Sofia Martinez
The flexural strength of plastic is a measure of its resistance to bending or deflection. It is one of the key mechanical properties of plastic, along with stiffness, hardness, and toughness. The flexural strength of a plastic material is determined by its ability to withstand stress up to a point of yield, which is when the material undergoes permanent deformation. The value of flexural strength varies depending on the specific type of plastic and the test methods used. ASTM D790 and ISO 178 are commonly used test methods, with slight differences in sample dimensions and testing procedures. Flexural strength is typically higher than tensile strength in plastics due to the long-chain molecular structure of polymers, making them more resistant to bending stress. Understanding the flexural strength of plastics is crucial for designing and selecting appropriate materials for various applications, ensuring that the plastic can withstand the expected loads and stresses without failing.
| Characteristics | Values |
|---|---|
| Definition | Flexural strength measures a plastic's resistance to bending or deflection. |
| Other names | Flexural modulus, stiffness, bending strength |
| Measurement | Measured in psi or MPa. |
| Compared to tensile strength | Flexural strength values are often higher than tensile strength values. |
| Testing | ASTM D790 or ISO 178 are the standard tests. |
| Test sample size | ASTM: 3.2mm(thick) x 12.7mm(wide) x 125mm(long). ISO: 4mm x 10mm x 80mm. |
| Test procedure | The sample is placed on two fixed anvils with a certain distance between them and then pressed at the center from the top at a speed of 15 mm/min until the sample breaks. |
| Flexural strength of specific plastics | Varies depending on the material. For example, PAI (Torlon®) has a flexural strength of 24,000 psi, Ultem® has 22,000 psi, and Homopolymer acetal (Delrin®) has 14,300 psi. |
| Temperature dependence | The flexural modulus increases with reinforcement content and temperature. Cross-linked polymers are excellent at retaining their elastic modulus at high temperatures. |
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What You'll Learn
- Flexural strength is used to determine plastic's resistance to bending or deflection
- Flexural strength values are often higher than tensile strength values
- Flexural strength is measured in psi or MPa
- The test method for flexural strength usually involves ASTM D790 or ISO 178
- Flexural strength is dependent on temperature
Flexural strength is used to determine plastic's resistance to bending or deflection
Flexural strength is a measure of a material's resistance to bending or deflection. It is one of the key mechanical properties of plastic, along with stiffness, hardness, and toughness. The flexural strength of a plastic material is determined by its ability to withstand stress without breaking when stretched or pulled.
The flexural strength of a plastic material is typically referred to as its tensile strength or ultimate strength. This value can vary depending on the specific type of plastic and the injection molding process used. The higher the flexural strength, the better the plastic's resistance to bending or deflection.
Flexural strength is an important consideration in plastic product design, as it helps determine the suitability of a particular plastic for a given application. For example, a plastic with high flexural strength may be ideal for applications requiring high stiffness and strength, such as engineering components. On the other hand, a plastic with lower flexural strength may be more suitable for applications requiring flexibility and sealability, such as wire coatings or tubing.
The test method for determining the flexural strength of a plastic material typically involves a three-point or four-point test. In the three-point test, a plastic strip sample is placed on two fixed anvils with a certain distance between them. A force is then applied from the top at the center of the two fulcra, pressing down at a specific speed until the sample breaks or exceeds a certain limit. The four-point test is similar, but with two additional points of contact at a fixed distance in the center of the two fulcra.
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Flexural strength values are often higher than tensile strength values
Flexural strength, also known as bend strength or transverse rupture strength, is a material property that defines the stress in a material just before it yields in a flexure test. The flexural strength of a material is measured in terms of stress and represents the highest stress experienced within the material at its moment of yield.
Tensile strength, on the other hand, measures a material's ability to withstand tensile forces or stretching without breaking. It indicates the maximum stress a material can withstand before fracturing and is a fundamental mechanical property that characterises a material's response to tension.
When a material is bent, only the extreme fibres experience the largest stress. Therefore, if those fibres are free from defects, the flexural strength will depend on the strength of those intact fibres. However, if the same material is subjected to tensile forces, all the fibres experience the same stress, and failure will occur when the weakest fibre reaches its limit. Hence, it is common for flexural strengths to be higher than tensile strengths for the same material. This is particularly true for nominally brittle materials, such as ceramics and certain composites, where larger components may exhibit lower strengths due to an increased statistical chance of larger defects.
For example, PAI (Torlon®) is a plastic that offers both stiffness and strength, with a flexural strength of 24,000 psi and a tensile strength of 18,000 psi. In this case, the flexural strength value is higher than the tensile strength value.
It is important to note that the stiffness of a material is distinct from its strength. While stiffness refers to the ability of a material to distribute a load and resist deformation, strength refers to how much stress a material can withstand without breaking.
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Flexural strength is measured in psi or MPa
Flexural strength is the measure of a plastic's resistance to bending or deflection. It is a key mechanical property of plastic, along with stiffness, hardness, and toughness.
