Maya Ellison
Strain-rate sensitivity is a fundamental principle of viscoelastic behaviour in plastic materials. It refers to how a plastic's strength and stiffness increase as the strain rate increases, while ductility decreases. This is observed in tensile tests, where the ultimate elongation is higher at slower deformation speeds. The strain rate sensitivity exponent, or strain rate hardening exponent, governs the slope of the stress-strain curve and determines how a material deforms. Materials with a positive exponent exhibit improved toughness and formability, while a negative exponent results in forming difficulties. Understanding this principle is crucial for engineers and designers working with plastic parts and raw materials.
| Characteristics | Values |
|---|---|
| Definition | Rate-sensitive plastics are materials that exhibit increased rate sensitivity of yield under the same strain rate/temperature conditions as the β-transition of the viscoelastic behavior. |
| Fundamental Principle | The equivalence of time and temperature determines the behavior of a material. |
| Mechanical Behavior | The mechanical behavior of polycarbonate and poly(methyl methacrylate) was observed at strain rates ranging from 10−4 to 104 s−1. |
| Tensile Tests | The property of elongation at break, or ultimate elongation, is considered a relative measure of ductility. |
| Impact Testing | Impact testing is a method to assess ductility using a higher spectrum of strain rates. |
| Slow Strain Rate Testing (SSRT) | SSRT is performed at a very low strain rate to encourage stress corrosion cracking or hydrogen embrittlement. |
| Stress-Strain Relationship | As the strain rate increases, a higher amount of flow stress is required to continue deforming the material, resulting in increased strength. |
| Strain Rate Sensitivity Exponent | A positive exponent improves toughness, while a negative exponent results in forming difficulties. |
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What You'll Learn
The impact of temperature on rate-sensitive plastics
Rate-sensitive plastics are those that exhibit a change in mechanical properties in response to alterations in strain rate and temperature. The strain rate is the speed at which a material is deformed, and it influences the strength and stiffness of the plastic. As the strain rate increases, the yield strength and modulus also increase, making the material stronger and stiffer. This relationship is described by the strain rate sensitivity exponent, which determines how the material deforms. A positive exponent indicates that properties such as yield strength and tensile strength increase with strain rate, improving the toughness of the plastic.
Temperature plays a crucial role in the behaviour of rate-sensitive plastics. According to the principle of time-temperature equivalence, changes in temperature have a similar effect to changes in strain rate. Lowering the temperature mimics the effect of increasing the strain rate, resulting in enhanced strength and stiffness. Conversely, raising the temperature has the same impact as decreasing the strain rate, leading to reduced strength and stiffness. This phenomenon is observed in tensile tests, where the ultimate elongation, a measure of ductility, decreases with increasing strain rate or decreasing temperature.
Additionally, the rate sensitivity of plastics is observed in their viscoelastic behaviour. Materials like polycarbonate and poly(methyl methacrylate) exhibit increased rate sensitivity of yield when subjected to specific strain rate/temperature conditions. This behaviour is characterised by a rate-dependent shift in material transition temperatures, highlighting the intricate relationship between temperature and the mechanical response of rate-sensitive plastics.
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Tensile testing
Rate-sensitive plastics are a type of polymer whose mechanical behaviour changes with the rate of strain and environmental conditions. Tensile testing is a critical method for characterising these materials and understanding their performance.
The test conditions, including temperature, humidity, and testing machine speed, are defined to maintain consistency. Elevated and reduced temperature tensile tests can be conducted to study the effects of temperature on the tensile properties of rate-sensitive plastics. Additionally, non-contact extensometers, such as the MTS Advantage™ Video Extensometer (AVX), are useful tools for capturing data during the test without affecting the specimen.
The data obtained from tensile testing is valuable for material selection, design optimisation, and quality control in manufacturing processes. It helps compounders and manufacturers make informed decisions about the characteristics of the materials they use, ensuring that their products meet the desired specifications, whether in toys, car headrests, or airplane seats.
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Viscoelastic behaviour
The strain rate sensitivity exponent (m) governs how a material deforms. If the exponent is positive, tensile properties such as yield strength, tensile strength, percent elongation, and reduction of area increase as the strain rate increases. This means that the material can better withstand deformation and results in improved toughness. On the other hand, a negative exponent leads to forming difficulties as the material cannot spread the deformation, resulting in immediate failure at the point of necking.
