Microstructural characterisation and modelling of fatigue in aluminium based plain bearing materials
Microstructural characterisation and modelling of fatigue in aluminium based plain bearing materials
Aluminium based plain bearings used in small automotive engines typically comprise several layers of different materials, with conventionally processed roll-bonded bearings constituting tri-layer systems. The fatigue performance of these bearing systems has been found to be strongly dependent on the relative layer properties and micro-mechanics of the A1-lining layer. Recent research in plain bearing failures consequently focuses on distinctly characterising the failure mechanisms occurring within the individual layers, with an aim of optimising the design and manufacture of the roll-bonded system as a whole. The roll-bonded bearings studied in this research are manufactured by Dana Glacier Vandervell Bearings (UK), and are typical shell bearings constructed in two parts or half-shells, which are clamped together within a housing to support the rotating journal. Each half shell comprises a thin layer of a complex A1, Sn-based lining material, roll-bonded onto a steel-backing layer via a thin layer of aluminium foil. This thesis presents the results of a three-year research program characterising the fatigue behaviour of A1-Sn based alloy linings used in modern plain journal bearings applications, where they offer an acceptable balance between load-bearing capacity and fatigue resistance, in combination with conformability and embeddability. The research predominantly focuses on a roll-bonded A1-Sn-Cu-Mn modern bearing design, and compares and contrasts the fatigue mechanisms with those occurring in an A1-Sn-Si-Cu alloy lining, studied previously.
A series of standard experimental material characterisation techniques and finite element parametric studies have been applied to investigate the material system’s macroscopic geometry and individual layer properties. Advanced statistical tessellation-based approaches were then used to characterise the lining material microstructure. Standard fatigue tests, in air and oil environment, were further used to determine preferential crack initiation sites by statistically identifying the microstructural features associated with initiation, and also to provide an overview of the effects of environment and microstructure morphology on the fatigue behaviour observed. Classification approaches using Adaptive Numerical Modelling (ANM) techniques were applied to the preferential crack initiation results (from tessellation) to provide possible predictions of microstructural sites most likely to initiate failure. These results were then analysed against those of the previously studied A1-Sn-Si-Cu alloy lining. The results so obtained were then combined with finite element modelling of simplified aspects of the lining microstructure, to investigate the correlation between crack initiation sites and the local stress-strain fields evolved within the microstructure under applied loading. It has been shown that in air and oil environments, fatigue initiation occurs by decohesion at larger Si particles, and in the absence of Si particles, by decohesion at larger Sn particles. Oil environment is seen to enhance the fatigue performance of only the A1-Sn-Si-Cu alloy, with no change observed in the A1-Sn-Cu-Mn alloy. For the lining alloy with high Sn content and therefore exhibiting more phase directionality, a heavier dependence of crack initiation on angular information was found by the ANM classifier. These initiation mechanisms have been explained qualitatively in terms of strain mismatch arising from differing elastic moduli between the secondary particles and surrounding A1 matrix. These results indicated that optimising alloy-lining performance could be achieved by refining secondary phase distribution.
Novel High Velocity Oxy-Fuel (HVOF) processes have also been used to manufacture bi-layer systems with refined secondary phase distributions. A comparator HVOF system (A1-Sn-Cu) has undergone a similar testing regime to that of the roll-bonded specimens. Despite the refinement of the Sn phase within the HVOF system, additional initiation sites are operative (e.g. pores and unmelt boundaries, which are also associated with larger areas of Sn, squeezed out of the deposited coating in molten form during the deposition process).
In all systems, cracks appear to follow the Sn/matrix interface as an easy crack path; directionality and distribution of the Sn phase therefore playing a role in the fatigue resistance for both initiation and crack propagate through the lining, towards the backing and then deflect sharply, travelling within the interlayer parallel to the applied stress in the roll-bonded systems, whereas in the HVOF system, the interface itself seems weak and crack deflection occurs there. This behaviour is discussed in terms of the dissimilar elastic-plastic properties of the layers. Strain-lifting analyses have shown that the fatigue performance of the bearing system is dependent on more than just the properties of the lining alloys alone. Mechanical property parametric studies of the bearing architecture demonstrate that stress-strain properties and thicknesses of the individual layers critically influence the performance of the bearing system as a whole, with the ‘constraint-providing’ steel layers having the larger effect on the magnitude of evolved stresses and strains across the lining top-surface of a loaded system. Provided the linings are relatively thin, their mechanical properties and thickness have a lesser influence on the evolved stresses and strains within the lining layer. In terms of purely materials’ fatigue resistance (Δεp/2 vs Nf) the HVOF lining appears to show the best fatigue resistance at highest strain levels (although the possibility of early interface failure may mean that the strain levels are significantly over-estimated for this system) with the A1-Sn-Cu-Mn system showing better performance than the A1-Sn-Si-Cu system.
