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310
Tekstilec, 2017, 60(4), 310-316
DOI: 10.14502/Tekstilec2017.60.310-316
Corresponding author/Korespondenčni avtor:
Ing. Petr Kulhavý
E-mail: petr.kulhavy@tul.cz
1 Introduction
High-strength constructions based on long bre com-
posite frames are becoming increasingly important
across all industrial sectors. Plastic materials rein-
forced by long bres are widely used because of their
high strength and excellent Young’s modulus to den-
sity ratio. While conventional materials show one
Petr Kulhavy, Martina Syrovatkova, Pavel Srb, Michal Petru, Alzbeta Samkova
Technical University of Liberec, Studentska 2, 461 17, Liberec 1, Czech Republic
Irregular Winding of Pre-preg Fibres Aimed at the Local
Improvement of Flexural Properties
Neenakomerno navijanje predhodno impregniranih vlaken za
lokalno izboljšanje upogibnih lastnosti
Short Scienti c Article/Kratki znanstveni prispevek
Received/Prispelo 08-2017 • Accepted/Sprejeto 10-2017
Abstract
The main undisputed benefi t of using long fi bre composite materials, whose properties could be targeted
for a particular application, lies in the effi cient utilisation of material. Using a method of pre-impregnated fi -
bre winding, a rod with a reinforced middle part was created through the local adjustment of the winding
angle in order to increase the local bending stiff ness. The aim of our work was to describe, experimentally
and subsequently using appropriate numerical models, the behaviour of two composite rods, one with a
locally variable winding angle and the other with a constant winding angle. The diff erence in the mechan-
ical behaviour of both structures was clearly evident during the experiment. By using a suitable composite
pre-processor and by choosing some multiple element sets, it was also possible to accurately simulate the
real behaviour of such components, which actually have several regions, each with diff erent mechanical pa-
rameters. Together with the expected diff erent fl exural strength, a traditional three-point bending test also
explored the diff erent shape of the resulting deformation in the two compared parts. Diff erences in the max-
imum strength and the mode of fi nal deformations were also identifi ed.
Keywords: composite, pre-preg, winding, bending, local reinforcement
Izvleček
Največja korist kompozitnih materialov, ojačenih s fi lamenti, so široke možnosti za učinkovito izrabo materiala,
tako da bi bile te lastnosti usmerjene v uporabo za posebne namene. Z uporabo metode navijanja predhodno im-
pregniranih fi lamentov je bila izdelana palica z ojačenim srednjim delom, kjer je bila za povečanje lokalne upogib-
ne togosti uporabljena možnost lokalne nastavitve kota navijanja. Namen raziskave je bil opisati in preizkusiti pri-
merne numerične modele obnašanja dveh kompozitnih paličastih elementov, enega izdelanega z lokalno
spremenljivim kotom navijanja in drugega s stalnim kotom navijanja. Eksperimentalno je bila dokazana razlika v
mehanskem obnašanju obeh struktur. Z uporabo ustreznega predprocesorja za defi nicijo strukture kompozitov in
z izbiro večelementnih nizov je bilo mogoče natančno simulirati realno obnašanje komponent z več predeli, od ka-
terih ima vsak drugačne mehanske parametre. Skupaj s pričakovano različno upogibno trdnostjo so bile s tradici-
onalnim tritočkovnim testom upogibanja proučene tudi različne oblike nastalih deformacij v dveh primerjanih pre-
delih. Ugotovljene so bile tudi razlike v maksimalni trdnosti in obliki končnih deformacij.
Ključne besede: kompozit, predhodno impregnirana vlakna, navijanje, upogibna togost, lokalna ojačitev
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Tekstilec, 2017, 60(4), 310-316
Irregular Winding of Pre-preg Fibres Aimed at the Local
Improvement of Flexural Properties
failure mode (i.e. cracking), composites may exhibit
one or a combination of failure modes, including -
bre rupture, matrix cracking, delamination, interface
de-bonding and void growth [1, 2]. Compared to
conventional materials, composite laminates o er
some unique engineering properties, while present-
ing interesting and challenging problems for analysts
and designers [3]. e aim of this presented work was
to study composite rods with local reinforcement
achieved through the local varying of the winding an-
gle in chosen layers during manufacturing, and com-
pare it with a constant winding angle. e winding
methods were based on the layering of carbon bres
from several spools spinning around a non-bearing
core. Information about the optimisation of surfaces
created in this way and the associated cross-sections
can be found in other works [4, 5]. Zu [4] studied the
strain energy criterion in order to create the most ap-
propriate shape of toroidal vessels, achieved through
a signi cantly lower weight and aspect ratio (i.e. the
height to width ratio). It was concluded that the struc-
tural e ciency of lament-wound material can be
determined through a condition of equal shell strains.
