Identification of fracture sequences during sharp indentation of polycrystalline Al 2 O 3
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The effects of loading and unloading rate on fracture features formed during sharp indentation of polycrystalline A12O3 have been studied. It was found that there is a transition between intergranular and transgranular fracture if the loading rate is increased by a factor of 104 during the whole indentation cycle. By varying the loading or unloading rate during indentation, and correlating the corresponding dependence of fracture features, the part of the indentation cycle can be identified in which specific segments of cracks are generated. Hence, the fracture sequence during indentation is established. The fracture sequence for polycrystalline A12O3 is compared with those reported from direct observation in optically transparent materials such as soda-lime glass and various single crystal ceramics.
I. INTRODUCTION Indentation techniques have been widely used on ceramics to characterize various mechanical properties, such as hardness, elastic modulus, and fracture toughness.1"9 In recent years, a large amount of effort has been made to study crack initiation and propagation during and after indentation.10"15 Five major types of cracks have been reported, namely, ring/cone cracks, radial cracks, median cracks, halfpenny cracks, and lateral cracks.16"20 Typically, ring/cone cracks occur during elastic contact by a spherical or flat-punch indenter, and other types of cracks result from elastic/plastic contact by a sharp indenter, such as Vickers or Knoop.1'2'20 Recently, it was reported that the cone crack system could also be found at sharp indentation sites.21 The cracks that form during sharp indentation are illustrated in Fig. 1. Soda-lime glass has often been used as a model material because it is transparent. In this way, crack initiation and propagation sequences during the loading and unloading segments of indentation cycles have been observed.1"3'16"21 Based on these observations in soda-lime glass, it has been suggested that the fracture sequences during sharp indentation are as follows.2-20 Median cracks are initiated on the loading half of the cycle as full circles beneath the deformation zone and grow in size as loading continues. Radial cracks initiate on the unloading half of the cycle, and grow to full length on complete unloading. The final crack geometry beneath the surface is often a half-penny shape, depending on the particular material and load. It has also been reported that the final crack shape could be semielliptical due to the different extension of the surface and base components of the crack.22 Lateral cracks are nucleated beneath the J. Mater. Res., Vol. 9, No. 7, Jul 1994
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deformation zone at high load on the unloading half of the cycle and grow on unloading to reach full size at complete unloading. This fracture sequence has been widely accepted as typical for most brittle materials. However, a recent review by Cook and Pharr20 has suggested that there is no unique fracture sequence during indentation in most ceramics. Based on ex
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