Failure Mechanisms and Metallography: A Review

Cyclic Fatigue Testing and Metallographic Analysis of Nickel-Titanium Rotary Instruments

“…Because inclusions in NRT files are much more concentrated and smaller than those in the other models, this may suggest differences in the manufacturing method. It is well known that crack propagation is much enhanced by large nonmetallic inclusions acting as nucleation sites of void in ductile fracture (22,23). By contrast, small inclusions could slow or stop propagation of the crack acting as pinning points.…”

Rotary NiTi Instrument Fracture and its Consequences

“…Some surface voids of rotary NiTi instruments are presumed to be because of small amounts of oxygen, nitrogen, carbon, and hydrogen dissolving in the alloy to form various precipitates (19,68). Cracks may then propagate along these boundaries (48). Furthermore, the manufacture and machining of rotary NiTi instruments often results in an instrument having an irregular surface characterized by milling grooves, multiple cracks, pits, and regions of metal rollover (19, 51, 69 -75).…”

“…2-4) usually reveals certain distinct features that help identify the type of fracture mechanism involved (47). In brittle fractures crack fronts create ridges that spread along different planes within the alloy and generally radiate away from the origin of the crack, producing the so-called chevron pattern (48). In ductile fractures microvoids are produced within the metal and nucleation, growth, and microvoid coalescence ultimately weaken the metal and result in fracture (45).…”