Blazejewski [5] studied several possible combinations
of surface textures and the associated properties that
derive from individual plies and the combination of
angles. Mertiny [6] studied the dependency of ply an-
gles on the global strength of composite structures,
and observed that appropriate structures created us-
ing this method are commonly subjected to complex
loading conditions.
Modern design so ware and computer controlled
machines allow us to create almost any winding an-
gle (i.e. the angle between the bre direction and
the axis of the mandrel). Fibres may be wound in
directions ranging from 0° (axial layering) to 90°
(hoop – practically impossible). Computer-control-
led winding machines also facilitate the adjustment
of the winding angle during an operation. is fa-
cilitates the production of multi-angle lament-
wound structures. Lea [7] described the bene ts of
multi-angle winding (e.g. improved tension and
bending characteristics) compared with winding at
the traditional angle of 54°. is led to signi cantly
higher functional and structural strength under
loadings with hoop-to-axial ratios of less than one.
However, all of the above mentioned works ad-
dressed the idea of a constant angle in each ply. In
the presented work, a method based solely on local
angle adjustment is introduced.
2 Experimental
2.1 Materials and methods
e used manufacturing method, referred to as
winding, is the simultaneous deposition of several
laments, described in the works of Chen [8] or
Petru [9]. In our case, carbon pre-preg tapes were
used instead of wet bre laments. is method
of manufacturing parts with rotational shapes
from pre-pregs is usually limited only to the
straight tubes. e presented method could han-
dle the problem, and through the segmentation of
the primary material (i.e. the simultaneously
wrapping of up to 20 thin laments instead of
wrapping one wide), we were able to create curved
shapes and even parts with uently changed cross-
sections, or to locally change the angles in any
place of the part.
e material used was an epoxy UD carbon pre-preg.
According to measurements taken, the nal thickness
a er polymerisation was approximately 85% of the
original thickness. Pre-preg was produced from rein-
forced high-strength carbon bres with a unidirec-
tional orientation (nominal area weight of 150 g/cm2
and nominal bre density 180 g/m3) and epoxy
resin. Nominal resin content was 38%, while nom-
inal area weight was 242 g/m2, using a cure cycle of
60 minutes at 120 °C.
e created tubes were wound from 16 thin tapes
and had four plies with a total thickness of 0.84 mm.
e bre layout in the case of the regular rod was
55/-55/55/-55, with a weight of 158 g. In the case of
the locally thickened rod, the global layout of the
rst and fourth plies was also 55/-55/55/-55, while
the global layout of the second and third plies in the
middle part was locally 55/-70/70/-55, with a weight
of 165 g (Figure 1).
Figure 1: CAD model of bre layout with visible:
a) constant and b) various winding angle
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Irregular Winding of Pre-preg Fibres Aimed at the Local
Improvement of Flexural Properties
e prediction of mechanical properties of trans-
versely isotropic composites has been the subject of
many studies and research in the past, and is also the
subject of current research [10–12]. It should be not-
ed that elastic constants are generally di erent for
each type of composite, making it di cult to deter-
mine all the constants using analytical models. e
sti ness matrix C of one ply in the laminate could be
described by equation 1 in the form of expressed en-
gineering constants, where the Ei represents the
Young modulus of elasticity in the i-th directions, Gij
represents the shear modulus in the ij-plane and µij
represents Poisson’s ratio in the speci ed planes.
1
E1
1
E2
1
G23
1
G31
1
G12
1
E3
µ12
–
E1
µ13
–
E1
µ21
–
E2
µ31
–
E3
µ32
–
E3
µ23
–
E2
C =
(1).
Because there were several plies with various angles,
it was necessary to transform the sti ness matrix of
each ply to the nal matrix of the entire composite
by using equation 2 for individual elements:
C
–
11 = cosθ4C11 + 2cosθ2sinθ2(C12 + 2C66) + sinθ4C22
C
–
12 = cosθ2sinθ2(C11 + C22 – 4C66) +
+ (sinθ4 + cosθ4)C12
C
–
13 = cosθ2C13 + sinθ2C23
C
–
16 = cosθsinθ[cosθ2(C11 – C12 – 2C66) +
+ sinθ2(C12 – C22 + 2C66)]
C
–
22 = sinθ4C11 + 2cosθ2sinθ2(C12 + 2C66) + cosθ4C22
C
–
23 = sinθ2C13 + cosθ2C23
C
–
26 = cosθsinθ[sinθ2(C11 – C12 – 2C66) +
+ cosθ2(C12 – C22 + 2C66)]
C
–
33 = C33
C
–
36 = sinθcosθ(C13 – C23)
C
–
44 = cosθ2C44 + sinθ2C55
C
–
45 = sinθcosθ(C55 – C44)
C
–
55 = sinθ2C44 + cosθ2C55
C
–
66 = cosθ2sinθ2(C11 – 2C12 + C22) +
+ (sinθ2 – cosθ2)2C66 (2),
where the line in the upper index represents the ele-
ment of the transformed matrix and θ represents
the angle of direction of individual plies.
e theoretical values of the basic engineering
constant a er the mutual summing of the individ-
ual transformed sti ness matrix for the two con-
cepts of layered rods are presented in polar graphs
in Figure 2, one with a constant regular winding
angle and the other for the irregular winding an-
gle, reinforced in the centre.
