Hardness/Tensile Strength

I am trying to find the ultimate tensile strength of a viscoelastic material. Access to the material is confined in that I have only access to its surface. I have looked into finding the shore hardness of the material and then converting this into an ultimate tensile strength. However I have read that this conversion is unreliable. Any advice on possible methods of obtaining reliable tensile strengths would be appreciated.

Thanks
MF