Concept of Elasticity:
Elasticity is a property of the material to regain the original shape or form after removing deforming force, or load given the elastic limit is not exceeded. There are certain terms utilized in connection with elasticity; stress and strain.
The stress on an elastic material is stated as force exerted on material per unit area. It can also be stated as:
Stress = Force/Area =F/A
Force is estimated in Newton's (N) and area is in meter square (m2) then unit of stress is thus Newton per meter square (Nm-2). If the object has original length of l, when it is stretch or compressed in such a way that it has the extension or compression value of e, then, strain on object is stated as
Strain = Extension produced on the object/ the original length of the object
Strain = e/l
Stress is connected to force producing deformation. Strain is associated to amount of deformation.
Let the helical spring of original length lo. When the weight is joined to it, it will be seen that spring will be stretched through the distance e. By using slotted weight, we can differ force F acting on spring and with the help of the ruler we can compute extension e for each equivalent weight.
When different force (F) are plotted against corresponding extensions (e) estimated, a linear relationship is attained, that is, provided elastic limit is not exceeded. It is on the basis of experiment that British Scientist Robert Hooke formulated law of elasticity that is called as Hooke's law.
Hooke's law defines that force applied on elastic material is directly proportional to extension produced if elastic limit is not exceeded.
F ∝ e
F = -ke
Where k is the constant of proportionality generally referred to as spring constant. K is property of elastic material. Negative sign is to illustrate restoring ability of material. Later a modified form of this law was given. According to which within elastic limit, stress on the elastic material is directly proportional to strain produced in material.
Stress & strain
Stress = M strain
Where M is constant, constant is known as a Modulus of elasticity of the material of the body.
Concept of Elastic Limit:
The maximum stress which can be applied to the metal without creating permanent deformation. When external forces apply on material they tend to form internal stresses inside it that cause deformation. If stresses are not very great material will return to the original shape and dimension when external stress is removed.
Concept of yield point:
Yield point or yield strength of the material is stated as stress at which a material starts to deform plastically. Before yield point the material will deform elastically and will return to the original shape when applied stress is removed. Once yield point is passed, few fraction of deformation will be permanent and non-reversible.
Substances that lengthen significantly and go through plastic deformation until they break are ductile substances. Examples of such substances are Copper (Cu), Lead (Pb), and Wrought Iron (Fe).
When substances break very soon after elastic limit is obtained, then, such substances are called as Brittle. Glass, high carbon steel brass, bronze and other alloys do not have yield point.
Ultimate tensile strength (UTS):
It is frequently shortened to tensile strength (TS) or ultimate strength, is a maximum stress that a material can endure while being stretched or pulled before failing or breaking. Tensile strength is not the similar as compressive strength and values can be fairly different.
Elastic modulus, or modulus of elasticity, is the number which estimate the object or substance's resistance to being deformed elastically (that is, non-permanently) when the force is applied to it. Elastic modulus of the object is stated as slope of its stress-strain curve in elastic deformation region. The stiffer material will have higher elastic modulus.
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