Omar Khalid
Tensile strength is a measure of the force that can be applied to a plastic before it stretches irreparably (yielding) or breaks. Tensile testing is important to ensure manufacturers can properly gauge the mechanical strength of their materials. There are various methods for testing tensile strength, including ASTM D638, ASTM D882, and ISO 527-1/-2. ASTM D638 is the most common testing standard for determining the tensile properties of plastics and is performed by applying a tensile force to a sample specimen and measuring various properties of the specimen under stress. ISO 527-1/-2 defines specimen shapes, accuracy for specimen dimension measurements, and permitted tolerances.
| Characteristics | Values |
|---|---|
| Testing standard | ASTM D638, ISO 527-1/-2, ASTM D882, ISO 16012, ASTM D5947 |
| Test method | Applying a tensile force to a sample specimen and measuring various properties of the specimen under stress |
| Test machine | Universal testing machine (tensile testing machine) |
| Tensile rates | 1 to 500 mm/min |
| Tensile force | Until the specimen fails (yields or breaks) |
| Tensile strength | The amount of force that can be applied to a plastic before it yields or breaks |
| Tensile modulus | Measurement of how much a material can deform (stretch) in response to stress before it yields |
| Elongation | Increase in gauge length after a break, divided by the original gauge length |
| Poisson's Ratio | Relationship between how far a material is stretched and how thin it gets during stretching |
| Specimen type | Rigid plastic samples between 1.00 mm and 14 mm in thickness |
| Specimen shape | Defined by ISO 527-1/-2 |
| Specimen size | Type I tensile bar |
| Specimen grip | Side-action pneumatic grips with serrated jaw faces |
| Grip force | Maintained by air pressure |
| Software | Bluehill Universal, testXpert |
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What You'll Learn
Tensile strength testing standards: ASTM D638, ISO 527-1, and ISO 527-2
ASTM D638 is the most common testing standard for determining the tensile properties of reinforced and non-reinforced plastics. It is performed by applying a tensile force to a sample specimen and measuring its different properties while under stress. ASTM D638 is conducted on a universal testing machine (also known as a tensile testing machine) at tensile rates ranging from 1 to 500 mm/min until the specimen fails (yields or breaks). ASTM D638 allows for five different specimen kinds, each with a different size based on the thickness of the specimen and the amount of available material. The most prevalent are Type I specimens, which are 3.2 mm thick and are often made by injection moulding.
ISO 527 is an international standard closely aligned with ASTM D638, providing procedures for tensile testing of plastics. ISO 527 has multiple parts covering different material forms: ISO 527-1 covers general principles, while ISO 527-2 covers test conditions for moulding and extrusion plastics. ISO 527-2 is intended for testing rigid and semi-rigid plastics, whether moulded, extruded, machined, or cast. It also applies to reinforced plastics, with the exception of fibre-reinforced plastics, which are covered under ISO 527-4 and ISO 527-5.
While ASTM D638 and ISO 527 are technically equivalent, they do not deliver fully comparable results due to differences in specimen shapes, test procedures, and determination of results. For example, ISO 527 emphasises consistency in testing speed, specimen size, and data collection methods, enhancing the comparability of results across international laboratories. ASTM D638, on the other hand, typically recommends test speeds of 5 mm/min to 50 mm/min, while ISO 527 provides broader guidance depending on material type and specimen dimensions. Additionally, ISO 527 includes more flexible data reporting requirements compared to ASTM D638, which emphasises detailed reporting of stress-strain curves, tensile strength at yield and break, elongation, and modulus of elasticity.
In terms of specimen dimensions, ISO 527 often provides more extensive guidance for various materials and product forms. The preferred specimen shape for ISO 527-2 is a dumbbell-shaped specimen with a nominal width of 10 mm and a thickness of 4 mm. ASTM D638 Type I specimens are also dumbbell-shaped, with a thickness of 3.2 mm and a gauge length of 50 mm.
