1. Failure Analysis for the Medical Device Engineer Case Study – Surgical Tool Blade

2. Case Study – Surgical Blades Background Two different blades used as part of a surgical tool were reported to have fractured in the field These blades had been in the field for only a few weeks, and were used and re-sterilized a few times The product had been subjected to load testing, fatigue testing, and aging studies prior to release The blades were made from 420 martensitic stainless steel, supplied in a “full-hardened” condition to enhance cutting properties and wear resistance

3. Case Study – Surgical Blades Due to the small size of the blades, little macroscopic failure analysis was conducted The returned blades were examined in the scanning electron microscope and showed intergranular fracture Exemplar blades, unused, were intentionally fractured, these blades showed microvoid coalescence initiation that transitioned to a mixed cleavage/microvoid coalescence mode

4. Case Study – Surgical Blades Field return blades:

5. Case Study – Surgical Blades Exemplar blades – intentionally fractured:

6. Case Study – Surgical Blades Metallurgical analysis: Indicated crack branching in field returns Confirmed intergranular fracture (no branching in unused exemplars) Hardness testing indicated average hardness of 58 on the Rockwell C scale

7. Case Study- Surgical Blades Analysis: Failure analysis examination of fractured field return blades indicated a hydrogen embrittlement (HE)/stress corrosion cracking (SCC) mechanism Comparison of field return history with life-testing indicated the following: –No exposure to saline (chlorides) in life testing –Cyclic loads only in life testing (no prolonged static loads) Altering life testing to include exposure to saline solution and prolonged static loads (blades in closed condition) reproduced field failures

8. Case Study – Surgical Blades Follow-up: Increased tempering temperature for 420 martensitic stainless steel was recommended Target hardness was 52 Rockwell C, to increase resistance to HE while maintaining cutting and wear performance Modified, conservative life-testing recreated fractures in hard blades, showed no failures or subcritical cracking in new blades