Page 23 - Mechanics Analysis Composite Materials

8 Mechanics and analysis of composite malerials
IF

t

t

Fig. 1.6. Dependence of force (a) and strain (b) on time.

the moment t = tl elastic strain disappears, while reversible part of the creep strain,
E:, disappears in time. Residual strain consists of the plastic strain, ep, and residual
part of the creep strain, E:.
Now assume that cp -=K which means that either material is elastic or the applied
load does not induce high stress and, hence, plastic strain. Then we can neglect cp in
Eq. (I .IO)and simplify the model. Furthermore let EC << EC which in turn means that
either material is not susceptible to creep or the force acts for a short time (ti is close
to zero). Thus we arrive at the simplest elastic model which is the case for the
majority of practical applications. It is important that the proper choice of the
material model depends not only on the material nature and properties but also on
the operational conditions of the structure. For example, a shell-type structure made
of aramid-epoxy composite material, that is susceptible to creep, and designed to
withstand the internal gas pressure should be analyzed with due regard to the creep
if this structure is a pressure vessel for long term gas storage. At the same time for a
solid propellant rocket motor case working for seconds, the creep strain can be
ignored.
A very important feature of material models under consideration is their
phenomenological nature. This means that these models ignore the actual material
microstructure (e.g., crystalline structure of metals or molecular structure of
polymers) and represent the material as some uniform continuum possessing some
effective properties that are the same irrespective of how small the material volume
is. This allows us, first, to determine material properties testing material samples
(as in Fig. 1.1). Second, this formally enables us to apply methods of Mechanics of