Short Answer:

Stress is the internal resisting force developed within a material when an external force or load is applied to it. It is the reaction of the material to the external force that tries to deform or change its shape. Stress is measured as the force per unit area acting inside the material and helps in determining the strength and safety of structures or machine components.

In simple words, stress represents how strongly the particles of a material resist deformation when subjected to external loads such as tension, compression, or shear. It plays a very important role in engineering design and analysis to prevent failure of materials.

Detailed Explanation:

Stress

Definition and Meaning:
In mechanical engineering, stress is defined as the internal resistance or opposing force per unit area that develops within a body when an external load is applied to it. When a material is subjected to forces, its particles tend to move or deform. However, internal forces arise between these particles to resist the deformation. The intensity of this internal resisting force is called stress.

Mathematically, it is expressed as:

The unit of stress in the SI system is Pascal (Pa) or N/m², and sometimes it is expressed in MPa (Mega Pascal) for larger values.

In simple terms, stress can be understood as the measure of load distributed over a given area. For example, if we pull a steel rod by applying a force, the rod will resist that force internally. The intensity of that internal resistance per unit area is the stress.

Concept of Stress

Whenever an external force acts on a body, it tries to deform the shape or size of the body. To counter this deformation, internal forces develop inside the material. These internal forces always act in the opposite direction of the applied load to maintain equilibrium. The magnitude of these internal forces, when divided by the cross-sectional area over which they act, gives the value of stress.

For instance, if a bar of steel is stretched by applying a pulling force, internal forces are generated within the bar to resist the elongation. These resisting forces are distributed uniformly across the cross-section if the material is homogeneous and the load is applied evenly.

Types of Stress

There are different types of stress depending on the type and direction of external forces applied. The main types are:

1. Tensile Stress

Tensile stress is developed when a material is subjected to pulling or stretching forces.
In this case, the body tries to elongate, and internal forces resist this extension.
Example: When a rod is pulled at both ends, the particles inside resist the tension, and tensile stress is developed.

2. Compressive Stress

Compressive stress is produced when a material is subjected to pushing or squeezing forces.
In this case, the body tends to shorten or compress, and the internal resisting forces act to oppose this compression.
Example: The stress developed in a column of a building under the weight of the structure is compressive stress.

3. Shear Stress

Shear stress occurs when two equal and opposite forces act tangentially to the surface of a body, trying to slide one layer of the material over another.
Example: When scissors cut paper or when a pin is subjected to double shear, shear stress is generated.

4. Bending Stress

Bending stress is produced when a beam or bar is subjected to bending or moment loads. It is a combination of tensile and compressive stresses.
The upper portion of the beam undergoes compression, while the lower portion undergoes tension.
Example: The stress developed in a bridge beam when vehicles pass over it is bending stress.

5. Torsional Stress

Torsional stress occurs when a circular shaft or object is twisted by applying a torque. The outer surface of the shaft experiences maximum stress, while the center has zero stress.
Example: The stress developed in the shaft of a motor or turbine when it rotates under load is torsional stress.

Importance of Stress in Engineering

The study of stress is one of the most important topics in mechanical engineering. Understanding stress helps engineers in:

• Designing structures and machine components that can safely carry loads.
• Preventing material failure due to excessive loading.
• Selecting proper materials for specific applications based on their strength.
• Determining safety factors in design calculations.
• Analyzing deformation, strain, and elastic behavior of materials.

Without the concept of stress, it would be impossible to design safe bridges, buildings, machines, or vehicles because we would not know how materials behave under forces.

Stress-Strain Relationship

Stress is closely related to strain, which is the measure of deformation produced in the material. The relationship between stress and strain is important in material testing and mechanical design. According to Hooke’s Law, within the elastic limit:

This means that the stress increases linearly with strain until the elastic limit is reached. Beyond that, permanent deformation begins.

Factors Affecting Stress

1. Magnitude of Applied Force: Larger forces produce higher stress.
2. Cross-sectional Area: Smaller area leads to higher stress for the same force.
3. Material Properties: Different materials resist stress differently depending on their elasticity and strength.
4. Type of Loading: Whether the load is static, dynamic, or impact changes the stress developed.
5. Temperature: Thermal expansion or contraction can create additional stresses.

Conclusion:

Stress is the internal resistance developed within a body when it is subjected to an external force. It is measured as force per unit area and plays a key role in determining how materials behave under different loading conditions. Various types of stresses such as tensile, compressive, shear, bending, and torsional are studied to design safe and efficient mechanical systems. Understanding stress helps engineers to predict material performance, prevent failures, and ensure the safety and reliability of engineering designs.