Alexei Volkov
The tensile and flexural strength of a material are two of the most commonly used values to compare materials. The tensile strength of a plastic material measures its resistance to stretching or tension, while the flexural strength measures its resistance to deformation under bending. The two values are not mutually exclusive, and a material can exhibit both tensile and flexural strength. 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.
Characteristics and Values of Plastic Joints
| Characteristics | Values |
|---|---|
| Tensile Strength | Measures a plastic material's resistance to stretching or tension |
| Flexural Strength | Measures resistance to bending or deflection |
| Flexural Modulus | Measures stiffness |
| Tensile Modulus | Measures stiffness |
| Ultimate Strength | How much stress plastic can withstand without breaking |
| Compressive Strength | Measures short-term loading capabilities |
| Impact Strength | Measured by Charpy or Izod impact tests |
| Hardness | Measure of resistance to deformation under concentrated compressive load |
| Toughness | Measure of energy absorption capacity during impacts |
| Flexural Strength Range | 40 to 1000 MPa for plastics |
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What You'll Learn
Reinforced plastics have high flexural strength
The flexural strength of a material measures its resistance to bending or deflection. This is distinct from tensile strength, which measures a material's resistance to stretching or tension.
The flexural strength of plastics can range from 40 to 1000 MPa, depending on the specific material type. Thermoset composite materials, specifically FR4, have remarkable flexural strength. FR4 is a popular grade of glass-reinforced epoxy laminate used extensively in the electronics industry due to its superior mechanical and electrical properties. Its flexural strength can be as high as 340 MPa, making it an excellent choice for applications requiring high mechanical stability.
The addition of carbon fiber and glass fiber reinforcement generally improves the tensile and flexural strength of plastics. However, the effect is more pronounced in injection-molded samples than in extruded materials. The improvement in creep properties results in a more noticeable macroscopic effect in the long term rather than short-term load performance.
The high flexural strength of reinforced plastics makes them suitable for a wide range of applications. For example, they are used in construction to improve load-bearing capacity and in high-performance vehicle components due to their strong, lightweight, and chemically resistant properties.
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Tensile strength vs. flexural strength testing methods
Tensile strength testing and flexural strength testing are two different methods used to determine the strength of materials. While tensile testing involves applying tension to a specimen until it fractures, flexural testing, or bend testing, measures the behaviour of materials when subjected to simple beam loading.
Tensile Strength Testing Methods
Tensile strength testing is a standardised process that involves applying tension to a specimen until it breaks. The test specimen is typically shaped like a long dumbbell, with a small cross-section or gauge length and two large ends called shoulders. The specimen is placed in a testing machine, and tension is applied until the specimen fractures. During this process, the elongation of the gauge section is recorded against the applied force. The data is then manipulated to calculate the engineering strain.
The most common testing machine used in tensile testing is the universal testing machine, which can be electromechanical or hydraulic. The choice of machine depends on the purpose, specification, and method of the test. For example, serrated faces are used for rigid substances with low ductibility, while pinned grips are suitable for plastic materials.
Flexural Strength Testing Methods
Flexural strength testing, also known as bend testing or transverse beam testing, is performed to determine the flexural modulus, flexural strength, and flexural yield strength of a material. Polymers are commonly tested with a 3-point bend test, while wood and composites are typically tested with a 4-point bend test.
A flex test creates tensile stress on the convex side of the specimen and compression stress on the concave side, resulting in shear stress along the midline. The span-to-depth ratio is controlled to ensure that primary failure comes from tensile or compression stress. Flexural strength is defined as the maximum stress in the outermost fibre of the specimen, calculated on the convex or tension side.
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Flexural strength in product design
Flexural strength is a fundamental concept in product design, especially in manufacturing and engineering. It is a critical property in the fields of additive manufacturing, 3D printing, and broader engineering disciplines. Essentially, it is a measure of a material's resistance to deformation under bending forces or loads. This measure of durability and resistance is essential in ensuring the safety, longevity, and performance of various products.
In product design, flexural strength is crucial in evaluating the ability of structural components to withstand bending-related failures and external pressures. For instance, in the automotive industry, flexural strength is vital for increasing the toughness of structural parts, reducing the likelihood of cracking, and improving impact resistance. This ultimately enhances the overall structural integrity under various bending loads.
The bending modulus is of utmost importance for product designers as it provides insights into the material’s response to bending forces, thereby aiding in the design of safe and effective products. A higher flexural modulus signifies that a material is more resistant to bending, thus making it more rigid. On the other hand, a lower flexural modulus suggests that the material is more flexible and has less resistance to bending, indicating its potential for deformation under stress.
