Alexei Volkov
Plastic strain refers to a type of mechanical strain that causes a permanent deformation in a material body. When two surfaces of ductile materials are placed in contact and the load exceeds the elastic limit of one of the materials, plastic deformation occurs, preventing the material from returning to its original size and shape. This phenomenon is observed in both single crystals and polycrystals, with the presence of grain boundary planar defects influencing the plastic flow. The transition from elastic to plastic deformation is characterised by critical resolved shear stress, which initiates dislocation migration along slip planes. The behaviour of plastic strain can be modelled using stress-strain relationships, considering factors such as stress, temperature, and time.
| Characteristics | Values |
|---|---|
| Definition | Plastic strain is a type of mechanical strain in which the distorted body does not return to its original size and shape after the deforming force has been removed. |
| Cause | External constraints or loads |
| Types | Time-independent and time-dependent (creep) |
| Stress-strain relationship models | Deformation theory of plasticity and incremental theory of plasticity |
| Ductile materials | Plastic deformation occurs when the load exceeds the elastic limit |
| Amorphous materials | Undergo plastic deformation due to the presence of free volume or wasted space |
| Crystallites | A single crystal with less than five independent slip systems exhibits greater plasticity than its polycrystalline form |
Explore related products
What You'll Learn
- Plastic deformation occurs when the load exceeds the elastic limit
- Stress-strain curves characterise material structural behaviour
- Plastic deformation is observed in the presence of lubricants
- The deformation theory of plasticity expresses strain as a function of stress, temperature, and time
- Inhibiting additives can affect the efficiency of protection under elastic-plastic strain
Plastic deformation occurs when the load exceeds the elastic limit
Plastic strain refers to the phenomenon of plastic deformation, which occurs when a load exceeds the elastic limit of a material. This is one of two types of mechanical strain, the other being elastic strain. Mechanical strain is a measure of deformation, representing the displacement between particles in a material body. When a material is placed under a load, it may undergo deformation, and the type of deformation depends on the magnitude of the load relative to the elastic limit of the material.
Elastic deformation occurs when the load is within the elastic limit of the material. In this case, the material will deform temporarily but return to its original shape and size once the load is removed. On the other hand, plastic deformation occurs when the load exceeds the elastic limit, causing the material to distort and not return to its original state.
The transition from elastic to plastic deformation behaviour is defined by the critical resolved shear stress (CRSS). This is influenced by factors such as the yield strength of the material, the Schmid factor, and the angle between the slip plane direction and the applied force. When the load exceeds the CRSS, dislocation migration occurs, leading to plastic deformation.
Plastic deformation is observed in both crystalline and amorphous materials. In crystalline materials, such as single crystals and polycrystals, dislocation migration occurs along slip planes. In amorphous materials like polymers, plastic deformation can occur due to the presence of free volume, resulting in a hazy appearance known as crazing.
The presence of lubricants can also influence the occurrence of plastic deformation. In some cases, lubricants can increase the deformability of solid surfaces through mechanisms like the Rehbinder effect. This results in plastic deformation even when lubricants are present between two surfaces in solid-state contact.
Protecting Plastic Car Mouldings: Tips for Longevity
You may want to see also
Explore related products
Stress-strain curves characterise material structural behaviour
Stress-strain curves are used to characterise the structural behaviour of materials. They reveal the relationship between stress and strain, which is obtained by gradually applying a load to a test coupon and measuring the deformation. This deformation is a geometric measure of how the material's body changes shape, representing the relative displacement between particles in the material.
Stress-strain curves provide insights into the material's strength, stiffness, ductility and failure limits. For example, a glass marble dropped to the ground would shatter immediately, while a rubber ball would return to its original shape after impact. This difference in behaviour is explained by their respective stress-strain curves.
The curves also reveal other properties of a material, such as Young's modulus, the yield strength and the ultimate tensile strength. Young's modulus is the slope of the linear portion of the curve, where the material undergoes elastic deformation. The upper yield point is where plastic flow initiates, and the lower yield point is where slip bands appear and propagate along the gauge length until deformation becomes uniform.
Beyond the Lüders strain, the stress increases due to strain hardening until it reaches the ultimate tensile stress. This is when a process of necking begins, which ends in a 'cup and cone' fracture characteristic of ductile materials. The stress-strain curve for a ductile material can be approximated using the Ramberg-Osgood equation.
Stress-strain curves can be classified into two categories depending on the choice of the cross-sectional area measured during a tensile test. For engineering purposes, it is commonly assumed that the cross-sectional area remains constant during deformation.
Shrinking Plastic with Boiling Water: A Quick and Easy Method
You may want to see also
Explore related products
Plastic deformation is observed in the presence of lubricants
Plastic deformation refers to the process by which an object changes its size or shape due to an applied force, and this change is irreversible. It is observed in many objects, such as steel rod bending, and occurs when the load exceeds the elastic limit of the material. This is also known as the yield strength of the material. When a specimen undergoes plastic deformation, energy is consumed and expelled from the system as the microstructure is irreversibly changed. As a result, when the load is removed, the specimen's shape or volume remains permanently altered.
