summary
Currently, the accuracy and precision testing of digital image-related (DIC) equipment in the industry is divided into two standardized technical routes: room temperature calibration of grating indicator calibrators and high temperature calibration of metrological tensile testing machines. The two schemes have completely different hardware, processes, applicable working conditions, and metrological indicators. Many material laboratories have problems with mixing or misusing the calibration schemes, resulting in mismatched working conditions of calibration certificates and invalid test data.
Based on the research findings of "Accuracy and Precision Verification Methods for Digital Image Correlation Devices," this paper comprehensively compares two DIC verification schemes from six dimensions: verification standards, applicable temperature, operating procedures, error sources, cost, and metrological effectiveness. It clarifies the optimal verification methods for room temperature cycling fatigue, high temperature tensile, and high temperature fatigue testing of carbon fiber composites. At the same time, it summarizes the general judgment criteria (ASTM E83 B-1 level) for the two verification schemes, providing a clear and practical guide for laboratories to build a DIC metrological system and formulate periodic verification plans.
I. Background and Applicable Scenarios of the Two Types of DIC Verification Schemes
With the widespread application of DIC three-dimensional full-field strain measurement systems in composite material mechanical testing, metrology institutions and materials laboratories have gradually developed two standardized verification methods to address the accuracy verification needs of DIC under both room temperature and high temperature conditions:
The room temperature calibration method for grating indicator calibrators: The benchmark is a 1μm-level grating displacement device, which is only applicable to a constant temperature laboratory of 20±1℃. It is specifically used to verify the accuracy and precision of 2D/3D DIC equipment under normal temperature conditions. It is the standard calibration method for room temperature cycle fatigue testing of carbon fiber and glass fiber composite materials.
B-1 grade metrological tensile testing machine high temperature verification method: adapted to high temperature furnace integrated testing conditions, relying on the crossbeam displacement of the qualified testing machine as the strain reference, covering the high temperature range of 200~1000℃, used for high temperature resistant composites, high temperature alloy high temperature fatigue, high temperature tensile DIC system verification.
A common misconception in the industry is that DIC equipment calibrated using room-temperature grating fixtures is directly used for high-temperature composite fatigue testing, assuming the calibration certificate is universally applicable. However, actual high-temperature operating conditions introduce additional errors such as thermal drift and window distortion, meaning room-temperature calibration results cannot represent high-temperature measurement accuracy, and metrology institutions do not recognize such high-temperature test data. The following comprehensive comparison of the core differences between the two solutions clarifies the boundaries of the application scenarios.
II. Complete Comparison of Six Core Dimensions (Raster Room Temperature Verification VS High Temperature Tensile Testing Machine Verification)
2.1 Measurement Standards and Upper Accuracy Limits
Grating-type calibrator: displacement resolution ≤0.1μm, displacement indication error ≤1μm, higher reference accuracy, theoretically can stably reach ASTM E83 Class A and Class B-1, with extremely small verification error for micro-strain (50~100με), perfectly matching early micro-damage strain verification of composite materials at room temperature;
The tensile testing machine has the highest accuracy of B-1 for beam displacement reference. However, due to the influence of the gaps in the loading cylinder and transmission mechanism, the dispersion of small strain verification is slightly higher than that of the grating tooling, making it more suitable for high-temperature working conditions in the medium-to-high strain range.
2.2 Applicable Temperature Range
Grating calibration fixture: only for constant temperature laboratory at 20±1℃. The equipment is not resistant to high temperature and must not be placed in a high temperature furnace.
Tensile testing machine calibration: room temperature to 1000℃ gradient temperature, can be directly integrated with high temperature furnace, covering the entire high temperature testing range.
2.3 Requirements for test samples/tooling
Grating solution: Standardized speckle calibration block, no mechanical sample required, tooling can be reused for a long time, and replacement cost is extremely low; speckle is a common industrial black and white coating, which is stable at room temperature;
High-temperature testing machine solution: Custom high-temperature alloy tensile specimens require re-coating with YSZ high-temperature resistant ceramic speckle for each test, resulting in higher costs for consumables and specimen preparation, and a longer test cycle.
2.4 Sources of Error Interference and Correction Logic
The grating room temperature calibration only involves lens distortion and pixel conversion ratio error, without thermal drift or glass refraction interference. The calibration data does not require additional compensation and the calculation logic is simple.
High-temperature tensile testing involves three major systematic errors: thermal radiation, quartz window distortion, and equipment thermal drift. The testing process requires collecting thermal drift images of blank samples to deduct spurious strain, making the data correction process complex and demanding a higher level of expertise from operators.
