When enough load is applied to a material or a structure, it will cause the material to change shape. This change in shape is known as deformation. As per the material science theory, when sufficient stress is applied to cause permanent deformation to the metal, it is called plastic deformation.
Also, the involvement of breaking of a limited number of atomic bonds by the movement of dislocations is known as plastic deformation. The force required is so huge to break the bonds of all the atoms in a crystal plane.
The process due to which an object changes its size or shape due to applied force, that is irreversible, is known as plastic deformation. It can be observed in many objects like
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Concrete
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Plastics
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Soils
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Metals
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Rocks
Example: steel rod bending.
Plastic Deformation Meaning
Plastic Deformation – Even after the removal of the applied forces when the deformation stays, it is irreversible.
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The property of the material to undergo enduring the deformation under pressure is known as plasticity. It is the state or quality of plastic material, especially the molding and altering capacity.
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The malleability and ductility of a material are directly proportional to plasticity of the material. For an ideal plasticity property of material, it is to undergo irreversible deformation without any increase in load or stress.
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Fracture or rupture of the material may be caused by plasticity. Plasticity also leads to plastic deformation, which happens in many metal forming processes like forging, pressing, rolling, swaging, etc.
Plastic Deformation of Metals
In experiments, plastic deformation is studied with the help of springs. Here, Hooke’s law is explained to differentiate between plastic and elastic materials. There are many mechanisms that cause plastic deformation. Dislocation plasticity is caused in metals, whereas for brittle materials like concrete, rock, and bone plasticity is caused due to slippage of microcracks.
Even when the initiating stress is removed, the plastic deformation and plastic strain is a dimensional change that doesn’t disappear. Deformation to the body will be permanent if the load exceeds the limit even after its removal. When applied stress exceeds the elastic limit or yield stress, the deformation of the material body occurs. This is due to the result of a slip or dislocation mechanism at the atomic level.
Slip and Twinning
Plastic deformation in a metal has two prominent mechanisms, and they are:
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Slip Twinning Slip and
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Twinning
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A prominent mechanism of deformation in metal is slip. It occurs in crystallographic planes or slip planes and involves the sliding of blocks of crystal over one another
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Whereas in twining, a portion of crystals takes up orientation related to the rest of the untwined lattice in a symmetrical and definite way.
A comparison between slip and twinning can be done based on defect type, change in crystal axis, visibility, seen in, stress requirement, and occurrence threshold value of stress.
Differences Between Slip and Twinning
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Slip |
Twinning |
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Line defect is known as crystal slip |
The surface defect grain boundary is known as twinning. |
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The atoms in the block move at the same distance during slip. |
The atoms in each successive plane in a block move through distances which are proportional to their distance from the twinning plane at the time of twinning. |
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It is commonly observed in face-centered cubic (FCC) and body-centered cubic (BCC). |
It is commonly observed in Hexagonal close packing (HCP) metals. |
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The crystal axis remains the same after slipping. |
The crystal axis is deformed in twinning. |
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The slipped crystal lattice has similar orientation. |
In the twinned crystal lattice is a minor image of the original lattice. |
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Comparatively, less stress is required. |
Comparatively, more stress is required. |
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The stress required to propagate slip is comparatively higher to the stress required to start slip. |
The stress required to propagate twinning is comparatively lower to the stress required to start slip. |
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When viewed under a microscope, slip can be seen as thin lines. |
When viewed under a microscope, slip can be seen as broad lines. |
Difference Between Plastic and Elastic Deformation
The difference between elastic and plastic deformation is given here below:
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Elastic Deformation
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When the material recovers its original dimension from a deformed body after the load is removed; it is known as elastic deformation.
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Elastic limit is the limiting load beyond which the material no longer behaves elastically.
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When a force applied to the body increases, there is more change in an object’s shape or size.
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E.g., when a rubber band is released, it regains its original shape.
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Plastic Deformation
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When the elastic limit of a body is exceeded, it will experience a permanent deformation or set when the applied load is removed. It is known as plastic deformation in which an object is permanently deformed.
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It happens when bonds between atoms are broken, and new ones are formed, making the reversal to original shape impossibl
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How much force is applied on the object (stress) is directly proportional to the object’s dimensions (strain).
