Single Side Stitching, an innovative textile ... - Mechanical Engineering

Computer Controlled, Automated Manufacture of 3D-Braids for Composites.
H. Langer, DaimlerChrysler AG, Research & Technology, Ottobrunn, (D); A. Pickett, Engineering
Systems International GmbH, Eschborn, (D); B. Obolenski, FEG Textiltechnik Forschungs- und
Entwicklungsgesellschaft mbH, Aachen, (D); H. Schneider, Herzog Flechtmaschinen GmbH & Co.
KG, Oldenburg (D); M. Schneider, Institut für Textiltechnik der RWTH Aachen, Aachen (D); E.
Jacobs, F.A. Kümpers GmbH & Co. KG Rheine (D) (Speaker)
Introduction
3D-Rotary braiding is a further development of the traditional 2D-braiding technique that allows far
greater flexibility in yarn placing and interlacing by moving the bobbins independently and selectively
in a flat array of horngears. In this way this technique allows the production of near netshaped
3D-textile preforms with a yarn architecture specifically designed to satisfy required design
criteria.
Today, a pneumatic/electrical driven prototype machine is available that can produce braids in labscale.
A great deal of practical experience has been gained with this machine which was used to
design a new industrial standard machine.
This paper presents the main results of a consortium of research and industrial partners developing
this technology in the frame of the MaTech project 3D-BRAIDING (03N3036A/0).
This includes the description of the new machine- and machine control concept, of a new CAE
(Computer Aided Engineering) platform for braid development inclusively the FEM braiding
simulation and the presentation of specific mechanical properties of 3D-braid reinforced composites
Principles of 3D-Rotary Braiding
3D-Rotary braiding is based on the well known 2D-Rotary braiding concept but now uses the
horngears arranged in a flat array as shown in Figure 1. Each horngear is equipped with a special
clutch-brake mechanism, which allow a controlled stop or rotation of each single horngear and the
attached bobbins. Groves in the machine working plate guide the bobbins driven by the horngears.
Switches however, located between each pair of horn gears can be activated to transfer the bobbin
to an adjacent horn gear or to be kept. According to the status of the clutch-brake mechanism and
switches any arbitrary movement of the bobbins is possible.
3D-braid
Bobbin path
Take-up
direction
0°-fibre
insertion
Bobbin
Fig.1: Principles of 3D-Rotary braiding and view of the prototype machine

For the insertion of standing ends into the braid, those yarns are led through input tubes positioned
between the horngears or through the horngear axles (see Fig.1). In this way it is possible with this
technique to produce braids with almost any fibre orientations and cross-section geometry in near
net-shape with minimum waste. Due to the possibility to change the number of "active" bobbins the
cross-section area and with that the geometry of the braid can be varied online.
CAE Tools for Braid Development Experience has shown that producing a good 3D-braid is such
a complex process that it must be supported by numerical tools. Consequently, four linked software
tools (modules) have been developed to optimise the bobbins movements, visualise and check a
chosen scheme, simulate the braiding process, and finally, analyse the mechanical performance of
the composite part. Using these tools the design engineer is able to ‘virtually’ design the braid and
generate an output file to control the 3D-Rotary braider. This approach to design allows the design
engineer to optimise ‘virtually’ both the manufacturing process and final part without resort to
difficult and time consuming testing methods.
The CAE concept with the interaction and the function of the different software tools is explained in
the diagram of Fig.2.
ROUTE
Automatic enumeration of
possible bobbin movements
under defined conditions
CAB
Visualisation of the bobbin
movements (e.g. paths). General
control by the user prints of bobbin
setups etc.
Unsatisfactory design - redesign
PAM-SOLID
1. Dynamic Finite element
simulation of the braiding
process
2. Static and failure analysis of
the textile braided composite
Satisfactory design
MANUFACTURE
Generation of output file for the
3D-rotary braider
Fig.2: Interactions of the different software tools and schematic representation of
the process to design virtually 3D-braids
The software tool ROUTE helps to select the best combination of bobbins movements and to activate
as many bobbins as possible and in this way to maximise the machine efficiency. After the
enumeration of all possible paths for each bobbins under the defined boundary conditions it produces
an overall solution based on the compatibility of all paths. The solution can be visualised by
the machine internal Computer Aided Braiding (CAB) simulation software, which acts manifold.
