Verification of Speed for Material Testing Machines: Best Practices and Methods

Accurate speed control is essential for material testing machines to deliver reliable and valid test results. Verifying the speed of these machines ensures that they operate within the specified limits, maintaining precision and consistency during testing. Whether it’s a tensile, compression, or flexure test, deviations in crosshead speed can significantly impact the material properties being measured. This article outlines best practices for speed verification, aligned with ASTM E2658 and other international standards like ISO 9513 and ISO/IEC 17025, which govern calibration procedures.

The Importance of Speed Verification

Verifying the speed of material testing machines ensures that they operate consistently, reducing the risk of errors and inaccuracies in test results. A material’s response to stress, strain, and loading is highly dependent on the rate at which these forces are applied. For materials sensitive to strain rate, even minor deviations in speed can cause significant variations in test outcomes. Regular speed calibration is, therefore, essential to ensure that your equipment meets the required performance standards.

Industries that rely on precise material testing, such as aerospace, construction, and automotive, must adhere to strict guidelines to ensure their products meet safety and durability standards. Speed verification ensures compliance with these standards, reduces the likelihood of material failures, and provides a traceable link back to recognized calibration standards.

Best Practices for Speed Verification

Regular Calibration and Traceability

Speed verification should be performed at regular intervals to ensure that the machine’s crosshead maintains the correct speed. According to ASTM E2658, speed verification should occur at least annually or every 18 months, depending on the frequency of machine use. Using traceable calibration devices ensures compliance with national and international standards such as ISO/IEC 17025, which defines the general requirements for the competence of testing and calibration laboratories.

These calibration devices must be traceable to recognized national standards, such as NIST (National Institute of Standards and Technology) in the U.S. or equivalent bodies elsewhere, ensuring that results are reliable and verifiable.

Methods for Verifying Speed

Start and Stop Method

One of the simplest and most widely used methods for verifying crosshead speed is the Start and Stop Method. This method uses a displacement calibration device and a time indicator. The device measures the displacement of the crosshead over a set period, and the speed is calculated using the displacement and time data. By comparing the measured speed with the set speed, you can verify whether the machine operates within acceptable limits.

Continuous Acquisition Method

For more precise verification, the Continuous Acquisition Method can be used. This method continuously records speed and displacement data as the machine operates, providing real-time feedback on performance. This is particularly useful for machines that must maintain a consistent speed over long periods or during complex tests. Continuous monitoring ensures that any variations are quickly detected and corrected.

Addressing Speed Errors

Once the verification process is complete, any discrepancies between the machine’s indicated speed and the measured speed must be addressed. According to ASTM E2658, the speed error is calculated using the following formula:

Percent Speed Error = (V_measured-V_set)/V_set ×100

Where:
V_measured – is the measured speed
V_set – is the expected set speed

If the error exceeds the allowable limits, adjustments to the machine’s control systems are necessary. Regular calibration and verification ensure that the machine’s speed remains accurate and that any potential errors are identified and corrected promptly.

International Standards for Speed and Displacement Verification

In addition to ASTM E2658, which is the primary standard for verifying the speed of material testing machines, other relevant international standards include ISO 9513, which covers the calibration of extensometers, and ISO/IEC 17025, which provides a framework for general calibration procedures. These standards ensure that the calibration and verification processes are consistent globally and meet quality assurance requirements.

This International Standard ISO 9513 specifies a method for the static calibration of extensometer systems used in uniaxial testing, including axial and diametral extensometer systems, both contacting and non-contacting. Extensometers indicate strain, either directly or indicating the change in gauge length of a specimen, which is converted into strain by dividing by the relevant gauge length.

Accurate displacement and strain measurements are critical in tests where crosshead speed and the resulting material deformation need to be tightly controlled. Verifying both speed and displacement ensures that the testing machine provides precise results across all testing conditions.

Conclusion: Ensuring Consistent Test Results

Regular speed verification is crucial for maintaining the accuracy and reliability of material testing machines. By following best practices outlined in ASTM E2658, ISO 9513, and ISO/IEC 17025, manufacturers can ensure that their equipment operates within specified parameters, reducing the risk of errors and ensuring compliance with international testing standards.

Whether using the Start and Stop Method or the Continuous Acquisition Method, consistent verification is key to delivering valid test results. Proper speed verification allows manufacturers to confidently perform tests that meet industry regulations, ensuring that their products are safe, durable, and reliable.

By adhering to these verification practices, material testing facilities can maintain a high standard of accuracy and performance, reducing downtime and improving the overall reliability of their test results.

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