. The coordinate axes x and y on the free unloaded surface of a linearly elastic isotropic body are principal directions. At what angle θ relative to the x axis must a strain gage be place in order...

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The coordinate axes
x
and
y
on the free unloaded surface of a linearly elastic isotropic

body are principal directions. At what angle
θ
relative to the
x
axis must a strain gage be

place in order that direct measurement of the principal stress
σx
be made with this gage,

that is, so that
σx
=
K_g? Assume that the elastic coefficients of the material are known.