On the one hand it allows the user to specify the boundary conditions (e.g. bobbin set-up, braid
cross-section geometry, etc.) of the bobbin movement and to create and visualise the bobbin tracks
interactively. On the other hand it creates the machine control files and it generates the informations
about bobbin set-up and movement, which are necessary for the FEM simulation. The tool used
therefore is the commercial FEM software PAM-SOLID, that has been adapted to simulate the
braiding process. A dynamic simulation of the braiding process give a detailed prediction of the
yarn architecture in the braid which then is used as a basis for a meso-mechanical stiffness analysis.
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FEM-Simulation of the 3D-Braiding Process
Simulation of the braiding process is a difficult application for finite element analysis. The calculation
must permit very large displacements and a highly efficient contact algorithm is essential to
treat the enormous amount of yarn interaction. Some special features are also needed such as an
element to model the ‘feed-in’ and ‘feed-out’ of yarn from the bobbin as it transverses the base
plate. The key features of a braiding FE model are shown in Figure 3. The yarns are represented as
a fine mesh of bar (or membrane) elements with a typical length of 2 mm so that fine details of the
braid may be captured. All nodes at the braid point are moved vertically with a constant ‘take-up’
velocity and each node at the lower end is assigned a velocity time history to describe the motion of
the bobbin it represents. Stationary yarns are similarly modelled except that their lower nodes are
held fixed. The yarns are given typical material properties and the special elements at the base,
which represent the bobbins, are given a force-deflection behaviour that corresponds to the ‘feedout’
and ‘feed-in’ of yarn from the bobbin. Finally, potential contact, with friction, is defined for all
yarns so that this may be identified and treated.
take-up
velocity
bobbin path
braiding point
(110 mm, 220 mm, 700 mm)
stationary thread
braiding thread
220 mm
bobbin element
bar elements
node j+1
node j+2, j+3
horn gear
velocity
node j
bar n+1
bar n
bar n-1
membrane elements
node j+1, j
membrane n+1
membrane n
membrane n-1
Fig.3: Key features for the finite element
modelling of 3D-braiding using bar and
membrane elements
Braiding
needles
Fig.4: Simulation of a rectangular braid using
membrane elements
The simulation result using membrane elements is shown in Figure 4 for a rectangular braid. A generally
good interaction of the yarns is obtained and the contact between the yarns is seen to function
well. These results allow the motion of yarns and their final positions to be studied and other valuable
information, such as the total feed-out of yarn from each bobbin, to be predicted. Variations in
bobbin tensile force, braid take-up velocity and alternative braiding schemes may be easily simulated
and assessed.
Mechanical analysis of braided composites
From the braiding simulation a detailed model of yarn architecture is obtained which could provide
a basis for a mechanical analysis of the textile reinforced composite. The approach being investigated
is to extract a representative section of the braid from the virtual model which is then overlaid
with a mesh of solid elements to approximate the matrix resin. These two independent finite element
meshes are modified and coupled via constraint equations.
This level of modelling is at the ‘meso-’ scale which can give a good local prediction of stress and
strain distribution within each constituent material. The model will correctly capture the effects of
the anisotropic yarn reinforcement and, furthermore, should provide a sound basis for failure pre-
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diction if appropriate failure criteria can be identified for the fibre and matrix parts. Indeed many of
the inherent difficulties of composite macro-failure models, which try to characterise different
failure modes of the combined constituents under different loading conditions, could be overcome
using this level of modelling. That is, each constituent has a meaningful measure of stress and strain
distribution that can be used in its own failure law.
The following Figure 5 shows typical results for the tensile loading to failure of the model .
Location where
critical fibres
break internally
Location of
final fibre and
matrix rupture
Fig.: 5 Meso-mechanical model for stiffness and failure prediction
Internal fibre breakage
(maximum load)
Subsequent
Matrix
failure
Advanced Machine Concept
The new 3D-braiding machine, developed during this project, still is based on the main principles of
3D-braiding as realised in the available prototype machine. Of course it was tried to eliminate all
technical deficits recognised during the operation of the prototype machine and to improve its functionality.
The main innovations are:
- vertical orientation of the machine working surface and resulting horizontal alignment of the
carriers and yarn bobbins axis
- the machine body consists out of 9 segments, each build up of 16 so called horngear modules,
i.e.: 144 horngear modules in total.
- each horngear is driven by its own servo gearmotor which allows a change of the direction of
rotation of each single horngear and a max speed up to. 60 rpm.
- all horngear modules are fitted with new developed double-torque-clutches which prevent any
damage of the gearmotor in case of a carrier crash.
- if necessary the 9 segments of the machine body easily can be arranged according to the
geometric shapes of the braids to be produced, e.g. L-, T- etc. or O-shape for overbraiding of
simple mandrels or cores
- the horizontal take-off of the braid is realised by a winch system with integrated planetary gear
to vary the speed.
