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# What is difference between stress and pressure?

## Question

What is difference between stress and pressure?

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On 5/18/2017 at 1:18 AM, MayankBambal said:

Sir DrD, it seems you are a professor!(always makes students to read,read and read more) that comment made me read my notes again(and some internet articles) to really understand the concept of tensors. i must say i did a foolish mistake.

Yes stress is a second order tensor while pressure is zero order tensor.

Thank you very much for correcting me.

Yes, I am a retired Prof.. of Mech Engr. I suppose I should say, in response to the comment about read, read, and read more, that is what we faculty are here to do!

You are welcome for the correction.

DrD

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Pressure is the force acting upon the surface of an body.(Action)

Stress is the resisting force developed in a body when an external force acts on a body.(Reaction)

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Sirazz92 has given a fairly good answer.

Pressure usually refers to a distributed external load applied to a body. Stress is the distributed internal loading associated with displacement under load.

DrD

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I would like to add a story here. There are a limited number of units in this world. Different properties share the same units and combination of units though the meanings are VERY different, as DrD and sirazz92 explained.

Two values that are very related are stress and modulus of elasticity (MoE). Both have the units of pounds per square inch. I once witnessed a complete misunderstanding of the relationship.

The engineer, a very proficient ProE user when it came to building models, ran an FEA routine. He was under the mistaken belief that as long as the stress was below the MoE the load was ok. Dr D is now holding his head.

The MoE for steels are usually taken as 30,000,000 psi. This is good enough for classroom work. When you are doing real world work you will find values vary, but not by large percentages.

Some steels are very strong but I am not aware of any that yield or fail at 30,000,000 psi.

On a somewhat related topic a group of us were discussing a stress issue and a question came up that also showed a lack of understanding the basics. I too had to stop and think for a while. Though I was long past confusing MoE with the strength of a material, a basic understanding had grown stale.

QUESTION: If all steels have about 30,000,000 psi for MoE, how can they have different strengths? I would like to see answers from students and recent graduates. Dr D knows the answers all too well so he is not allowed to answer.

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On 13/02/2016 at 8:32 AM, saurabhjain said:

What is difference between stress and pressure?

1. pressure represent intensity of external forces acting at a point. but stress represent intensity of internal resisting forces develop at a point.
2. pressure is always acts normal to the surface. but but stress may also act either normal or parallel to the surface.
3. magnitude of pressure at a point in all direction remain same. but magnitude of stress at a point in all the direction are unequal.
4. pressure can be measure by using measuring device.like pressure gauge. but stress can't be measure directly by using any device.

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Most of above answers are good and almost explains the difference.

1. pressure is a scalar while stress is a vector.

On 5/11/2017 at 10:41 PM, JAG Engineering LLC said:

I would like to add a story here. There are a limited number of units in this world. Different properties share the same units and combination of units though the meanings are VERY different, as DrD and sirazz92 explained.

Two values that are very related are stress and modulus of elasticity (MoE). Both have the units of pounds per square inch. I once witnessed a complete misunderstanding of the relationship.

The engineer, a very proficient ProE user when it came to building models, ran an FEA routine. He was under the mistaken belief that as long as the stress was below the MoE the load was ok. Dr D is now holding his head.

The MoE for steels are usually taken as 30,000,000 psi. This is good enough for classroom work. When you are doing real world work you will find values vary, but not by large percentages.

Some steels are very strong but I am not aware of any that yield or fail at 30,000,000 psi.

On a somewhat related topic a group of us were discussing a stress issue and a question came up that also showed a lack of understanding the basics. I too had to stop and think for a while. Though I was long past confusing MoE with the strength of a material, a basic understanding had grown stale.

QUESTION: If all steels have about 30,000,000 psi for MoE, how can they have different strengths? I would like to see answers from students and recent graduates. Dr D knows the answers all too well so he is not allowed to answer.

Don't know much, but i will try to answer.
First of all some definitions:

1. MoE: It is the ratio of stress to the strain developed. strain is dimensionless and hence the units of MoE are same as that of Stress i.e. N/m2 or Psi.

2.Strength: or i can say maximum strength or Ultimate strength. is the maximum force that a material( in our case steel) can withstand just before fracture.

3.Yield strength: it is the value of stress at which sample starts yielding. i.e. going plastic deformation.

as seen from the above definitions all three are not same.

e.g. for pure aluminum. the MoE is 10^7 psi, Ultimate strength : 110*10^6 psi  and Yield strength: 95*10^6 psi

for different steel there is same MoE but different strength as shown in the image.

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14 hours ago, MayankBambal said:

1. pressure is a scalar while stress is a vector.

Well, not really. Stress is a second order tensor while vectors are first order tensors. Confused enough? How about if we say that stress requires a square matrix for full representation, while a vector can be fully represented by a single column (or row) matrix.

To JAG above ...

Well, you're no fun at all. I want to play as well.

DrD

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### MayankBambal that diagram makes it very clear. It provides the answer and I must say I have never seen it or one like it before that states it so clearly. If you think of steel as a spring they all have about the same spring constant. Once you reach the yield limit the slope of the graph deviates.  Different steels have different yield stress values. Since most structures are controlled by the allowable deflection, the respective yield points are never reached. Often people will think a strong steel will provide a different deflection. If you look at a beam equation they do not include a yield stress value. If you are allowed a deflection that would result in reaching or exceeded the yield of low carbon then going the the strong steel is a solution.

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On 2016-2-13 at 8:02 AM, saurabhjain said:

What is difference between stress and pressure?

Pressure is a force which acts on a body external and stress is force which acts on a body internally.

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6 hours ago, DrD said:

Well, not really. Stress is a second order tensor while vectors are first order tensors. Confused enough? How about if we say that stress requires a square matrix for full representation, while a vector can be fully represented by a single column (or row) matrix.

To JAG above ...

Well, you're no fun at all. I want to play as well.

DrD

Sir DrD, it seems you are a professor!(always makes students to read,read and read more) that comment made me read my notes again(and some internet articles) to really understand the concept of tensors. i must say i did a foolish mistake.

Yes stress is a second order tensor while pressure is zero order tensor.

Thank you very much for correcting me.

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Strees applies on single point

Preesure applies on multi point

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Yup ,,,

Pressure can causes stress but stress can't ....

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pressure is an acting force, on the other hand stress is a reacting force.