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.
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:
- tensile strength
- yield strength
- yield strain
- strain at break
- modulus of elasticity
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.
Test System
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 34SC series or 68SC 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 in order to test until break. Instron's 6800 series is ideal for labs desiring advanced functionality in their test systems.
