Alexei Volkov
The flexural modulus, also known as the bending modulus or modulus of rigidity, is an intensive property used to describe the stiffness of a material and its resistance to bending. It is determined by the ratio of stress to strain in flexural deformation and is calculated using the slope of a stress-strain curve. In the context of plastics, the flexural modulus is a critical parameter that indicates how much a plastic material can flex before experiencing permanent deformation. This value is influenced by factors such as temperature and the specific type of plastic used. Various testing methods, such as ASTM D-790 and ISO 178, are employed to determine the flexural modulus of plastics, providing insights into their mechanical behaviour and suitability for different applications.
| Characteristics | Values |
|---|---|
| Definition | The flexural modulus, also known as bending modulus or modulus of rigidity, is an intensive property that defines a material's capacity to resist deformation under bending. |
| Calculation | The flexural modulus is computed as the ratio of stress to strain in flexural deformation. It is determined from the slope of a stress-strain curve produced by a flexural test. |
| Units | The units of the flexural modulus are force per area, which can be Kg/cm^2 or MPa. |
| Testing Methods | ASTM D790, ISO 178:1993, and other standard testing methods involve applying a load to the center of a specimen to produce three-point bending at a specified rate. |
| Factors Influencing Flexural Modulus | The flexural modulus can be influenced by the choice of thermoset composite substrate, resin system, type of plastic (unreinforced or reinforced), and the type and volume of fibers used in reinforcement. |
| Applications | Flexural modulus is important in material selection for structural durability and safety in various fields, including aerospace, automotive, and construction. It is particularly relevant when comparing unreinforced and reinforced plastics. |
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What You'll Learn
Flexural modulus testing methods
The flexural test is a mechanical test that determines the flexural properties of a material, namely its flexural strength, flexural strain, and modulus of elasticity in bending. The test measures the force required to bend a beam of plastic material, and the data is used to select materials that can support loads without flexing.
There are three types of flexural tests: three-point, four-point, and simple cantilever loading. The three-point test is the most common type for plastic materials. This test involves placing a test bar on a three-point bend fixture and applying force to the centre until it reaches 5% strain or breaks. The three-point test concentrates the load at a single point under the loading nose, resulting in maximum bending moment and axial fibre stress at the centre of the sample. The four-point test, on the other hand, distributes the load over two points, resulting in a more uniform bending moment.
The ASTM D790 standard is widely used to determine the flexural modulus of plastic materials. This method involves testing plastic material on a three-point loading condition. The specimen size for this test is typically 127 mm by 12.7 mm by 3.2 mm for thermoplastic extrusion material. The test bar is set across two supports and pushed down in the middle until it breaks or bends a specified distance. The force on the bar and the deflection are continuously measured.
The flexural modulus is determined from the slope of a stress-strain curve produced by the flexural test. It is calculated using a specific mathematical equation that considers various parameters such as the support span, the speed of loading, and the maximum deflection. The flexural modulus provides valuable insights into the material's properties, such as its stiffness and resistance to bending.
However, it is important to note that current testing methods have limitations. For instance, flexural modulus testing may not accurately measure the flexural strength of specific materials, and inconsistencies in testing procedures can impact the results.
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Flexural strength
The ASTM D790 test is commonly used to evaluate the flexural strength of plastics and composites, including reinforced and unreinforced plastics, thermoplastics, thermosets, and high-modulus composites. This test measures the force required to bend a beam of the material and determines its resistance to flexing or stiffness. The dimensions of the specimen and the test setup may vary depending on the thickness of the material being tested.
The ISO 178 test is similar to ASTM D790 but has some differences. For example, in the ISO 178 test, the specimen is typically thicker, and the test is stopped only when the specimen breaks. If the specimen does not break, the test is continued, and the stress at 3.5% deflection is reported.
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Stiffness of plastics
The stiffness of plastics is a critical property that determines a material's resistance to deformation under applied force. It is described by the flexural modulus, which is the ability of a material to bend. The higher the flexural modulus, the stiffer the material. This property is important for selecting the right material for a given application.
