In the piping industry, the term Pressure and Stress are always used and most of the time it creates huge confusion among users as the unit of both Pressure and Stress is the same (Pounds per square inch or Newtons per square meter or Pascal). However, Both Pressure and Stress are different.
Pressure Vs Stress
The pressure is an intrinsic property and most of the time related to fluids. It depends on the momentum transfer between the atoms of a liquid or gas volume (molecules constrained inside the volume) on a micro-scale.
While Stress is a consequence of the tendency of a body undergoing arbitrary deformation to spring back to its own reference state; To simplify stress is generated whenever a force is applied on a body to deform it.
In this article, I will list down the major differences between Pressure and Stress in a Tabular format.
Difference Between Pressure and Stress
|1||Pressure can be defined as the intensity of external forces acting at a point. It is the amount of external force applied per unit area. The pressure is exerted on the body.||Stress can be defined as the intensity of internal resistance force that is developed at a point due to application of force or pressure. It is the amount of internal force being exerted per unit area. Stress is produced inside the material body.|
|2||The pressure is a unique property of thermodynamics or physics.||Stress is a material property.|
|3||Pressure always acts normal to the surface on which it acts.||The stress can be in any direction and any angle depending on the application of force.|
|4||Pressure always try to compress the surface on which it is applied.||Stress can be tensile or compressive depending on the load type.|
|5||Pressure can be exerted on both liquids and gases.||Stress can only be exerted on solid materials.|
|6||Pressure is physically measured (measurable quantity) using pressure gauges, manometers and other pressure measuring devices or instruments.||There is no device to measure to stresses (not a measurable quantity). These are only calculated mathematically. Stress is calculated by measuring strain or elongation.|
|7||Pressure is a positive unit and can be represented as F/A (F positive as direct force)||Stress can be either a positive or a negative force, this can be represented as Stress= – F/A. (-F as internal resisting force)|
|8||The pressure is independent of the area of the contact surface. It remains constant and does not vary with change in surface area. For example, Assume a 500 Pa pressure is applied to 100 m^2 area. Now even if you change the area to 10 m^2 the pressure will remain constant. However, the calculated force will vary depending on the area.||On the other hand, stress varies with change in surface area. Here, Force remains constant but increase is area causes the force to reduce and vice versa. Magnitude of Stress varies inversely with surface area on which it applies.|
|9||The pressure is a scalar field variable and does not depend on the direction as it always acts normal to surface. The magnitude of the pressure at a point in all the directions remains the same.||Stress is a Vector variable and depends on direction of applied force. The magnitude of stress at a point in different direction is different.|
|10||Pressure is the cause for stress||Stress is the result of pressure.|
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