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Papers by Michael Fitzpatrick

Journal of The Mechanical Behavior of Biomedical Materials

The failure of an orthopaedic implant can be initiated by residual strain inherent to the hydroxy... more The failure of an orthopaedic implant can be initiated by residual strain inherent to the hydroxyapatite coating (HAC). Knowledge of the through-thickness residual strain profile in the thermally sprayed hydroxyapatite coating/substrate system is therefore important in the development of a new generation of orthopaedic implants. As the coating microstructure is complex, non-destructive characterization of residual strain, e.g. using neutron diffraction, provides a useful measure of through thickness strain profile without altering the stress field. This first detailed study using a neutron diffraction technique, non-destructively evaluates the through thickness strain measurement in nanostructured hydroxyapatite plasma sprayed coatings on a titanium alloy substrate (as-sprayed, heat treated, and heat treated then soaked in simulated body fluid (SBF)). The influence of crystallographic plane orientation on the residual strain measurement is shown to indicate texturing in the coating. This texturing is expected to influence both the biological and fracture response of HA coatings. Results are discussed in terms of the influence of heat-treatment and SBF on the residual stress profile for these biomedical coatings. The results show that the through thickness residual strain in all three coatings was different for different crystallographic planes but was on average tensile. It is also concluded that the heat-treatment and simulated body fluid exposure had a significant effect on the residual strain profile in the top layers of HAC.► Texturing in hydroxyapatite coatings. ► Residual stress dependency on orientation of planes. ► Tensile residual stress on (030) & (211) planes. ► Compressive residual stress on (022) & (213) planes. ► Neutron diffraction through-thickness strain measurements.

Materials Science Forum, 2010

Page 1. Neutron Diffraction Residual Strain Measurements in Plasma Sprayed Nanostructured Hydroxy... more Page 1. Neutron Diffraction Residual Strain Measurements in Plasma Sprayed Nanostructured Hydroxyapatite Coatings for Orthopaedic Implants R. Ahmed 1,a , NH Faisal 1,b , SM Knupfer 1,c , AM Paradowska 2,d , ME Fitzpatrick 3,e , KA Khor 4,f and J. Cizek 4,g ...

Journal of Thermal Spray Technology

The aim of this investigation was to nondestructively evaluate the residual stress profile in two... more The aim of this investigation was to nondestructively evaluate the residual stress profile in two commercially available alumina/substrate coating systems and relate residual stress changes with the fracture response. Neutron diffraction, due to its high penetration depth, was used to measure residual strain in conventional air plasma-sprayed (APS) and finer powder high velocity oxy-fuel (HVOF (θ-gun))-sprayed Al2O3 coating/substrate systems. The purpose of this comparison was to ascertain if finer powder Al2O3 coatings deposited via θ-gun can provide improved residual stress and fracture response in comparison to conventional APS coatings. To obtain a through thickness residual strain profile with high resolution, a partially submerged beam was used for measurements near the coating surface, and a beam submerged in the coating and substrate materials near the coating-substrate interface. By using the fast vertical scanning method, with careful leveling of the specimen using theodolites, the coating surface and the coating/substrate interface were located with an accuracy of about 50 μm. The results show that the through thickness residual strain in the APS coating was mainly tensile, whereas the HVOF coating had both compressive and tensile residual strains. Further analysis interlinking Vickers indentation fracture behavior using acoustic emission (AE) was conducted. The microstructural differences along with the nature and magnitude of the residual strain fields had a direct effect on the fracture response of the two coatings during the indentation process.

First published 2003 by Taylor & Francis 11 New Fetter Lane, London EC4P 4EE ... more First published 2003 by Taylor & Francis 11 New Fetter Lane, London EC4P 4EE Simultaneously published in the USA and Canada by Taylor & Francis Inc, 29 West 35th Street, New York, NY 10001 Taylor & Francis is an imprint of the Taylor & Francis Group This edition published ...

