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32 results on '"Shollock, B.A."'

8. Realistic microstructure-based modelling of cyclic deformation and crack growth using crystal plasticity

Author

Farukh, F., Zhao, L.G., Jiang, R., Reed, P., Proprentner, D., and Shollock, B.A.

Subjects
Crack growth, Extended finite element, Chemistry(all), Crystal plasticity, TN, Physics and Astronomy(all), Realistic microstructure, Computational Mathematics, TA, Materials Science(all), Mechanics of Materials, QD, Cyclic deformation, QC, Computer Science(all)
Abstract

Using crystal plasticity, finite element analyses were carried out to model cyclic deformation for a low solvus high refractory (LSHR) nickel superalloy at elevated temperature. The analyses were implemented using a representative volume element (RVE), consisting of realistic microstructure obtained from SEM images of the material. Monotonic, stress-relaxation and cyclic test data at 725°C were used to determine the model parameters from a fitting process and their sensitivity to RVE size and random grain orientation. In combination with extended finite element method (XFEM), the crystal plasticity model was further applied to predict surface crack growth, for which accumulated plastic strain was used as a fracture criterion. Again, realistic microstructure, taken from the cracking site on the surface of a plain fatigue specimen, was used to create the finite element model for crack growth analyses. The prediction was conducted for a pseudo-3D geometrical model, resembling the plane stress condition at specimen surface. The loading level at the cracking site was determined from a viscoplasticity finite element analysis of the fatigue specimen. The proposed model is capable of predicting the variation in growth rate in grains with different orientations.

Published
2016
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13. Cautionary note on use of focused ion beam sectioning as technique for characterising oxidation damage in Ni based superalloys

Author

Cruchley, S., Sun, J.F., Taylor, M.P., Evans, H.E., Bowen, P., Sumner, J., Nicholls, J.R., Simms, N.J., Shollock, B.A., Chater, R.J., Foss, B.J., Hardy, M.C., Stekovic, Svjetlana, Cruchley, S., Sun, J.F., Taylor, M.P., Evans, H.E., Bowen, P., Sumner, J., Nicholls, J.R., Simms, N.J., Shollock, B.A., Chater, R.J., Foss, B.J., Hardy, M.C., and Stekovic, Svjetlana

Abstract

Previous observations on Ni based superalloys, obtained through the use of focused ion beam (FIB) sample preparation and imaging, have reported the presence of subsurface voids after oxidation. In this present study, oxidised specimens of the Ni based superalloy, RR1000, were subjected to conventional sample preparation as well as both dual and single beam FIB preparation, with the aim of re-examining the previous observations of subsurface void formation. It is clear from FIB preparations that features previously interpreted as networks of voids have been demonstrated to be internal oxides by varying the sample tilt angles and imaging signal using either secondary electrons (SEs) or secondary ions (SIs). Conventional preparation methods illustrate the presence of subsurface alumina intrusions and the absence of voids, supporting previous evidence. The positive identification of voids and oxides by FIB can be complex and prone to misinterpretation and thus, the use of several imaging conditions and tilt angles must be used, along with conventional preparation methods, to confirm or refute the presence of voids underneath oxides.

Published
2014
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27. The Effect of a Friction Modifier Additive on Micropitting.

Author

Lainé, E., Olver, A.V., Lekstrom, M.F., Shollock, B.A., Beveridge, T.A., and Hua, D.Y.

Abstract

Previous studies have shown that many anti-wear additives have a tendency to aggravate micropitting damage in hard steels subjected to rolling sliding contact at low lambda values. This is apparently because anti-wear additives suppress the gradual smoothing of the rough surfaces that takes place when a pure base stock is used under mild conditions. However, in practice, it is common to combine anti-wear and friction modifier additives in formulated oils, a combination that leads to reduced boundary friction. In the work described in this article, we added a common friction modifier agent, a molybdenum bis-diethylhexyl dithio-carbamate (MoDTC), to an oil containing an anti-wear additive, a secondary zinc dialkyl-dithio-phosphate (ZDDP) in a mineral base-stock. We examined micropitting behavior using a disc tester. The oil with both MoDTC and ZDDP showed initial micropitting that gradually disappeared with continued running, whereas ZDDP alone led to continuing severe damage. Analysis of the counter-discs after the test showed the presence of MoS2 deposits on the asperity crests. Reduced boundary friction was confirmed using a tribometer test. It is speculated that the improved micropitting behavior resulted from the effect of a reduction in local tensile stress due to reduced asperity friction. This may have reduced the opening of the surface cracks and inhibited their extension. [ABSTRACT FROM AUTHOR]

Published
2009
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