The title of your post asks a question. In the context of ANSYS, there is no such thing as dynamic impact strength.
The body of your post says you are doing a ball drop test in Transient Structural and you will compare the Total Strain with Elongation at Break to decide when failure has occurred. This is a good start. The question is what material properties are you using?
Some materials exhibit a significant sensitivity to strain rate. That means the material may have a low stiffness and a large Elongation at Break for low strain rates, and a high stiffness and a small Elongation at Break for high strain rates.
You can explicitly model strain rate in the Anand Viscoplasticity material model, which I added to a Nylon 6/6 material. I don't know how to use this material model, I just show it as an example of how strain rate can be included in the model.