It is important to note that the choice between ASTM D638 and ISO 527 standards often depends on geographical location. North American manufacturers usually test to ASTM D638, while those in Europe and Asia primarily test to ISO 527-2. Customers in China and Malaysia test to both standards.
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Specimen preparation and gripping
Specimen preparation is a critical aspect of tensile testing for plastics, as it ensures the accuracy and consistency of the test results. Here are the key considerations and procedures for preparing and gripping specimens for tensile testing:
Specimen Selection and Requirements
The ASTM D638 standard, which is commonly used for tensile testing of plastics, specifies five allowable specimen types, including Type I tensile bars, which are the most commonly used. The choice of specimen type depends on the thickness of the material and the amount of material available. It is important to follow the standard's guidelines for specimen dimensions, shape, and thickness to ensure accurate results.
Gripping Techniques and Equipment
Secure gripping of the specimen inside the tensile testing machine is crucial. Side-action pneumatic grips with serrated jaw faces are often recommended for rigid plastics. Pneumatic grips maintain a constant gripping force using air pressure, ensuring stability even if the specimen thickness changes during the test. For reinforced materials requiring forces above 10 kN, manual wedge action grips are typically preferred.
Test Setup and Normalization
After the specimen is securely gripped, it is important to normalize any unwanted forces that could interfere with the test results. Instron's Specimen Protect feature, available on some universal testing machines, automatically adjusts the crosshead to keep unwanted forces under control during the setup phase. Additionally, software like Bluehill Universal can normalize forces across multiple specimens, ensuring consistent results.
Environmental Considerations
When conducting tensile tests, the environment and equipment used can impact the results. ASTM standards and ISO 527 provide guidelines for the test environment and equipment requirements. For example, the ISO 527-1/-2 standard defines specimen shapes, accuracy requirements for dimension measurements, permitted tolerances, and initial gauge length. Refer to the specific standards for detailed instructions.
Pre-Testing Procedures
Before conducting the tensile test, it is essential to follow any pre-test procedures specified in the chosen standard. For example, the ISO 527-1/-2 standard recommends using a specified pre-load to ensure reproducible results, independent of the operator. Additionally, the ASTM standards may refer to ISO 16012 and ASTM D5947 for defining requirements and methods of dimensional measurements.
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Measuring tensile modulus and Poisson's ratio
When measuring the tensile modulus and Poisson's ratio of plastic, it is important to consider the unique properties of this material. Poisson's ratio is a measure of the Poisson effect, which is the phenomenon where a material expands in directions perpendicular to the direction of compression. Conversely, when stretched, the material tends to contract in the transverse direction. This is evident when a rubber band is stretched and it becomes thinner.
The Poisson's ratio is calculated by dividing the cross-over strain (change in width) by the hub strain (change in length). It is represented as a lower-case Greek letter nu (v). For most materials, the Poisson's ratio falls between 0.0 and 0.5, with elastic materials tending towards 0.5 due to their high compressibility, and less compressible materials like steel having a ratio of around 0.3. The Poisson's ratio of plastics falls within this range of 0 to 0.5, with natural rubber at 0.5 and reinforced plastics ranging from 0.1 to 0.4.
Young's modulus, on the other hand, evaluates the relationship between stress and strain in a material, measuring its elasticity or ability to withstand deformation under stress. It is determined using the pressure-strain relationship, where stress is the applied power per unit region, and strain is the subsequent deformation ratio. Young's modulus is represented by the letter E.
The tensile modulus, or Young's modulus, can be determined through tensile tests, which directly measure both E and v. These tests are often used to study the behaviour of plastics under tension. The shear modulus, G, can also be estimated from rheological measurements and used to calculate E.
It is important to note that the temperature changes can significantly impact the tensile modulus and Poisson's ratio of plastics. Additionally, the magnitude of stresses and strains, as well as the direction of loading, can influence the Poisson's ratio. These factors should be carefully considered when designing experiments to measure the tensile modulus and Poisson's ratio of plastics.
