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(PSI) or Newtons per square meter or Pascal). However, Both Pressure and Stress are different. So, the differences between stress and pressure i.e, Stress vs Pressure must be known to clearly understand the terms.
Pressure Vs Stress
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 to a body to deform it.
In this article, I will list down the major differences between Pressure and Stress in a Tabular format.
Pressure vs Stress
Difference Between Pressure and Stress
Sr. No
Pressure
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 the 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
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 tries 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, barometers, manometers, and other pressure-measuring devices or instruments.
Pressure is a positive unit and can be represented as “p”= F/A (F positive as direct force)
Stress can be either a positive or a negative force, this can be represented as Stress “Ϭ”= – 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 changes in surface area. For example, Assume a 500 Pa pressure is applied to a 100 m2 area. Now even if you change the area to 10 m2 the pressure will remain constant. However, the calculated force will vary depending on the area.
On the other hand, stress varies with changes in surface area. Here, Force remains constant but an increase in the area causes the stress to reduce and vice versa. The magnitude of Stress varies inversely with the 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 the surface. The magnitude of the pressure at a point in all directions remains the same.
Stress is a vector variable and depends on the direction of the applied force. The magnitude of stress at a point in a different direction is different.
10
Pressure is the cause of stress
Stress is the result of pressure.
11
Pressure is a thermodynamic property.
Stress is a material property.
Table showing the Differences Between Pressure and Stress
I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.
7 thoughts on “Pressure vs Stress: Differences Between Pressure and Stress”
Hello sir ,
Please send me pdf notes of NDT-2 level as your post which is very useful to me and my colleages.
Thanking you.
With regards,
Ajit tborat.
No.1
It might be easier to understand if explained like this. Pressure is action force on material/area, while stress is reaction force from material/area.
No.9 on stress
Stress is not a vector. It is a tensor. Stress is very good example of tensor. Other example of tensor is electric filed and magnetic field. A tensor is somewhat difficult property to understand comparing to scalar/vector. It is the property that is changed and defined with direction such as sigma_xy, sigma_xx(double subscript).
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As written, #8 makes no sense. I could just as easily say Magnitude of Pressure varies inversely with surface area on which it is applied and if stress is 500 Pa, then area doesn’t matter just the force changes. Both equal F/A.
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Hello sir ,
Please send me pdf notes of NDT-2 level as your post which is very useful to me and my colleages.
Thanking you.
With regards,
Ajit tborat.
Very good article!!
Some advice.
No.1
It might be easier to understand if explained like this. Pressure is action force on material/area, while stress is reaction force from material/area.
No.9 on stress
Stress is not a vector. It is a tensor. Stress is very good example of tensor. Other example of tensor is electric filed and magnetic field. A tensor is somewhat difficult property to understand comparing to scalar/vector. It is the property that is changed and defined with direction such as sigma_xy, sigma_xx(double subscript).
Stress is Tensor and not vector
pressure can create stresses on the surface expose on the surface
The blog focuses on a why I broke my scale .This article is attracting many people and I also enjoyed reading this article. I would surely recommend others too. Great blog indeed, will visit again future to read more!!
As written, #8 makes no sense. I could just as easily say Magnitude of Pressure varies inversely with surface area on which it is applied and if stress is 500 Pa, then area doesn’t matter just the force changes. Both equal F/A.
nice content