Acta Crystallographica Section A Foundations of Crystallography

Powder Diffraction

The harsh environment in the core of a nuclear reactor imposes high constraints for the choice of... more The harsh environment in the core of a nuclear reactor imposes high constraints for the choice of materials for structural components: materials need high corrosion resistance, good mechanical properties at high temperatures and must sustain their properties under high irradiation. Zr-2.5%Nb is a zirconium-based alloy which fulfils these criteria and has a low neutron cross section. Therefore it is used for the manufacture of pressure tubes in CANDU reactors, for example. It is now well understood that Zr crystals, because of their hexagonal close packed structure at room temperature, present a strong plastic anisotropy [1]. In polycrystals, this anisotropy is responsible for the heterogeneity of grain-to-grain behaviour and the generation of high intergranular misfits after deformation. Moreover, the strong texture usually found in processed materials brings the anisotropy to a macroscopic level. It is therefore important to understand the mechanisms of deformation. The use of neutron diffraction during in-situ mechanical deformation has proven to be a real asset for such studies and much work has been performed on various alloy systems [2]. However some gaps remain in our understanding of how to link, model and predict the behaviours at the macro and micro scales. The influence of some parameters such as initial texture, grain size/shape and the initial intergranular stress state is also not very well identified. This paper presents the results of a set of in-situ thermo-mechanical processes on coupons cut from a Zr2.5%Nb pressure tube. The experiment was performed in-situ at ENGIN-X, the engineering neutron diffractometer at ISIS, UK. A first set of samples were compressed at room temperature along the three processing directions and up to 10% deformation. Another two samples were briefly annealed in-situ up to 560!C and 620!C respectively and then compressed at room temperature. During compression in the as received state, the macroscopic as well as the microscopic anisotropy were evidenced. As expected from the texture, the hoop direction was stronger than the other directions. The neutron diffraction results show that the 0002 reflections generally bear most of the load in the alpha hcp phase. In the rolling direction no such reflections are present and during compression along this direction the second phase undergoes very high strains. In the Poisson's directions the intergranular strains are particularly high (up to 5000 microstrain). During annealing substantial changes in the diffraction spectra are evidence of the Beta phase decomposition. By comparing the initial and the post heat treatment d-spacings, it was found that some strain relief occurred.

International Journal of Fatigue

ABSTRACT

The Journal of Engineering, 2015

Finite element modeling can be a powerful tool for predicting residual stresses induced by laser ... more Finite element modeling can be a powerful tool for predicting residual stresses induced by laser peening; however the sign and magnitude of the stress predictions depend strongly on how the material model captures the high strain rate response. Although a Johnson-Cook formulation is often employed, its suitability for modeling phenomena at very high strain rates has not been rigorously evaluated. In this paper, we address the effectiveness of the Johnson-Cook model, with parameters developed from lower strain rate material data (∼10 3 s -1 ), to capture the higher strain rate response (∼10 5 -10 6 s -1 ) encountered during the laser peening process. Published Johnson-Cook parameters extracted from split Hopkinson bar testing were used to predict the shock response of aluminum samples during high-impact flyer plate tests. Additional quasi-static and split Hopkinson bar tests were also conducted to study the model response in the lower strain rate regime. The overall objective of the research was to ascertain whether a material model based on conventional test data (quasi-static compression testing and split Hopkinson bar measurements) can credibly be used in FE simulations to predict laser peen-induced stresses.

Journal of The Mechanical Behavior of Biomedical Materials

The failure of an orthopaedic implant can be initiated by residual strain inherent to the hydroxy... more The failure of an orthopaedic implant can be initiated by residual strain inherent to the hydroxyapatite coating (HAC). Knowledge of the through-thickness residual strain profile in the thermally sprayed hydroxyapatite coating/substrate system is therefore important in the development of a new generation of orthopaedic implants. As the coating microstructure is complex, non-destructive characterization of residual strain, e.g. using neutron diffraction, provides a useful measure of through thickness strain profile without altering the stress field. This first detailed study using a neutron diffraction technique, non-destructively evaluates the through thickness strain measurement in nanostructured hydroxyapatite plasma sprayed coatings on a titanium alloy substrate (as-sprayed, heat treated, and heat treated then soaked in simulated body fluid (SBF)). The influence of crystallographic plane orientation on the residual strain measurement is shown to indicate texturing in the coating. This texturing is expected to influence both the biological and fracture response of HA coatings. Results are discussed in terms of the influence of heat-treatment and SBF on the residual stress profile for these biomedical coatings. The results show that the through thickness residual strain in all three coatings was different for different crystallographic planes but was on average tensile. It is also concluded that the heat-treatment and simulated body fluid exposure had a significant effect on the residual strain profile in the top layers of HAC.► Texturing in hydroxyapatite coatings. ► Residual stress dependency on orientation of planes. ► Tensile residual stress on (030) & (211) planes. ► Compressive residual stress on (022) & (213) planes. ► Neutron diffraction through-thickness strain measurements.

Materials Science Forum, 2010

Page 1. Neutron Diffraction Residual Strain Measurements in Plasma Sprayed Nanostructured Hydroxy... more Page 1. Neutron Diffraction Residual Strain Measurements in Plasma Sprayed Nanostructured Hydroxyapatite Coatings for Orthopaedic Implants R. Ahmed 1,a , NH Faisal 1,b , SM Knupfer 1,c , AM Paradowska 2,d , ME Fitzpatrick 3,e , KA Khor 4,f and J. Cizek 4,g ...

