Alexei Volkov
Yield strength is a critical factor in understanding the mechanical behaviour of materials, including plastics. It is defined as the stress point at which elastic behaviour ends and plastic deformation begins. Determining the yield strength of plastics is essential for designing plastic parts and understanding their performance under various loading conditions. The yield point can be identified through methods like the divider technique, observing the stress-strain curve, and considering factors such as material properties, crystallinity, processing history, and strain rate. The yield strength of plastics can be enhanced by adding fibres or fillers, resulting in high-strength plastics with improved characteristics.
| Characteristics | Values |
|---|---|
| Yield strength | Peak of the stress-strain curve, where slope is zero |
| Determining yield point | Divider method - observing visible elongation between two gauge marks on the specimen |
| Elastic limit | Stress at which plastic deformation begins |
| Determining elastic limit | Successive increasing loads with intervening complete unloading to detect first plastic deformation |
| Yield strength of filled plastics | Add fibres to plastic to increase strength |
| Flexural strength | Load required to cause 5% deformation/strain of outer surface |
| Tensile strength | Stress plastic can withstand without breaking when stretched or pulled |
| Izod impact strength | Resistance of material to impact, energy required to cause fracture upon collision |
| Brittle fracture | Large load applied rapidly may result in brittle fracture |
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What You'll Learn
The yield point is the peak of the stress-strain curve
In engineering and materials science, a stress-strain curve illustrates the relationship between stress and strain. The curve is obtained by gradually applying load to a test coupon and measuring the deformation, from which the stress and strain can be determined. These curves can reveal many of a material's properties, such as Young's modulus, yield strength, and ultimate tensile strength.
The yield point is the point on a stress-strain curve that indicates the limit of elastic behaviour and the beginning of plastic behaviour. Yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically. In contrast, the yield point is where nonlinear (elastic + plastic) deformation begins. The stress component of this point is defined as the yield strength or upper yield point (UYP).
The upper yield point is associated with the pinning of dislocations in the system, and permanent deformation occurs once dislocations are forced to move past pinning points. The lower yield point results from the formation and propagation of Lüders bands, which are heterogeneous plastic deformation bands. After the sample is uniformly deformed, the increase in stress with the progress of extension results from work strengthening, where dense dislocations induced by plastic deformation hinder the further motion of dislocations.
The yield point derived from the point where the slope goes to zero is a reference point for comparing materials. It represents the formation of a stable neck in the specimen, after which gross yielding occurs.
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The divider method can be used to determine the yield point
The divider method is a simple and direct way to determine the yield point, and it can be used for a variety of materials, including plastics. It is important to note that the yield point as commonly used does not exist as a precise value, but rather it is a convenient way to quickly characterize and describe material properties. The definition of the yield point may vary depending on the specific application or material being tested.
When using the divider method, it is crucial to have a clear understanding of the material being tested and its properties. The specimen should be prepared and marked with gauge marks, and the observer must be experienced in identifying the visible elongation between these marks. The load and corresponding stress values are recorded during the test, and these data points are used to calculate the yield point.
To enhance the accuracy of the divider method, standardized tensile sample geometries can be utilized to avoid geometrical variations. Additionally, the use of calibrated equipment and controlled testing conditions is essential to minimize errors and ensure consistent results. The divider method can be complemented with other characterization techniques, such as tensile testing, to further validate the material properties and behavior.
Overall, the divider method is a valuable technique for determining the yield point of plastics and other materials. By following the procedure outlined above and adhering to standardized practices, this method can provide insightful information about the yield behavior of different materials, aiding in their selection and application in various engineering contexts.
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Yield strength depends on the material and its crystallinity
The yield strength of a material is the stress corresponding to the yield point at which the material begins to deform plastically. It is a crucial factor in engineering and design, as it is often used to determine the maximum load a mechanical component can bear without permanent deformation.
For crystalline materials, the yield strength is closely related to crystallographic factors such as crystalline morphology, crystal structure, and crystallinity (Xc). Crystallinity is the degree to which a material is organised into a crystal lattice, and it can be altered by various processing techniques. For example, an isothermal crystallisation process can be introduced after the routine sintering process to increase the crystallinity of sintered parts.
The yield strength of a crystalline material can be fine-tuned by altering dislocation density, impurity levels, and grain size. Dislocations in the crystal lattice can increase yield strength as they require greater stress to move. Similarly, introducing impurities or defects can increase yield strength as a larger stress is required to plastically deform the material.