Plastic deformation can occur in ductile materials, which exhibit a phenomenon called necking. Necking occurs when a tensile load is applied to a material, causing it to elongate and stretch, achieving large deformations. The center portion of the specimen thins significantly before failure.
Plastic deformation can occur in the presence or absence of lubricants. Lubricants can increase the deformability of solid surfaces through mechanisms such as the Rehbinder effect. This effect facilitates the generation of slip lines for dislocation flow in the solid surface. Dislocations are line defects in the solid with higher energy states, making them more reactive to certain chemical agents.
The use of lubricants in plastic deformation offers several advantages. Firstly, it helps to avoid seizing or friction, which can damage the final product and the machinery. By creating a resistant film between the machinery and the material being processed, lubricants enable smooth sliding and prevent unwanted friction. Additionally, the performance of plastic deformation processes is strongly influenced by the choice of lubrication and the methodology employed.
In some cases, lubricants are essential for the protection of the materials being deformed. For example, in the case of rubbed metal, inhibitors are introduced to lubricating systems to prevent corrosion. Different inhibiting additives have been studied for their efficiency in protecting steel under continuous deformation, with some inhibitors showing lower efficiency under elastic-plastic strain conditions.
Plastic PV Value Calculation: A Step-by-Step Guide
You may want to see also
Explore related products
The deformation theory of plasticity expresses strain as a function of stress, temperature, and time
Plastic strain is a type of mechanical strain, which is a measure of deformation representing the displacement between particles in a material body. When a deforming force is removed, a body that has undergone elastic strain will return to its original size and shape. Plastic strain, on the other hand, is irreversible.
The deformation theory of plasticity is based on the Cauchy stress tensor, which is a function of the strain tensor. This theory accurately describes the behaviour of materials under increasing loading, such as strain loading, but it cannot account for irreversibility. For example, ductile materials can sustain large plastic deformations without fracture, but they will eventually fracture when the strain becomes large enough due to work hardening.
The unified stress-strain model, based on the incremental theory of plasticity, treats plastic and creep strain increments as one inelastic strain increment. In this model, if a specimen is held at a constant strain for a period of time, the stress will relax slowly. If the straining is resumed, the specimen will behave as though it had been unloaded elastically. Similarly, if a specimen is subjected to a constant stress, it will continue to deform plastically, although the plastic strain will increase very slowly. This phenomenon is known as "creep".
The mathematical theory of plasticity, or flow plasticity theory, uses a set of non-linear, non-integrable equations to describe the changes in strain and stress with respect to a previous state and a small increase in deformation. If the stress exceeds a critical value, the material will undergo plastic deformation. This critical stress can be tensile or compressive, and it can be determined using criteria such as the Tresca and von Mises criteria.
Time-independent plastic flow in both single crystals and polycrystals is defined by a critical or maximum resolved shear stress (τCRSS), which initiates dislocation migration along parallel slip planes, marking the transition from elastic to plastic deformation. The behaviour of crystalline materials under plastic deformation can be further understood through the temperature-dependent characteristics of τCRSS.
Install Plastic Garden Edging: A Step-by-Step Guide
You may want to see also
Explore related products
$37.99 $39.99
Inhibiting additives can affect the efficiency of protection under elastic-plastic strain
Plastic strain refers to the permanent deformation of an object when an external force is applied. When the load exceeds the elastic limit of a material, plastic deformation occurs, and the object does not return to its original size and shape after the force is removed.
Inhibiting additives are used to prevent corrosion in metals under elastic-plastic strain. The efficiency of these inhibitors depends on their mechanism of action and the type of metal they are protecting. For example, in an investigation of the tribochemical behaviour of steel under continuous deformation, three inhibiting additives (In1, In2, and In3) were tested at a strain rate of 34%/min. The inhibitors varied in their efficiency due to their distinct mechanisms of action.
Inhibitor In1, n-decyl-3-hydroxy-pyridinium chloride, demonstrated lower efficiency under elastic-plastic strain, leading to the destruction of the coating layer. Its mechanism of action involves steel surface passivation, potentially due to the presence of OH" groups in its molecule.
Inhibitor In2, alkyl hexamethyleniminium bromide, on the other hand, completely eliminated the tribochemical effect. Its protective properties are a result of the synergistic effect of organic cations and anions working together.
The third inhibitor, In3, sodium bromide, exhibited corrosion protection effects in plastic deformation. This inhibitor prevents tribochemical dissolution by forming a strong elastic adsorbed film on the metal.
The effectiveness of inhibiting additives in protecting against elastic-plastic strain is influenced by their specific mechanisms of action and interactions with the metal surface.
The Plastic-Oil Connection: Understanding the Byproduct Relationship
You may want to see also
Frequently asked questions
Plastic strain is a type of mechanical strain, which is a measure of deformation representing the displacement between particles in a material body. Plastic strain occurs when the distorted body does not return to its original size and shape after the force causing the deformation is removed.
Plastic strain is caused when an external force or load exceeds the elastic limit of a material. This can occur in the presence or absence of lubricants.
Plastic strain is measured using stress-strain curves, which characterise the material's structural behaviour. The stress-strain model based on the deformation theory of plasticity expresses strain as a function of stress, temperature, and time.
