Ethan Clarke
Tensile strength is a critical factor in determining the overall performance and functionality of plastic materials. It refers to the maximum stress a plastic material can withstand before it breaks under tension or yields (stretches irreparably). The tensile strength of plastic is influenced by several factors, including the type of plastic, its molecular structure, and the processing conditions used during production. ASTM D638 is the most common testing standard for determining the tensile properties of plastics, and it is performed by applying a tensile force to a sample specimen to measure its strength and deformation characteristics.
| Characteristics | Values |
|---|---|
| Tensile strength | The ability of plastic material to withstand a maximum amount of tensile stress without failure |
| Strength | Measure of the resistance of a material to external stress |
| 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 |
| Elastic elongation | The percent elongation that can be reached without permanent deformation |
| Modulus | Measure of how well a material resists deformation |
| Yield strength | The stress a material can withstand without permanent deformation |
| Ultimate strength | The maximum stress a material can withstand |
| Breaking strength | The stress coordinate on the stress-strain curve at the point of rupture |
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What You'll Learn
Tensile strength definition
Tensile strength is a key measurement used by researchers, engineers, and quality control departments to evaluate the mechanical properties of a material, product, or component. It is defined as the maximum mechanical tensile (pulling) stress that a specimen can withstand before failure. The definition of failure depends on the type of material and its design.
Tensile strength is calculated by dividing the cross-sectional area of the specimen by the maximum achieved tensile force. In other words, it is the force per unit area of the material. In the International System of Units (SI), the unit is the Pascal (Pa) or a multiple thereof, often megapascals (MPa). In the American system, it is expressed in pounds per square inch (lbf/in² or psi).
When a material is subjected to tensile stress, its atomic bonds are stretched and eventually broken as the stress increases. When the stress is removed, the behaviour of the material depends on whether its atomic bonds were merely stretched or broken. If the atomic bonds were only stretched, the material is within its elastic region and will return to its original shape. If the atomic bonds have been broken, the material has entered its plastic region and will not return to its original shape. This process is permanent and irreversible.
The tensile strength of a material can be evaluated at the point where it enters the stage of plastic deformation (yield point) or at the point where the material ruptures (ultimate tensile strength). Ductile materials such as metals are typically evaluated at the point of yield, whereas brittle materials such as composites are often evaluated at the point of rupture.
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Tensile strength testing methods
Tensile strength is a measure of the force required to break a material and the extent to which it stretches before reaching that breaking point. Tensile testing is used to determine how plastics perform under controlled tension. The choice of testing methods depends on the plastic material's composition and intended use.
One of the most common plastic strength specifications is ASTM D638, which covers the tensile properties of unreinforced and reinforced plastics. This test method uses standard "dumbbell" or "dog-bone" shaped specimens under 14mm of thickness. A universal testing machine (UTM) or tensile testing machine is needed to perform this test. The test begins by separating the tensile grips at a constant rate of speed, which depends on the specimen's shape and can range from 0.05 to 20 inches per minute. The target time from the start of the test to break should be between 30 seconds and 5 minutes.
Another standard for tensile testing of plastics is ISO 527, which has different parts for specific applications, such as ISO 527-2 for injection and extrusion-molded plastics and ISO 527-3 for plastic films and sheet material with a thickness of up to 1 mm.
ASTM D638 and ISO 527 are technically equivalent, but they do not deliver fully comparable results due to differences in specimen shapes, test procedures, and result determination.
Other ASTM standards relevant to tensile testing of plastics include ASTM D882 for plastic film and sheeting material, ASTM D412 for vulcanized rubber and thermoplastic elastomers, ASTM D1708 for microtensile specimens, ASTM D5323 for polyethylene geomembranes, and ASTM D3039 for polymer matrix composite materials.
In addition to the standardized tests mentioned above, there are other mechanical tests performed on plastics, such as impact testing, which measures the energy absorbed by a material during fracture, and flexural testing, which determines the force required to bend and break the material. These tests are conducted to detect possible quality issues before selling products to clients and to ensure the optimal choice of materials for specific applications.
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Factors influencing tensile strength
Tensile strength is the ability of a plastic material to withstand a maximum amount of tensile stress without failure while being stretched or pulled. It is a critical property for designing and manufacturing reliable parts.
