What is Extensometer System Verification?
Extensometer system verification, as defined by ASTM E83-10a, is the standardized procedure for determining the accuracy and classifying the performance of devices used to measure strain (change in length) during tensile or compressive testing. Unlike force verification which checks the load cell, this practice focuses on the “strain measurement” chain, including the extensometer itself, signal conditioning electronics, and readout devices (recorders, digital displays). It covers Type 1 (self-defining gauge length), Type 2 (specimen-defined gauge length), and Type 3 (intrinsic strain sensing, e.g., video) systems. The process involves comparing the extensometer’s output against known displacements applied by a high-precision verification apparatus. This ensures that critical material properties like Young’s Modulus, yield strength (offset method), and elongation are measured with defined error limits, which is essential for material certification and quality control.
What is a typical Extensometer Verification Result?
The results classify the extensometer system into specific accuracy grades based on maximum permissible errors:
① Accuracy Class: The system is assigned a class (A, B-1, B-2, C, D, or E) based on the magnitude of strain error and resolution. Class A represents the highest precision (±0.1% relative error), typically required for modulus determination, while Class E (±1.0%) is suitable for general elongation measurements at large strains.
② Strain Error: The difference between the strain indicated by the system and the true strain applied, expressed as both a fixed value (e.g., 0.0001 in./in.) and a relative percentage of the reading. The allowable error is the greater of these two values.
③ Resolution: The smallest change in strain the system can detect. The standard sets strict limits; for example, Class B-1 requires resolution not to exceed 0.00005 in./in. or 0.25% of reading. Resolution often dictates the lower limit of the verified range.
④ Gauge Length Error: For Type 1 systems, the deviation of the actual gauge length from the specified value must not exceed the class limit (e.g., ±0.25% for Class B-1). This is critical because strain is ΔL/L0 ; an error in L0 scales all results.
⑤ Repeatability: The algebraic difference between errors from two separate verification runs (simulating removal and re-mounting) must not exceed the class tolerance. This tests the consistency of mounting and system stability.
⑥ Verification Range: The specific span of strain over which the class is valid. An extensometer may be Class B-1 for small strains (0.5% to 5%) but drop to Class C for larger strains due to linearity or resolution limits.
Discovery and Evolution of Extensometer Verification
Early tensile testing relied on crosshead displacement, which includes machine compliance and grip slip, leading to inaccurate strain data, especially for modulus and yield. As precise strain measurement became critical, dedicated extensometers were developed. However, inconsistent calibration methods led to data variability between labs. ASTM E83 was introduced to harmonize these procedures, evolving from simple linearity checks to a comprehensive classification system. The 2010 revision (E83-10a) refined the criteria for multi-range systems, clarified the treatment of resolution as a limiting factor for lower range limits, expanded guidance on non-contact (Type 3) systems, and added detailed appendices on temperature effects, frequency response, and measurement uncertainty calculation. It shifted focus to ensuring the entire “system” (sensor + electronics + readout) meets the class requirements.
ASTM E83-10a distinguishes itself from general calibration guides by:
System-Based Approach: Verifying the entire chain (extensometer + electronics + readout) rather than just the sensor.
Dual Error Criteria: Using both fixed and relative error limits to define classes, ensuring accuracy at both low and high strains.
Resolution Enforcement: Explicitly linking the minimum verified strain to the system’s resolution (100x rule), preventing misuse at low loads.
Type Differentiation: Providing specific protocols for Type 1, 2, and 3 systems, acknowledging the unique challenges of non-contact and specimen-dependent gauges.
Environmental & Dynamic Guidance: Including informative appendices on temperature effects (X2), frequency response (X3), and uncertainty calculation (X4).
The key contents covered by the standard include:
Scope and definitions of extensometer types and system components.
Requirements for verification apparatus (accuracy, calibration interval).
Detailed verification procedures: gauge length measurement, positioning, temperature control, zeroing, and loading sequences.
Classification criteria (Table 1) with specific limits for error, resolution, and gauge length.
Protocols for multi-range systems and bidirectional testing.
