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

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

Cable-Stop Fractures

Cable-Stop Fractures

Cable-Stop Fractures

Intentional fracture of new device

Cable-Stop Fractures

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