Material Composition
The elements that make up an alloy greatly influence its mechanical properties, including tensile strength. For instance, adding carbon to iron creates a carbon-steel alloy, which has greater strength than pure iron. The presence of additives, such as fillers and plasticizers, also affects the tensile strength of plastics. Fillers can increase the stiffness and strength of plastic but reduce its elongation at the break. Conversely, plasticizers improve flexibility and elongation but decrease tensile strength by making the polymer chains more susceptible to breaking under tension.
Heat Treatment and Processing
The way a material is processed significantly impacts its mechanical properties. For example, the manufacturing process and purity or composition of a material influence its tensile strength.
Grain Size and Orientation
The size and orientation of a material's grains or crystallites play a crucial role in determining its mechanical properties. Finer grains generally result in higher strength due to the increased number of grain boundaries impeding dislocation movement. Additionally, the orientation of grains influences a material's response to stress, as some orientations facilitate slip and dislocation movement more than others.
Cost and Availability
Budget constraints and the availability of materials are also factors that influence the selection of materials for specific applications. While more expensive materials may offer superior performance, it is necessary to weigh the benefits against the increased cost. Some materials may have long lead times or be challenging to source, impacting production schedules.
Environmental Conditions
Environmental conditions, such as temperature, can also affect the tensile strength of a material. For example, PEEK (polyether ether ketone) is a popular thermoplastic with excellent temperature resistance, contributing to its high tensile strength.
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Tensile strength and deformation
Tensile strength is the ability of a plastic material to withstand a maximum amount of tensile stress without failure. The stress occurs while the material is being pulled or stretched. It is the point when a material goes from elastic to plastic deformation.
Plastic deformation is when a material does not return to its original dimensions. This process is permanent and irreversible. Elastic deformation, on the other hand, is reversible and non-permanent. The material returns to its original dimensions.
The tensile strength of a plastic specimen can be measured by applying a stretching or tensile force to it. This force causes the object to extend, and its behaviour can be understood using a stress-strain curve in the elastic deformation region, known as Hooke's Law. The extension produced by the force depends on the plastic material and dimensions of the object. Stress is defined as the force per unit area of plastic and is measured in N/m2 or Pa.
Tensile strength is also referred to as ultimate tensile strength, which is the maximum stress a material can withstand while being stretched or pulled before breaking. The yield point is the onset of plastic deformation, where the material can no longer recover its original shape.
The tensile properties of plastics are crucial for engineering design purposes and are often included in material specifications to ensure quality and meet requirements.
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Tensile strength in engineering design
Tensile strength is a fundamental concept in engineering design, particularly when working with plastic materials. It refers to the ability of a material to withstand tension loads without failing. In other words, it measures the maximum stress a material can endure while being stretched before breaking.
When it comes to plastics, tensile strength is critical. Plastic materials undergo elastic deformation when stressed, meaning they return to their original dimensions once the stress is removed. However, if the stress exceeds the material's tensile strength, it undergoes plastic deformation, resulting in permanent and irreversible changes.
For engineers, understanding the tensile strength of plastics is vital for several reasons. Firstly, it helps in selecting the appropriate material for a specific application. Different plastics have varying tensile strengths, and the choice depends on the expected loads and requirements of the final product. For instance, if designing a mechanical part that will be exposed to mechanical loading, knowledge of tensile strength is crucial to ensure the material can withstand the expected stresses without failing.
Additionally, tensile strength data is often included in material specifications to guarantee quality and meet requirements. Engineers can use this information to determine the breaking strength of various polymers used in the plastics industry. This is particularly relevant in 3D printing, where the tensile strength of a material may differ from its theoretical strength due to the printing technology used. By understanding these variations, engineers can make informed decisions about material selection and design optimization.
Furthermore, tensile strength is essential when considering the directional properties of materials. Some 3D printing technologies produce parts with isotropic structures, where properties are uniform in all directions. In contrast, other materials may exhibit different tensile strengths in various directions, such as the Z and XY directions. Engineers must consider these directional variations to ensure the final product performs as expected, regardless of the loading direction.
In summary, tensile strength plays a pivotal role in engineering design, especially when working with plastic materials. It provides valuable data that guides material selection, ensures quality, and influences design choices. By considering tensile strength, engineers can create reliable products that meet the demands of their intended applications.
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Frequently asked questions
Tensile strength is the ability of a plastic material to withstand a maximum amount of tensile stress without failure while being pulled or stretched.
Plastic deformation is when the material does not return to its original dimensions. This process is permanent and irreversible.
ASTM D638 is the most common testing standard for determining the tensile properties of reinforced and non-reinforced plastics. It involves applying a tensile force to a sample specimen and measuring various properties of the specimen under stress.
