Page 126 - Fiber Fracture

FRACTURE CHARACTERISTICS OF SINGLE CRYSTAL AND EUTECTIC FIBERS 111

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Strain, XIO~
Fig. I. Stress-strain curves for (0001) A1203 using Yamaguchi’s laser-speckle technique. Data have been
shifted an arbitrary amount of pre-load to clearly show the data. Individual points represent the experimental
data; the straight lines represent the least-squares fit.

among the observed errors for the laser-speckle strain measurement system and proved
to be typical of these sapphire fibers, Fig. 1. The average Young modulus for (1 1 1 )
Y3Al5OI2 was 290 GPa and was in a good agreement with the literature data, but
slightly lower than reported values (Corman, 1991; Sayir and Matson, 1991). There was
less scatter in the data; f21 GPa. The change in the scatter of the data for the different
crystals indicates that these crystal interact differently with the incoming argon laser,
i.e. backreflection characteristics for each crystal type are different. The average Young
modulus of (111) Y203 was 164 GPa along the axis of the fiber. This information
could not be independently confirmed by literature values since single-crystal Y2O3
has not been grown in bulk form. An independent study was initiated by Palko et
al. (2001) to determine bulk and shear moduli of single-crystal Y2O3 using Brillouin
spectroscopy. This technique was able to measure the elastic constants, which form a
fourth rank tensor for Y2O3. Young’s modulus along the (1 11) direction of the fiber was
150 GPa. This value is well within the range of data of the present work obtained using
Yamaguchi’s speckle-shift technique.
The Young moduli of the single-crystal fibers (Table 1) differ strikingly due in part to
their crystal structure and the packing density of the oxygen lattice. Y2O3 has a simple
cubic structure, hence the lowest packing of atoms, and exhibits a very low stiffness
value in the range of 164 to 180 GPa. The sapphire hexagonal closely packed structure
gives rise to a much higher modulus, 450 GPa. Y3A15012 does not have a closely packed
sublattice yielding a moderate value of Young’s modulus, 290 GPa.
The single-crystal fibers tested had distinct fracture characteristics and different
strengths (Table 1). Single crystals of (1 11) Y2O3 were the weakest fibers and strength