Flexural strength is measured in pounds per square inch (psi) or megapascals (MPa). Psi is a common unit of pressure in the US, defined as pound-force per square inch. MPa is the metric unit of pressure, equivalent to one million pascals (Pa). Pascal is the fundamental unit of pressure in the International System (SI).
When measuring the flexural strength of plastics, the ASTM D790 standard is used to measure the flexural properties of a material while under bending strain or deflection. The ISO 178 standard is used as a European equivalent.
Some examples of plastics and their flexural strength values include:
- Homopolymer acetal (Delrin®) with a flexural modulus of 14,300 psi
- PAI (Torlon®) with a flexural strength of 24,000 psi
- PEEK with a flexural modulus of 24,000 psi
- Ultem® with a flexural modulus of 22,000 psi
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The test method for flexural strength usually involves ASTM D790 or ISO 178
The flexural strength of a plastic refers to its stiffness, which is described by its flexural modulus (the ability of a material to bend). The higher the flexural modulus, the stiffer the material. The test method for flexural strength usually involves ASTM D790 or ISO 178.
ASTM D790 is a testing method to determine the flexural (bending) properties of reinforced and unreinforced plastics, high-modulus composites, and electrical insulation materials. ASTM D790 testing can be done on either a tabletop or floor model universal testing machine with a variety of accessories. ASTM D790 describes two different test procedures intended for different types of material. Procedure A, which is the preferred method, employs a strain rate of 0.01 mm/mm/min. Procedure B employs a strain rate of 0.10 mm/mm/min and is intended for materials that may not break at 5% strain if tested at the lower rate. ASTM D790 allows strain measurement to be taken from either crosshead displacement or the readings of an extensometer, described as Type 1 and Type 2 testing, respectively.
ISO 178 is a test method for determining the flexural properties of rigid and semi-rigid plastics by performing a three-point bend test on a universal testing system. A three-point bend test applies force at the midpoint of a rectangular specimen, which is freely supported at either end. The force applied is measured by a load cell, and the resulting deflection is measured by either the system's crosshead displacement or by a direct strain measurement device. There are four test types outlined in ISO 178, each specifying a deflection measurement method and an associated calibration accuracy requirement.
The choice between using ASTM D790 or ISO 178 depends on various factors such as the specific material being tested, the equipment available, and the requirements of the customer. ASTM D790 and ISO 178 differ in terms of specimen size, test speed, and the use of a deflectometer or compliance correction. ASTM D790 specimens have a preferred depth of 3.2 mm, while ISO 178 specimens have a preferred depth of 4 mm. ASTM D790 allows only one test speed, while ISO 178 allows a second, faster test speed to be used after the modulus is measured. Additionally, ISO 178 requires the use of a deflectometer or compliance correction to determine the modulus, while ASTM D790 only recommends it and allows the modulus to be calculated by crosshead displacement alone.
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Flexural strength is dependent on temperature
Flexural strength is the ability of a plastic material to resist bending or deflection. It is a critical property in determining the mechanical performance of plastics. The flexural strength of plastics is highly dependent on temperature, and understanding this relationship is essential for various applications.
At low temperatures, plastics tend to become more brittle and rigid. This decrease in temperature can lead to reduced flexibility and impact resistance in plastic materials. Consequently, they become more susceptible to breakage under bending or deflection. As a result, the flexural strength of plastics generally decreases at lower temperatures.
Conversely, as temperatures rise, plastics tend to soften and become more malleable. This increase in temperature can lead to improved flexibility, making the material more pliable and less prone to breaking under bending or deflection. Therefore, higher temperatures often result in increased flexural strength for plastics.
However, the relationship between temperature and flexural strength is not always linear. Different types of plastics exhibit varied responses to temperature fluctuations. For instance, thermoplastics undergo significant changes in rigidity and tenacity with changes in temperature. On the other hand, thermoset plastics tend to maintain their properties, including flexural strength, across various temperature ranges.
Additionally, the presence of fillers or reinforcements in plastics can also influence the temperature dependence of flexural strength. For example, TORELINA™ PPS, a commonly used plastic, exhibits varying flexural strength at different temperatures. The flexural modulus of PPS increases with reinforcement content, making it well-suited for applications requiring high elastic modulus in high-temperature environments.
In summary, the flexural strength of plastics is highly dependent on temperature. Understanding how flexural strength varies with temperature is crucial for selecting the appropriate plastic materials for specific applications, ensuring their reliability and functionality across different temperature ranges.
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Frequently asked questions
Flexural strength is a measure of a material's resistance to bending or deflection.
Flexural strength and stiffness are two different mechanical properties of plastic. Flexural strength refers to the resistance to bending, while stiffness refers to the ability of a material to distribute a load and resist deformation or deflection.
Flexural strength is typically measured using ASTM D790 or ISO 178 testing standards. The test method involves placing a plastic strip sample on two fixed anvils and pressing at the center to determine the bending strength.
Flexural strength is typically measured in pounds per square inch (psi) or megapascals (MPa).
The flexural strength of plastics can depend on temperature. For example, the flexural modulus of unreinforced elastomers is low at and near room temperature, exhibiting excellent flexibility.