Standardized testing methods such as 3-point and 4-point bending tests are used to determine a material’s flexural strength, with variations in testing to suit different materials. The main difference between tensile and flexural strength is that tensile strength measures a material's resistance to tension or stretching, while flexural strength measures resistance to bending or deflection. It is important to note that flexural strength values are often higher than tensile strength values, as polymers are long-chain molecular structures that resist bending stress more along the Y-axis.
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Stiffness and strength in plastics
The strength of a plastic material is typically referred to as its tensile strength or ultimate strength. This is a measure of how much stress a plastic can withstand without breaking when it is stretched or pulled. On the other hand, the stiffness of a plastic is its ability to distribute a load and resist deformation or deflection. Stiffness is described by the flexural modulus (the ability of a material to bend), which is measured in pounds per square inch (psi). The higher the flexural modulus, the stiffer the material.
Tensile strength and flexural strength are two different testing methods for determining the strength of a material. The former measures the resistance of a plastic material to stretching or tension, while the latter measures its resistance to bending or deflection. In practice, the load on a material is usually a combination of tension and bending, and other factors like compression or torsion may also be added to the mix.
The stiffness and strength of plastics can be increased in several ways. One way is by using a plastic material with higher strength and stiffness within the part. For example, glass fiber reinforcement can be introduced into a polypropylene resin to increase both the strength and stiffness of the polymer. However, this can also decrease ductility and result in lower part toughness. Another way to increase stiffness and strength is through the design of the plastic part itself. For instance, a common initial design recommendation to increase part strength is to increase the wall thickness. However, adding material can also increase the potential for manufacturing concerns and defects like sink marks and voids, which can negatively affect part strength.
Some examples of plastics with high stiffness and strength include Ultem® – PEI resins, which offer high stiffness and strength, superior elevated thermal resistance, and broad chemical resistance, with a flexural modulus of 22,000 psi; and PAI (Torlon®), which offers both stiffness and strength, with certain grades measuring a flexural strength of 24,000 psi and a tensile strength of 18,000 psi.
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The impact of temperature on plastic strength
The impact of temperature on the strength of plastics is a critical consideration in material selection and design. Plastics, like most materials, undergo thermal expansion as temperature increases, which can induce stresses and lead to unexpected failures if mated with another material that has a different coefficient of thermal expansion. This is particularly important when considering the dimensional stability and thermal resistance of plastic components in applications such as the automotive industry, where they encounter diverse temperature ranges.
At elevated temperatures, plastics can lose strength and toughness, becoming more susceptible to cracking, chipping, and breaking. Prolonged exposure to high temperatures can result in substantial degradation, with the rate of degradation proportional to the temperature and duration of exposure. Additionally, exceeding the heat deflection temperature (HDT) of a plastic material can cause it to distort, deform, or "creep" over time, especially when subjected to loads or forces. Most thermoplastics have an HDT of less than 500 degrees Fahrenheit, but this provides limited information about the long-term effects of continuous high-temperature exposure on their physical, mechanical, thermal, and electrical properties.
The mechanical properties of plastics, such as yield strength, modulus, and stiffness (flexural modulus), are significantly influenced by temperature. For example, at 85°C, the yield strength of a plastic material can decrease to less than 60% of its room-temperature value. This reduction in strength can impact the structural behaviour of the material, including its stress-strain relationship. Understanding the stress-strain curve at the application temperature is crucial for accurate modelling of the material's behaviour.
While thermoplastics are more susceptible to changes in temperature, with significant alterations in rigidity and tenacity, thermoset plastics generally maintain their properties across various temperature ranges. Polymers, in particular, exhibit non-uniform responses to temperature fluctuations, with materials rapidly losing their high-modulus properties as temperatures increase. This highlights the importance of selecting the appropriate plastic material for the specific operating environment and considering factors such as part geometry, material thickness, and expected loads or forces at different temperatures.
In summary, temperature has a significant impact on the strength and mechanical properties of plastics. Elevated temperatures can lead to reduced strength, distortion, and degradation, while low temperatures can affect flexibility and impact resistance. By understanding the thermal behaviour of plastics, designers and engineers can make informed decisions about material selection and ensure the long-term reliability of plastic components in various applications.
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Frequently asked questions
Tensile strength measures a plastic material's resistance to tension or stretching, while flexural strength measures its resistance to deformation under bending.
To determine a material's flexural strength, a force is applied and the resulting bending stress is calculated. The method of measurement can vary depending on the material type. For example, for wood, flexural tests are used, while the 4-point bending test is the preferred method for ceramics.
The flexural strength of plastics typically falls between 40 to 1000 MPa, while metals like steel and aluminium exhibit higher flexural strength. Steel's flexural strength ranges from 370 to 520 MPa, and aluminium's ranges from 70 to 700 MPa.
The addition of carbon fibre and glass fibre reinforcement can improve the flexural strength of plastics. Reinforced plastics have a higher flexural strength due to the redistribution of stress from the less robust polymer matrix to the more resilient fibres.
