Alexei Volkov
Yield strength is the maximum stress a material can withstand before it starts to deform. It is an important factor to consider when designing plastic parts, as plastic commonly has lower strength compared to metal. The yield strength of plastics varies depending on the type of plastic, with values ranging from 4 MPa for plasticized PVC to 300 MPa for carbon-fiber-filled polyamide. The yield strength of a plastic material can be determined by consulting its stress-strain curve or using simulation tools. However, it is important to note that the actual yield strength of a plastic material is often lower than the specified value, and engineers need to make informed trade-offs when working with plastics.
Explore related products
$31.99
$19.12 $20.11
What You'll Learn
Plastic deformation
In crystalline materials, plasticity is caused by slip and twinning. Slip is a shear deformation that moves atoms through many interatomic distances relative to their initial positions. Twinning is the plastic deformation that takes place along two planes due to a set of forces applied to a given metal piece. In brittle materials such as rock, concrete, and bone, plasticity is caused predominantly by slip at microcracks. In cellular materials such as liquid foams or biological tissues, plasticity is mainly a consequence of bubble or cell rearrangements.
In amorphous materials, the discussion of "dislocations" does not apply since the entire material lacks long-range order. These materials, such as polymers, still undergo plastic deformation. When the bending moment exceeds the fully plastic moment, these materials plastically deform. This applies to open-cell foams where the bending moment is exerted on the cell walls.
In engineering, the transition from elastic behaviour to plastic behaviour is known as yielding. The deformation that occurs here is not recoverable. The stress-strain relation does not follow a linear line. The stress that separates the two regions is called the yield stress. The ideal curve, however, is not applicable for actual design use. In most plastic materials, the border between the elastic and plastic regions is not clearly stated.
Plastic Membrane: A Concrete Slab's Best Friend?
You may want to see also
Explore related products
Stress-strain curve
Plastics are widely used in engineering design due to their cost efficiency, robustness, and visual appeal. However, they generally have lower strength than metals, making it crucial to determine their strength capabilities to ensure the functionality and success of the final product.
The yield strength of a material is the point at which it transitions from elastic deformation to plastic deformation. In other words, it is the maximum stress a material can withstand without experiencing permanent deformation. Beyond this point, the material will not return to its original shape when the load is removed.
The stress-strain curve is a graphical representation of how a material responds to applied stress, typically tensile stress. It plots the relationship between the stress applied to a material and the resulting strain or deformation. The curve typically starts with a linear section, where the stress and strain are proportional, indicating elastic deformation. As the stress increases, the curve deviates from linearity and enters the plastic deformation region, where the material undergoes permanent deformation. The maximum point on the curve represents the material's tensile strength, after which it moves towards the fracture point where the material ultimately breaks.
For plastics, the stress-strain curve is more complex than for other materials due to their nonlinear elasticity and time-dependent behaviour. The curve is influenced by various factors, including the degree of crystallinity, processing history, and strain rate. Additionally, the behaviour of polymers is temperature-dependent, with different types of polymers exhibiting distinct stress-strain curves at different temperatures.
While the stress-strain curve provides valuable information for engineering design, it is important to note that the ideal curve may not be applicable to actual design use. Determining the yield stress of plastics can be challenging, and engineers must consider the trade-offs between strength and other factors, such as cost and functionality.
Plastic Spoons: Convenience or Environmental Threat?
You may want to see also
Explore related products
Nonlinear elasticity
Plastics are known to have lower strength than metals, and determining their strength capability upfront is crucial for the success of their functionality. This is because plastics flow even at very low forces and can undergo deformation, which is not recoverable.
In the context of plastics, nonlinear elasticity is particularly relevant because plastics often exhibit nonlinear behaviour even before yielding. This means that the stress-strain curve for plastics is not a straight line, but rather a curve that deviates from linearity. As a result, using linear elastic parameters to analyse plastic parts can lead to inaccuracies.