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E.g., a hanger doesn’t regain its original shape when it is bent.
Plastic Deformation is the permanent alteration of shape, form or texture of a material due to the action of stress. In other words, it’s the change of shape and dimensions brought on by forces.
Plastic deformation is the most common type of deformation and happens to the most materials and in most circumstances. There are three basic categories of plastic deformation of which this is the simplest and most common type: elongation, contraction and expansion. A fourth type, referred to as thermoelastic deformation (or creep) involves a change in material dimension with temperature change (usually increases) with the material behaving elastically at low temperatures and more plastically (in an extended manner) with increasing temperature.
Elongation
Elongation refers to the process of increase in length and decrease in thickness of a material (known as ‘drawing’ or ‘drawing in’), when stretched under constant pressure and temperature conditions.
Elongation deformation is the most common type of deformation because it is often a response to high stress. When you put a rubber band around a bunch of bananas, for example, the bananas are pulled and the rubber band stretches. In this case the bananas are being pulled out of the bunch and lengthened to the width of the rubber band.
Elongation does not take place in a material if the stress is too low or the pressure is too weak.
Plastic deformation occurs when the deformation is in the elastic limit. When the load is too high in comparison to the amount of strain plastic deformation starts.
Plastic deformation is characterized by uniform flow of the metal material and no change in its volume. The amount of load required to cause plastic deformation is called the plastic limit stress (PLS).
Plastic deformation occurs when the deformation is in the elastic limit. When the load is too high in comparison to the amount of strain plastic deformation starts.
Types of Plastic Deformation
Plastic deformation can be divided into two types
Ductile: Plastic deformation is said to be ductile if the material is able to undergo permanent deformation by a stress greater than the yield stress. Ductile materials are able to recover their original shape without any residual stress after the load is removed.
Irreversible Plastic: Plastic deformation is said to be irreversible when the plastic flow cannot be recovered. The plastic flow stops after yielding, at this point no stress is able to restore the structure back to its original state.
Process of Plastic Deformation: A plastic deformation occurs as a result of the interaction between the load and the strain of the material. When the strain is increased to an amount exceeding the yield stress of the material, it undergoes plastic deformation.
When the load on the material is removed the material will retain a permanent deform, because the material has exceeded its elastic limit. The amount of stress which causes plastic deformation is called the plastic limit stress or the plastic limit strain.
When a stress is applied to the material in its elastic range the plastic deformation occurs in the material. If the amount of strain in the material is increased beyond the yield point the deformation will continue up to the failure point. At this point the deformation will stop.
The maximum deformation occurs in the region known as necking. In this process an area known as the neck is formed where deformation has been maximum. The maximum deformation is known as the necking deformation.
If we increase the amount of strain further than the yield point, the material will deform up to the point of failure. Here the deformation will stop. This is known as the complete deformation. The material will retain a permanent strain after the load is removed. The stress at which this permanent strain is retained is called the ultimate strength of the material.
In order to understand plastic deformation we need to understand yield points and strengths. For our purposes, ‘elastic’ or ‘yield’ means that when a load is applied to the material, that the material will remain undeformed.
‘Yield point’ is the point at which the material fails to return to its original position. If the material is loaded to a greater amount than the yield point, then deformation will occur. ‘Yield strength’ is the load at which the material reaches its yield point.
‘Ultimate strength’ is the maximum load at which the material can sustain a permanent deformation. So, in simple terms, yield means that the material returns to its original shape, and strength is how much force we use to deform the material.
The plastic deformation of steel, which is the deformation of the steel at the yield point, can be up to three percent for mild steel (lower quality steel). Plastic deformation is a reversible process so when we remove the stress and allow the steel to recover it returns to its original shape.
If we are to use the plastic deformation of steel as an analogy, imagine the shape of the steel is a pot, and we pour water into the pot. If we start to pour, the water level rises quickly and then stops. This is because we need to reach the yield point to begin to deform the steel.
However, if we drain the water back out, the shape of the steel will not return to its original shape, and the water will remain. The steel will retain the shape of the pot, and it will be permanent. If we consider plastic deformation of steel, this is how the steel is acting – it is trying to deform the water back into a round shape – but it can’t.