Drill Pipes Tool Joints Torsional Strength & Makeup Torque
Drill Pipe Tool Joint Torsional Strength

- Most standard API tool joints are weaker in torsion than the Drill Pipe tubes to which they are welded. 

- API sets tool joint torsional strength arbitrarily at 80 percent of tube torsional strength. This tool joint torsional strength ratio (TSR) of 0.8 is the basis for establishing tool joint ID and OD. 

- As the drill pipe tool joint OD  wears the torsional strength decreases. Tool joints with TSR's less then 0.8
https://www.drillingmanual.com/torsional-strength-of-tool-joints/