Figure 2: Engineering properties of the a) regular and
b) irregular rod
2.2 Experiment
e exural strength of a material is the maximum
stress that a material subjected to bending load is
able to resist before failure. A traditional three-point
bending test was used to compare the homogenous
and locally variable tube. A hydraulic circuit with an
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Irregular Winding of Pre-preg Fibres Aimed at the Local
Improvement of Flexural Properties
attached force-meter and a cylindrical indenter were
used as the source of the loading force. e use of a
hydraulic system is particularly advantageous with
regard to constant speed regulation, shock absorp-
tion and the smoothness of movement without
“jumps” that are typical, for example, for pneumatic
systems due to the compressibility of the used medi-
um. Devices based on electric power are usually lim-
ited by their size and the range of the operating val-
ues. e applied quasi static loading was increased
in increments of 0.5 mm/s until it caused the nal
displacement of the used indenter of 40 mm or until
total failure of the tested samples occurred. It is evi-
dent from the pictures in Figure 3 that the two types
of rods showed signi cantly di erent shapes of de-
formation during loading. e distance between the
supports was 380 mm in this case.
It is possible to determine exural stress based on
the measured deformation and force response
(equation 3, where D and d [m] represent the outer
and inner tube diameters, F [N] represents the max-
imal bending force and l [m] represents the distance
between supports). It should be emphasised that it
is not possible to use the additive law for the cross
section module Wo, simply using the di erence of
values of individual diameters.
σo =
Momax
Wo =
8 D Fmax · l
π(D4 – d4)
(3)
Figure 3: Shape of deformation during the bending
test for: a) homogenous and b) locally reinforced rod
2.3 Model
e prediction of the behaviour of composite mate-
rials is a very complex problem because the process
induces the orientation of bres, the interface of
plies, etc. e nite-element method (FEM) is a
powerful tool, without which it is impossible to e -
ciently design composite parts today. Numerical
analysis allows us to derive the di erent strain ener-
gies stored in the material directions of the constit-
uents of composite materials [13]. In our case, the
models of the shell composite plate of the two tubes
were created using an ANSYS ACP pre-post proces-
sor. e model was solved as a fully contact task.
e pure penalty formulation with the nodal-nor-
mal detection of integration points was used for the
combination of solid and shell elements. Frictional
support with asymmetric behaviour was also set.
e simplest way of handling an initially uncon-
strained model (i.e. a rod simply lying on solid sup-
ports) was to add weak springs as mentioned by
Gruber or Whitney [14, 15]. e spring constant
was dependent on the loading parameter, thus the
e ect could only be seen in the beginning of the
simulation. e scheme of the created model with
boundary conditions is shown in Figure 4, which
also presents the results of equivalent stress in the
simulation of the regularly wound tube (±55°).
Figure 4: Layout of the solved shell/solid model (re-
sults of stress distribution for regular winding)
e irregular winding (local reinforcement) in the
middle third of the modelled rod was created using
a combination of several local coordinate rosettes
and oriented elements sets. In order to double the
density of the central laminate, the de ection of the
inner rosette BETA is equal to equation 4:
β =
π
2 – α
2
(4),
where alpha represents the global winding angle rel-
ative to the actual central axis.
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Irregular Winding of Pre-preg Fibres Aimed at the Local
Improvement of Flexural Properties
e computed values for both cases are illustrated
in Figure 5, while the reference vectors in the sec-
ond ply of the model can be seen in Figure 6.
Figure 5: Scheme of ply orientation with an irregular
centre part
Figure 6: Reference directions in one of the elementa-
ry plies in the created FEM model
e eventual drop-o places in the boundaries be-
tween the various sectors was lled by material with
the same properties as the primary composite in or-
der to simplify the model solution. e equivalent
stress in the entire locally reinforced rod can be seen
in Figure 7.
