Abstract :
In fiber reinforced composites (FRCs), exhibiting
heterogeneous structure at multiple length scales, the
interphase phenomena at various length scales were shown
to be of pivotal importance for the control of the performance
and reliability of such structures. Various models
based on continuum mechanics were used to describe
effects of the macro- and meso-scale interphase on the
mechanical response of laminates and large FRC parts,
satisfactorilly. At the micro-scale, the interphase is considered
a 3D continuum with ascribed average properties.
Number of continuum mechanics models was derived over
the last 50 years to describe the stress transfer between
matrix and individual fiber with realtively good success. In
these models, the interphase was characterized by some
average shear strength, sa, and elastic modulus, Ea. On the
other hand, models for tranforming the properties of the
micro-scale interphase around individual fiber into the
mechanical response of macroscopic multifiber composite
have not been generally successfull. The anisotropy of
these composite structures are the main reasons causing the
failure of these models. The strong thickness dependence
of the elastic modulus of the micro-scale interphase suggested
the presence of its underlying sub-structure. On the
nano-scale, the discrete molecular structure of the polymer
has to be considered. The term interphase, originally
introduced for continuum matter, has to be re-defined to
include the discrete nature of the matter at this length scale.
The segmental immobilization resulting in retarded reptation
of chains caused by interactions with solid surface
seems to be the primary phenomenon which can be used to
re-define term interphase on the nano-scale. Thus, the
Rubinstein reptation model and a simple percolation model
were used to describe immobilization of chains near solid
nano-particles and to explain the peculiarities in the viscoleastic
response of nano-scale ‘‘interphase.’’ It has also
been shown that below 5 nm, Bernoulli–Euler continuum
elasticity becomes not valid and higher-order elasticity
along with the proposed reptation dynamics approach can
provide suitable means for bridging the gap in modeling
the transition between the mechanics of continuum matter
at the micro-scale and mechanics of discrete matter at the
nano-scale.
