Is strain uniform in a material?

[jendrix]

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

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

 

[JBA]

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.

 

[jrmichler]

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?

 

[Chestermiller]

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.

 

[miltos]

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.

 

[miltos]

If anyone interested, please have a look at the following somewhat simplified image, regarding reinforced concrete
https://i.ytimg.com/vi/xn69MLh7Pl0/maxresdefault.jpg

 

[jrmichler]

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.

 

[miltos]

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)