Characterisation of short fatigue cracks in titanium alloy IMI 834 using X-ray microtomography

Abstract

A first attempt at the three-dimensional evaluation of naturally initiated surface connected and internal fatigue cracks is presented. Fatigue crack initiation and growth in air and vacuum environments have been investigated through Xray microtomography in air and vacuum environments at elevated temperatures (350• C), accompanied by post-mortem electron microscopy of the fracture surfaces. In vacuum (< 10 −5 mbar), multiple internal and surface-connected crack initiation was observed, but only the surface-connected cracks grew. In contrast, fewer cracks formed in air, these were mostly surface-connected and all were observed to grow. In all instances the initiation features were associated with globular primary α. An improved fatigue life was found in vacuum, which was mostly a consequence of delayed initiation, but was also due to slower fatigue crack propagation. The non-propagation of internal cracks was taken to imply that even the good laboratory vacuum obtained here was insufficient to simulate the conditions obtained for an internal crack in a component. The crack shape evolved towards a semi-circular shape a/c = 1 in air during fatigue crack growth, whilst the vacuum cracks remained semi-elliptical (a/c 1.4). This was taken to imply that oxide-induced crack closure played a role in fatigue crack growth in air.

Location, location & size: defects close to surfaces dominate fatigue crack initiation

“…These results are puzzling as they tend to indicate that, for equivalent K Imax values, the internal cracks propagate with crack growth rates similar to those of surface cracks. However, many literature results111314172627282937, as well as our CT-specimen results, indicate that internal cracks which are assumed to propagate in a vacuum like environment, grow with rates about one order of magnitude lower than surface cracks (ambient air propagation).…”

“…It has been observed29 for titanium alloys, where internal crack nucleation is induced by the microstructure itself and not by defects, that internal propagation exhibits slower crack growth rates than surface propagation occurring under the highest vacuum level possibly attained using state-of-the-art pumps. This might indicate that for titanium alloys the internal vacuum is purer than that produced at the pumped free surface.…”

“…Internal failure in cast Al alloys is however rarely observed during high cycle fatigue (10 5  < N cycles failure  < 10 7 ) and, when it occurs, it does not entails a reduction of the expected fatigue life2627. As for the environmental conditions of internal cracking, they are generally considered to be close to those of vacuum as their fracture surface features look similar262829.…”

In situ synchrotron ultrasonic fatigue testing device for 3D characterisation of internal crack initiation and growth

“…Surface cracks prevail over internal ones in LCF and HCF regimes, that is the main reason why only restricted observations of internal crack propagation 9,11,12 have been done so far but this is different in very high cycle fatigue (VHCF) regime where internal cracks dominate. Surface cracks prevail over internal ones in LCF and HCF regimes, that is the main reason why only restricted observations of internal crack propagation 9,11,12 have been done so far but this is different in very high cycle fatigue (VHCF) regime where internal cracks dominate.…”

Engineering Science at the I12 Beamline at Diamond Light Source

“…To mimic the fatigue life of such a component in the lab, lowcycle fatiguing (LCF) is applied, where, at each cycle, the material undergoes severe plastic deformation. Chapman et al [23] used in-situ tomography to image the crack growth of an areograde Ti-alloy during LCF testing. In all instances of crack initiation, the mechanism on the fracture surface was by facet formation, on the scale of primary grain size (15-50 µm diameter).…”