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Low‐cycle fatigue of NiTi rotary instruments of various cross‐sectional shapes
Author(s) -
Cheung G. S. P.,
Darvell B. W.
Publication year - 2007
Publication title -
international endodontic journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.988
H-Index - 119
eISSN - 1365-2591
pISSN - 0143-2885
DOI - 10.1111/j.1365-2591.2007.01257.x
Subject(s) - nickel titanium , materials science , fracture (geology) , curvature , amplitude , structural engineering , ductility (earth science) , deformation (meteorology) , cyclic stress , bending , cross section (physics) , composite material , mathematics , geometry , optics , engineering , physics , creep , quantum mechanics , shape memory alloy
Aim To compare the low‐cycle fatigue (LCF) behaviour of some commercial NiTi instruments subjected to rotational bending, a deformation mode similar to an engine‐file rotating in a curved root canal, using a strain‐life analysis, in water. Methodology A total of 286 NiTi rotary instruments from four manufacturers were constrained into a curvature by three rigid, stainless steel pins whilst rotating at a rate of 250 rpm in deionized water until broken. The number of revolutions was recorded using an optical counter and an electronic break‐detection circuit. The surface strain amplitude, calculated from the curvature (from a photograph) and diameter of the fracture cross‐section (from a scanning electron micrograph), was plotted against the number of cycles to fracture for each instrument. A regression line was fitted to the LCF lives for each brand; the value was compared with that of others using one‐way analysis of variance ( anova ). The number of crack origins observed on the fractographic view was examined with chi‐square for differences amongst various groups. Results A linear strain‐life relationship, on logarithmic scales, was obtained for the LCF region with an apparent fatigue‐ductility exponent ranging from −0.40 to −0.56. The number of crack‐initiation sites, as observed on the fracture cross‐section, differed between brands ( χ 2 , P < 0.05), but not LCF life (one‐way anova , P > 0.05). Conclusions The LCF life of NiTi instruments declines with an inverse power function dependence on surface strain amplitude, but is not affected by the cross‐sectional shape of the instrument.