Sofia Martinez
There are several methods to test the strength of plastic, each of which is used to measure different qualities. ASTM D638 is one of the most common plastic strength tests and is used to determine the tensile properties of unreinforced and reinforced plastics. This test involves using a standard dumbbell or dogbone specimen under 14mm of thickness. The specimen is then placed in a tensile tester, which pulls the material apart until it breaks. This test can be used to measure the yield strength, breaking strength, and modulus of elasticity of the plastic. Another test is ASTM D882, which is used to quantify the tensile strength of thin plastic sheeting and films less than 1.0 mm in thickness. This test involves cutting a 1 wide specimen from the material and loading it into a tensile tester. The tester then pulls the material apart, and a load cell measures the force and distance to calculate the yield strength, breaking strength, and modulus of elasticity. Other tests include flexural strength, Izod impact strength, and tensile modulus or Young's modulus.
| Characteristics | Values |
|---|---|
| Tensile modulus | Stiffness, elasticity, or rigidity |
| Rockwell hardness | |
| Elongation at break | |
| Flexural strength | Resistance to deformation under load |
| Impact | |
| Hardness | |
| Flammability | |
| Shear strength | |
| Tensile strength | |
| Yield point | |
| Tensile impact energy | |
| Flexural stress at 5% strain | |
| Izod impact test | Energy required to break notched specimens under standard conditions |
| Puncture resistance | |
| Anti-fog characteristics | |
| Thickness | |
| Number of film layers |
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What You'll Learn
Tensile strength
The most common testing standard for determining the tensile properties of reinforced and non-reinforced plastics is ASTM D638. This test is performed by applying a tensile force to a sample specimen and measuring various properties of the specimen under stress. ASTM D638 is conducted on a universal testing machine (also called a tensile testing machine) at tensile rates ranging from 1 to 500 mm/min until the specimen fails. The most common specimen for ASTM D-638 is a Type I tensile bar.
There are five allowable specimen types for ASTM D638, which differ in size depending on the thickness of the specimen and the amount of material available. ASTM D638 only applies to rigid plastic samples between 1.00 mm and 14 mm in thickness. If your sample is a sheet or film less than 1.00 mm thick, it should be tested using ASTM D882. ASTM D638 provides similar results to ISO 527-2, but they are not considered technically equivalent due to differences in specimen size and test requirements.
ISO 527-1/-2 first defines the specimen shapes, the accuracy of specimen dimension measurements, the permitted tolerances, and the initial gauge length. The easiest way to perform the test is by using the testXpert standard test program for the ISO 527-1/-2 standard. All settings for performing tests to ISO 527 have already been preset with guaranteed standard compliance.
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Flexural strength
There are three types of flexural tests: three-point, four-point, and simple cantilever loading. The three-point flexural test is the most common type for testing plastic materials. This is because the highest or maximum bend stress occurs under the loading anvil, which is suitable for homogeneous materials such as plastic. The three-point test is also used to evaluate resins and laminated fibre composites. The test can be performed according to ASTM D790, which recommends evaluating at least five specimens of each sample. Another standard for flexural properties is ISO 178, which focuses on three-point bending tests and covers thermoplastic molding and extrusion materials.
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Impact testing
There are various methods of impact testing, including the Charpy impact test, the Izod impact test, and the Tensile Impact Test. The Charpy impact test, first introduced in 1900, is one of the most widely used methods for determining the impact strength of materials. It involves dropping a pendulum from a certain height to strike a notched specimen, which absorbs energy upon impact. The energy absorbed is determined by measuring the pendulum's height before and after the fracture.
The Izod impact test is similar to the Charpy test but uses a different arrangement of the specimen under test. In this test, a pivoting arm is raised to a specific height and then released, swinging down and hitting a notched sample. The energy absorbed by the sample is calculated from the height the arm swings to after hitting the sample. The ASTM International standard for this test is ASTM D256, and the results are expressed in energy lost per unit of thickness.
The Tensile Impact Test is another method for testing the impact resistance and strength of plastic materials. In this test, a notched specimen is held between two grips, and a sudden force is applied to measure the energy required to break the material. This test is useful for evaluating plastic materials for applications that may experience sudden and repeated impacts, such as automotive or aerospace components. It can also be used to optimise the design of plastic parts and in quality control to ensure that the plastic material meets the minimum standards and requirements.
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Hardness
The hardness of a material refers to its level of resistance to permanent indentation. Testing the hardness of plastic is important to assure its quality and meet customer expectations.
There are three types of hardness tests used for testing the hardness of plastic: shore-durometer hardness, Rockwell hardness, and Knoop & Vickers test. The Rockwell hardness test is the most commonly used test for plastics. It involves applying a test force in two stages to eliminate the impact of surface roughness and measuring errors. The test force is selected from values of 49 N, 132 N, 358 N, and 961 N, and the indentation depth is measured after a dwell time of 30 seconds. If the depth is outside the range of 0.15 to 0.35 mm, the test force needs to be adjusted. The Rockwell hardness (HR) is then calculated using the residual indentation depth and a formula defined by the standard.
The ASTM D785 and ISO 2039-1/-2 standards also describe methods for determining the hardness of plastics using a loaded ball indenter. The ISO 2039-1 standard calculates the hardness value by measuring the surface of the indentation caused by the ball indenter. The ISO 2039-2 standard uses a Rockwell hardness testing machine with scales E, L, M, and R.
The hardness of plastic can also be determined by applying an external force with an indenter at the center of the sample material. If the indenter deeply penetrates the plastic, it is considered soft, while if it fails to penetrate, the plastic is extremely hard. The hardness level can be calculated by measuring the indenter depth. However, this method requires proper training to perform correctly and obtain accurate results.
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Flammability
The flammability of plastics is determined by various test methods from Underwriters Laboratories (UL), FMVSS, ASTM, and ISO. UL is an independent organization that provides product safety testing and certification. Their UL94 testing procedures and rating systems for thermoplastics flammability are the generally accepted standard throughout most of the world.
The UL94 standard testing method consists of two preselection test programs conducted on plastic materials to measure flammability characteristics. The first program determines the material's tendency to either extinguish or spread the flame once the specimen has been ignited. The second program measures the ignition resistance of the plastic to electrical ignition sources.
The UL94 test also includes two types of burn tests: the Horizontal Burn test and the Vertical Burn test. In the Horizontal Burn test, a specimen less than 3 mm thick is oriented horizontally and ignited. It passes the test if it burns at a rate of less than 76 mm/min. In the Vertical Burn test, the specimen is placed vertically and must self-extinguish within 10-30 seconds. The Vertical Burn test also considers whether the specimen dripped flaming particles. Depending on the test results, the material is given a V2, V1, or V0 rating, with V0 being the best rating.
Other flammability test methods include the Glow Wire test and the Needle Flame test, which are popular in Europe. These tests simulate thermal stresses that materials may be subjected to in failure situations.
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Frequently asked questions
Some standard measurements for plastic strength include flexural strength, tensile strength, and Izod impact strength.
Tensile strength is the measure of how much stress a plastic can withstand without breaking when stretched or pulled. The ASTM D882 standard test is conducted by cutting a 1" wide specimen from the material and loading it into a tensile tester. The tester then pulls the material apart until it breaks, and a load cell measures the force and distance.
Flexural strength is the measure of how well a plastic resists deformation under load. This number indicates the load required to cause a given test sample to show a 5% deformation of its outer surface.
