• Iron is the metal of choice for mechanical properties and unit cost, but performance requirements will drive the choice of a specific iron alloy.
  
• For mechanical performance, the iron alloy has to be both strong and resistant to cyclic fatigue, because stresses are high and brittle failure has to be avoided.
  
• The design engineers set mechanical specifications of 80 ksi ultimate tensile strength, 50 ksi tensile yield strength, and a fatigue limit of 30 ksi.
  
• The coupler requires precision machining, so the selected alloy must be machinable with a baseline machining speed of 60m/min.
  
• The coupler is finished with a black phosphate corrosion protection coating, so corrosion resistance is not a factor in alloy selection.

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