PREDICTION OF ELASTIC CONSTANTS AND THERMAL EXPANSION
stochastic, it always reduces the symmetry of the textile composite. In particular, the
translational invariance of nominally periodic textiles will be violated; and periodic
boundary conditions applied to a unit cell will no longer be strictly valid. Broken
symmetry has a relatively minor effect on estimates of macroscopic elastic constants
[5.22], but it can have very important implications for predicting the onset of failure,
which will be taken up in Section 6.
L
L
2t
2t
(a)
(b)
SC.4188T.011896
Figure 5-10. Schematics of buckling of an aligned tow under compression. (a)
When relatively soft through-thickness reinforcement acts as a soft elastic
foundation, allowing deflections over relatively long wavelengths. (b) When stiff
through-thickness reinforcement restricts lateral deflections to the intervals between
successive through-thickness tows.
5.2 Summary of Available Codes for Analyzing Stiffness
This handbook presents annotated documentation for various computer codes that
can predict the properties of textile composites. Some of the codes were developed by the
Textiles Working Group within the Advanced Composites Technology (ACT) Program
sponsored by NASA Langley Research Center. Others were developed outside the ACT
program. All are available for public use. The codes represent a wide range of approaches
and capabilities and cover most of the classes of textile composites currently perceived to
be valuable in aircraft manufacture. This section provides prospective users with a guide
to selecting the best codes for analyzing the stiffness or thermal expansion of a given
