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Chapter VIII Micro-hardness studies…
concluded that the effect of isotropic assumption on the extracted fracture
toughness was 33% with 23% contribution resulting from the elastic anisotropy
and 10% from the slip system plastic anisotropy.
Vickers indentation was used to access the fracture toughness on the
(100) faces of pure and amino acids doped KDP crystals at room temperature.
Well formed straight radial cracks emanate from the corners of the indents for
load exceeding ~ 0.25 N were observed, which is in agreement with Fang and
Lambropoulos [112]. The present author employed load in the range of 0.294 N
to 0.98 N and each load was applied for 15 sec. Five indents were made at
each load for better averages. The indentation diagonal d and crack size 2c (tip
to tip) were measured with an optical microscope with filar eye piece as
described in section (8.3).
However, no analysis is available to convert measured micro-indentation
crack size to fracture toughness in tetragonal system like KDP. The only
available analysis is for isotropic materials such as glasses or polycrystalline
ceramics [113]. To convert the direct measurements of indentation crack size to
a fracture toughness, it is assumed that KDP possesses an equivalent isotropic
young’s modulus E = 38.7 GPa [114]. The KDP crystals have been studied by
Evans [115] and Anstis et al [116].
Evans [115] applied dimensional analysis and curve fitting over a range
of c/(d/2) from 1.5 to 7. As per this model,
Kc = H (d/2) 1/2 (E/H) 0.4 10 f(x)
f(x) = -1.59 – 0.34 – 2.02 x 2 + 11.23 x 3 -23.97 x 4 + 16.23 x 5
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(8.19)
(8.20)
Where, Kc is the fracture toughness, H is the hardness, d is the indentation
diagonal, E is the Young’s modulus and C is the half crack size.
According to Anstis et al model[119],