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.