Omar Khalid
Tensile strength is a critical factor in determining the durability of plastic products. It is a measure of the force required to break a material and the extent to which it stretches. Manufacturers in the plastic product industry conduct tensile strength tests to ensure product quality and safety. ASTM D638 is the most common test specification, using standard dumbbell or dogbone shaped specimens under 14mm in thickness. The specimen is placed between two jaws and pulled until it breaks, with the speed of jaw separation depending on the specimen's shape. This test helps manufacturers determine the durability of their products and their ability to withstand tensile loads. Tensile strength testing is a continuous process, performed at every level of production to ensure the high quality and versatility of plastic products across various applications, from automobiles to household goods.
| Characteristics | Values |
|---|---|
| Test Type | Tensile Test |
| Test Purpose | To measure the tensile strength of plastic materials |
| Test Machine | Universal Testing Machine (tensile testing machine) |
| Test Speed | Constant rate of speed, ranging from 0.05–20 inches per minute |
| Target Time | 30 seconds to 5 minutes |
| Test Specimen | Standard "dumbbell" or "dogbone" shape under 14mm thickness; Type I tensile bar |
| Test Temperature | Internal electric heaters for elevated temperatures and external carbon dioxide gas as coolant for reduced temperatures |
| Test Result | Indicates durability of plastics against tensile force |
| Test Frequency | Continuous procedure at every level of production |
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Tensile testing machines
To measure the tensile strength of plastic, a tensile testing machine, also known as a pull tester or universal testing machine (UTM), is required. This electromechanical test system applies a tensile (pull) force to a material to determine its tensile strength and deformation behaviour until it breaks. ASTM D638 is one of the most common plastic strength specifications, covering the tensile properties of unreinforced and reinforced plastics. This test method uses standard "dumbbell" or "dogbone" shaped specimens with a thickness of under 14mm.
The ASTM D638 test procedure begins by separating the tensile grips at a constant rate of speed, which depends on the specimen's shape and can range from 0.05 to 20 inches per minute. The target time from the start of the test to break should be between 30 seconds and 5 minutes. The capacity of the machine should also be sufficient for the materials being tested. For instance, a 1,000 lbf single-column system is typically adequate for most non-reinforced plastics, while a 2,000 lbf dual-column system is also commonly used.
Various companies offer tensile testing machines with different features and capacities. Instron, for instance, provides a range of tensile grips, including the Mechanical Wedge Action Grip, which is simple, robust, and perfect for plastics, with a force capacity of up to 250kN. They also offer the Advanced Screw Side Action Grips, which are commonly used for plastic film applications and have a force capacity of up to 10kN.
ZwickRoell is another manufacturer of tensile testing machines, including the zwickiLine, ProLine, AllroundLine, and E-series machine. These machines are designed to meet specific customer requirements, such as testing capacity, types of materials, applications, and industry standards like ASTM D638 for plastics. The E-series machine can handle large test forces starting at 330 kN.
Overall, tensile testing machines play a crucial role in determining the tensile strength of plastics, ensuring that materials entering the supply chain have predictable mechanical properties and are unlikely to fail in their intended use.
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Specimen shape
The ASTM D638 test utilises standard "dumbbell" or "dogbone" shaped specimens with a thickness of less than 14mm. The exact shape used depends on the material's rigidity and thickness. The speed at which the tensile grips are separated during the test depends on the specimen's shape and can vary from 0.05 to 20 inches per minute. The target time from the start of the test to breakage should be between 30 seconds and 5 minutes.
The dumbbell-shaped specimen is not uniform in width, and as a result, errors can occur when both the wide and narrow sections elongate at different rates. To address this issue, it is essential to use a servo-controlled mechanism to maintain a constant rate of speed during the test.
The Type I tensile bar is the most common specimen shape for the ASTM D-638 test. This shape allows for standardised testing across different plastic materials, facilitating comparisons of tensile strength between different samples or types of plastics.
By selecting the appropriate specimen shape and controlling the test parameters, accurate measurements of the tensile strength of plastics can be obtained.
