A revolution in the strain measurement
of filled plastics

The use of reinforced plastics is growing across many industries, but the mechanical testing of these materials places many challenges on both R&D and QC testing labs. To help deal with these growing challenges Instron has developed the second generation Advanced Video Extensometer (AVE 2). Challenges in filled plastic testing: high energy breaks. Filled plastics are more brittle and stronger than their virgin counterparts; therefore, when they break, they often release more energy. This excess energy can harm standard clip-on extensometers and can damage the bonded strain gauge element. This damage requires the device to be sent back to a service repair center, which costs money and can leave your machine unable to test. To protect these extensometers, many labs will remove the extensometer before failure, but removing the extensometer causes other problems in your lab. Since the extensometer needs to be removed for every specimen tested, the overall test will take longer making your lab less efficient. Furthermore, since there’s no extensometer on the specimen at break, the strain at break calculations won’t be measured according to the most common standards. Finally, removing the extensometer during the test may be easy at room temperature, but can prevent difficult challenges when wearing thick safety gloves while testing in a temperature chamber.

To simplify your procedure and protect your investment, the AVE 2 uses an optical measurement principal. The test can be started after placing two dots on the specimen; there’s no need to pause the test to remove the extensometer, allowing the operator to keep his hands outside of the test space. This helps to keep your operator safe and allows for more samples to be tested in less time. Since the device uses no moving parts it can be used to measure strain through failure without risking damage. This keeps your system running smoothly and your service costs low.

New materials, more testing In order to use a new material, many more tests are required to
validate and qualify the material for new applications. These tests can include tensile and flex over a variety of temperatures, typically from 608C up to 2508C. This poses some safety concerns when using a contacting device. When using a contacting extensometer, the operator must reach into the hot temperature chamber and place the extensometer onto the specimen. And once the test ends, the operator must reach into the chamber again to remove the device. This creates additional risk for the lab operators. Video devices don’t have this issue as there is no need to attach it to the specimen, reducing the number of times an operator has to reach into your chamber and saving time per each test. Furthermore, most bonded strain gauge extensometers can’t work properly over the full range. To complete this testing, many labs will purchase additional special-purpose extensometers to use inside the chamber, resulting in higher capital expenses and additional verifications each year. The Instron AVE 2 doesn’t have this challenge and can be used to measure strain through the window of a temperature chamber. As the glass of the chamber is kept clean, the AVE 2 will measure strain accurately, allowing operators to use just one device for all their

New standards, higher accuracy
The standards for measuring strain on plastics have changed and now require stricter accuracy than ever before. The latest version of ISO 527 now states that in order to use modulus on a plastic
specimen you need have an absolute measurement accuracy of 1 mm over the modulus range. This error is extremely small (about 1/100th the thickness of the typical human hair) and is very difficult to measure accurately. Most commercially available extensometers can’t measure this accurately and require a special extensometer with very low travel. These low-travel extensometers will measure modulus properly, but may not have the travel necessary to measure total elongation of many plastics. To measure these properties you may need to run two tests (one for modulus and a separate test for elongation), causing your lab to run slower and less efficiently. The AVE 2 can achieve the 1 mm accuracy with a travel of up to 200 mm. This allows you to use one extensometer and run one test
to gain both elongation and modulus, saving you time and money.

New behaviors, new techniques
Non-reinforced plastics are isotropic, meaning their mechanical properties are the same in each direction. This makes them very easy to model for engineers in finite element modeling software
and easy to predict behavior. However, reinforced plastics are often anisotropic and their anisotropic behavior makes modeling them difficult to predict. To better model them engineers started using Digital Image Correlation (DIC) techniques. DIC takes images of the specimen while it’s being tested and creates a 2D map of strain on the surface of the specimen. This 2D strain map can then be compared to the finite element modeling and errors in the modeling can be seen clearly. The DIC techniques are very powerful, but can be difficult to use. Most DIC systems come from third party vendors and are not designed for ease of use. They use a tripod mounting system for the camera, which makes it difficult to access the test space, and which can’t be bumped or else the system will need to be recalibrated. They also require a separate PC and set of electronics to control the camera, requiring interaction with multiple systems in order to get data. The biggest challenge is to pair the data from the testing
frame and the DIC system. The images have to be properly synchronized to the testing machine to make the data meaningful, but this process is entirely manual and allows for mistakes to occur.
If the operator makes any mistakes in this process then the results from the DIC test will be wrong and could cause errors in your analysis.

Instron provides a fully integrated DIC solution for Instron testing frames. This uses the AVE 2 as the DIC camera and it is mounted firmly to the frame. This mounting prevents the camera from getting in your way and because it’s rigid, it removes the possibility of being bumped out of position. The AVE 2 will use the same PC to run the DIC Replay software program, making it simpler for your operators. Since Instron provides every piece of the testing system, the images and the test data are automatically
synchronized making it faster to get data and making it impossible to create errors in your analysis.