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Fatigue testing of a NiTi rotary instrument. Part 2: fractographic analysis
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.1365-2591.2007.01256.x
Subject(s) - nickel titanium , materials science , fracture (geology) , dimple , cyclic stress , crack closure , brittleness , amplitude , fractography , radius , fatigue testing , fracture mechanics , composite material , bending , structural engineering , engineering , optics , physics , computer security , computer science , shape memory alloy
Aim To examine the topographic features of the fracture surface of a NiTi instrument after fatigue failure, and to correlate the measurements of some features with the cyclic load. Methodology A total of 212 ProFile rotary instruments were subjected to a rotational‐bending test at various curvatures until broken. The fracture surface of all fragments was examined by SEM to identify the crack origins. The crack radius, i.e. extent of the fatigue‐crack growth towards the centroid of the cross‐section, was also measured, and correlated with the strain amplitude for each instrument. Results All fracture surfaces revealed the presence of one or more crack origins, a region occupied by microscopic striations, and an area with microscopic dimples. The number of specimens showing multiple crack origins was significantly greater in the group fatigued under water than in air ( P < 0.05). A linear relationship between the reciprocal of the square root of the crack radius and the strain amplitude was discernible ( P < 0.001), the slopes of which were not significantly different for instruments fatigued in air and water. Conclusions The fractographic appearance of NiTi engine‐files that had failed because of fatigue is typical of that for other metals. The fatigue behaviour of NiTi instruments is adversely affected by water, not only for the low‐cycle fatigue life, but also the number of crack origins. There appears to be a critical extent of crack propagation for various strain amplitudes leading to final rupture (akin to the Griffith's criterion for brittle materials).