Omar Khalid
The mechanical properties of plastic are important to understand for any structural applications. When considering the use of plastic in construction, it is crucial to know how it behaves under tension and compression. Tension and compression refer to the forces applied to a material that cause it to stretch or shorten, respectively. Plastic has different tensile and compressive strengths, which refer to how much stress the material can withstand without breaking. The compressive strength of a plastic material is its ability to withstand being pushed together, while tensile strength refers to its ability to withstand being pulled apart.
| Characteristics | Values |
|---|---|
| Tensile strength | Refers to how much stress a plastic can withstand without breaking when it is stretched or pulled |
| Compressive strength | Refers to the ability of a plastic material to withstand compressive forces before it deforms or breaks |
| Flexural strength | One of the most commonly used values to compare materials |
| Stiffness | Measure of the resistance of a material to deformation |
| Hardness | Measure of the resistance of a material to deformation under concentrated compressive load |
| Toughness | Measure of the energy absorption capacity of a material during impacts |
| Impact strength | The higher the value, the better the impact resistance |
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What You'll Learn
Plastic tensile strength
The tensile strength of a material refers to how much stress it can withstand without breaking when stretched or pulled. This is an important consideration for materials that will be stretched or under tension.
Some plastics have a higher tensile strength than others. For example, PAI (polyamideimide) has a tensile strength of 21,000 psi, making it the ultimate high tensile strength plastic. PAI has good wear and radiation resistance, low flammability and smoke emission, and high thermal stability. It is used in jet engines, internal combustion engines, thrust washers, and printed circuit boards.
Other plastics with high tensile strength include PPS (polyphenylene sulfide), with a tensile strength of 12,500 psi, and Nylon, with a tensile strength of 12,400 psi. PPS stands out from other thermoplastic resins in heat resistance, dimensional stability, rigidity, chemical resistance, flame retardance, and moisture-heat resistance. It is used in car parts, mechanical parts, and electrical and electronic parts. Nylon is used in clothing, rope, reinforcement in rubber materials, and substituting for low-strength metals.
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Plastic rigidity
The rigidity of a plastic refers to its resistance to deformation or deflection under load. In other words, it is a measure of how well the plastic can maintain its shape when subjected to external forces. The rigidity of plastics is influenced by factors such as crystallinity level, fibre orientation, and the addition of reinforcement materials like carbon fibre or glass fibre.
In robotics, the choice between rigid and soft materials is influenced by the specific requirements of the application. While traditional robotic systems have predominantly used rigid materials like metals and hard plastics, there is a growing interest in using soft materials and fabrics to create more flexible and adaptable systems. Soft polymers, elastomers, and fluids, for instance, can provide advantages in terms of conformity to irregular shapes and the creation of artificial muscles, despite introducing challenges in monitoring and controlling the system's behaviour.
The rigidity of plastics can be quantified through various mechanical tests, such as the tensile test, flexural modulus test, and compressive strength test. These tests help determine the plastic's ability to withstand tension, bending, and compression, respectively. By understanding the rigidity of different plastics, engineers and designers can make informed choices to select the most suitable materials for their specific applications, balancing the need for structural integrity, flexibility, and performance.
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Plastic compressive strength
The compressive strength of plastic gives an indication of its short-term loading capabilities. Compressive strength is expressed by the maximum strength (or stress) exhibited when a compression load is applied at a constant rate in a direction perpendicular to the cut cross-section of the test piece.
Plastic deformation occurs after the yield point is exceeded, but when a certain strain is exceeded, buckling occurs without breakage, causing compressive strength to decrease. This buckling is unique to compression tests and is a result of the elastic modulus of the material and the strain in the test piece. For instance, in unreinforced PPS, the yield strength is maximized, but in materials prone to brittle fracture, such as reinforced PPS, buckling occurs before the yield point is reached.
The temperature also affects the compressive strength of plastics. Generally, the higher the reinforcement content, the higher the compressive strength at high temperatures. The addition of carbon fibre and glass fibre reinforcement improves the compressive strength of the polymer, with the effect being more visible in the long term due to the improvement of creep properties.
