Stress in Beams - Same Force, Same Area, Different Lengths

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    Beam Stress
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Discussion Overview

The discussion revolves around the behavior of stress in beams of different lengths but with the same cross-sectional area and applied force. Participants explore the implications of beam length on maximum stress, particularly in the context of finite element analysis (FEA) versus traditional beam theory.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants question the definition of "max stress" and whether it varies with beam length, suggesting that stress should not be affected by the length below the point of force application.
  • Others argue that beam theory is an approximation and that a 3-D finite element model represents a different approximation, implying that results may not align perfectly.
  • One participant notes that maximum stress occurs at the area around the top fixing point, raising questions about how changing the length of the beam affects stress in that region.
  • There is a discussion about the types of stress involved, specifically bending stress versus shear stress, indicating a need for clarification on these concepts.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between beam length and stress, with no consensus reached on whether the length affects maximum stress at the fixing point. The discussion remains unresolved regarding the implications of FEA results compared to traditional beam theory.

Contextual Notes

Limitations include potential misunderstandings of stress definitions and the assumptions underlying beam theory versus finite element analysis. The discussion does not resolve how these factors influence the observed stress results.

Victor.wong
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if you look at the attached photo.

we have 2 beams, 1) is shorter, 2) is longer. cross-sectional area is the same. same force is applied X cm from the fixed end.

what is the max stress in each case? why are they the same ( and why not)?

Take gravity out of the equation.
FEA shows different result.
 

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victor.wong
what is the max stress in each case? why are they the same ( and why not)?

Hello victor, is this a lab experiment you are writing up?

You surely don't need a finite element analysis to distinguish between types of stress, this is more fundamental than FEA.

So what do you mean by max stress?

Do you know how to derive stress as a function of distance?
 
How different is "different"? Beam theory is only an approximation. A 3-D finite element model is a different approximation. You wouldn't expect them to be "exactly" the same.
 
I know it is a very simple question. but FEA is telling me sth i don't expect. so ask this equation and see who can explain.

first of all, stress only caused if a block of material is under 2 force. action force and reaction (the fixture). since the length under the point of force of action is not under another reaction force. these length should not affect the stress produced by this action force

in both cases, max stress appeared at the area around the top fixing point. if i let the applied force and location constant, changing the length of the block will change the max stress at the fixing area. why??
 
Well what do you mean by max stress?

Or if you like, what type of stress?

And how about answering my other questions, they were intended to help.
 
OK. may be a pic will explain more

if you see the new attachment, my question is, why is the length of the block below the applied force affect the stress at the circled area.

my thinking behind is, no matter how long the rod is below the force application. it should not affect the portion on top of the applied force.

right? and why?
 

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Do you know the difference between bending stress and shear stress?
 

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