Unexpected fractures in surgical dilators were investigated to determine the cause of spontaneous tip breakage after ultrasonic cleaning. The study combined geometric reconstruction, microscopic examination, and metallurgical analysis to identify the failure mechanisms and propose solutions.
The Problem: Unexpected Fractures in Surgical Dilators
The failure analysis of the surgical dilators focused on understanding the origin and mechanism of spontaneous tip fractures that occurred after automated cleaning in an ultrasonic bath. Each batch provided both fractured and intact specimens made from austenitic stainless steel grades 1.4301 or 1.4307. A complete geometric reconstruction of the dilators, performed using 3D computed tomography at a voxel resolution of 100 µm, enabled an accurate representation of the tip geometry and revealed no macroscopic defects beyond the fractured regions.
Root Cause #1: Fatigue Cracks from Manufacturing Defects
The macroscopic and fractographic evaluation of the broken tips consistently demonstrated the hallmarks of fatigue failure. Scanning electron microscopy revealed distinct regions of crack initiation at the inner, manually ground surfaces of the tips, where pronounced grinding burrs and longitudinal grinding grooves were present. These features acted as sharp micro‑notches that served as preferential initiation sites. The fracture surfaces displayed typical fatigue morphology: partially developed and locally interrupted striations, secondary parallel micro‑cracks aligned with the crack propagation direction, and regions of severe plastic deformation at the crack origins, which appeared heavily hammered. Particularly notable were areas showing densely spaced striations characteristic of high‑cycle fatigue (HCF). These observations strongly indicate that the tips were subjected to cyclic loading conditions with extremely high load-cycle counts.
Root Cause #2: Brittle Microstructure from Over-Work-Hardening
Metallographic examinations provided further insight into the underlying susceptibility of the material. Etching with Beraha I revealed a strongly work‑hardened microstructure dominated by deformation‑induced martensite with residual austenite, accompanied by significantly elevated hardness values (452 ± 8 HV1). Such microstructural transformation is typical of metastable austenitic steels subjected to severe mechanical deformation—in this case, introduced during the manual shaping and sharpening of the tips. While the resulting high strength increases resistance to plastic deformation, it simultaneously reduces toughness and fatigue resistance. Thus, the combination of deformation‑martensite and mechanically induced surface defects created a microstructural and geometric environment highly conducive to fatigue crack initiation.
Root Cause #3: The Trigger—Resonance During Ultrasonic Cleaning
The operating environment during cleaning further amplified this vulnerability. Ultrasonic cleaning systems typically run at frequencies between 30 and 400 kHz, which can generate an enormous number of load cycles—on the order of 1.8 to 24 million cycles per minute. Given the slender geometry of the dilator tips, it is plausible that their natural frequencies fall within or near the excitation spectrum of the ultrasonic bath. A resonance condition would lead to localized vibration amplitudes far exceeding those expected from standard cleaning processes, thereby accelerating fatigue crack growth from the already highly stressed notch-like defects. This combination of geometric notch sensitivity, a critically hardened microstructure, and high‑frequency excitation creates a coherent and technically well-supported multi-factorial failure mechanism.
Evidence: Manufacturing Process Creates Universal Surface Defects
Even the intact specimens exhibited pronounced grinding burrs, visible grooves, and small edge breakouts at the tip, confirming that these surface conditions were not isolated anomalies but inherent to the manufacturing process. Their presence on both fractured and unfractured parts strengthens the conclusion that the surface finish represents a contributing factor rather than the singular cause of failure. Nevertheless, it plays a decisive role in determining crack initiation behavior under HCF loading.
The Solution: Electropolishing Eliminates Surface Defects
To mitigate these issues, electrolytic polishing was evaluated as a potential surface improvement method. A short treatment (6 s at 14 V in a perchloric acid electrolyte using a platinum electrode) produced significantly smoother surfaces with fully removed burrs and substantially reduced roughness. Although the material removal rate was somewhat excessive in the initial trials, optimization of electrolyte composition and process parameters is expected to produce a controlled, reproducible finish. Such a polished surface would lower the effective notch sensitivity and reduce fatigue initiation risk while simultaneously improving cleanability and sterilization performance—an important side benefit in medical applications.
To further validate the proposed failure mechanism and guide design or process modifications, a future finite‑element‑based study is planned. Using CT‑derived geometric models converted into voxel‑based meshes, numerical simulations will determine the eigenfrequencies and modal shapes of the dilators, quantify the vibration amplitudes required to trigger fatigue failure, and define safe operating windows for ultrasonic cleaning equipment. This analysis will also support recommendations for optimal surface quality (including polished or modified geometries) and identify frequency ranges or bath settings that minimize resonant amplification.
In summary, the combined evidence clearly shows that the fractures are fatigue-related and originate from manufacturing-induced surface defects on heavily work-hardened martensitic tip regions, which are subsequently exposed to extremely high-frequency cyclic loading during ultrasonic cleaning. The synergy of geometric notches, microstructural embrittlement, and vibrational excitation constitutes the root cause scenario. Improved surface finishing—particularly via optimized electrolytic polishing—together with resonance-aware cleaning process parameters represent the most effective and technically justified pathway to preventing recurrence.
Figure 1: Fracture at tip of vascular dilatator – arrows mark crack initiation sites
Figure 2: Tip after manual grinding (left) – and after 6s electropolishing with perchloric acid at 14V (right)
Figure 3: Forceps tip after manual grinding (left) and after 6s electropolishing with perchloric acid at 14V (right)