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Speed of test
The speed of a tensile strength test depends on the specimen's shape and can range from 0.05 to 20 inches per minute. The target time from the start of the test to breakage should be between 30 seconds and 5 minutes. The test must be servo-controlled to maintain a constant speed. The capacity must be sufficient for the materials being tested.
A 1,000 lbf single-column system is usually adequate for most non-reinforced plastics, whereas a 2,000 lbf dual-column system is more common. Sometimes, a high-capacity 10,000 lbf model is required for larger samples and/or stronger materials such as reinforced plastics or composites.
The ASTM D638 test method, one of the most common plastic strength specifications, uses standard "dumbbell" or "dogbone" shaped specimens under 14mm in thickness. The specimen is placed between two jaws and pulled until it breaks. This test method is used to determine how much tensile force a plastic product can withstand without deformation.
A tensile testing machine is required to perform the ASTM D638 test. This machine helps manufacturers test the safety characteristics of their products during use.
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Elongation
In the context of tensile testing, elongation is one of the mechanical properties that can be determined. Tensile testing involves applying an ever-increasing tensile load to a specimen until it reaches its breaking point. By measuring the resulting deformation of the specimen, properties such as elongation can be calculated. Specifically, elongation measures the amount of stretching a material can undergo before breaking.
Tensile testing machines are used to conduct these tests, and they apply force to the sample while also measuring the force and elongation. There are several recognised standards for tensile testing, such as ASTM International's ASTM E8/E8M for tension testing of metallic materials and ASTM D638 for plastic tensile strength testing. ASTM D638 is one of the most common plastic strength specifications and uses standard "dumbbell" or "dogbone" shaped specimens under 14mm of thickness.
When conducting a tensile test, it is important to consider the sample preparation, fixing the sample securely in the machine, applying a tensile load, recording data, and then calculating the tensile strength and elongation. By measuring the force applied and the corresponding elongation throughout the test, one can gain valuable insights into the material's behaviour and make informed decisions about its suitability for specific applications.
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ASTM D638 test
ASTM D638 is a standard test method for determining the tensile properties of plastics, both reinforced and non-reinforced. It is one of the most common testing standards for plastics and is used to generate data for the control and specification of plastic materials. The test is performed on a universal testing machine (tensile testing machine) by applying a tensile force to a sample specimen and measuring its behaviour under stress.
The ASTM D638 standard outlines the specific specimen shapes and sizes that can be used, with five allowable specimen types. The most common is Type I, which is typically used for rigid plastics and is "dumbbell" or "dogbone"-shaped with a total length of 165 mm, a thickness of 3.2 mm, and a gauge length of 50 mm. This shape ensures that the break occurs in the centre of the specimen. The test method is designed for specimens with a thickness between 1.00 mm and 14 mm; specimens thinner than 1.00 mm should be tested using ASTM D882, while thicker specimens must be reduced through machining.
The ASTM D638 test involves separating the tensile grips at a constant rate of speed, which can range from 0.05 to 20 inches per minute, depending on the specimen shape. The target time from the start of the test to break should be between 30 seconds and 5 minutes. It is important that specimens are held securely inside the tensile machine, and side-action pneumatic grips are often used for this purpose.
The test measures various mechanical properties of the specimen, including tensile stress, strain, tensile modulus, yield point, point of break, and Poisson's ratio. Tensile strength, for example, measures the amount of force that can be applied to a plastic specimen before it yields (stretches irreparably) or breaks. ASTM D638 provides valuable data for manufacturers to properly gauge the mechanical strength of their plastic materials.
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Frequently asked questions
Tensile strength is a measure of the durability of plastic. It indicates how much tensile force a plastic product can withstand without deforming.
Tensile strength is important because it is a measure of the durability of plastic. This is a crucial factor in the quality of plastic products.
Tensile strength is measured using a tensile testing machine. The specimen is placed between two jaws and pulled until it is damaged.
ASTM D638 is one of the most common plastic strength specifications. It covers the tensile properties of unreinforced and reinforced plastics. This test uses standard "dumbbell" or "dogbone" shaped specimens under 14mm in thickness.
The target time for the test is between 30 seconds and 5 minutes.
