M.R. Joyce

Elevated temperature short crack fatigue behaviour in near eutectic Al-Si alloys

This 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.

The effect of environment and orientation on fatigue crack growth behaviour of CMSX-4 nickel base single crystal at 650 °C

Abstract The 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.

Numerical modelling of crack shielding and deflection in a multi-layered material system

Finite 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.

Microstructural characterisation of fatigue crack initiation in Al-based plain bearing alloys

The fatigue initiation behaviour of two Al-based bearing lining alloys has been assessed by image analysis tessellation techniques and subsequent adaptive numerical classification approaches. Fatigue initiation occurs by decohesion at Si particles, and in the absence of Si particles decohesion occurs at Sn particles. In both cases, relatively unclustered, large particles initiate fatigue, and the relative position of the nearest neighbour particle seems to have significance. This can be explained qualitatively in terms of strain mismatch arising from differing elastic moduli between the particles and surrounding Al matrix and local stress concentration and superposition effects. The lining alloy containing no Si and high Sn content exhibits more directionality, and a heavier dependence on angular information is found by the classifier.

Fatigue crack initiation and early growth in a multiphase Al alloy included in a multilayer material system

Abstract The 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.

Elevated temperature fatigue of Al-Si piston alloys

Long 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.