Page 346 - Mechanics Analysis Composite Materials

Chapter I. Environmental, special loading, and manufacturing e-ecrs 33 I

The corresponding creep diagram is shown in Fig. 7.19.
Under relatively high stresses, polymeric composites demonstrate nonlinear
viscoelastic behavior. The simplest approach to study nonlinear creep problems is
based on experimental isochrone stress-strain diagrams of the type shown in
Fig. 7.12. Using the curves corresponding to time moments tl < a? < t.3 etc. we can
solve a sequence of nonlinear elasticity problems for these time moments and thus
determine the change of strains and stresses in time. This approach sometimes
referred to as the aging theory is approximate and can be used to study the
structures loaded with forces that do not change in time or change very slowly.
There exist also several variants of nonlinear hereditary theory described, e.g. by
Rabotnov (1980). According to the most common versions, Eq. (7.31) is generalized
as

~(t)= - ~(t)+ CI(t - Z)o(r)dz
El[ J0

J J
0 0
r I k

r I 1

I

In conclusion, it should be noted that properly designed composite structures (see
the next chapter) in which material behavior is controlled by fibers usually do not
exhibit pronounced time-dependent behavior. For example, consider a filament
wound glass-epoxy pressure vessel studied in Section 6.3 (see Fig. 6.23 and the
second row in Table 6.1 for parameters of the vessel). The vessel consists of $36"
helical plies and circumferential plies, and has structural parameters that are close
to optimal (see Section 8.1). Experimental dependence of circumferential strain on
time for step-wise loading with internal pressure p presented in Fig. 7.20 does not
indicate any significant creep deformation. It should be emphasized that this
conclusion is valid for normal conditions only - under elevated temperature
composite structures can exhibit significant creep deformation.