Journal of Thermal Spray Technology

The aim of this investigation was to nondestructively evaluate the residual stress profile in two... more The aim of this investigation was to nondestructively evaluate the residual stress profile in two commercially available alumina/substrate coating systems and relate residual stress changes with the fracture response. Neutron diffraction, due to its high penetration depth, was used to measure residual strain in conventional air plasma-sprayed (APS) and finer powder high velocity oxy-fuel (HVOF (θ-gun))-sprayed Al2O3 coating/substrate systems. The purpose of this comparison was to ascertain if finer powder Al2O3 coatings deposited via θ-gun can provide improved residual stress and fracture response in comparison to conventional APS coatings. To obtain a through thickness residual strain profile with high resolution, a partially submerged beam was used for measurements near the coating surface, and a beam submerged in the coating and substrate materials near the coating-substrate interface. By using the fast vertical scanning method, with careful leveling of the specimen using theodolites, the coating surface and the coating/substrate interface were located with an accuracy of about 50 μm. The results show that the through thickness residual strain in the APS coating was mainly tensile, whereas the HVOF coating had both compressive and tensile residual strains. Further analysis interlinking Vickers indentation fracture behavior using acoustic emission (AE) was conducted. The microstructural differences along with the nature and magnitude of the residual strain fields had a direct effect on the fracture response of the two coatings during the indentation process.

First published 2003 by Taylor & Francis 11 New Fetter Lane, London EC4P 4EE ... more First published 2003 by Taylor & Francis 11 New Fetter Lane, London EC4P 4EE Simultaneously published in the USA and Canada by Taylor & Francis Inc, 29 West 35th Street, New York, NY 10001 Taylor & Francis is an imprint of the Taylor & Francis Group This edition published ...

Acta Crystallographica Section A Foundations of Crystallography

Powder Diffraction

The harsh environment in the core of a nuclear reactor imposes high constraints for the choice of... more The harsh environment in the core of a nuclear reactor imposes high constraints for the choice of materials for structural components: materials need high corrosion resistance, good mechanical properties at high temperatures and must sustain their properties under high irradiation. Zr-2.5%Nb is a zirconium-based alloy which fulfils these criteria and has a low neutron cross section. Therefore it is used for the manufacture of pressure tubes in CANDU reactors, for example. It is now well understood that Zr crystals, because of their hexagonal close packed structure at room temperature, present a strong plastic anisotropy [1]. In polycrystals, this anisotropy is responsible for the heterogeneity of grain-to-grain behaviour and the generation of high intergranular misfits after deformation. Moreover, the strong texture usually found in processed materials brings the anisotropy to a macroscopic level. It is therefore important to understand the mechanisms of deformation. The use of neutron diffraction during in-situ mechanical deformation has proven to be a real asset for such studies and much work has been performed on various alloy systems [2]. However some gaps remain in our understanding of how to link, model and predict the behaviours at the macro and micro scales. The influence of some parameters such as initial texture, grain size/shape and the initial intergranular stress state is also not very well identified. This paper presents the results of a set of in-situ thermo-mechanical processes on coupons cut from a Zr2.5%Nb pressure tube. The experiment was performed in-situ at ENGIN-X, the engineering neutron diffractometer at ISIS, UK. A first set of samples were compressed at room temperature along the three processing directions and up to 10% deformation. Another two samples were briefly annealed in-situ up to 560!C and 620!C respectively and then compressed at room temperature. During compression in the as received state, the macroscopic as well as the microscopic anisotropy were evidenced. As expected from the texture, the hoop direction was stronger than the other directions. The neutron diffraction results show that the 0002 reflections generally bear most of the load in the alpha hcp phase. In the rolling direction no such reflections are present and during compression along this direction the second phase undergoes very high strains. In the Poisson's directions the intergranular strains are particularly high (up to 5000 microstrain). During annealing substantial changes in the diffraction spectra are evidence of the Beta phase decomposition. By comparing the initial and the post heat treatment d-spacings, it was found that some strain relief occurred.

International Journal of Fatigue

ABSTRACT

The Journal of Engineering, 2015

Finite element modeling can be a powerful tool for predicting residual stresses induced by laser ... more Finite element modeling can be a powerful tool for predicting residual stresses induced by laser peening; however the sign and magnitude of the stress predictions depend strongly on how the material model captures the high strain rate response. Although a Johnson-Cook formulation is often employed, its suitability for modeling phenomena at very high strain rates has not been rigorously evaluated. In this paper, we address the effectiveness of the Johnson-Cook model, with parameters developed from lower strain rate material data (∼10 3 s -1 ), to capture the higher strain rate response (∼10 5 -10 6 s -1 ) encountered during the laser peening process. Published Johnson-Cook parameters extracted from split Hopkinson bar testing were used to predict the shock response of aluminum samples during high-impact flyer plate tests. Additional quasi-static and split Hopkinson bar tests were also conducted to study the model response in the lower strain rate regime. The overall objective of the research was to ascertain whether a material model based on conventional test data (quasi-static compression testing and split Hopkinson bar measurements) can credibly be used in FE simulations to predict laser peen-induced stresses.