Based on my experience, eviscerate has a 100% activation chance on a damaging hit. In this fight log, I stalemated with fly prior to this log, and evis was applied in said fight. However, evis did not get applied again on the first hit with my sword. That being said, eviscerate appears to have activated TWICE within the same fight…
https://www.torn.com/page.php?sid=attackLog&ID=f9ad978195780204fc08c50f5cd6a75e