Stress-Strain Curve is explained along with detailed diagram and explanation of each point in the curve to better understanding.
Here, we will learn the curve, along with a diagram, stress strain diagram of many material, their characteristics.
Stress-Strain Curve, as the name suggests, it’s basically related to material’s stress and strain. Stress strain curve is defined as the curve or a graphical representation of a material’s stress and its strain and understood the relationship between stress and strain.
The importance of stress strain curve is very crucial, as it establishes the relationship between stress and strain. We can easily understand the behavior of any material with respect to the application of stress.
However, before going to the stress strain curve, we will try to understand what is stress and strain and relation between stress vs strain.
We have already seen that we can stretch a rubber very easily and if we stretch is more at certain point it will be broken.
But, what about if we try to stretch an iron rod? Will the iron rod be stretched? Or if it stretched will it be broken?
Here, the concept of stress strain comes into picture.
If force is applied to a material, it can be stretched or compressed. Now, stress is defined as the forced applied per unit area.
It is denoted as σ (Sigma) and written as,
σ = Forced / Area
The unit of stress is N/m2
Remember the followings,
Now, application of force incurs deformation on the body. Strain is related to the deformation and it is defined as the ratio of deformation of the body in the direction of force application to the initial dimensions of the body.
The strain is written, as follows,
Remember the followings,
Stress vs strain relation is stated with the use of Hooke’s law,
Stress is proportional to strain and it is known as Hooke’s Law. Hence, we can write as per Hooke’s Law,
Stress ∝ Strain
σ ∝ ε
σ = E x ε
E = σ / ε
E = (Fn / A) / (dl / lo)
E = Young’s Modulus (N/m2) (lb/in2, psi)
Modulus of Elasticity, or Young’s Modulus, is commonly used for metals and metal alloys and expressed in terms 106 lbf/in2, N/m2 or Pa. Tensile modulus is often used for plastics and is expressed in terms 105 lbf/in2 or GPa.
Let’s try to understand stress-stress curve with a stress strain diagram. Let’s take an example of steel to study the deformation in the application of stress to have a clear idea.
There are several points of interest in the diagram above:
Different material has different stress-strain graph and it looks different based on the characteristics. We will learn all these in brief later.
Now, we will discuss all the points in detail. So, how does this curve generated?
The stress strain curve is plotted in Universal Testing Machine (UTM). In this machine, there are two claws to hold the material and apply uniform pressure to deform.
The force applied and the strain produced is recorded until a fracture occurs. The two parameters are then plotted on an X-Y graph to get the familiar graph.
Here, stress is applied to the steel and provide a gradual tension up to its failure. The entire process is plotted in a curve and the same is known as stress-strain curve of steel.
Stress strain curve of steel, aluminum, cast iron, elastomers etc., are described.
Refer below diagram for stress strain curve of steel.
The stress strain curve describes many engineering parameters, like
In this graph, as the stress increases, the strain also increases. Steel is considered for the example of stress-strain curve as it is a little complex rather than brittle material. Stress-strain curve for brittle is so simple & we will learn after ductile material.
Stress is proportional to strain, as per Hooke’s Law. That means,
Based on the application of stress, various points are created in the stress strain curve, let’s describe.
The points in the stress strain curve are as follows,
All the metals behave like an elastic material up to a specific range. It means after the application of stress; the material returns back to its original dimension or shape. The point up to which the material behaves this proportionality is called the proportional limit.
This proportional limit depends on the followings,
In this proportional limit,
When the machine pulls the object at the ends, the object experiences tensile stress. Up to the proportionality limit, it obeys Hooke’s law and the ratio of stress and strain is constant.
This constant is called as Young’s modulus of elasticity in material science.
Young modulus of elasticity = longitudinal stress / strain
Hence, Young modulus of elasticity is defined as the ratio of longitudinal stress to strain within the proportional limit of a material.
