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Cable-Stop Fractures
Background• 420 stainless steel cable-stops are used as part of
a re-useable, cable-driven mechanical surgical tool apparatus
• Cable stops were observed to have fractured shortly after use in service
• Cables and stops are exposed to blood and saline• Only stops from one of two manufacturers
fractured
Cable-Stop Fractures
Testing• Wedge-load testing of stops in saline– From manufacturer that exhibited breaks– From manufacturer that did not exhibit breaks
Results• Testing replicated field experience, stops from
“bad” manufacturer broke, others didn’t
Cable-Stop Fractures
Hardness Testing:• Specification: 250 ksi tensile strength (~50 HRC)• “bad” manufacturer: ~260 ksi (51 HRC)• “good” manufacturer: ~210 ksi (45 HRC)
Cable-Stop Fractures
What Gives?• Manufacturer that meets tensile strength
specifications has stops that fracture in service• Manufacturer with below-specified strength
does not fracture!?
Cable-Stop Fractures
Conclusions/Recommendations• Cable stops fractured due to hydrogen
embrittlement from exposure to body fluids/saline• 250 ksi specified strength is too high for 420
stainless steel cable stops subjected to saline environments
• Significantly reduce specified strength or change alloys (to something like Nitronic 60)
• Validate performance with long-term static testing in aggressive environment