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

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Presentation transcript:

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

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

Intentional fracture of new device

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 Post wedge-load testing

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

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