Please help me solve these two questions Explain the criteri

Please help me solve these two questions:

Explain the criteria to select the best angle for a strain rosette.

Solution

(1) since strain gage can measure strain only in one direction and to dtermine strain we need extra two gages are needed

all three gages are used together at one location on a stressed object is called strain rosette

to increase the accuracy of strain rosette large angles are used.a common rosette of three gages is where the gages are saperated 45 DEGREE

(2)since hydrostatic stresses alone does not cause yielding we can find a material plane called octahedral plane where the streses are decoupled in to dilation strain energy. on the octahedral plane the octahedral normal stresses solely contributes to the dilation strain energy.