Fracture and R -curves in high volume fraction Al 2 O 3 /Al composites
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Fracture toughness and fracture mechanisms in Al2O3/Al composites are described. The unique flexibility offered by pressureless infiltration of molten Al alloys into porous alumina preforms was utilized to investigate the effect of microstructural scale and matrix properties on the fracture toughness and the shape of the crack resistance curves (R-curves). The results indicate that the observed increment in toughness is due to crack bridging by intact matrix ligaments behind the crack tip. The deformation behavior of the matrix, which is shown to be dependent on the microstructural constraints, is the key parameter that influences both the steady-state toughness and the shape of the R-curves. Previously proposed models based on crack bridging by intact ductile particles in a ceramic matrix have been modified by the inclusion of an experimentally determined plastic constraint factor (P) that determines the deformation of the ductile phase and are shown to be adequate in predicting the toughness increment in the composites. Micromechanical models to predict the crack tip profile and the bridge lengths (L) correlate well with the observed behavior and indicate that the composites can be classified as (i) short-range toughened and (ii) long-range toughened on the basis of their microstructural characteristics.
I. INTRODUCTION
Ductile phase reinforced ceramics,1–5 interpenetrating phase composites,6–8 and metal matrices with high volume fraction of ceramic reinforcements (>50%)9,10 are emerging as important classes of materials with potential for structural applications requiring high specific modulus, strength, and toughness. The high strength and toughness in these materials are shown to be a direct consequence of energy dissipating toughening mechanisms that reduce the crack driving force at the crack tip.11 Although fracture behavior has been studied extensively, the contribution of any single mechanism to increasing the crack resistance is rather difficult to estimate, and a dominant mechanism is often known to mask the effect of other operating mechanisms. The predominant mechanism of strengthening the toughness enhancement in these classes of composites has been identified as bridging of flaws and cracks by an intact ductile phase in the wake zone behind the crack tip.12,13 The bridging ligaments exert closure stresses which reduce the stress intensity at the crack tip and offer resistance to further crack opening or propagation (R-curves). Thus, in such materials that exhibit wake controlled toughness behavior, it is obsered that the applied load and the crack opening displacement (COD) are strongly coupled and the COD measurement for predicting crack length becomes functionally dependent on specimen geJ. Mater. Res., Vol. 15, No. 5, May 2000
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ometry and distance from the crack tip. As shown in Fig. 1 a single crack is observed to start from the notch with bridges in the wake of the crack tip. Initially, the length of such a wake zone (Lo) is identical wi
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