When a solid object (such as an iron rod or a metal plate) is gradually subjected to force, it either stretches or deforms slightly. The amount of this strain or deformation depends on the amount of force applied.
The graph that explains this entire behavior is called Stress and Strain Curve.
This curve fully reflects the mechanical behavior of a material, how strong it is, when it begins to deform permanently, and when it breaks.
Stress and Strain Curve is the identification of strength, flexibility and durability of any material.
What is Stress and Strain Curve?
Stress and Strain Curve is a graph that shows the relationship between the applied force (stress) and the resulting strain (strain) on a material.
This curve tells us that,
- How long does a material remain elastic,
- when does it become permanently deformed (plastic),
- and when does it reach the point of fracture.
In simple words,
“Stress-Strain Curve is a graph that shows how much a material can stretch and where it will break when a given force is applied on it.”
How Stress-Strain Curve is Obtained?
1. Tensile Test: A gradually increasing tensile force is applied to a specimen of a material.
2. Measurement: Force and length change are measured.
3. Graph Plot: The relationship between Stress (vertical axis) and Strain (horizontal axis) is plotted.
This graph shows the complete mechanical behavior of the material from inception till breakage.
Different Regions of Stress and Strain Curve
The stress-strain curve for a ductile material (such as mild steel) has five main parts,
1. Proportional Limit: Stress is directly proportional to strain (Hooke’s law applies). Beyond this point, the linearity is lost.
2. Elastic Limit: Up to this point, if the force is removed, the material returns to its original state. Beyond this point, the deformation becomes permanent.
3. Yield Point: At this point, the material begins to deform significantly even with a slight increase in stress. Plastic deformation begins here.
4. Ultimate Tensile Strength: This is the stress where the material reaches its maximum strength. It is the highest point on the curve.
5. Fracture Point: After the Ultimate Strength, the material’s length increases, but the cross-section decreases (necking), and eventually fractures.
Factors Affecting Stress and Strain Curve
- Material Type: Materials like steel, copper, aluminum behave differently.
- Temperature: Ductility increases with higher temperatures.
- Strain Rate: A material may become brittle when subjected to a rapid load.
- Impurities: Alloying elements or defects alter the curve.
- Specimen Shape: Thickness and length affect the slope of the curve.
Importance of Stress-Strain Curve
- The most important test for material selection.
- It helps design engineers determine when a structure will yield or fail.
- It is used for quality control and safety verification.
- It helps determine the Factor of Safety (FOS) for a structure.
- It is the basis for measuring a material’s ductility, strength, and toughness.
Applications
- Civil Engineering: Beam, column, and bridge design.
- Mechanical Design: Shaft, spring, rod, and fastener analysis.
- Automobile & Aerospace: Body materials and load-bearing parts.
- Research & Testing Labs: For material certification.
Advantages
- Easy to understand the strength and behavior of materials.
- Identification of elastic and plastic limits.
- Comparison of different materials possible.
- Useful for determining Young’s modulus, yield strength, ultimate stress, ductility, toughness.
Limitations
- Shows only uniaxial stress; in real life, there are multidirectional stresses.
- Changing temperature and speed can cause the curve to vary.
- Very brittle materials (such as cast iron) require a different graph.
Conclusion
The stress and strain curve is the mechanical characteristic of any material. It tells us how long the material will remain elastic, when it will yield, and when it will break.
This curve helps the engineer determine which metal to use where, how much load will be safe, and how much material deformation it can withstand.
Stress-Strain Curve is not a graph, but the complete story of the material.