Hello Osama,

I understand tensile tesing machines now have an optical camera to image the flat face of the gauge portion of the test specimen.  On that flat face, a black paint is sprayed onto the surface creating a random speckle of black dots. The imaging system can perform analysis on the image to measure the relative deformation of those black dots. The analysis of the set of images computes the Engineering Strain vs time. The tensile testing machine has a force transducer which can be converted to Engineering Stress vs time. Those two quantities can be plotted to give the Stress-Strain curve for the sample being tested.

When you build a finite element model of that tensile test geometry, the meshing software creates a random or uniform set of nodes on the surface of the geometry, depending on the type of mesh created.  When the software computes the solution for the relative displacement of the nodes, that is converted into True Strain, and the True Stress is computed from the Stress-Strain curve that was input into the Engineering Data for that material.

I don't think it is necessary to apply the black dots to the surface of your Ansys model. I suggest the nodes will be sufficient.