1,720,974 research outputs found
Short fatigue crack growth behaviour in plain bearings
No full textPlain bearings have been in use for many years, over this time their design and the choice of materials used in their construction has been continuously improved. They are currently used as main and con-rod big end bearings in automotive engines. The service conditions for automotive bearings are becoming more severe with new engine designs, and it is important that new bearing materials are designed to provide the longest possible operating life at these higher loads, and at an economic cost. In order to do this it is necessary to study both the loading conditions and the fatigue failure behaviour of plain bearings, with a view to optimising their design and the use of appropriate materials. The aim of our research is to evaluate early fatigue initiation and short crack growth behaviour in bearing materials to establish the key parameters controlling early crack propagation, hence providing material optimisation data. The approach taken combines experimental fatigue evaluation of the bearing lining and finite element modelling of the local microstructure
Assessment of mixed mode loading on macroscopic fatigue crack paths in thick section Al-Cu-Li alloy plate
Get PDFHigh strength, wrought 7xxx (Al-Zn-Mg) and Al-Li based alloys show a propensity for fatigue macroscopic crack deflections aligned along grain boundaries. The present work reports a study on a 3rd generation Al-Li based alloy in the form of a thick AA2297 (Al-Cu-Li alloy) plate, where it was found that although the lithium containing material may indeed be more susceptible to mixed mode grain boundary failure (and by implication crack deflection in conventional tests), the AA2297 alloy fatigue behaviour is mechanistically and functionally equivalent to 7xxx alloy behaviour. It is shown that crack paths are controlled by a combination of crack loading mixity (KII/KI ratio) and maximum strain energy release rates (expressed as Keq.max). Increasing KII/KI ratio is seen to favour sustained grain boundary failure. Crack growth rate behaviour is discussed in terms of extrinsic and intrinsic components of crack growth resistance. It is shown that the present approach can be successfully applied to predict crack deflection / crack paths for a range of sample geometries and orientations on a range of high strength orthotropic aluminium alloys, including 3rd generation Al-Li based alloys and well-established 7xxx alloys
Elevated temperature short crack fatigue behaviour in near eutectic Al-Si alloys
No full textThis paper considers two candidate automotive piston alloys and highlights the influence of microstructural features on fatigue behaviour. Fatigue initiation and subsequent short crack growth was assessed at 20, 200 and 350 °C. It is shown that both temperature and test frequency have a strong influence on the fatigue performance of the materials tested. The microstructure was quantitatively characterised in terms of the primary Si distribution. Together with post failure analysis, this allowed identification of critical microstructural features affecting both fatigue crack initiation and early growth. Large primary Si particles were found to act as preferential initiation sites by cracking or decohesion (dependent on test temperature) and are also sought out preferentially during short crack growth
Fatigue crack initiation and early growth in a multiphase Al alloy included in a multilayer material system
No full textThe fatigue properties of a multilayer material system employed in the manufacture of small automotive plain bearings are assessed. This comprised three layers; a multiphase Al - Sn - Si lining, a bonding Al interlayer, and a steel backing. The structure was found to exhibit complex fatigue behaviour; including multiple crack initiations, highly microstructural crack growth, and complex crack interaction, coalescence, and deflection events. Key microstructural features are identified for both initiation and early propagation of fatigue cracks within the lining material. The effects of both the layered structure and service environment on fatigue crack propagation behaviour are assessed. This provides valuable information for the development of new lining alloys for future automotive bearing designs
Numerical modelling of crack shielding and deflection in a multi-layered material system
Get PDFFinite element analysis has been used to investigate the fatigue behaviour observed in testing a layered structure (representative of an automotive journal bearing). The aim of the analysis was to explain the deflection or bifurcation observed as a fatigue crack propagates through the multi-layered structure of a bearing. A fracture mechanics approach was adopted using detailed evaluations of the J-integral to assess and monitor both crack tip driving force and directional propensity with crack growth. Crack shielding or anti-shielding as well as deflection or bifurcation were conclusively linked to the difference between the fundamental elasto-plastic properties of the various constituent materials
Effects of mixed mode loading on fatigue and creep–fatigue in SRR-99 single crystals
No full textFatigue tests have been carried out at 20, 650 and 850 °C over a range of frequencies on SRR-99 single crystals under both pure mode I and mixed mode I/II loading conditions in vacuum at a stress-ratio of 0.5. At low temperatures, predominantly slip band crack growth (which remained effectively in-plane) was observed, although the scale of the facets increased under mixed mode loading. At high temperatures mixed mode loading resulted in deflected co-operative slip crack growth along a direction that experienced the maximum opening mode (analogous to Stage II crack growth). Some pore cavitation and interlinking was observed at the highest temperatures and lowest frequencies. The effect of increased dwell time at 850 °C was to increase crack growth rates compared with the highest frequency test, although differences in the magnitude of these effects were seen for the two different loading conditions. The increased constraint experienced by the deflected crack offers a partial explanation for this
The effect of environment and orientation on fatigue crack growth behaviour of CMSX-4 nickel base single crystal at 650 °C
No full textThe fatigue behaviour of nickel base single crystal CMSX4 at 650 °C in air and vacuum has been investigated for two orientations with differing nominal crack propagation orientations but the same (001) tensile axis. The orientation containing a 110 crack growth direction shows better fatigue crack propagation resistance. This has been linked to differing orientations and amounts of interdendritic porosity (giving increased crack path roughness and shielding) increased stiffness along the crack growth direction (with implications for shear band decohesion) and lower resolved shear stresses along active slip systems, limiting faster Stage I crack growth. Faster crack growth rates are generally seen in vacuum compared with air, indicating the homogeneising effect of oxidation on slip, suppressing faster Stage I crack growth and possibly increasing oxidation-induced closure. The results of this study indicate that the effect of secondary orientation and environment in single crystal components will affect both failure mode and fatigue crack propagation rates
Elevated temperature fatigue of Al-Si piston alloys
No full textLong crack fatigue tests were carried out on two cast Al-Si alloys used in the manufacture of pistons for small automotive engines. These were performed at a frequency of 15Hz at room temperature, 200 degreesC and 350 degreesC, in addition further tests were performed at 50Hz at the highest temperature. The fatigue performance of both alloys was found to improve with increasing temperature. This was linked to a change in fatigue crack propagation mechanism in both alloys with respect to the primary Si phase. It was seen that the transition between a given Si phase decohering or fracturing ahead of a fatigue crack occurred at a temperature and strain rate dependent critical stress intensity factor. The relation between critical stress intensity factor, temperature and strain rate differentiated the two materials studied
Microstructural influences on fatigue crack initiation and early growth behaviour in plain bearing Al-based linings
No full textFatigue initiation and subsequent short and long crack growth has been investigated in an typical automotive bearing system, focussing on the behaviour of the aluminium-tin-silicon lining material. Crack initiation appears to be associated with decohesion of the interface between the primary, globular silicon and the aluminium matrix. Short crack growth, and crack growth at low Delta K levels in the long crack case, appears to be tortuous and highly microstructurally dependent, deviating towards secondary phase/matrix interfaces. At higher Delta K levels crack growth occurs principally through the aluminium matrix, and classical striations are seen. Multiple crack initiation is seen in the multiphase bearing lining material and crack coalescence gives rise to the dominant crack. Uncoalesced cracks appear to arrest at a "critical" surface crack length, whose dimension has been linked to sub-surface crack deflection within the lining material once the crack reaches the steel backing
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