In the case of polyethylenes (PEs), research has shown a positive correlation between yield strength and crystallinity. However, the specific relationship between yield strength and crystallinity can vary between different materials. For instance, while some materials exhibit a direct correlation between yield strength and crystallinity, others may show a more complex relationship influenced by additional factors.
It is important to note that the behaviour of plastics, such as polymers, differs significantly from that of metals. Plastics exhibit nonlinear elasticity and flow even at very low forces. Therefore, it is crucial to design plastic parts that experience only a fraction of their nominal yield stress to avoid permanent deformation.
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Yield strength depends on the processing and strain rate
The yield strength of a plastic material is the peak of its stress-strain curve, where the slope is zero. This is a reference point that is useful for comparing materials. The yield strength of plastics depends on various factors, including processing and strain rate.
Processing plays a crucial role in determining the yield strength of plastics. The way a plastic material is processed, including factors such as temperature, pressure, and cooling rate, can significantly influence its mechanical properties, including yield strength. Different processing techniques can lead to variations in the microstructure and crystallinity of the plastic, affecting its ability to withstand stress without permanent deformation.
Strain rate, which is the rate at which deformation occurs in a material, also has a significant impact on yield strength. As the strain rate increases, the yield strength of plastics tends to increase. This relationship is known as the strain rate effect. At higher strain rates, the material appears stronger and stiffer due to the increased resistance to deformation.
Conversely, at lower strain rates, the viscous or energy-damping aspects of material behaviour become more prominent. This favours the energy management capabilities of the plastic rather than its load-bearing performance. It is important to note that the strain rate is closely linked to temperature. Increasing the temperature has a similar effect as decreasing the strain rate, while decreasing the temperature mimics the behaviour of increasing the strain rate.
Additionally, the rate of loading, which is related to the strain rate, also influences the perceived performance of plastic materials. High strain rates, or events that occur over a short period, tend to favour the elastic properties of plastics, contributing to their load-bearing capacity. On the other hand, low strain rates, or events that occur over a longer period, may result in creep, plasticizer loss, oxidation, and physical ageing, impacting the overall yield strength of the material.
In summary, the yield strength of plastics is influenced by a combination of processing conditions and strain rate. The interaction between these factors can lead to complex behaviour, and a thorough understanding is essential for designing and optimizing plastic parts in various applications.
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Yield strength can be increased by adding fibres to the plastic
The yield strength of a material is the peak of its stress-strain curve, where the slope is zero. Plastics, as polymer materials, have very different properties from metals, and so their yield strength is determined differently.
Plastics flow even at very low force, and so a plastic part should be designed to experience no more than 1/4 of its nominal yield stress. The yield strength of plastics can be increased by adding fibres to the plastic. Nylon, for example, is a synthetic thermoplastic that can be melted into fibres and has excellent mechanical properties. It is highly effective at reinforcing plastics, and its uniform dispersion allows for effective stress transfer throughout the matrix.
Nylon fibres can be easily incorporated into plastic formulations, and its lightweight reinforcement properties can improve the strength-to-weight ratio of parts, leading to fuel efficiency. The hardness of nylon also improves abrasion resistance, increasing the lifetime of parts.
Other fibres can also be used to reinforce plastics and increase yield strength. For instance, glass fibre reinforcement can be introduced into a polypropylene resin to increase the strength and stiffness of the polymer. However, ductility decreases with the addition of glass fibres, which can result in lower part toughness.
The orientation and distribution of fibres within the plastic part are critical to optimising its strength. Design features such as ribs, gussets, and fillets can also be incorporated to increase the strength of the plastic part while maintaining manufacturability and an overall lighter part.
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Frequently asked questions
Yield strength is the peak of the Stress-Strain curve, where the slope is zero. It is the point where elastic behaviour ends and plastic behaviour begins.
The yield point can be determined by the divider method. An observer uses a pair of dividers to watch for visible elongation between two gage marks on the specimen. When visible stretch occurs, the load is recorded, and the stress corresponding to that load is calculated.
The material, degree of crystallinity, processing method, and strain rate all influence the yield strength of plastic.
Yield strength specifically refers to the point at which a material begins to deform, whereas tensile strength, for example, measures the stress a material can withstand without breaking when stretched or pulled.
The yield strength of plastic can be increased by adding fibres to the material. This is a common method used to create high-strength plastics.
