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Chapter 2
2.4 Microcrack formation in the shear zone
2.5 Gross fracture phenomenon ahead of tool-tip
2.6 Workpiece microstructure effect
2.7 Conclusions from the literature review
The size effect in micro-cutting has
been studied by various researchers and it is attributed to different phenomena
such as material strengthening, subsurface deformation, process parameters,
fracture, workpiece microstructure, etc. Orthogonal micro-cutting process has
been modeled using various numerical formulations within different frameworks. Microcrack
formation, gross fracture phenomenon and workpiece microstructure also cause
the size effect in micro-cutting. The literature review shows that these
aspects have not been adequately considered to quantify the size effect in micro-cutting.
The following specific conclusions can be drawn from the literature review:
·
Researchers have observed formation of microcracks during
micro-cutting experimentally and attributed their occurrence to various
phenomena. But, the phenomenon of microcrack formation along the shear plane is
difficult to capture experimentally. In literature, there is no specific
analytical or numerical formulation to estimate the number, the location and
the contribution of the microcracks to the size effect during micro-cutting.
·
As suggested by Atkins,
specific work of fracture (R) has
been widely used to quantify the energy consumed in the formation of new
surfaces during metal cutting. Atkins’s
model doesn’t take into account the effect of tool edge radius. Therefore, Karpat proposed another model to
evaluate R taking into account the
tool edge radius. Researchers observed that R
becomes a significant portion of total cutting energy in micro-cutting thereby
influencing the phenomenon of the size effect. An elaborate analysis of R as a function of processing parameters
and its contribution to the size effect is not available in the literature.
·
Fracture in cutting of ductile as well as brittle materials,
in various zones of deformation has been characterized analytically using
parameters such as K, G, R and J-integral. Review of literature shows
that the energy release rate (G),
which is used by few researchers to evaluate fracture energy during metal
cutting typically uses LEFM theory, whereas metal cutting is a nonlinear
dynamic problem. Similarly, evaluation of K
depends on material constant and fracture modes. It is also observed that the R value doesn’t represent the true
fracture energy as its evaluation depends on the energy balance considered
during metal cutting. Ueda et al. [74]
used J-integral to characterize ductile-brittle
fracture during cutting of ceramics. J-integral
is also widely used in the continuum fracture mechanics but finds extremely
limited use in the metal cutting.
·
Most of the past studies have either used phenomological
models to predict work material properties or developed approximate multi-phase
models without taking into account actual nature, size and distribution of
grains and grain boundaries. Moreover, there are various challenges to model
actual grain and grain boundaries using traditional numerical formulations due
to very small sizes of these microstructural aspects. Moreover, there is hardly
any literature on grain and grain boundary modeling that uses real life
micrographs of workpiece microstructure.
2.8 Objective and scope of the research