Flexural modulus is an intensive property that is computed as the ratio of stress to strain in flexural deformation. It is determined from the slope of a stress-strain curve produced by a flexural test, such as the ASTM D790 and ISO 178:1993. The ASTM test is stopped when the specimen reaches 5% deflection or breaks before, whereas the ISO test is stopped only when the specimen breaks. If the specimen does not break, the test is continued, and the stress at 3.5% is reported. The flexural modulus is measured in pascals (Pa), with practical units of megapascals (MPa) or gigapascals (GPa).
The stiffness of plastics is important in structural plastic parts required to support or absorb a load, such as a bumper beam, plastic rod, or plastic extruded profiles. For example, a support beam must be rigid enough to bear a load without flexing. In contrast, a locking mechanism must be flexible enough to allow for the proper seating of a connection, then flex back into position to lock the connection.
The stiffness of plastics can be increased by adding fillers, especially polyolefins. The addition of a thermotropic liquid crystalline aromatic polyester, for example, can improve the tensile strength and modulus of blends with polyether ketone. Some plastics with high stiffness include homopolymer acetal (Delrin®), PAI (Torlon®), PEEK, and Ultem® – PEI resins.
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Bending modulus
The flexural modulus, also known as the bending modulus or modulus of rigidity, is a measure of a material's ability to resist bending without deformation. It is an intensive property calculated as the ratio of stress to strain in a flexural deformation. In other words, it indicates how much a material can flex before it undergoes permanent deformation.
For plastics, the flexural modulus is an important property, as it determines the material's stiffness and strength. The higher the flexural modulus, the stiffer the plastic; the lower the flexural modulus, the more flexible the plastic. This property is particularly crucial in applications where the plastic needs to support or absorb a load, such as in structural plastic parts like bumper beams, plastic rods, or extruded profiles.
The flexural modulus of plastics can be determined through standardised tests, such as ASTM D-790 and ISO 178:1993. These tests involve applying a load to a plastic specimen, typically in a three-point bending configuration, and measuring the amount of deflection or strain the material can withstand before permanent deformation or breakage occurs. The results are used to calculate the flexural modulus, which is typically measured in pounds per square inch (psi) or kilogram-force per square centimetre (Kg/cm2).
The flexural modulus of different plastics can vary significantly. For example, homopolymer acetal (Delrin®) has a flexural modulus of 14,300 psi, while PAI (Torlon®) can reach 24,000 psi. The flexural modulus is also dependent on temperature, with some plastics, like PAI, maintaining their properties even at extreme temperatures. Therefore, it is important to test the flexural modulus of plastics at temperatures that simulate their intended end-use environment.
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Plastic testing
The flexural modulus is a measure of a material's stiffness and bending resistance. It is indicative of how much a material can flex before permanent deformation. In plastic testing, the flexural modulus is a critical parameter that determines the suitability of the material for various applications.
For ASTM D790, the test is stopped when the specimen reaches 5% deflection or breaks before 5%. For ISO 178, the test continues until the specimen breaks, and if it does not, the stress at 3.5% deflection is reported. These tests provide data on the flexural strength, flexural strain, and modulus of elasticity in bending.
The flexural modulus is calculated from the slope of the stress-strain curve produced by the test. It represents the ratio of stress to strain in flexural deformation, indicating the material's resistance to bending. This value is crucial in determining the suitability of plastics for specific applications, such as structural plastic parts that need to support or absorb loads.
By understanding the flexural modulus, manufacturers can ensure that the plastic used in their products has the required flexibility and stiffness. This testing process is essential for quality control and product development, ensuring that plastic components perform as expected under various conditions.
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Frequently asked questions
The flexural modulus of a plastic is a measure of its stiffness and resistance to bending. It is an intensive property that indicates how much a material can flex before permanent deformation.
The flexural modulus is important for design engineers when selecting materials for structural durability and safety. It helps determine the bending resistance of plastics, which is key for applications such as support beams, where the load-bearing capacity is crucial.
The flexural modulus of plastics is commonly tested using ASTM D790 or ISO 178:1993 methods. The specimen is placed on a support span and loaded in the centre, creating three-point bending. The test is stopped when the specimen reaches 5% deflection or breaks before 5% for ASTM D790, while for ISO 178, it is stopped only when the specimen breaks.
The flexural modulus of plastics can be influenced by various factors such as the choice of resin system, the type of plastic (unreinforced or reinforced), and the use of reinforcing fibres or materials. The thermoset composite substrate and the presence of fibres in plastics can also significantly impact the flexural modulus.
