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Paper Summary: Design of and with thin-ply non crimp fabric as
building blocks for composites
Sci Eng Compos Mater 2018; 25(3): 501-516
Rajath Anand Iyengar
Department of Mechanical Engineering, California State University Northridge, Northridge CA
Corresponding author: rajath-anand.iyengar.104@my.csun.edu
1.Introduction
This paper discusses a new set of non-crimp fabric which is used to create a ply that is thin and
lightweight than traditional pre-peg ply. This is achieved by laying the fibers in various orientation
that does not require crimping and can be laid in one axis which significantly lowers the cost of
manufacturing, scrap, and labor. A homogenized laminate without ply clusters is formed by
different fiber orientations of well-dispersed plies created by thin-ply NCF stacking. Another way
of creating a thin-ply NCF is called the bi-angle method which offers two different orientations
with one being an on-axis (0°) and the other being an off-axis (at any ɸ°) forming (0/ ɸ) assembly
as shown in figure 1. A modern technique of tow-spreading allows the thickness of the ply to be
lowered to one-third of a commercial high-quality ply thickness. Evaluation of the structural and
mechanical properties has been done by various experiments and multi-scale modeling.
Figure1: (-45/0) assembly
on left & (0/45) assembly
of bi-angle thin-ply NCF
pre-peg C-plyTM with
Aldila’s AR2527 resin on
right.
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2. Methods
At first, the thin-ply concept was put in place as individual unidirectional plies because
microscopic damage accumulation in UD composite fibers is sensitive to the thickness of fiber
orientation. By utilizing the technique of tow spreading (Figure 2), the fibers are spread without
inducing any stress thus eliminating the risk of fiber damage. Air is blown across a 12 k or 24 k
tow to create thin plies of about 1/6 of the conventional ply thickness of 0.12mm.
Figure2: Difference between original tow and spread tow (left) and
Tow-spreading technique (right)
This way of producing thin-plied NCF eliminates the issue of large ellipsoid tow bundles separated
by resin pockets to a greater extent. These fibers are then put under tension, compression, and
transverse loading in various experiments. Under computational stress analysis, it was observed
that there is an increase in strength by 23% with a significant reduction in crack density. The
simulations also recorded that the stresses leading to crack progression were smaller within the
plies in the area adjacent layers.
Homogenization at the laminate level can be clearly understood by normalizing the laminate
stiffness matrices as the components would have the same unit.
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Where A, B, and D are the laminate’s extensional, extension-bending coupling, and bending
stiffness matrices respectively and h is the total laminate thickness. Figure 3 shows how plies of
well-defined orientations can be dispersed so that the laminate becomes homogenized throughout
its thickness. Repeat index ‘r’ increases the dispersion of the distinct ply orientation or total
thickness relative to building blocks. Due to this, the laminate becomes equivalent homogenized
material, such that A and D matrices are equal, [A] = [D*].
Figure 3: Dispersion of distinct fiber orientation and anticipated crack-stopping
characteristics are shown in red curves as the index ‘r’ increases for
homogenized laminates. (Top)
Homogenized laminate thickness and building block approach (Bottom)
3. Results and discussion
This method resulted in increased resistance to the matrix crack accumulation and enhanced the
fatigue life of thin-ply fiber-reinforced laminates in tension. Another experiment showed a
standard ply laminate subjected to out-of-plane transverse loading by indentation led to increased
delamination and matrix cracks, whereas thin-ply laminated withstood the loading and ultimately
failed by sudden fiber fracture. The study of failure surfaces or envelopes of the homogenized
laminates is done graphically. For laminated composites, the FPF (first-ply failure) and LPF (lastply failure) envelopes predictions are performed by ply-by-ply analysis. To get accurate analysis
results for several sub-laminates of bi-, tri-, and quad-ply orientation are compared.
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Figure 4: Sub-laminate FPF
and LPF envelopes (in
MPA): ply-by-ply (-P) vs
Homogenized (-H)
A: Quasi-isotropic
B: Balanced and orthotropic
like
C: Anisotropic
Out of three orientations of plies, the bi-angle NCF proved to be better. This type of configuration
of ply can be used where a structure is heavily loaded with stress such as wind turbine blades or
wing designs. The stress plane can be improved for reduced weight and increased strength.
4. Conclusion and future work
By introducing the tow-spreading technique, an innovative bi-angle thin-ply non-crimp fabric we
get an alternative for laminate production. With the prior analysis for laminate stiffness and
strength, a building block or a sub-laminate can be designed and replaced for traditional options
like quasi-isotropic NCFs. Other than bi-angle NCF, anisotropic NCF building blocks can be
considered as they also offer isolation for the deformation modes under combined loading.
Stacking/ repeat of the NCF building block or sub-laminates is the basis of homogenized laminates.
As the repeat increases, the laminate can be treated as if it was made up of homogeneous material.
By this method, equivalent material properties can be easily predicted for further use. The
properties can be stiffness and strength. Homogenized FPF and LPF envelopes were analyzed by
ply-ply analysis of the failure to compare different orientations of plies. The FEA analysis was
shown to be consistent and concluded that the accuracy of sub-laminate-based analysis is sufficient
to use for continuous homogenized laminate thickness variables in the design optimization of
structures made using thin-ply non-crimp fabric (NCF).
Efficient and smart staggering schemes may be explored in order to withstand varieties of loading.
In order to determine the properties of ply-by-ply analysis, we can opt basic testing scheme for the
back-calculation of ply data.