Ouch...
If you don't have access to the full stress/strain curve for your material, see if you can download the CAMPUS database...it has material data from a number of polymer suppliers, for many of their specific resin types, and will have full stress /strain curves.
If I understand where you're going with this, you need to define the elastic and plastic regions separately in your non-linear analysis code, so you need to know where your yield point is (the transition point between the elastic and plastic portion of the curve).
I would define yield point as the limit of the elastic region. IE: the point at the onset of plastic deformation. Up to this point, if you stretch the material, then release the load, the material will return to it's unloaded shape. If you stretch the material beyond this point, you will have permanent deformation of the part. Plastics don't have as well defined a yield point as metals, so you can estimate the value if you have the stress/strain curve to look at. I would define the tensile strength as the ultimate tensile strength of the material, the point at which it breaks under load. This is often shown on data sheets, but isn't as useful if you have to design for deflection, not structural failure (breakage).
Without having a good idea of the general shape of your curve for the resin type you're working with, I wouldn't feel that comfortable with estimating the yield point if all you are given is the ultimate tensile strength (IE: from a material data sheet). If you can't get access to the CAMPUS data base, then try the material suppliers web site...they may have stress strain curves you can use, or if you're careful, you may be able to scale the yield point if you find a material curve of the same shape...but this is not very reliable if you don't have some experience doing this.
