Fracture toughness of selectively reinforced Al2124 alloy: Precrack tip in the composite side
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3/4/04
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Fracture Toughness of Selectively Reinforced Al2124 Alloy: Precrack Tip in the Composite Side M.T. MILAN and P. BOWEN In addition to the inherent fracture toughness of each bimaterial component, KQ(5 pct) values of the Al2124/Al2124 SiC bimaterials are largely affected by the thermal residual stresses, the elastic/ plastic mismatch, and the precrack tip position. Regardless of the precrack tip distance to the interface, KQ(5 pct) values are increased in general above “composite only” values. This is deduced to be due to the compressive residual stresses and despite the amplification of the crack driving force from the elastic/plastic mismatch. Additionally, KQ(5 pct) values of the bimaterials increase if the precrack tip is positioned closer to the interface. When the crack propagates, it extends to the interface, bifurcates, and arrests. The load then has to be increased to promote further crack growth in the unreinforced Al2124 alloy side and the subsequent onset of plastic collapse. The crack tip blunting and deflection mechanism increase the toughness attained at the onset of plastic collapse of the Al2124 based bimaterials above both the composite only and “Al2124 only” values.
I. INTRODUCTION
THE selective reinforcement concept consists of reinforcing specific parts of the component to achieve the desired mechanical/physical properties and therefore resulting in a reduction of cost and weight. This concept can be found in Al alloy components of combustion engines (such as piston crowns and cylinder liners) selectively reinforced with short alumina fibers in order to result in high strength and fatigue resistance at moderate temperatures (200 °C).[1] Moreover, if the near surface region of a component remains unreinforced, the processes of joining and machining are largely facilitated.[2] This concept may allow radical changes in design, such as observed in the “bladed ring” where fiber reinforcement is used in the circumferential orientation.[3] However, when a crack approaches perpendicular to a macroscopic planar interface between dissimilar materials, many factors can affect the crack tip driving force: the elastic mismatch, the plastic mismatch, interfacial bonding strength, residual stresses, and crack tip position with relation to the interface. The mismatch in elastic properties of the components of the bimaterial will produce both elastic stress singularities and load partitioning that can affect the value of the local stress intensity factor (K).[4–10] Essentially, when the crack approaches the interface from the higher modulus material, there can be a sharp increase in K near the interface. Conversely, when the crack grows initially from the lower modulus material, there can be a strong decrease in K, as the crack approaches the interface. In metallic materials, although this effect is minimized by the plastic zone ahead of the crack tip, effects are still observed.[9] In addition, load partitioning is predicted to be significant only in cases where there is a high el
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