Hi Celina,

I guess you’re trying to apply Rosette Analysis using three strain gauges. If so, you have to read only the normal strains (not shear strains) of the three strain gauges at 0, 45, and 90 degrees.

Subsequently, the principal strains and the corresponding principal directions can be calculated. You can look up online for “Rosette Analysis” and you will be able to find the associated relations.