2.5 Operation Cycle and Usage Costs
Grating fixture calibration: A single complete gradient strain calibration takes 1.5 to 2 hours. The fixture can be purchased once and reused for a long time. There are no consumable costs for daily calibration, making it suitable for high-frequency periodic self-inspection in laboratories.
High temperature testing machine calibration: Single temperature range calibration takes 3-4 hours, and multi-temperature gradient complete calibration takes 1-2 days. High temperature speckle and high temperature sample consumption result in higher labor and material costs for calibration. It is suitable for special high temperature calibration.
2.6 Validity of Measurement Certificates
The certificate issued for grating calibration only indicates that it is valid under room temperature conditions and can only be used for cyclic fatigue and tensile testing of composite materials at room temperature.
The high-temperature tensile test certificate indicates the corresponding test temperature range and can only be used for high-temperature composite mechanical testing at that temperature. It cannot be used for room temperature conditions.
III. Guidelines for Selecting Optimal Verification Schemes for Different Composite Material Testing Scenarios
Scenario 1: Cyclic fatigue test of carbon fiber/glass fiber composite material at room temperature (-10℃~60℃)
Optimal solution: Calender indicator calibrator calibration at room temperature, target level B-1.
Demand Logic: No additional error at room temperature, grating tooling has significant advantages in micro-strain calibration accuracy, can accurately verify the early interface debonding of composite materials and the accuracy of micro-strain measurement of microcracks; low calibration cost and short cycle, suitable for periodic self-inspection.
Scenario 2: High-temperature resistant carbon fiber composite materials undergo high-temperature cyclic fatigue and high-temperature tensile testing (200~800℃).
Optimal solution: High-temperature gradient verification of B-1 grade universal testing machine
Requirement logic: Room temperature grating fixtures cannot simulate the distortion and thermal drift interference of high temperature furnaces. Gradient strain verification must be completed under real high temperature test conditions to ensure that the strain measurement data of micro-delamination and thermal cracks at high temperatures have metrological traceability.
Scenario 3: Simultaneous mechanical testing of composite materials at room temperature and high temperature in the laboratory.
Combined solution : Two verification processes run in parallel. The grating of the room temperature equipment is verified every six months; the high temperature dedicated DIC system is verified quarterly for the high temperature testing machine, and corresponding working condition verification certificates are issued. The corresponding working condition certificates are matched and archived during the test.
IV. Common Core Judgment Standards for Both Verification Schemes (Uniformly aligned with ASTM E83 B-1 Level)
Regardless of whether room temperature grating calibration or high temperature tensile calibration is used, the criteria for DIC equipment compliance are completely unified. Only when all four mandatory criteria are met can it be recognized as a B-1 level high-precision equipment:
The relative error of the gauge length is ≤ ±0.25%;
Absolute strain resolution ≤ 0.00005 (50 με);
The absolute strain indication error for all strain levels is ≤0.0001 (100με), and the relative indication error is ≤0.5%.
The standard deviation of repeatability for multiple measurements of the same strain is ≤20με (room temperature) / ≤30με (high temperature).
The two schemes differ only in the testing environment and benchmark equipment; their metrological grading standards are fully compatible, facilitating a unified equipment accuracy control system for laboratories.
V. Engineering risks arising from the misuse of verification plans (common industry issue)
The high-temperature test only has a room temperature grating calibration certificate: the third-party metrology audit directly rejected the test report, the test data has no legal effect, and the project test must be redone;
The data from the room temperature fatigue test using the high temperature testing machine shows that the dispersion of small strains in the high temperature test is higher, which cannot prove the accuracy of DIC micro-damage measurement at room temperature, and the reliability of early damage data of composite materials is insufficient.
Only rigid body translation verification was performed, without using grating/tensile machine gradient strain verification: ASTM E83 strain indication error data is missing, and the metrology agency does not recognize the verification results.
VI. Conclusion
The room temperature calibration of grating indicator gauges and the high temperature calibration of tensile testing machines are currently the only two standardized and feasible solutions for verifying the accuracy and precision of DIC equipment. Neither is inherently superior; they differ only in applicable operating conditions, cost, and emphasis on precision. Materials laboratories need to match the appropriate calibration route to their specific carbon fiber and glass fiber composite material testing temperature ranges. Room temperature cyclic fatigue testing should utilize grating fixture calibration, while high-temperature composite material testing should employ high-temperature gradient calibration using a tensile testing machine. The applicable scope of the two calibration certificates must be strictly distinguished, and both should be uniformly aligned with the ASTM E83 B-1 grade judgment standard to construct a complete and compliant metrological control system for DIC equipment.
