How to extract the strain component on a surface location of a solid?

Hi all,

My model was built with tetra solid elements, and a structural analysis was solved.

In the test, the strain of some points on the solid structure surface were gotten by strain gauges. I want to compare the strain component from my simulation with the test result. However, my problem is how to extract the strain result on the surface of a tetra solid model in optistruct?

Thank you