Accuracy Verification and Strain Calibration for High-Temperature DIC Systems

summary

High-temperature tensile testing and high-temperature cyclic fatigue testing are core mechanical testing items for aerospace high-temperature alloys and high-temperature resistant carbon fiber composites. Under high-temperature conditions, digital image correlation (DIC) systems are subject to multiple interferences such as thermal radiation, window glass distortion, equipment thermal drift, and high-temperature speckle failure, resulting in a significant decrease in measurement accuracy. Room temperature grating calibration fixtures cannot be directly used for high-temperature environment calibration.

Based on the "Accuracy and Precision Verification Methods for Digital Image Correlation Devices," this paper establishes a DIC accuracy verification system adapted to high-temperature furnace conditions. Using the crossbeam displacement of a universal tensile testing machine with B-1 level accuracy, verified through metrology, as the standard strain benchmark, gradient strain verification is conducted using high-temperature stable alloy samples. The paper clarifies the high-temperature DIC image acquisition parameters, insulation specifications, and error calculation logic, distinguishing between room temperature and high-temperature testing schemes. This addresses the industry challenge of lacking metrological traceability and verifying the accuracy of DIC data in high-temperature composite material testing, providing a standardized process for issuing high-temperature operating condition verification reports for high-temperature DIC strain measurement systems.

I. Sources of error and challenges in high-temperature DIC measurement

Compared to room temperature environments, DIC full-field strain measurement under high-temperature conditions involves multiple additional errors, which is the core reason why the room temperature grating fixture cannot be directly reused for high-temperature DIC accuracy verification.

Thermal radiation optical interference: Infrared radiation from samples above 300℃ will overwhelm the visible light speckle signal, reduce image grayscale contrast, and significantly increase related calculation errors;

Quartz window glass distortion error: The refractive index of the observation window glass in the high-temperature furnace changes when heated, introducing a systematic pixel shift that cannot be eliminated by conventional camera calibration;

Equipment thermal drift spurious strain: The camera bracket, lens, and calibration fixture expand due to heat, producing rigid displacement without real deformation, which is superimposed on the strain data of the sample;

High-temperature speckle stability issues: Ordinary black paint speckles carbonize and peel off at temperatures above 600℃, resulting in the loss of speckle characteristics and the failure of DIC-related calculations;

No standard high-temperature strain measurement fixture: The grating indicator calibrator is not heat resistant and cannot be placed inside a high-temperature furnace, so it cannot be used directly as a high-temperature strain reference.

In high-temperature cyclic fatigue testing of high-temperature glass fiber/carbon fiber composites, interlaminar separation and matrix thermal cracking both generate minute local strains. If the high-temperature DIC measurement error exceeds the B-1 limit, it will directly lead to a misjudgment of the material's high-temperature fatigue life. Therefore, it is necessary to establish an independent and adaptable method for verifying the accuracy and precision of DIC in high-temperature furnace cavities. This method should rely on the crossbeam displacement of a calibrated universal tensile testing machine as the strain benchmark to achieve quantitative verification of the DIC system under high-temperature conditions.

II. Core Standard for High-Temperature DIC Verification: B-1 Grade Measuring Tensile Testing Machine

For high-temperature verification, a grating calibrator is not used. Instead, a universal testing machine calibrated by the Metrology Institute and achieving ASTM E83 B-1 grade accuracy for beam displacement is selected as the standard strain generating device. The core basis is:

The displacement value of the crossbeam of the testing machine is traceable. The theoretical standard strain ε=ΔL_standard/L₀ can be accurately calculated by the crossbeam displacement and the original gauge length of the specimen.

The testing machine can be integrated with a high-temperature furnace to achieve gradient loading across the entire temperature range from room temperature to 1000℃;

It can stably output multi-level tensile strain, fully covering the elastic and plastic deformation ranges of high-temperature alloys and high-temperature resistant composites.

The test specimens are made of deformed high-temperature alloy thin plates with mature technology and highly stable mechanical properties, avoiding additional errors caused by the dispersion of material mechanical properties; the surface of the specimens is sprayed with YSZ high-temperature resistant ceramic speckle, which can withstand temperatures above 1000℃, ensuring the stability of image correlation calculations throughout the process and avoiding interruption of the test due to speckle falling off.

III. Complete Operating Procedures for Standardized Testing of High-Temperature DIC Accuracy and Precision

3.1 Preparations before verification

Equipment status verification: The universal tensile testing machine has a valid metrological certificate, and the beam displacement is grade B-1; the temperature uniformity of the high-temperature furnace is ≤±2℃; the DIC camera is equipped with a narrow-band blue light filter to suppress heat radiation;

Sample preparation: High-temperature alloy plate tensile specimens, with the original gauge length L0 fixed, and the surface coated with high-temperature resistant ceramic speckle.

System setup: The sample is clamped in the testing machine fixture, the high-temperature furnace is closed, the binocular DIC camera is aligned with the quartz observation window of the furnace body, and the system distortion correction calibration with glass window is completed;

Insulation specifications: After heating to the target test temperature (200℃/400℃/600℃ gradient), maintain the temperature for no less than 20 minutes to eliminate the temperature gradient inside the furnace and the thermal drift of the equipment.

