Quantitative assessment of preferential fatigue initiation sites in a multi‐phase aluminium alloy

Abstract

Complex multi-phase Al–Sn–Si alloys are commonly employed in the manufacture of small automotive plain bearings. The fundamental fatigue initiation behaviour of this class of alloys is currently not well understood. A range of analytical techniques were applied to investigate preferential initiation site location and to attempt to identify critical microstructural features. It was apparent from experimental studies that points of fatigue crack initiation are associated with the Si secondary phase. Using tessellation approaches and subsequently both adaptive numerical modelling and micro-scale finite element modelling allowed the identification of features affecting the probability that a given Si phase would initiate a fatigue crack

Fatigue crack shielding and deflection in plain bearings under large-scale yielding

“…The bifurcation within the interlayer of a tri-layer system shown in Figure 2(a), observed during a previous experimental investigation on the fatigue performance of bearings [11], can be attributed to a sudden drop in material resistance as soon as the crack crosses the lining-interlayer boundary. This is confirmed by the large CDF gradients noted in Figure 5 on either side of that boundary, especially at higher loads.…”

“…This architecture consists of elastically and plastically mismatched materials. Crack shielding has been shown to cause deflection and bifurcation in such specimens despite the absence of far-field mixed-mode loading [10,11], as shown in Figure 2(a). J integral estimations based on finite element modeling of the three-point bending tests [10] reported a strong tendency for highly deflected bifurcated crack paths, corresponding to a sharply increasing J integral, representing the crack driving force as the crack tip approaches a stiffer layer.…”

“…The analyses are based on two-dimensional finite element (FE) models of a multi-layered flat strip and appropriate fracture mechanics concepts applicable to monotonic and fatigue loading. The three-point bending loading is applied to replicate the experimental conditions in previous fatigue tests [11,15,20]; this generates pure opening mode at the central section of the strip just opposite to the applied load P as shown in Figure 3. Bifurcated and deflected paths have been experimentally observed to evolve from originally straight cracks at this location.…”

“…The prediction of small deflections led to the consideration of deflections possibly arising from the irregular, multi-phase micro-structure of the lining material observed in tested bearings [11,20]. This kind of heterogeneity would modify locally the solution based on problem symmetry into a mixed-mode stress field triggering deflection.…”

Microstructural modelling of fatigue initiation in aluminium-bearing alloys

“…Microstructural fatigue initiation studies carried out on two lining alloys, Al-12Sn-4Si-1Cu and Al-20Sn-1Cu-0.25Mn, designated AS1241 [9] and AS16 [2], respectively, showed that coarser Sn and Si particles were responsible for early initiation of microstructurally short fatigue cracks. It was also shown that the actual initiation behaviour was associated with a complex combination of particle distribution and geometries [9].…”