Page 332 - Mechanics Analysis Composite Materials

Chapter 7. Environmental, special loading, and manufacturing effects 317

1-

0.8 .

0.6 .

0.4 '

-
0.2
0 T,"C
0 40 a6 120
Fig. 7.8. Experimental dependencies of normalized stiffness (solid lines) and strength (broken lines)
characteristics of unidirectional glass-polypropylene composite on temperature.

composite under heating up to 2500°C demonstrates only 7% reduction of tensile
strength and about 30% reduction of compressive strength without significant
change of stiffness.
Analysis of thermoelasticdeformation for materials whose stiffnesscharacteristics
depend on temperature presents substantial difficulties because thermal strains
are caused not only by material thermal expansion, but also by external forces.
Consider, for example, a structural element under temperature TOloaded with some
external force POand assume that the temperature is increased up to the level TI.
Then, the temperature change will cause the thermal strain associated with material
expansion, and the force PO,being constant, also induces additional strain because
material stiffness at temperature TIis less than stiffness at temperature TO.To
determine the final stress and strain state of the structure, we should describe the
process of loading and heating using, e.g., the method of successive loading (and
heating) presented in Section 4.1.2.

7.2. Hygrothermal effects and aging

Effects that are similar to temperature action, i.e., expansion and degradation
of properties are also exerted by moisture. Moisture absorption is governed by
equation analogous to Eq. (7.2) in which T should be changed for moisture
concentration W and 1. - for moisture diffusion coefficient p. The principal
difference that exists between temperature and moisture diffusion processes is
associated with high differencebetween 1and p, the first being much higher than the
second one. As shown by Shen and Springer (1976), the temperature increasing in
time inside the surface-heated shell approaches the steady state temperature about
lo6 times faster than the moisture content approaches the stable state under the
same conditions. Moisture content in the material depends on environmental
conditions, temperature, pressure, fiber and matrix materials, levels of porosity, and