3.2 Accuracy (Indication Error) Inspection Procedure

Temperature gradient verification: Verification is performed separately in three typical high temperature ranges of 200℃, 400℃ and 600℃, covering the mainstream high temperature test conditions for aerospace composites.

Strain gradient loading: At each temperature setting, the crossbeam of the testing machine is controlled to move at a constant speed, and 8 equally spaced displacement settings are set to calculate the corresponding theoretical standard strain.

Image synchronous acquisition: After each displacement stabilizes, 20 to 50 speckle images are acquired at a constant acquisition frequency within 1 to 3 minutes, and the average full-field strain within the gauge length is calculated using DIC software.

Error quantification calculation: Compare the measured strain of DIC with the theoretical strain of beam displacement to calculate the absolute strain error and relative indication error;

Grading determination: If the absolute strain error of all strain levels is ≤0.0001 and the relative error is ≤0.5% at the same temperature, the DIC system is determined to have reached the B-1 level of accuracy at that temperature.

3.3 High-Temperature Operating Condition Accuracy (Repeatability) Inspection Procedure

Three strain points (low, medium, and high) were selected for each calibration temperature.

Each strain point completes 3 full cycles of "load to target strain - unload and reset";

After each cycle stabilizes, 30 sets of DIC strain data are collected, and the standard deviation of the data is calculated.

Acceptance criteria: The repeatability standard deviation under high temperature conditions is ≤30με, which meets the stable data acquisition requirements for long-term high temperature fatigue cycle testing.

3.4 High-Temperature Specific Error Compensation Processing (Verification Data Correction)

During the verification process, high-temperature images of blank samples (without load) are acquired simultaneously, and the rigid spurious strain caused by thermal drift of the equipment is calculated. The system thermal drift component is subtracted from the final DIC strain data to eliminate the additional error caused by the thermal expansion of the support and lens, ensuring that the verification results only reflect the actual deformation measurement error of the sample, which meets the requirements of ASTM E2208 for the correction of optical measurement system errors.

IV. Comparison of High-Temperature DIC Verification and Room-Temperature Grating Verification Schemes

The two testing schemes complement each other, fully covering the metrological requirements of DIC equipment under all operating conditions of material mechanics at high and low temperatures. Laboratories need to conduct two sets of verifications, one for room temperature and one for high temperature, according to their own test temperature range. They cannot be substituted for each other.

V. Engineering Significance of DIC Verification in High-Temperature Composite Fatigue Testing

Cyclic high-temperature fatigue testing of high-temperature resistant carbon fiber composites requires capturing minute strain concentrations at high temperatures, such as matrix-fiber interface debonding, interlaminar thermal stratification, and microcrack propagation. If the DIC system has not undergone high-temperature accuracy calibration, the following test failures may occur:

Quartz window distortion causes an overall shift in strain across the entire domain, making it impossible to identify local damage strain peaks.

Thermal drift spurious strain masks the true micro-damage strain, leading to the misjudgment that the composite material has no early damage;

Since there is no unified measurement standard for the test data from different batches at different temperatures, the test conclusions do not have the value for cross-comparison.

Using the high-temperature tensile testing machine calibration scheme described in this paper, after completing the accuracy verification of DIC by temperature gradient, the full-field strain data collected by the high-temperature DIC system can be used for fatigue life assessment and failure mechanism analysis of high-temperature composites.

VI. Common Problems and Optimization Solutions in High-Temperature DIC Verification Practice

Overexposed thermal radiation images with insufficient speckle contrast: Add a blue light narrowband filter, shorten the camera exposure time, increase the intensity of the external cold light source, and collect multiple frames of images for grayscale averaging and noise reduction before verification.

Localized detachment of high-temperature speckle patterns leads to the failure of related calculations: Replace with platinum metal micro-marker speckle patterns, which can withstand up to 1600℃ and are suitable for ultra-high temperature testing;

Introducing fixed system errors into the viewing window glass: During calibration, retain the high-temperature furnace quartz glass, conduct binocular distortion calibration with medium, and correct the glass refraction error;

Poor thermal drift repeatability: Extend the heat preservation time to 30 minutes and use Invar camera bracket with low thermal expansion coefficient to reduce the deformation of the equipment due to heat.

VII. Conclusion

For high-temperature DIC equipment accuracy and precision verification, room-temperature grating fixtures cannot be reused. This paper establishes a standardized high-temperature verification process based on a universal tensile testing machine calibrated to B-1 level. The process standardizes the entire operation, including temperature holding, image acquisition, error compensation, and grading, for high-temperature DIC metrological verification. This verification method can be directly used for periodic metrological verification of various binocular three-dimensional high-temperature DIC systems, and is compatible with cyclic fatigue and high-temperature tensile mechanical testing of high-temperature carbon fiber and glass fiber composite materials, providing complete metrological traceability support for full-field strain measurement data under high-temperature conditions.