PREDICTION OF ELASTIC CONSTANTS AND THERMAL EXPANSION
codes such as ยตTEX and SAWC use an externally generated idealized geometry. This
should be adjusted to the ensure the correct overall fiber volume fraction.
Standard measurements of fiber volume fraction, e.g. by matrix dissolution,
include all fibers without distinguishing their orientation or host tow. This leaves the
problem of apportioning the fibers to the various components of the architecture.
Fortunately, the proportions of fibers in each type of tow (e.g., weaver, stuffer, warp
weaver in an interlock weave; or warp and weft in a 2D weave) can be deduced from the
manufacturer's setup and they may be regarded as unchanged during processing, even if
the overall fiber volume fraction changes. If the predicted proportions are combined with
a measurement of total fiber volume fraction, the textile is well specified for the purpose
of calculating elastic constants.
5.4.4 Calibrating Fiber Waviness
Fiber waviness is difficult to calibrate. Unless it is excessive, in which case
changes should be made to the processing methods, it leads to knockdowns in composite
stiffness that are too modest to be clearly distinguished from other sources of variance.
Direct measurements of waviness require destructive inspection, which is laborious and
expensive.
Nevertheless, waviness will always be present to some degree and its effects
should always be monitored. A viable approach might include stiffness knockdown
estimates based on simple formulae such as Eq. (3.1); combined with occasional
destructive inspection of parts and control of relevant processing parameters, such as yarn
tensioning.
References
5.1 S.W. Tsai and H.T. Hahn, "Introduction to Composite Materials," (Technomic, Lancaster,
Pennsylvania, 1980).
5.2 M.R. Piggott, "Load-Bearing Fibre Composites," (Pergamon, Oxford, 1980).
5.3 Z. Hashin, "Analysis of Properties of Fiber Composites with Anisotropic Constituents," J. Appl.
Mech.,
, 543-550 (1979).
5.4 R. Hill, "Theory of Mechanical Properties of Fiber-Strengthened Materials-III. Self-Consistent
Model, J. Mech. Physics Solids,
, 189-198 (1965).
5.5 R.M. Christensen, "A Critical Evaluation for a Class of Micromechanics Models," J. Mech. Phys.
Solids,
, 379-404 (1990).
