Ali, Muhammad Sarfraz
Microstructural modelling of fatigue in layered bearing architectures.
University of Southampton, School of Engineering Sciences,
Small automotive plain engine bearings are used to provide the relative motion between the engine block and the crankshaft via the connecting rod. Under rapidly changing engine loads, these bearings may suffer fatigue damage during service. In modern multilayered bearing designs, fatigue resistance is a complex function of engine loading coupled with the layer architecture and a multiphase lining alloy. This research has mostly focussed upon micro-scale fatigue damage initiation on thin (0.2-0.3mm thickness) lining surface and its subsequent growth leading towards gross failure. The systems examined comprise Al alloys and sintered bronze as relatively soft and conformable lining layers. The weight percent composition of Al lining alloy was Al-6.5Sn-2.5Si-1Cu-1Ni-0.25Mn roll bonded to a stiffer and thicker backing steel layer (1.5-1.8mm thick) via an even thinner Al foil (0.04mm) as an interlayer. The other system comprised an Al lining (Al-20Sn–1Cu) alloy spray coated on to a medium carbon steel layer in the form of a flat bar. All these systems were compared with the previously investigated Al based designs with lining compositions: Al-12Sn-4Si-1Cu and Al-20Sn-1Cu-0.25Mn (manufactured by roll bonding processes). The performance evaluation was based upon the investigation of microstructural features involved in early fatigue initiation and their effect upon short crack growth on the surface. Subsurface crack growth through the layers has also been assessed and finally the observed fatigue life of various components linked to these behaviours. A 3-point bend test configuration was adopted for laboratory fatigue tests. Fatigue comparison was made on the basis of lining surface plastic strain amplitude vs. number of cycles to failure according to a uniform predefined criterion for all the systems. Maximum plastic strains developing at the lining surface were estimated using a combination of finite element analysis (FEA) and strain gauge measurements so that the fatigue life of all systems studied was presented as strain-life data. Specimens in the form of both finished bearings and flat bars were tested. Similar fatigue behaviour was observed for the two testing geometries, giving greater confidence in the fatigue evaluation process and allowing detailed observations of small crack initiation and growth processes in flat bars to be related to behaviour of the actual bearing geometry.
In the previous research, the coarser Si particles in the Al-12Sn-4Si-1Cu lining and Sn particles in the Al-20Sn-1Cu-0.25Mn alloys were identified as potential crack initiation sites, though the relationship between particle geometry and arrangement/clustering was found to be important. The newly developed Al-6.5Sn-2.5Si-1Cu-1Ni-0.25Mn lining alloy with finer and fewer Sn and Si particles showed a delayed initiation of short fatigue cracks compared to the previous systems. However, a large number of widely scattered intermetallics in the new linings were observed to fracture causing early fatigue initiation at the micro-scale level with some more complex processes of detaching Sn layers from harder intermetallics and Si particles. Using the mechanical property data for bulk lining and secondary phase particles obtained from tensile testing and instrumented hardness testing, stress fields were investigated within the hard particles (intermetallics), surrounding thin layers and the matrix on the basis of the analytical and numerical modelling. On the basis of these modelling results, optimum particle shapes were defined to minimize tensile stresses (within the particles) and hydrostatic stresses (at the particle matrix interfaces). The experimental growth data of a dominant crack when combined with a Hobson type growth model based upon measured particle distributions and experimental crack growth rates, helped in predicting fatigue life of a similar component at different stress levels. Surface crack driving force reduces considerably when subsurface crack deflection occurred within softer Al interlayer. Replacing this interlayer with a harder brazed sheet did not give any significant difference in the observed fatigue life. In the HVOF systems, crack initiation was observed to be from the weaker interface between a harder matrix and softer circular unmelts as well as from various scattered pores. The overall fatigue life of the HVOF systems was comparable to the previous roll bonded systems; however subsurface deflection of crack at the lining-backing interface resulted in the debonding of the lining and hence the observed lining fatigue resistance may not be a good indication of the overall performance in a bearing system.
At similar lining surface plastic strain levels, the bronze bearing with very thin Sn and Ni as overlay layers (~7 microns each) showed comparable fatigue resistance to the currently investigated RB Al based designs. However annealing this system resulted in the formation of hard Ni3Sn intermetallics at the Sn-Ni interface, and the observed fatigue resistance of this system was higher than the RB systems. This has been linked to very fine scale local crack deflection in the overlay layers (although these have not been observed clearly).
All these layered bearing systems provide a complex fatigue problem. Factors which reduce initiation /early growth behaviour are likely to offer the best service performance enhancements in view of the relatively HCF nature expected in service.
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