Guidelines for verifying multiple readouts and data acquisition systems.
Recommended verification intervals (annual, max 18 months) and spot-check procedures.
Reporting requirements including all raw data, errors, and class assignment.
Appendices on examples, temperature effects, frequency effects, and uncertainty.
Referenced ASTM Standards
ASTM E6: Terminology Relating to Methods of Mechanical Testing.
ASTM E21: Test Methods for Elevated Temperature Tension Tests of Metallic Materials (references E83 for high-temp verification).
ASTM E251: Test Methods for Performance Characteristics of Metallic Bonded Resistance Strain Gauges (for bonded gauges not covered by E83).
ASTM E8/E8M: Test Methods for Tension Testing of Metallic Materials (mandates E83 verification for extensometers).
ISO 9513: Metallic materials — Calibration of extensometer systems used in uniaxial testing (International equivalent, highly harmonized).
ASTM E2309: Standard Practices for Verification of Displacement Measuring Systems and Devices used in Material Testing Machines.
ISO 7500-1: Metallic materials — Calibration and verification of static uniaxial testing machines (covers force, complements E83).
VAMAS TWA26: Projects related to standardization of optical strain measurement methods.
Analysis of Accuracy Classes and Error Sources
A core aspect of ASTM E83 is the interplay between fixed error, relative error, and resolution:
Class Hierarchy: Class A allows only ±0.1% relative error and 0.00002 fixed error, making it essential for modulus ( E ) where small strain errors cause large modulus deviations. Class B-1 (±0.5%) is the common requirement for yield strength. Class E (±1.0%) is acceptable for total elongation.
Resolution Limit: The standard dictates that the minimum verified strain ( ϵmin ) is limited by resolution ( r ): ϵmin ≥ 100×r. If a system has a resolution of 0.0001 in./in., it cannot be verified below 0.01 in./in. strain, regardless of linearity.
Gauge Length Sensitivity: For Type 1 systems, a 0.25% error in gauge length (Class B-1 limit) directly introduces a 0.25% error in all strain calculations. Precise measurement of L0 is mandatory.
Repeatability: The requirement to remove and re-mount the extensometer between runs tests the user’s ability to consistently position the device. Large differences between runs indicate mounting issues or mechanical instability.
Temperature Effects: Thermal expansion of the extensometer arms or changes in transducer sensitivity can shift zero or span. Appendix X2 highlights that verification at room temperature may not validate high-temperature performance; in-situ verification is recommended for critical high-temp tests.
Environment:
Temperature: Verification should be performed at the temperature of intended use. For room temperature tests, stable lab conditions are required. For elevated/cryogenic tests, verification should ideally occur in the same environmental chamber to account for thermal gradients, window refraction (optical), and material expansion.
Vibration: The setup must be isolated from vibrations that could affect resolution, especially for high-sensitivity Class A systems.
Test Procedure:
- Inspection: Visually inspect the extensometer and system components for wear, damage, or dirt. Ensure smooth operation.
- Gauge Length Measurement: Measure the actual gauge length ( L0 ) using direct (calipers/microscope) or indirect (marking on soft rod) methods. Calculate % error vs. specified
- Setup: Mount the extensometer on the verification apparatus exactly as used on specimens. Allow thermal stabilization.
- Zeroing: Displace slightly negative and return to zero to set the zero point. Repeat until stable.
- Loading Sequence (Run 1): Apply at least five displacement steps (excluding zero) covering the intended range. Steps should be spaced such that no gap exceeds 1/3 of the range. Record indicated vs. true strain.
- Simulated Remounting: Remove the extensometer (or move it away) and re-mount it to simulate a new test.
- Loading Sequence (Run 2): Repeat the displacement steps and record data.
- Data Analysis: Calculate strain error (fixed and relative) for each point. Determine resolution. Check repeatability (difference between Run 1 and Run 2 errors).
- Classification: Compare max errors and resolution against Table 1 limits. Assign the class. Determine the valid lower limit based on resolution (100×r ).
- Reporting: Generate a report including all raw data, errors, resolution, gauge length error, and the assigned class/range.
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