The shape of the plastic part also affects its deformation under load. Large, flat parts will deform less under the same load compared to taller parts. This is due to the "Shape Factor", which is the ratio between the area of the perimeter and the area of the top and bottom surfaces.
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Plastic impact strength
The impact strength of a plastic is a measure of its toughness or its ability to resist high-impact forces without breaking, fracturing, or deforming. Tough materials are naturally more impact-resistant than other materials. However, various factors can affect a material's impact resistance. For instance, plastics tend to be more brittle at cooler temperatures and tougher at warmer temperatures. Prolonged exposure to elevated temperatures and UV light tends to decrease a plastic's toughness.
The impact strength of thermoplastics is measured either by Charpy or Izod impact tests. In the Charpy test, a small rectangular rod is struck by a pendulum at high speed and the energy absorbed while breaking the specimen is measured; the higher the value, the better the impact resistance. The Izod test is similar, but the specimen is notched to obtain information under more severe conditions. The notched Izod test measures the energy absorbed by a plastic, which helps determine how much energy it can stand before deformation.
Some common impact-resistant plastics include acrylonitrile butadiene styrene (ABS), which is known for its high impact resistance and mid-range cost, polyphenylsulfone (PPSU), PEEK, TPE or thermoplastic elastomer, and even some nylons and acetals. Polyamide-imide (PAI) is another example of an impact-resistant plastic. It is extremely tough, strong, and stiff, with good chemical resistance, electrical grade insulation, and low thermal expansion.
The impact resistance of a plastic is also influenced by its design. For example, notches and sharp corners tend to increase the chance that a part will break under force, whereas rounded corners tend to make parts more durable.
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Plastic stability in tension and compression
Plastic Stability in Tension
Tensile strength is a critical factor in assessing the stability of plastics under tension. It refers to the ability of a plastic material to resist breaking when stretched or pulled. The tensile strength of plastics is influenced by factors such as crystallinity level, fiber orientation, and the addition of reinforcements like carbon fiber or glass fiber. High tensile strength plastics, such as PAI (polyamide-imide), Ultem, PEEK (polyetheretherketone), PPS (polyphenylene sulfide), and Nylon, are widely used in structural applications. These plastics can withstand high stress without breaking, making them suitable for components in jet engines, car parts, and mechanical parts.
Plastic Stability in Compression
Plastic stability in compression, also known as compressive strength, refers to the ability of a plastic material to withstand compressive forces before deforming or breaking. Unlike tension, compression involves pushing materials together, resulting in a decrease in length. Compressive strength is measured in stress units like pounds per square inch and is crucial for understanding the short-term loading capabilities of different plastic materials. The addition of carbon fiber or glass fiber reinforcements can improve the compressive strength of plastics, although the effect is more pronounced in the long term due to improved creep properties.
The stability of plastics in tension and compression can be assessed through standardized test methods. For tension, the DIN EN ISO 527 test briefly applies a load in one direction to measure tensile strength and rigidity. In compression testing, a mechanical apparatus applies controlled compressive forces to a specimen to evaluate its resistance to deformation or failure. These tests provide valuable insights into the mechanical properties of plastics, helping engineers select the most suitable materials for specific applications.
In conclusion, plastic stability in tension and compression is determined by the plastic's ability to resist external forces without deforming or breaking. Tensile strength and compressive strength are key metrics used to assess these capabilities, with reinforcements and specific testing methods employed to enhance and evaluate stability, respectively. Understanding plastic behaviour under tension and compression is essential for optimizing material performance and ensuring the safe use of plastics in a wide range of applications.
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Frequently asked questions
Tensile strength is the ability of a material to resist breaking under tensile stress, or in other words, how much stress a plastic can withstand without breaking when it is stretched or pulled.
Compressive strength is the ability of a material to withstand compressive forces before it deforms or breaks. It is the measure of a material's resistance to loads reducing its size.
Some plastics with high tensile strength include PAI, Ultem, PEEK, PPS, and Nylon.
The addition of carbon fibre or glass fibre reinforcement generally improves the compressive strength of plastics. This is because it improves the creep properties of the material, leading to better long-term load performance.