University of Southampton
2004
Mwanza, Mathew Chuchu
(2004)
Microstructural characterisation and modelling of fatigue in aluminium based plain bearing materials.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
Aluminium based plain bearings used in small automotive engines typically comprise several layers of different materials, with conventionally processed roll-bonded bearings constituting tri-layer systems. The fatigue performance of these bearing systems has been found to be strongly dependent on the relative layer properties and micro-mechanics of the A1-lining layer. Recent research in plain bearing failures consequently focuses on distinctly characterising the failure mechanisms occurring within the individual layers, with an aim of optimising the design and manufacture of the roll-bonded system as a whole. The roll-bonded bearings studied in this research are manufactured by Dana Glacier Vandervell Bearings (UK), and are typical shell bearings constructed in two parts or half-shells, which are clamped together within a housing to support the rotating journal. Each half shell comprises a thin layer of a complex A1, Sn-based lining material, roll-bonded onto a steel-backing layer via a thin layer of aluminium foil. This thesis presents the results of a three-year research program characterising the fatigue behaviour of A1-Sn based alloy linings used in modern plain journal bearings applications, where they offer an acceptable balance between load-bearing capacity and fatigue resistance, in combination with conformability and embeddability. The research predominantly focuses on a roll-bonded A1-Sn-Cu-Mn modern bearing design, and compares and contrasts the fatigue mechanisms with those occurring in an A1-Sn-Si-Cu alloy lining, studied previously.
A series of standard experimental material characterisation techniques and finite element parametric studies have been applied to investigate the material system’s macroscopic geometry and individual layer properties. Advanced statistical tessellation-based approaches were then used to characterise the lining material microstructure. Standard fatigue tests, in air and oil environment, were further used to determine preferential crack initiation sites by statistically identifying the microstructural features associated with initiation, and also to provide an overview of the effects of environment and microstructure morphology on the fatigue behaviour observed. Classification approaches using Adaptive Numerical Modelling (ANM) techniques were applied to the preferential crack initiation results (from tessellation) to provide possible predictions of microstructural sites most likely to initiate failure. These results were then analysed against those of the previously studied A1-Sn-Si-Cu alloy lining. The results so obtained were then combined with finite element modelling of simplified aspects of the lining microstructure, to investigate the correlation between crack initiation sites and the local stress-strain fields evolved within the microstructure under applied loading. It has been shown that in air and oil environments, fatigue initiation occurs by decohesion at larger Si particles, and in the absence of Si particles, by decohesion at larger Sn particles. Oil environment is seen to enhance the fatigue performance of only the A1-Sn-Si-Cu alloy, with no change observed in the A1-Sn-Cu-Mn alloy. For the lining alloy with high Sn content and therefore exhibiting more phase directionality, a heavier dependence of crack initiation on angular information was found by the ANM classifier. These initiation mechanisms have been explained qualitatively in terms of strain mismatch arising from differing elastic moduli between the secondary particles and surrounding A1 matrix. These results indicated that optimising alloy-lining performance could be achieved by refining secondary phase distribution.
Novel High Velocity Oxy-Fuel (HVOF) processes have also been used to manufacture bi-layer systems with refined secondary phase distributions. A comparator HVOF system (A1-Sn-Cu) has undergone a similar testing regime to that of the roll-bonded specimens. Despite the refinement of the Sn phase within the HVOF system, additional initiation sites are operative (e.g. pores and unmelt boundaries, which are also associated with larger areas of Sn, squeezed out of the deposited coating in molten form during the deposition process).
In all systems, cracks appear to follow the Sn/matrix interface as an easy crack path; directionality and distribution of the Sn phase therefore playing a role in the fatigue resistance for both initiation and crack propagate through the lining, towards the backing and then deflect sharply, travelling within the interlayer parallel to the applied stress in the roll-bonded systems, whereas in the HVOF system, the interface itself seems weak and crack deflection occurs there. This behaviour is discussed in terms of the dissimilar elastic-plastic properties of the layers. Strain-lifting analyses have shown that the fatigue performance of the bearing system is dependent on more than just the properties of the lining alloys alone. Mechanical property parametric studies of the bearing architecture demonstrate that stress-strain properties and thicknesses of the individual layers critically influence the performance of the bearing system as a whole, with the ‘constraint-providing’ steel layers having the larger effect on the magnitude of evolved stresses and strains across the lining top-surface of a loaded system. Provided the linings are relatively thin, their mechanical properties and thickness have a lesser influence on the evolved stresses and strains within the lining layer. In terms of purely materials’ fatigue resistance (Δεp/2 vs Nf) the HVOF lining appears to show the best fatigue resistance at highest strain levels (although the possibility of early interface failure may mean that the strain levels are significantly over-estimated for this system) with the A1-Sn-Cu-Mn system showing better performance than the A1-Sn-Si-Cu system.
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Published date: 2004
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Local EPrints ID: 465639
URI: http://eprints.soton.ac.uk/id/eprint/465639
PURE UUID: d405084d-311f-47f2-9555-1e995c7d0417
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Date deposited: 05 Jul 2022 02:14
Last modified: 05 Jul 2022 02:14
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Author:
Mathew Chuchu Mwanza
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