured separately.

Frequently Asked Questions

▪ high-temperature DICDoes the Camera Need to Go Inside the High-Temperature Furnace?

Usually not necessary. The camera is generally placed outside the high-temperature area, capturing specimen images through the furnace mouth or observation window, with thermal insulation, protection and filtering designs ensuring stable imaging.

▪ Can ordinary black-and-white paint be used for high-temperature speckle?

Not necessarily. Ordinary coatings may discolor, crack or fall off after heating. Speckle materials need to be verified according to the specimen material, maximum temperature, test duration and atmosphere.

▪ Why does displacement still fluctuate when the specimen is static?

Common causes include hot air disturbance, heated camera mounts, deformation of the observation window, exposure changes and vibration from heating equipment.

▪ Can high-temperature DIC only measure in 2D?

No. Depending on the specimen deformation mode and system configuration, 2D DIC, single-camera 3D DIC or stereo 3D DIC can be used.