Refer young’s modulus value of few commonly used materials,
|Material||Young’s Modulus Pa (N/m2)|
|Steel||200 x 109|
|Copper||110 x 109|
|Iron||91 x 109|
|Brass||91 x 109|
|Glass||55 x 109|
When the tensile force is increasing further, stress will increase beyond the proportionality limit. After this limit, a small portion of stress strain curve acts elastically. That means, in this region, if tensile stress is removed from the material, it will return to its original length.
If we increase the tensile force on the metal further, tensile stress will increase and the material will cross the elastic limit and start to deform or yield. This point where deformation starts is called as yield point and the stress which creates this deformation is known as yield strength.
After yield strength, Hooke’s law is deviated and different material acts differently. To avert this problem, a new term, proof stress is introduced.
Proof Stress is nothing but a parallel line to the straight portion of stress strain curve at a strain value of 2%.
The ultimate tensile strength is the maximum stress value before its failure and it is the highest stress value in any curve.
After the elastic point, the elasticity of the material is lost and strain hardening region in the curve started.
Material experiences a very high rate of plastic deformation after yield point. After ultimate tensile strength, ductile material can be able to support load up to a very small extent. Once the material yields, strain harden starts, it increases the strength of the material.
In strain hardening, molecules are obstructing each other and rearranged. This rearrangement helps the metal to increase its strength.
Necking means simply the formation of neck. When the plastic deformation continues, with increasing the tensile stress, the material starts to form its neck, that is narrowing its cross section.
This phenomenon is called as necking.
In the necking region, plastic deformation happens. In this region of the stress-strain curve, no need to increase the load further, to experience the plastic deformation. A fracture occurred at the neck. The point at which fracture happens is known as the fracture point.
Stress-strain curve of other materials like aluminum, concrete, cast iron, elastomers, perfectly plastic materials are required for their formation or making other materials are necessary.
We will discuss, this curve of few widely used material, as follows,
We have already learned that yield stress is measured by drawing a proof stress if a material doesn’t have distinct yield stress. In case of aluminum or aluminum alloys, there is no distinct yield stress so proof stress is adopted.
The curve for rubber is linear within proportionality limit and it is with high strain value around 0.1 to 0.2.
After proportionality limit, stress diagram depends on the type of rubber,
To know the behavior of concrete for the use of construction, this curve is required to study.
Cast Iron is not a ductile material, instead it is a brittle material. We have already learned that for all brittle materials, material fracture suddenly.
So, tensile strength is the main driving parameter for brittle materials to get the behavior of the materials. Cast iron is falling under this category. Refer the diagram for cast iron.
Elastomers mean a material that can be stretched up to double of its length at normal conditions without any permanent deformation.
There are many elastomers used in various industries, which exhibits the same curves,
We have already learned strain hardening or work-hardening. In the case of perfectly plastic materials,
Look at the stress-strain curve for,
If a material breaks without any plastic deformation, the material is known as a brittle material. On the other hand, all ductile materials experience plastic deformation before failure.
We can distinguish ductile vs brittle – stress-strain curves, as below,
|Description||Ductile Material||Brittle Material|
|Plastic Deformation||Undergo plastic deformation before failure||No plastic deformation or negligible plastic deformation before failure|
|Elongation||Elongation is more||Very less elongation|
|Energy absorb||Energy absorb before fracture is high||Less energy absorbs|
|Wire||Wire can be made from this kind of material||Not recommended|
|Bending||It can bend||Difficult due to failure|
|Shear Fracture||Better shear fracture||Not good|
|Crack||Crack propagates slowly||Crack propagates fast|
|Forming Process||Forming process can be done||Very difficult|
|Life||Longer life if it is subjected to fatigue loading.||Comparatively short life if it is subjected to fatigue loading.|
|Examples||Mild steel |
|Cast iron |
Hence, we have got a basic concept of stress, strain, and stress-strain curve along with a detailed explanation. Any doubt, please feel free to write.
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