How to Perform a Tear Strength Test on Vulcanized Rubbers and Thermoplastic Elastomers
Written by Kayla Thackeray
ASTM D624 is a common test method used to determine the tear strength of vulcanized rubber and thermoplastic elastomers. Due to the specimen shapes often used, this test is sometimes called a trouser, angle, or crescent test. This guide is designed to introduce you to the basic elements of an ASTM D624 test and will provide an overview of the testing equipment, software, and specimens needed. However, anyone planning to conduct ASTM D624 testing should not consider this guide an adequate substitute for reading the full standard.
Is ASTM D624 The Right Standard for Me?
ASTM D624 provides similar results to ISO 34, another common test that measure the tear strength of elastomeric materials. Though the ASTM and ISO organizations are currently working toward diminishing the differences between these two standards, the results cannot yet be considered comparable, and care must be taken to ensure the correct test standard is being used for a given application. ASTM D624 is intended to measure tearing strength only: anyone seeking to determine the tensile properties of elastomers should refer to ASTM D412.
Testing to ASTM D624 can be performed on a tabletop (dual column) or single column universal testing system due to its low crosshead travel. Laboratories that test to ASTM D624 are typically also running tensile tests to ASTM D412, and for the sake of convenience will often use the same test system for both. Because the maximum force applied during elastomer tear testing is significantly lower than the maximum force applied during elastomer tensile testing, load cells of differing capacities may be used. However, a 2580 series load cell can accommodate both types of tests for most applications because of their exceptional force range and accuracy. Instron’s 2580 series load cells are designed for 5900 series testing systems.
Software is a critical element of any testing system and can greatly simplify the testing process. Bluehill Universal comes pre-loaded with all of the most common ASTM and ISO standards, including ASTM D624, ASTM D412, and ISO 34.
The best types of grips to use for ASTM D624 are grips that supply a constant pressure like pneumatic side action or self-tightening roller grips. Both of these grips will prevent slipping during the test and will allow for quick specimen insertion. For some materials, basic screw action grips can be used. An important element of ASTM D624 testing is to align specimens vertically and in the same position for each test. Because tear tests are known to produce wide variation in results, it is important to eliminate as many variables as possible. Pneumatic side action grips have the advantage over other grips in that they help to align the specimen. It is also important to pair the grips with jaw faces to make visual alignment easier: jaw faces should be the same width as the specimen.
In order to properly align your specimen, set the initial position of the crosshead such that the top and bottom edge of the specimen line up with the top edge of the jaw face on the upper grip, and the bottom edge of the jaw face on the lower grip. This will ensure that the center of the specimen is aligned with the center of the grip separation.
When testing to this standard, the specimen is first nicked or cut, and a tear is propagated through this area of damage, which becomes the site of stress concentration. ASTM D624 includes two different definitions for tear strength based on the specimen type being tested.
There are 5 acceptable test specimens described in this standard: Types A, B, C, T, and CP. When testing specimen types A, B, or C, the tear strength is defined as the maximum force divided by the thickness of the specimen. When testing specimens type T or CP, the tear strength is the average or median force of the portion of the curve generated during the tear of the specimen divided by the thickness of the specimen. Either average or median is used depending on the type of stress/strain curve generated by the tested material.
Tear testing of elastomers is particularly important because most elastomers that break or fail in their intended application do so because of the initiation and spread of a tear. The five different specimen types in ASTM D624 testing provide a variety of options for geometries and stress concentrations that might lead to the initiation of a tear in a real-world application. The results from this test aren’t meant to quantify the tear strength of a material in service, but simply to understand how it behaves under the specified test conditions.
Types A, B, C and T specimens should all be prepared using a cutting die according to the dimensions in the specification. Type CP specimens should be molded. For types A and B, a nicking device or die should also be used to make the small cut.
Since the results of this test are highly dependent on the thickness of the specimen, it is critical to ensure that you measure your specimen precisely using a micrometer in accordance with ASTM D3767. ASTM D3767 specifies the tolerance and accuracy requirements of the specimen measurement device, along with the geometry of the contact surface and the force that is exerted onto the specimen during measurement. Specimen measurement device accuracy is often overlooked and can have a significant influence on your test results. For a seamless workflow, opt for an automatic specimen measuring device that will input the necessary dimensions into your testing software.
Elastomers tested under ASTM D624 are often destined for future use under non-ambient conditions. Because ambient conditions have a great impact on the tear strength of elastomers, care must be taken to ensure that test conditions mimic those of the intended end-use application. Test speed, temperature, humidity, specimen dimensions, and pretest conditions all have notable effects on test results and must be controlled in order for the test to produce useful data. To ensure that these elements simulate the material's end-use application, ASTM D624 is frequently performed inside an environmental chamber where heating or cooling (LN2 or CO2) can be used.
Instron's 600 series environmental chambers enable operators to monitor the temperature inside the chamber throughout the duration of the test. In addition, soaking time and temperature can be set up within Bluehill Universal to ensure that all specimens are conditioned appropriately before the start of the test.
For labs looking to increase their throughput, several modifications to the system setup can be made. Automatic specimen measuring devices and pneumatic grips all increase test efficiency by reducing the amount of manual input needed from the test operator. Although the test time for ASTM D624 is generally low, laboratory that also test to ASTM D412 might want to increase the amount of specimens tested per day. In this case, multi-station test frames may provide the best throughput because the operator can run up to five tests simultaneously. Fully automated test systems are also available and are designed to incorporate specimen measurement, specimen loading, testing, and removal. These systems can run for hours without requiring any operator interaction. In addition, these systems help reduce variability due to human error.