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Cable-Stop Fractures

Cable-Stop Fractures. Brad James Ph.D., P.E., FASM. Cable-Stop Fractures. Background 420 s tainless 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

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Cable-Stop Fractures

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  1. Cable-Stop Fractures Brad James Ph.D., P.E., FASM

  2. 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

  3. Cable-Stop Fractures

  4. Cable-Stop Fractures

  5. Cable-Stop Fractures

  6. Cable-Stop Fractures Intentional fracture of new device

  7. Cable-Stop Fractures

  8. 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

  9. Cable-Stop Fractures Post wedge-load testing

  10. Cable-Stop Fractures Hardness Testing: • Specification: 250 ksi tensile strength (~50 HRC) • “bad” manufacturer: ~260 ksi (51 HRC) • “good” manufacturer: ~210 ksi (45 HRC)

  11. 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!?

  12. 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

  13. Cable-Stop Fractures Hydrogen Embrittlement! • Sensitivity to stress corrosion cracking/hydrogen embrittlement increases with strength • Stronger cable stops were more susceptible to the chloride environments

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