The speed or strain rate at which a plastic is deformed has a significant impact on its mechanical behaviour. As the strain rate increases, the yield strength and modulus also increase, making the material appear stronger and stiffer. This effect can also be achieved by keeping the strain rate constant and reducing the temperature. Conversely, increasing the temperature has the same effect as decreasing the strain rate.
The relationship between strain rate and temperature is crucial in understanding the viscoelastic behaviour of plastics. For instance, in slow strain rate testing (SSRT), a tensile sample is pulled at a very low strain rate to observe stress corrosion cracking or hydrogen embrittlement. The ductility measured at failure is usually directly proportional to the strain rate.
Additionally, the stress level required to maintain deformation during plastic deformation is known as flow stress. Multiple flow stress tests are performed at different strain rates and temperatures to quantify the deformability of a material. The results show that as the strain rate increases, higher flow stress is needed, indicating that the material has a higher strength at faster deformation speeds.
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Flow stress
The flow stress can be found anywhere within the plastic regime of a stress-strain curve. As deformation proceeds, the flow stress changes and usually increases as strain accumulates due to work hardening. However, the flow stress could decrease due to any recovery process. The flow stress for a given material will vary with changes in temperature, strain rate, and material properties.
Accurate and reliable flow stress models are important for describing the deforming behaviour of materials during machining processes. The widely used constitutive model of flow stress is the JC strength model, which represents the flow stress of a material as the product of strain, strain rate, and temperature.
In plastic deformation, the stress level required to keep the deformation continuing is called the flow stress. This property is typically measured in a compression test, as opposed to tension tests used for other properties associated with strain hardening. Multiple flow stress tests are typically run on a material to quantify its deformability, usually at different strain rates and temperatures. The results of these tests show that a higher amount of flow stress is needed to continue deforming a material as the strain rate increases, effectively increasing the material's strength when deformation is faster.
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Stress-strain curves
Rate-sensitive plastics, also known as polymers, are materials that exhibit viscoelastic behaviour, meaning their mechanical properties are influenced by both time and temperature. This is important for engineers who design plastic parts, as changes in temperature and time can impact the strength and stiffness of the material.
Polymer stress-strain curves are a way to visualise the relationship between stress and strain in these materials. To generate a stress-strain curve, a sample of the polymer is stretched at a constant rate, applying a tensile force. The curve illustrates the amount of deformation (strain) that occurs in response to an applied force (stress).
The specific shape of the stress-strain curve depends on the type of polymer being tested and the temperature at which the test is conducted. For example, a semi-crystalline polymer will exhibit different curves when tested above and below its glass transition temperature. This temperature dependence is a unique characteristic of polymers, setting them apart from other materials.
Additionally, the inherent time-dependence of polymers also influences their stress-strain behaviour. For instance, when a constant stress is applied to a polymer, it may exhibit a phenomenon called "creep", where the strain increases over time without any change in the applied stress. This time-dependent behaviour is crucial to understanding how polymers respond to long-term loading.
The stress-strain curve is a valuable tool for engineers and designers working with rate-sensitive plastics. By understanding the mechanical behaviour of these materials, they can make informed decisions about material selection, design optimisation, and failure analysis. Furthermore, the stress-strain curve can provide insights into the ductility of the material, which is an important consideration in tensile testing.
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Frequently asked questions
Rate-sensitive plastics are those that exhibit strain-rate sensitivity, meaning their mechanical properties are dependent on the rate at which they are deformed. The strain rate is the speed at which the material is deformed, and it affects properties such as strength, stiffness, and ductility.
As the strain rate increases, the yield strength and modulus also increase, making the material appear stronger and stiffer. However, as strength and stiffness increase, ductility decreases. This relationship is described by the stress-strain curve, which illustrates how the deformation of a material changes with the applied stress.
Strain-rate sensitivity is quantified by the strain rate sensitivity exponent, which governs how a material deforms. If the exponent is positive, tensile properties such as yield strength and tensile strength increase with increasing strain rate, leading to improved toughness. A negative exponent results in forming difficulties as the material cannot effectively spread deformation.
Strain-rate sensitivity can be tested through various methods, including tensile tests, impact testing, and slow strain rate testing (SSRT). Tensile tests measure the elongation at break, providing a relative measure of ductility. Impact testing assesses ductility using a higher spectrum of strain rates. SSRT is performed at very low strain rates to encourage stress corrosion cracking in the test environment.