Figure 7: Equivalent stress distribution in the irregu-
larly wound rod
3 Results and discussion
Behaviour in terms of the exural loading of both
types of rods was experimentally measured and si-
multaneously modelled. Experimental results are
shown in Figure 8. Even if the values of the maximum
forces are almost the same, the distinctly di erent
course of displacement could be seen. While the rup-
tures and delamination were uent in the case of the
regular rod, the reinforced rod was extremely durable
until the nal moment of sudden total collapse.
1400
1200
1000
800
600
400
200
0
1600
Force [N]
0.00 20.0010.00 30.00 40.00
Displacement [mm]
Reinforced
Regular
Figure 8: Graph of experimental results of the bend-
ing test for: a) regular and b) reinforced rods
Basic statistics from the resultant bending stress and
absolute deformation in the direction of the acting
load are presented in Table 1 below for several cre-
ated samples.
Table 1: Experimental results of the three-point bend-
ing test
Sample Stress, σ [MPa] Deformation,
dy [mm]
x
–
µx
–
µ
Regular 768.41 59.08 14.25 1.73
Reinforced 930.39 28.58 26.00 1.83
e results obtained in the created model (Figure 9)
showed similar trends for approximately the rst 10
mm of deformation. is is also the point where the
results of our experiment start to di er signi cantly.
1400
1200
1000
800
600
400
200
0
1600
Force [N]
0.00 25.0010.005.00 15.00 20.00
Displacement [mm]
Reinforced center Reinf entire
Figure 9: Graph of the model results of the bending
test for: a) regular and b) reinforced rods
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Irregular Winding of Pre-preg Fibres Aimed at the Local
Improvement of Flexural Properties
e following course, in which several di erent
deviations occurred, could not be simply explained
by the stress/strain relationship for the reinforced
layered material. We therefore performed a model
comparison of occurring failure criteria.
Figure 10: Tsai-Hill and Puck failure criteria in a)
regular and b) irregular rods
Figure 11: Micro-pictures of the place of rupture in the
centre of a rod: a) regular b) irregular winding angle
One of the chosen criteria was Tsai-Hill. Capela, for
example, studied this criterion in bending and tor-
sion loading in his work [16]. He claimed that the
Tsai-Hill criterion could su ciently predict the load-
ing e ect on the static strength of specimens. e
second criterion used was the Puck criterion, which
is probably the most frequently used criterion today
because of its universal application. Quite interest-
ing research was conducted in this eld by Francis
[17] who stated that the matrix shear or tension
cracking modes were always observed in the rst ply
for carbon/epoxy thin-walled tubes with [0/90]s and
[±45]s. It is evident from Figure 10 that the modes of
the layer failure are slightly di erent. In Figure 10b,
the “undamaged” green elements are precisely in the
location of the supports and all other elements of the
composite rod are fully loaded. In the case of ulti-
mate loading, this means that the stress will be still
transferred through the entire part, not just locally,
as could be seen in Figure 3a. is is the right way to
create composite parts because there is no reason to
use reinforced composite materials when the con-
centration of acting stresses is not high.
4 Conclusion
e unconventional method of pre-preg winding
was introduced in the rst part of this study. Even if
there are numerous bene ts (compared to traditional
“wet” methods), the main problem lies in the fact
that the stickiness of material causes a signi cant in-
crease of forces in the entire mechanism and the oc-
currence of the imperfect alignment of bres and
their mutual storage in several places.
e aim of this study was to use this method to cre-
ate and subsequently compare the behaviour of two
rods, one with a regular winding angle in all plies
and the other locally reinforced by local adjust-
ments to the angle in two of the four total plies.
During our experiment, we identi ed a signi cantly
di erent behaviour between the two types of rods
tested, particularly at the moment of part rupture.
e experimental results were evaluated using a nu-
merical model, applying an advanced composite
pre-post processor. Based on the work performed,
we can conclude that local changes in the winding
angle may not only locally increase (or decrease)
exural strength, but may also change the shape of
part deformation and the resulting material failure
process (Figure 11). rough the targeted local ad-
justment of the winding angle, it is possible to save
the material and concentrate it solely in places that
actually require reinforcement. is could be seen,
for example, in Figure 10, which illustrates a mutual
comparison of failure criteria. is is one of the
most important ndings of our work, as one of the
basic principles of designing composite structures is
to provide bres only where are they need.
e presented work introduced a topically important
area in the eld of advanced high-strength materials.
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Irregular Winding of Pre-preg Fibres Aimed at the Local
Improvement of Flexural Properties
Future work will concentrate not only on straight
tubes, but also on certain curved and closed compos-
ite frames. e question of how to adequately describe
imperfections in layered plies, such as the wrapping
and twisting of the bres, remains unanswered.
Acknowledgement
e results of this project (LO1201) were made possi-
ble with co-funding from the Ministry of Education,
Youth and Sports, as part of targeted support from
the programme “Národní program udržitelnosti I”.
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