What is Displacement Verification?
Displacement verification measures the actual length of travel by the machine's crosshead or actuator over a defined range of operation. Verification is necessary in order to ensure that the displacement reported by the testing machine is traceable back to a national or international standards laboratory, which means that devices used in the verification process have themselves been calibrated with adequate calibration standards.
Do I Need to Verify Displacement?
The benefit of displacement verification varies with the use and application of your testing instrument. You should calibrate your machine's displacement measuring system if any of the following apply to you:
- Displacement is critical to your testing needs and is reported in the test results
- You use the crosshead extension to characterize strain when the use of an extensometer is impractical or impossible
- You use the crosshead extension to characterize displacement of a specimen or component.
- You need to know that your displacement reading or crosshead/actuator position is precise and repeatable.
- LVDTs commonly found in servohydraulic systems are often not linear throughout their range.
- Displacement or position readings are critical to the control of your system and could impact the safety of either the test operator or test specimen
What are the Basic Requirements of Testing to ASTM E 2309?
ASTM E 2309 requires that speed sensing devices are verified at least once per year as a system, in place, and operating as in actual use (Except that during verification there should be no specimen installed in the machine). At least two verification runs must be performed within the displacement ranges selected, and each run consists of 5 displacement values (points) within the specified range. The difference between any two successive points must be no larger than 1/3rd the difference between the selected minimum and maximum test displacement values. System verification is invalid if the displacement sensing devices are removed and checked independently of the testing machine. If calibration adjustments are made during testing, the testing process must begin again with two verification runs and issuance of a new report.
What are the Limitations of Displacement Verification to ASTM E 2309?
ASTM E 2309 verification is done under "no-load" conditions. A no-load displacement verification shows that the displacement transducers and the related electronics and software are working properly and are capable of making accurate measurements. Operators need to use caution, however, when comparing no-load displacement data to data taken during a test. There are several potential sources of additional displacement during a loaded test that are not measured during ASTM E 2309 verification. This means that users cannot always make a direct comparison between no-load data and actual specimen displacement. Typical contributors to discrepancies between loaded and no-loaded displacement readings are:
For Screw-Driven Machines: Crosshead, screw, and load cell deflections; gear or drive belt stiffness and backlash; grip or fixture deflections; specimen/grip interaction; bearing and drive nut deflections.
For Servohydraulic Machines: Crosshead, column, and load cell deflections; grip or fixture deflections; specimen/grip interaction; piston rod and actuator body deflections; compressibility of oil.
The displacement calibration under no-load conditions must be correct as a minimum requirement before verifying performance under load. Poor accuracy at no-load also means poor accuracy during testing.
The use of extensometry is often the best solution for determination of precise measurement of specimen strain in "under load" conditions.
The Role of Crosshead Displacement in Testing
When determining specimen strain in "under load" conditions, the use of extensometry is often the best solution. There are, however, some instances where crosshead displacement can be a valid source of test data.
As long as the load frame is of high quality and in good condition, crosshead displacement readings can often be valid when testing is done at low loads and/or high elongations. Non-specimen deflections should be either predicted or measured, then compared to the accuracy requirements of the application or standard.
If testing is done at higher loads or low elongations, displacement readings from the crosshead may still be valid. Care must be taken to understand what the data means and how it relates to the deflection of the specimen. Use of an extensometer is usually recommended.
Some materials testing software packages feature compliance correction, which automatically subtracts non-specimen deflections from the displacement reading. This will often allow a very good approximation of strain, depending on the testing conditions and quality of the load frame. The displacement measuring system should still be verified first for this technique to be valid.
Can Instron Verify my Machine to ASTM E 2309?
Yes. Instron is at the leading edge of test machine calibration and verification and was an active participant in the development of ASTM E 2309. Instron is accredited by NVLAP (lab code 200301-0) to perform displacement verifications that are fully compliant with ASTM E 2309 and ISO/IEC 17025 using measurement standards traceable back to NIST. In Europe and Asia we continue to calibrate displacement to Instron methods, accredited to NVLAP and NATA.
To perform a verification, Instron will compare the crosshead or actuator positioning of your testing machine to a precision high-resolution glass scale displacement standard with calibration directly traceable to NIST. This instrument attaches to the machine and feeds its measurement data automatically into the calibration software used by the Instron Service Engineer. Displacement accuracy is verified to 0.5% for electromechanical systems 1% for servohydraulic systems. The transducers used typically have an uncertainty of measurement ranging from 5 to 30 microns, but this will vary depending on the displacement measured.
Instron offers two different services for displacement verification to ASTM E 2309 (these services are combined with speed verification to ASTM E2658).
Instron's Standard speed and displacement meets all ASTM requirements by providing:
- Verification of 3 customer-selected test system speeds via 3 data runs.
- Verification of any displacement range between 0.8" and 40" of total displacement. Instron will verify 5 total points between 10% and 100% of the range displacement desired, with data being collected via 3 data runs. If required, Instron will verify up to 20 data points.
Instron's Enhanced Speed and Displacement verification exceeds the requirements of the applicable ASTM standards and mitigates additional risk by providing:
- Verification of 5 speeds, including 3 customer-selected speeds for their most common test speeds as well as verification at 1mm/min and 1270 mm/min - the reasonable minimum and maximum test speeds. This data is collected via 3 data runs.
- Verification of any displacement range between 0.8" and 40" of total displacement. Instron will verify 5 points for decade of measurement between 1% and 100% of the customer-specified range.
Though ASTM D2658 only requires two data runs, both our Standard and Enhanced verification services perform three data runs. This is because three runs are required to determine the repeatability of measurement. Repeatability results are reported on your verification certificate.
Do I Need Enhanced Services if the ASTM Requirements are met with the Standard Service?
Instron's Enhanced Verification Services are helping organizations mitigate additional risk by providing more thorough verifications. This service may benefit you if:
Your test methods change frequently. If new test methods are frequently being written, you may be at risk of collecting data at speeds you haven't had verified.
Your requirements change frequently.If your testing standards or auditing entities are frequently changing or becoming more strict, additional displacement verifications may help prevent unwanted interruptions in your testing.
You collect test data near the boundaries of your verifications.If data is being collected near the boundary of your most recent verification, you may be at risk of capturing data that falls outside of the verified range.