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It's about shear stress and strain. We are talking about Newtonian fluids, a situation where we are lubricating a rotating shaft with oil.

I cannot understand what we are measuring that we use as the strain.

SOLIDS. As far as I know we can say: Stress (F/A) = G x (Δ l / l)

Where G is shear modulous, Δl is the displacement, parallel with the force, due to the stress (or force) - probably in fractions of a millimeter - and l is the height of the zone where displacement is taking place, again probably in fractions of a millmeter. You get the strain by measuring two distances - Δl and l - and obtaining the ratio of them, which is a unitless number.

FLUIDS: Okay, can we say?: Stress (F/A) = η x (Δl / Δt)

Where η is the coefficient of dynamic viscosity, Δl is the distance travelled by the oil at the rotating shaft side, Δt is unit time of 1 second.

With fluids strain is actually the rate of strain. Of course, the layers of oil are circling around, there no movement on one side of the boundary, and where the shaft is, the oil there is moving at the speed of the shaft. So, there is a velocity gradient in the oil. The thin layers within the oil are sliding alonside each other, which must be if there is a gradient in the velocity of the oil across the height or length of the film of oil.

But, in the above formula, there is no l. Only Δl.

Thinking about shear in solids, I want to see things in these terms: Stress (F/A) = η x (Δl /t ) / l).

The above formula is measuring the distance, at the shaft side, the oil traveles in 1 second (rate of shear?) and l is the thickness of the oil film.

There are probably errors of some description of the above. Anyway, I'm not really sure what is rate of strain. Practically, what would you be measuring to get rate of strain? Obtaining strain is easy with solids. It's simple distances, with no reference to time. Thanks.

I cannot understand what we are measuring that we use as the strain.

SOLIDS. As far as I know we can say: Stress (F/A) = G x (Δ l / l)

Where G is shear modulous, Δl is the displacement, parallel with the force, due to the stress (or force) - probably in fractions of a millimeter - and l is the height of the zone where displacement is taking place, again probably in fractions of a millmeter. You get the strain by measuring two distances - Δl and l - and obtaining the ratio of them, which is a unitless number.

FLUIDS: Okay, can we say?: Stress (F/A) = η x (Δl / Δt)

Where η is the coefficient of dynamic viscosity, Δl is the distance travelled by the oil at the rotating shaft side, Δt is unit time of 1 second.

With fluids strain is actually the rate of strain. Of course, the layers of oil are circling around, there no movement on one side of the boundary, and where the shaft is, the oil there is moving at the speed of the shaft. So, there is a velocity gradient in the oil. The thin layers within the oil are sliding alonside each other, which must be if there is a gradient in the velocity of the oil across the height or length of the film of oil.

But, in the above formula, there is no l. Only Δl.

Thinking about shear in solids, I want to see things in these terms: Stress (F/A) = η x (Δl /t ) / l).

The above formula is measuring the distance, at the shaft side, the oil traveles in 1 second (rate of shear?) and l is the thickness of the oil film.

There are probably errors of some description of the above. Anyway, I'm not really sure what is rate of strain. Practically, what would you be measuring to get rate of strain? Obtaining strain is easy with solids. It's simple distances, with no reference to time. Thanks.

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