The ASTM D638 test is the most common standard for determining the tensile properties of reinforced and unreinforced plastics. The introduction of the video extensometer in this context represents a significant improvement in terms of accuracy and detail of the data collected.
Cermac is the official dealer for Italy of X-Sight video-extensometers. These video-extensometers integrate perfectly with our universal testing machines.
What is a Video Extensometer?
Definition and Operation
A video extensometer is an instrument that uses video technology to precisely measure the strain of a material during a tensile test. Unlike traditional extensometers, which require physical contact with the specimen, the video extensometer measures strain without touching the material, instead using images captured by a high-resolution camera.
Advantages over Traditional Methods
The main advantages of the video extensometer include:
- Increased precision: More accurate measurements and less prone to contact errors.
- Non-intrusiveness: No direct contact with the specimen, reducing the risk of damage.
- Detailed data: Ability to capture data on minimal strain and progressive changes in the sample.
Applications in Tensile Testing
Video extensometers are particularly useful in tensile testing for fragile materials or those with delicate surfaces, where the use of traditional extensometers could compromise the integrity of the specimen.
Preparation for ASTM D638 Testing
Specimen Selection For the ASTM D638 test, it is important to select appropriate specimens according to the standard specifications. “Type I” specimens are commonly used for this test, but the choice of type depends on the material and available dimensions.
Preliminary Measurement Before performing the test, specimens must be accurately measured in accordance with ASTM D5947, using precision micrometers to ensure that the specimen dimensions are exact.
Mounting the Specimen on the Test Machine Once measured, the specimen is mounted on the universal test machine. It is essential to ensure that the specimen is correctly aligned to avoid errors in the test results.
Setting Up the Video Extensometer
Installation and Calibration The installation of the video extensometer requires initial calibration to ensure accurate measurements. This process includes setting up the camera and ensuring that the entire field of view is covered.
Correct Positioning of the Video Extensometer The video extensometer must be positioned so that the camera has a clear view of the specimen. This usually involves using specific lights and backdrops to improve contrast and visibility of the reference measures.
Checking initial settings
Before starting the test, it is important to check that all settings of the video extensometer are correct, including focus and illumination, to avoid measurement errors.
Performing the tensile test
Applying tensile force
The test starts with the gradual application of the tensile force on the specimen, which is monitored in real time by the video extensometer.
Real-time monitoring by video extensometer
The video extensometer continuously captures and analyses images of the specimen, measuring strain with high accuracy without physical contact.
Collection of strain data
The collected data includes the deformation of the sample in relation to the applied force, providing a detailed representation of the material’s behaviour under stress.
Analysis of results
Interpretation of strength data
Tensile strength data are analysed to determine the yield point and breaking point of the material. These data are crucial for understanding the strength of the material.
Calculation of modulus of elasticity
The elastic modulus is calculated from the slope of the stress-strain curve, representing the stiffness of the material.
Evaluation of elongation
The elongation of the material is evaluated by measuring the change in length of the specimen during the test, providing information on its ductility.
Advantages of using the video extensometer
Increased accuracy
Video extensometers offer higher accuracy than conventional devices due to their ability to measure without physical contact.
Less interference with the specimen
The absence of contact eliminates any interference that could alter test results, making measurements more reliable.
More detailed data capture
The ability to capture high-resolution images allows video extensometers to capture data on minimal strain, providing a more detailed picture of material behaviour.
Challenges and Solutions
Common problems when using the video extensometer
Some common problems include incorrect calibration, poor lighting and incorrect camera positioning. These can be solved with proper preparation and regular checks.
Tips for improving accuracy
To improve accuracy, it is important to maintain regular calibration, use adequate lighting and make sure the camera is positioned correctly.
Regular maintenance and calibration
Regular maintenance and calibration of the video extensometer are essential to ensure accurate and reliable measurements over time.
Conclusions
The use of the video extensometer in ASTM D638 testing represents a significant improvement in data accuracy and reliability. This advanced technology offers numerous advantages for plastics manufacturers, improving the quality of results and providing more detailed information on material behaviour.
Frequently Asked Questions (FAQ)
What is a video extensometer?
A video extensometer is an instrument that uses video technology to measure the strain of a material during a tensile test without physical contact.
Why use a video extensometer in tensile testing?
The video extensometer offers greater accuracy and reduces interference with the specimen, providing more detailed and reliable data.
How is a video extensometer calibrated?
Calibrating a video extensometer includes installing the camera, setting the field of view and checking the illumination and focus settings.
What are the main advantages over traditional extensometers?
The main advantages include higher accuracy, non-intrusiveness and the ability to acquire detailed data without physical contact with the specimen.
What are the common challenges in using a video strain gauge?
Common challenges include incorrect calibration, poor lighting and incorrect camera positioning, which can be overcome with proper preparation and regular checks.
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