The deformation theory of plasticity attempts to formulate a nonlinearly elastic function for both loading and unloading. This theory provides a simplified approach to understanding the nonlinear behaviour of plastics, but it is limited to specific load histories. The Ramberg-Osgood function (RO) is often used to approximate the nonlinear stress-strain curve, but it is important to note that there is generally no closed-form solution for the stress-strain relationship in plasticity.
It is worth noting that the distinction between nonlinear elastic materials and elastic-plastic materials can be blurred due to their similar behaviour upon loading. This similarity allows for the use of certain analytical techniques, such as the J-integral, which is applicable to both types of materials. However, it is important to consider the unloading behaviour as well, which can differ between nonlinear elastic and elastic-plastic materials.
Black Plastic Pipe: What is it Called and Why?
You may want to see also
Explore related products
$9.69 $11.59
$59.95 $59.95
Tensile strength
Plastic parts are known to have lower strength than metal. However, they are cost-efficient, robust, and have a good visual appearance. They are also easy to machine and fabricate. The tensile strength of a plastic is its ability 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. This is an important consideration for materials that will be stretched or under tension.
The yield strength of a plastic is the point at which it will undergo gross yielding, and this is typically considered irrelevant. The yield point is used as a reference point for comparing materials. The yield strength of a plastic part should be designed to experience no more than 1/4 of the nominal yield stress. The actual yield stress value is often misunderstood and is usually lower than the specified value.
Some plastics have a higher tensile strength than others. For example, Nylon has a tensile strength of 12,400 psi, while PPS (polyphenylene sulfide) has a tensile strength of 12,500 psi. The ultimate high tensile strength plastic is PAI (polyamideimide), with a tensile strength of 21,000 psi. Ultem, a high-performance thermoplastic, has a tensile strength of 15,200 psi.
The tensile strength of a plastic is an important factor to consider when selecting materials for a project. It can affect the functionality and success of a part.
Flipshelf Folding Shelves: Plastic Connectors for Easy Assembly
You may want to see also
Explore related products
Plastic design
When designing with plastic, it is crucial to understand the concept of yield strength. Yield strength refers to the stress level at which a material undergoes permanent deformation and no longer returns to its original shape and size after the release of the applied stress. This point is known as the yield point. In the case of plastics, the deformation that occurs beyond the yield point is not recoverable, and the stress-strain relation does not follow a linear line.
To keep plastic parts from fracturing, it is common practice to design the operating load within its elastic region. The stress that separates the elastic and plastic regions is called the yield stress. However, the ideal curve is not applicable for actual design use, and the border between the two regions is often unclear. Therefore, it is recommended that plastic parts be designed to experience no more than 1/4 of the nominal yield stress.
Additionally, performing a static simulation on plastic parts is crucial to verify the design before prototyping. Special attention should be given to the material's tensile stress, as plastic flows even at very low forces. By consulting the stress-strain curve, engineers can determine the actual yield stress, which is typically lower than the specified value. This information allows engineers to make informed trade-offs and modifications to the design, such as allowing some extent of plastic deformation.
How to Prevent Discoloration of White Plastic by W4-40
You may want to see also
Frequently asked questions
Yield strength is the maximum stress a material can tolerate before plastic deformation begins.
Yes, plastics do have a yield strength.
The yield strength of plastics varies depending on the type of plastic. For example, the yield strength of a plasticized PVC is around 4 MPa, while that of a carbon-fiber-filled polyamide is approximately 300 MPa. In comparison, aluminium alloys typically have a yield strength ranging from 24 to 483 MPa, and stainless steels can start at around 250 MPa, reaching up to 1,000 MPa for certain types.
The yield strength of a plastic material can be determined by consulting its stress-strain curve. This curve illustrates the relationship between stress and strain, and the yield strength is found at the point where the curve begins to deviate from linearity.
The yield strength of plastics is influenced by various factors, including the specific type of plastic material, its degree of crystallinity, the processing methods used, and the strain rate applied. Additionally, the operating conditions and design considerations can also impact the yield strength of plastics.
