How to Perform a Tensile Test on Thin Plastic Film to ASTM D882
Written by Charlie Pryor
Thin plastic films and sheeting are a popular choice for packaging and can be found protecting many everyday items from leftover food to consumer goods. It is critical that plastics manufacturers and packaging companies obtain an accurate mechanical profile of the plastic films they are producing to ensure that they meet the required expectations. ASTM D882 is a common testing standard that is used to determine the tensile properties of thin plastic films and is commonly used for in-line quality control purposes. This guide is designed to introduce you to the basic elements of an ASTM D882 tensile test and will provide an overview of the equipment, software, and samples needed. However, anyone planning to conduct ASTM D882 testing should not consider this guide an adequate substitute for reading and following the full standard.
What Does it Measure?
ASTM D882 is specifically designed for testing plastics that are less than 1 mm in thickness, and can be used to test any type of plastic specimen within this defined thickness range. Plastics thicker than 1 mm should be tested to ASTM D638. ASTM D882 testing is conducted on a universal testing machine (also called a tensile testing machine) at a constant crosshead speed. The following are the most commonly measured tensile properties:
Test Method and Specimens
During ASTM D882 testing, specimens are pulled in tension until failure. The crosshead speed is dependent on the length of the specimen and can be calculated by multiplying the initial grip separation (length of the specimen) by the initial strain rate in mm/mm x min. The initial strain rate is conditional on the maximum elongation of the specimen at break. Because plastic films and sheeting tend to be highly elastic, higher crosshead speeds are commonly required. One critical factor in testing to ASTM D882 is using an appropriate preload. Thin film specimens are usually relaxed and not taut when loaded into grips for testing. A preload, which can be easily configured in Bluehill Universal testing software, helps to remove any slack in the specimen prior to recording data during testing, ensuring that results are accurate and repeatable.
This test is meant for specimens uniform to within 10% of the thickness over the length of the specimen between the grips. Thicker specimens approaching the 1 mm thickness limit must be uniform to within 5%. Specimens need to be between 5.0 and 25.4 mm wide, and at least 50 mm longer than the grip separation used during testing. The standard specimen gauge length for testing is 250 mm. However, if this is not possible, then a minimum specimen length of 100 mm (50 mm gauge length) is acceptable as long as it is established that this does not affect the results. If the sample is of isotropic material, the standard requires at least five specimens to be tested from each sample. If the sample is suspected of being anisotropic, however, then at least ten specimens must be tested, five along their long axis, and five along the supposed anisotropic axis. Furthermore, specimens used to determine modulus cannot be used to determine tensile strength or any other tensile properties, which means that anyone testing to ASTM D882 would be wise to produce a large number of specimens in order to appropriately measure all of the relevant properties.
Due to the low break forces of these specimens, ASTM D882 testing is typically performed on a single column universal testing machine such as Instron's 3340 series or 5940 series systems. Complications can arise, however, due to the fact that many thin film specimens exhibit high elongation at break. When pairing these specimens with the standard 250 mm gauge length, it can sometimes require an extra height single column frame or possibly a dual column frame such as the 5960 series in order to test until break. Instron's 5900 series is ideal for labs desiring advanced functionality in their test systems.
Due to the flexible, delicate nature of these plastic specimens, gripping can be a challenge. Pneumatic side action grips provide an adjustable inlet air pressure to ensure that specimens can be clamped sufficiently throughout the test. Pneumatic grips are preferable to screw action or hand-tightened grips when testing thin films, as the latter grips adds variability to the clamping pressure and can allow for more inconsistency in test results and a higher chance of jaw breaks or slippage. In some cases for very thin film specimens, self-tightening roller grips can also be used.Because ASTM D882 specimens are fragile, jaw face selection is also critical to successful testing. Serrated jaw faces are not recommended because they can cause damage to the specimen when gripping, leading to jaw breaks or inaccurate results. Rubber faces do not damage thin film specimens and have been shown to prevent specimen slippage as they thin out in the faces during the testing process, making rubber faces paired with pneumatic grips the preferred combination. This combination also produces the most repeatable results.
Rubber jaw faces should match the width of the gripped or tabbed end of the specimen. The most common face size for this type of material is 25 mm x 25 mm (1 in x 1 in). The matching width allows for ease of specimen alignment, which is critical for good breaks and accurate test results. In the event that the specimen width is on the smaller side of the tolerance for the standard, it is necessary to have jaw faces at least as wide as the specimen, but wider is also acceptable in this case.
Line contact gripping also is a possibility for film specimens that are too thin for rubber faces. Line contact faces include one smooth jaw face and an opposite face that is rounded, helping distribute the clamping force along one single line perpendicular to the crosshead’s movement. The most critical aspect of gripping is to avoid slippage when testing, as well as maintain an even stress distribution throughout each specimen while testing.
While ASTM D882 does not require extensometry in order to measure strain, the use of an extensometer always result in the most accurate strain measurement. Non-contacting video extensometry is often the best solution for thin film testing, as typical clip-on extensometers weigh down the specimen, while knife edges have the possibility of tearing the specimen during testing. Instron's AVE2 video extensometer is able to measure strain down to a single micron without physically contacting the specimen.
Instron's Bluehill® Universal testing software makes it easy to report calculations in compliance with the standard. The software also comes with a pre-configured test method for ASTM D882 testing, complete with specific test parameters and calculations to help ensure compliance.