In practical stress and strain analysis it is
the strain value which is first obtained by measurement using strain gauges.
With these measured data, stress values are then obtained analytically.
Strain measurements are possible because it is simple and highly reliable
techniques have been established.
Strains are measured along several closely clustered gauge line as shown below,
by lines a-a, b-b, c-c
inclined at 
,
and 
respectively to the x-axis. Such arrangement are called strain rosettes.
If 3 measurements are taken, it is sufficient to determine the complete state of
plane strain at a point. It can be shown (Ref
8: Popovers P430-P432) that:
Where n is the number of strain gauge. Note that the unknowns are 
and 
.
Hence, 3 measurements producing 3 equations are sufficient to solve for the
unknowns.
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