Dear all, I have problem on defining first principal stretch, second principal stretch and third principal stretch. Does it means in x axis we definite it as first, y axis is second, third is z axis? what if my load is applied on a cubic from the -y direction in a uniaxial testing tension, so my first principal stretch remains as x axis, or will it change? I hope someone can understand my statement, if not, do post it. i try to make myself clear.