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My friend texts me, what she heard in one of her lectures today:

"Do you know stress can't be measured. It is only the strain that can be measured directly from a strain gauge."

I have seen something similar before when I studied Mechanics of Materials for the first time. I never understood it.

The reasoning sounds very unsatisfactory to me. You say strain gauge measures strain directly, but after all, it measures first a fractional change in resistance and then calculates the fractional change in length to give a 'displayed value' of strain isn't it? Also, if just displaying a value on a screen makes a quantity 'directly' measurable then, in a UTM, I think we can program the software to divide the load by the initial area and then display the stress value. Wouldn't in this manner I would measure stress directly?

So my question is,

Is it true that stress can't be measured directly unlike strain? If yes, then in what context it is true.

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    $\begingroup$ I disregard broad, general statements like that if a rationale is not immediately provided. They tend not to be useful anyways and almost always involve unstated nuances and perspectives. $\endgroup$
    – DKNguyen
    Jul 18, 2022 at 18:27

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Turning strain to stress requires an equation, and also knowing where you are on the stress-strain curve. There isn't a device like a strain gauge to directly measure stress.

But if I know the force, I can also measure stress directly. I can count bowling balls sitting on a beam, measure the beam, and determine the stress that way.

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This is probably a technicality from the fact that all common ways to measure forces depend on things like strain gauges as you pointed out. Other ways to measure forces are springs (as in scales) where you are again measuring deformation and relating to stress, hydraulic apparatuses, where you measure the pressure on a fluid and relate it to stress, etc.

In short, most (if not all) ways to measure stresses depend on measuring a deformation and then relating it to stress.

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  • $\begingroup$ It's taken me a while, but there is one device that can measure load - and therefore stress - not as strain: a lever balance where you can put known weights on the other end. $\endgroup$
    – Daniel Hatton
    Aug 6, 2022 at 16:08
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    $\begingroup$ I thought the same, but I'm not completely sure. Technically what you measure is the displacement of the discs (or some needle or something like that) and aim it to match it to a known 0. It's a philosophical point, but still. $\endgroup$
    – Zegpi
    Aug 9, 2022 at 20:58
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I don't think the statement is correct as both are measurable with conversion. You should check out "Load Cell Sensors" and "Strain Gauge Load Cell Sensors" (for both force/stress and strain measurements).

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