What Are the Standards for Displacement Verification?
ASTM E 2309 Standard Practices for Verification of Displacement Measuring Systems and Devices Used In Material Testing Machines was released in March of 2005. 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 to NIST. In Europe and Asia we continue to calibrate displacement to Instron methods, accredited to NVLAP and NATA.
Do I Need to Verify Displacement?
The benefit of displacement verification varies with the use and application of your 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.
- LVDT’s 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 safety of either the operator or item under test.
Understand the Limitations
The use of extensometry is often the best solution for determination of precise measurement of specimen strain in “under load” conditions.
The standard displacement verification service is done under “no-load” conditions. A no-load displacement verification shows that the displacement transducer 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 displacement data taken during a test. There are several potential sources of displacement during a loaded test in addition to that of the specimen. This means that you cannot always make a direct comparison of “no-load” data to actual specimen displacement.
Typical contributors to differences in no-load displacement readings vs. loaded displacement readings are:
- For Screw-Driven Machine: Crosshead, screw and load cell deflections, gear or drive belt stiffness and backlash, grip or fixture deflections, specimen/grip interaction and bearing and drive nut deflections.
- For Servohydraulic Machine: Crosshead, column and load cell deflections, grip or fixture deflections, specimen/grip interaction, piston rod and actuator body deflections and compressibility of oil.
The displacement calibration under “no-load” must be correct as a minimum requirement before verifying performance under load. Poor accuracy at no load also means poor accuracy during the test.
Use of Crosshead Displacement ReadingsTesting is done at low loads and/or high elongations: This condition is normally where the use of crosshead displacement readings is most valid as long as the load frame is high quality and in good condition. Non-specimen deflections should be either predicted or measured, then compared to the accuracy requirements of the application.
Testing is done at higher loads or low elongations: Displacement data may still be valid if care is taken to understand what the data means and how it relates to the deflection of the specimen. Use of an extensometer is usually recommended.
Testing done with compliance correction: This is a feature available in some materials testing software packages that subtracts non-specimen deflections from the displacement reading. This will often allow a very good approximation of strain, depending on the testing conditions and the quality of the load frame. The displacement measuring system should be verified first for this technique to be valid.
Measuring actual strain in the specimen is critical: Use of a calibrated extensometer is normally required.
Specifications and Description
The standard verification includes five data points within the range specified by the customer, and if required, up to 20 data points can be verified. The crosshead or actuator position reading is compared to a precision high-resolution glass scale displacement standard with calibration directly traceable to NIST. This standard is attached 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 and 1% FS for servohydraulic systems. The transducers used typically have an uncertainty of measurement ranging from 5 to 20 microns, but this will vary depending on the displacement measured.
- Accuracy: 0.5% of reading (screw-driven machines)
1.0% of full scale (servohydraulic machines)
- Minimum Displacement: normally 2mm to 8mm
- Maximum Displacement: none. Data typically taken in 100 mm or 250 mm blocks
- Uncertainty of Measurement of the Calibration Standard: 5 to 20 microns
- Verification to Higher Accuracy: consult factory
How Can I Improve the Management of My Lab's Calibration Program?
Instron can make life easier for you. We will store all relevant information about your equipment and site in our database. That allows us to keep track of recalibration dates (specified by you), so we can call ahead of time, and schedule convenient and timely recalibrations. When the verification is complete, the Instron engineer can print the certificate and data report on-site before he leaves. The data and certificate are electronically backed up in a secure database at the Instron factory and can be retrieved, if necessary. Instron's highly trained calibration engineers know your equipment. They can identify potential problems early, perform preventive maintenance, and know how to handle and use it properly.