In mechanical engineering, pressure and stress are two important concepts that describe the behavior of materials under various loads and forces.
Definition
Pressure is defined as the force per unit area acting on a surface, perpendicular to the surface.
Stress is defined as the internal resisting force per unit area acting on a material.
Characteristics of pressure
Pressure is a scalar quantity: Pressure has only magnitude and no direction, which means it is a scalar quantity. It is usually expressed in units of pascal (Pa), which is defined as the force per unit area.
Pressure is always compressive in nature.
Pressure is isotropic: Pressure is the same in all directions at a point within a fluid at rest. This characteristic is known as isotropy.
Pressure varies with depth: Pressure increases with depth in a fluid due to the weight of the fluid above it, known as hydrostatic law.
Pressure can be measured using various instruments: Pressure can be measured using instruments such as pressure gauges, manometers, and pressure transducers. The choice of instrument depends on the specific application and the range of pressures involved.
Characteristics of Stress
Stress is a tensor quantity.
Stress can be classified into different types: Depending on the direction of the applied force and the orientation of the material, stress can be classified into different types such as normal stress, shear stress, and bearing stress.
Distribution of stress at a particular cross section within a body depends upon its distance from point of application of load.
Stress can cause deformation: Stress can cause a material to deform, either elastically or plastically, depending on the material properties and the magnitude of the applied stress.
Stress can be measured using various instruments: Stress can be measured using instruments such as strain gauges, extensometers, and deflectometers. The choice of instrument depends on the specific application and the range of stresses involved.
In summary, while pressure and stress are related, they have distinct definitions, units, directions, types, formulas, and effects. Pressure is the force per unit area acting perpendicular to a surface, while stress is the internal resisting force per unit area. Understanding the difference between these two concepts is important in designing mechanical systems and structures that can withstand various loads and forces.
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