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Software for normalising forces and removing errors
Software plays a crucial role in normalising forces and removing errors during tensile strength testing of plastics. Tensile testing involves measuring the force required to break a plastic sample and the extent of its elongation until that breaking point. This data is plotted on a stress-strain curve, which is used to determine the tensile modulus and other mechanical properties of the material.
To ensure accurate and consistent results, software like Bluehill Universal can be used to normalise forces across multiple specimens. This software removes any slack or compressive force, ensuring that the results are not affected by minute forces that can interfere with the test if not properly treated. By using software to normalise forces, test operators can avoid the need to manually balance these forces, which can cause an offset in results.
In addition to force normalisation, software is also used to manipulate and output data during tensile testing. Newer testing machines have digital measurement systems that use electronic sensors to collect data, which is then processed by software. This data manipulation ensures that the results are not specific to the geometry of the test sample, allowing for standardised comparisons. Software can also be used to automatically adjust settings to prevent damage to the specimen or system during the setup phase, such as Instron's Specimen Protect feature for their 6800 Series universal testing machines.
Furthermore, software can aid in the calculation of important metrics such as engineering stress and engineering strain. As tensile testing machines apply increasing force, software can perform real-time calculations to determine the engineering stress and the corresponding elongation, generating data points that can be graphed into a stress-strain curve. This automation ensures accuracy and repeatability in the measurement and analysis of tensile strength.
Overall, software is essential for normalising forces, removing errors, and processing data during tensile strength testing of plastics. By utilising software like Bluehill Universal and advanced digital measurement systems, operators can obtain consistent and accurate results, facilitating the characterisation of plastic materials' mechanical properties.
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Test equipment: tabletop machines, extensometers, and more
Tensile testing machines are available in a variety of sizes and force capacities, ranging from 0.02 N to 2,000 kN. Low-force testing is usually performed on an electromechanical single- or dual-column tabletop machine, while higher-force applications require floor model frames. Tabletop universal testing machines, such as Instron's 6800 Series, are commonly used for ASTM D638 testing, with 5 kN or 10 kN systems being the most frequently used. However, as reinforced plastics and composites increase in strength, higher-capacity units, such as 30 kN or 50 kN systems, may be required.
Specimen Protect is a feature available on the 6800 Series universal testing machines that helps prevent damage to the specimen or system during the setup phase of a test. When turned on, Specimen Protect automatically adjusts the crosshead to keep any unwanted forces under a certain limit. Bluehill Universal Software can also be used to normalize forces across multiple specimens and remove any slack or compressive force, ensuring consistent results.
Instron also offers the 3400 Series Universal Testing Systems for tensile, compression, bend, and other material property tests. This system uses Bluehill Universal Software, which is designed for touch interaction and provides an intuitive user experience.
An extensometer is an accessory to a tensile strength tester and is commonly used in materials testing. It places a sample under stress, causing it to physically deform, and measures these differences. Extensometers are typically used in industries where manufacturers need to test the tensile strength of their materials to determine if they can withstand typical strain. They are also used in field settings, such as mines, to test the risk of structural collapse.
There are two types of tensile testing machines: those that require contact with the sample and those that do not. Traditional contact extensometers may not be suitable for testing delicate or small samples due to the risk of damage if the sample breaks. Newer models use a digital sensor or 'feeler' arm to compensate for bending stresses and ensure more precise measurements.
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Frequently asked questions
Tensile strength is the amount of force that can be applied to a plastic sample before it stretches irreparably (yields) or breaks.
ASTM D638 is the most common testing standard for determining the tensile properties of reinforced and non-reinforced plastics. ASTM D638 is performed by applying a tensile force to a sample specimen and measuring its properties under stress.
ASTM D638 applies to rigid plastic samples between 1.00 mm and 14 mm in thickness. ISO 527-1/-2 applies to sheets or films less than 1.00 mm thick.
The modulus of elasticity is how much a specimen stretches or deforms in response to tensile force. It is one of the most important types of data gathered by ASTM D638 plastic tensile testing.
Poisson's Ratio is a measurement of the relationship between how far a material is stretched and how thin it gets during the stretching process.
