Is strain uniform in a material?

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Discussion Overview

The discussion centers around the concept of strain in materials, particularly in the context of using strain gauges for measurement. Participants explore whether strain is uniform throughout a material, the implications of strain gauge placement, and the variability of strain in different scenarios, such as cantilever beams and concrete structures.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions if the strain measured by a strain gauge at a single position can represent the strain in the entire material, suggesting a need for clarity on this point.
  • Another participant emphasizes that strain measurements should only be applied to the region covered by the strain gauge and along the line of action of the device.
  • A participant notes that in a cantilever beam, strain varies along its length, with an exception for pure moment loading, and mentions that longer strain gauges can be beneficial for installation and reducing self-heating.
  • It is pointed out that strain is not uniform through the thickness of a beam and can change sign, indicating tension on one side and compression on the other.
  • A later reply challenges the generality of the statement about strain changing sign, specifically mentioning that it does not apply to all materials, using concrete as an example.
  • Participants discuss the orientation of strain gauges, highlighting that they only measure strain components parallel to their grid and that incorrect orientation can lead to inaccurate measurements.
  • One participant briefly mentions the use of strain gauges for measuring torsion, suggesting that this point may be off-topic for the current discussion.

Areas of Agreement / Disagreement

Participants express differing views on the uniformity of strain in materials, with some asserting that strain varies in specific contexts while others suggest that this may not apply universally. The discussion remains unresolved regarding the general applicability of strain behavior across different materials.

Contextual Notes

Participants highlight the importance of understanding mechanics of materials for effective strain gauge application, indicating that assumptions about strain uniformity may depend on specific loading conditions and material properties.

jendrix
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Hello,

I am working on a project for an EE class and am brushing back up on the basics of strain and stress. My question was to do with how strain works in a material and if it is constant throughout. Say for example you had a setup like this
VHDO4Rr.png


The change in resistance the strain gauge measures can be used to calculate the strain. Is this the same as the strain in the entire test specimen? So you can measure the strain at one position and this can be used to find the strain in the entire material?

If so is there an advantage to having a longer strain gauge?Thanks
 

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For safety, the strain measurement should only be applied for the region covered by the strain measuring item i.e strain gage and along the line of action of the device.
 
In a cantilever beam, the strain varies down the length of the beam. There is one exception, and that is a pure moment loading on the tip of the beam (does not apply to your setup). The strain gauge is bonded to the object, and measures the average strain under the grid portion of the strain gauge. Longer strain gauges are less susceptible to self heating and are easier to install.

Trying to apply strain gauges without first taking a course in mechanics of materials is an exercise in frustration and futility. That background is necessary to choose the location for the gauge and to estimate the expected strain. You need an estimate of the maximum strain to know if your strain gauge signal conditioner is capable of delivering a signal large enough to measure and small enough to not saturate.

Do the ME students do a similar project? Could you team up with one of them?
 
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In addition to what jrmitchler said, I will mention that the strain isn't uniform through the thickness of the beam. In fact it changes sign at the center of the beam, such that, if the top of the beam is in tension, the bottom of the beam is in compression.
 
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Chestermiller said:
In fact it changes sign at the center of the beam

Hi!

The above statement does not apply to every material. A very good example is concrete.
 
Last edited by a moderator:
In addition to what @Chestermiller said, strain gauges only measure the component of strain parallel to the grid. If you apply the strain gauge with the wrong orientation, you could find yourself measuring nothing.
 
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You can also use strain gauges to measure torsion but there is not need to mention that at this discussion.
(I mean that you are a little off topic, by my point of you, as the above message is completely irrelevant with the discussion as far)
 

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