Stress compound cylinder problem

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Discussion Overview

The discussion revolves around a homework problem involving the analysis of stresses in a compound cylinder subjected to internal pressure and the effects of shrink fitting. Participants explore the application of Lame's equations to determine maximum stresses and the relationship between the dimensions of the tubes before shrinkage.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant presents a calculation resulting in a negative maximum stress, questioning its validity in the context of cylindrical structures.
  • Another participant suggests that a negative stress could indicate compression, but this raises further questions about its applicability to cylinders.
  • A subsequent reply provides revised calculations for hoop stress, yielding values of 83.3 MPa for the inner cylinder and 33.3 MPa for the outer cylinder, suggesting a more realistic outcome.
  • Concerns are raised about the assumption that both tubes are made of the same material, based on the single value of Young's modulus provided.
  • One participant mentions recalculating the stress due to the interference fit before considering the internal pressure, indicating a change in approach to the problem.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the interpretation of negative stress values and whether the assumption of uniform material properties is valid. There is no consensus on the correctness of the initial calculations or the implications of the results.

Contextual Notes

Participants note the need to consider both the interference fit and internal pressure in their calculations, highlighting potential dependencies on assumptions about material properties and the conditions of the problem.

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Homework Statement



A tube 40mm inside and 60mm outside dia is to be reinforced by shrinking on a tube of 80mm outside diameter. The compound tube is subjected to an internal pressure of 50MPa and the shrinkage allowance is such that the final maximum stress in each tube is the same. Determine the maximum stress and the difference in tube diameter before shrinkage. E=207000 N/mm^2


Homework Equations



Lames equations:
Radial stress σr=A-B/r^2
Hoop stress σθ=A+B/r^2

The Attempt at a Solution



Ive attempted the first part of the question however i have ended up with a negative maximum stress which cannot be. If someone could point out my mistake or give advice that would be great :)
 

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Usually, a negative stress indicates the member is in compression.
 
Ok thanks, so my answer is an acceptable one? I didn't think that would apply to cylinders.
 
I have just worked the calculation again and got A=16666687.5 and B=26666.7 and subbing into lames eqautuon for hoop stress in and out

σθin= 83.3MPa

σθout= 33.3MPa

this seems a more realistic answer, showing max stress is in the inside of the cylinder. ?
 
just looked at your working how did you now that the compound cylinder was made up of the same material, is that from the one value of young's modulus? also did you work out the stress produced from the shrink fit not just the internal pressure?
 
Last edited:
Yes as there is only one youngs mod given i would of thought it is the same material?

Ive been working on it and i have worked out the stress due to interference fit first and then calulated with internal pressure. not realising i had to to this at first.

:)
 

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