What is Force Verification of Testing Machines?
Force verification, as defined by ASTM E4, is the critical calibration procedure used to determine the accuracy of the force-indicating system of universal testing machines (tensile, compression, and fatigue testers). Unlike simple “calibration” which might imply adjustment, verification is the process of comparing the machine’s indicated force against known standard forces to quantify error. This practice ensures that mechanical property data (such as yield strength, tensile strength, and fracture toughness) derived from these machines are traceable to national standards (like NIST) and are reliable for engineering design, quality control, and material certification. It applies to both hydraulic and electromechanical testing systems across a wide range of capacities.
What is a typical Force Verification Result?
The results provide a quantitative assessment of the machine’s performance at specific test points:
① Percent Error: The difference between the force indicated by the machine and the actual force applied by the standard, expressed as a percentage of the indicated value.
② Accuracy Class: Based on the maximum percent error observed, the machine is assigned an accuracy class (e.g., Class A, B-1, B-2, C, or D). Class A represents the highest precision (±0.5% error), while Class D allows for larger errors (±5.0%).
③ Resolution: The smallest change in force that the machine’s indicating system can detect and display. This is crucial for determining the lower limit of the machine’s usable range.
④ Verification Range: The specific span of forces (from minimum to maximum) over which the machine meets the criteria for a specific accuracy class. Forces below the minimum verified load are considered unverified.
⑤ Linearity/Repeatability: While not always explicitly reported as a single number, the consistency of errors across increasing and decreasing loads indicates the linearity and hysteresis of the load cell or proving ring system.
⑥ Correction Factors: In some cases, if errors are consistent but outside tolerance, correction factors may be derived, though ASTM E4 primarily focuses on pass/fail classification per class.
Discovery and Evolution of Force Verification
ASTM E4-21 distinguishes itself from general calibration guides by:
Classification System: Providing a clear, tiered accuracy class system (A through D) that allows users to match machine capability to test requirements.
Resolution-Based Limits: Explicitly linking the minimum verified load to the indicating system’s resolution, preventing the misuse of machines at loads where they cannot read accurately.
Traceability Chain: Mandating that all standardizing devices be traceable to national metrology institutes, creating an unbroken chain of confidence.
Specific Loading Procedures: Defining exact protocols for approaching test points (always approaching from the lower force direction) to eliminate backlash errors.
The key contents covered by the standard include:
Scope and terminology specific to force verification (e.g., indicated force, true force, resolution).
Requirements for standardizing devices (proving rings, load cells) and their handling.
Detailed procedures for conducting the verification test (setup, loading sequence, data recording).
Calculation methods for percent error and determination of accuracy class.
Criteria for establishing the verification range (minimum and maximum loads).
Guidelines for reporting results, including certificates of verification.
Appendixes with examples of calculation and tables for common error limits.
Referenced ASTM Standards
ASTM E6: Terminology Relating to Methods of Mechanical Testing.
ASTM E74: Practice of Calibration of Force-Measuring Instruments for Verifying the Force Indication of Testing Machines (the companion standard for calibrating the standards used in E4).
ASTM E8/E8M: Test Methods for Tension Testing of Metallic Materials (references E4 for machine requirements).
ASTM E9: Test Methods of Compression Testing of Metallic Materials.
ASTM E2546: Practice for Instrumented Indentation Testing (references force verification).
ISO 7500-1: Metallic materials — Calibration and verification of static uniaxial testing machines — Part 1: Tension/compression testing machines (International equivalent).
ISO 376: Metallic materials — Calibration of force-proving instruments used for the calibration of uniaxial testing machines.
NIST Technical Note 1333: Guidelines for force calibration.
A2LA R205: Specific requirements for accreditation of calibration laboratories performing force calibrations.
Environment:
Temperature: Verification should be conducted in a controlled environment, typically 23±5 ℃ (or similar lab standard), to prevent thermal drift in load cells and proving rings.
Vibration: The testing area must be free from significant vibrations that could cause noise in the force signal, affecting resolution and stability.
Test Procedure:
- Preparation: Warm up the testing machine and electronic indicators for the manufacturer-recommended time (usually 15-30 mins).
- Standard Device Setup: Install the calibrated proving ring or load cell in the machine. Ensure axial alignment to avoid side-loading errors.
- Zeroing: Zero the machine’s force indicator and the standard device (if applicable) with no load applied.
- Loading Sequence: Apply force smoothly and continuously. Approach each target test point from the lower force direction (do not overshoot and come back).
- Data Recording: At each test point (minimum 5 points distributed across the range), record the force indicated by the machine and the true force from the standard device.
- Unloading (if required): For certain verifications or diagnostic checks, reduce load and record values to check for hysteresis.
- Calculation: Compute the percent error at each point. Identify the maximum error.
- Resolution Check: Determine the resolution at low loads to establish the minimum verified load limit.
- Classification: Assign the accuracy class based on the maximum error observed within the proposed range.
- Reporting: Generate a verification report stating the class, range, standard device ID, and date. Affix a calibration label to the machine.
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