- updated CAB software package for development of machine control files and visualisation of
bobbin movement and track; additional function for generation of basic bobbin set-ups for free
selectable braid cross sections.
For large scale braid production the bobbin carriers must work perfect. Beside the prevention of
fibre damage, which especially occurs if braiding brittle multifilament carbon rovings, easy and
time saving set-up, bobbin change and threading has to be possible. The most important innovations
realised to fulfil this requirements are:
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- optimised thread guides minimise fibre damage
- easy to handle 4 step yarn tension regulation (250, 400, 550 and 800g)
- integrated carrier guide shoes for rapid carrier feed in during set-up
- time saving bobbin change and easy threading of fibres
- yarn breakage and bobbin run out control by integrated optical sensor
In the following figure 6 the design drawing of the complete new "horizontal" machine is shown.
Fig. 6: Advanced machine concept
Braiding-Die- Take-Off-
Braiding machine Braiding-Die-Unit Take-Off Unit
Multi-Sensoric Control System
For efficient automatic operation of the 3D-braiding machine a suitable control system is necessary.
Under this aspect a control system was developed for the supervision of the carrier position, for
detection of breakage of all yarns -standing ends as well as all moving yarns and the control for run
out bobbin at each carrier. The special challenge regarding the development of this new control
system was the large number of elements to be controlled. Furthermore the modular machine concept
should not be restricted in any kind.
An analysis of various control techniques soon showed the advantages of an intelligent solution on
optical basis. Figure 7 shows the basic principles and the realised hardware components of the new,
non-contact multi-sensoric control system.
frame of the
3D-braider
FEG
Camera
Framegrabber
PC
CANcontroller
nominal data
error message
horn gears resp.
standing- and braiding threads
96 light guides
per machine segment
picture recording
image analysis; comparison of
nominal / actual data
FEG-Camera
and optical
system
Fig. 7: Principle and components of the developed multi-sensoric control system
Compact IPC with
integrated integrated CAN-Bus
Light
Guides
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For each machine segment compact units were developed with 96 flexible light guides which are
led to the predestinied positions in the machine frame. If the carriers move over this stationary
measuring points they cast shadow upon the open ends of the optical guides. This signals are used
to control the carrier positions. Information about the actual condition of the yarn (ok or broken
respectively. run out) on each moving carrier are transferred in the same way from the new developed
carrier with integrated light guides to the stationary measuring points.
All 96 single sensors are joined in a specially developed adapter and brought on the high-resolution
matrix-chip of an asynchronous CCD-camera via an optical input element. With the aid of a frame
grabber this picture is digitised and evaluated in a compact industry PC. With a special learning
program the position of each sensor can be detected fully automatically, so that an assignment of
the single sensors resp. light guide ends is not necessary. The interface to the machine control has
been realised by CAN-Bus. The nominal data of each single machine segment are transferred to the
control system via CAN-Bus. After the internal comparison of nominal/actual data in the multi-sensoric
system only the error message with detailed information of kind and position of error is
returned to the machine control.
Examples of realised 3D-braids
After a phase of intensive work to solve basic problems (fibre damage, occurrence of pre-braids)
recognised during braiding technical fibres as carbon or glass, braids were realised for different
applications. The most serious restriction in the selection of the application examples were based on
the limited cross-section areas to be realised with the available prototype machine. Areas of 200 to
300 mm² are in general are to low for aeronautic or automotive structural structures .
Two examples of 3D-braided fibre preforms are shown in figure 8.
87 mm
40 mm
Fig.8: 3D-braided axle with varying cross-section for a compressor stator vane
and T-profile for application in stiffened panel
In the braided fibre preform for an axle of a compressor stator vane two transition zones of the
cross-section geometry were realised. This was done by taking out part of the braiding yarns of the
process and changing the geometry online from round to rectangular. The unused fibres have to be
removed before impregnation. In this way a better coupling of the aerodynamic blade to the axle is
guaranteed. Furthermore different T-profiles were manufactured for reinforcement for panels as
they are used in aeroplane structures. An example too is shown in fig 8. The profile was realised
with a set-up of 111 braiding yarns (24K carbon fibre roving) and 50 standing yarns (2x24K carbon
fibre rovings) yarns. For the actual prototype machine bobbin set-ups of this range at the moment
are the limit with respect to profile dimension.
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Impregnation
For cost saving production of composites efficient impregnation process are as important as the
development of efficient textile processes. For the impregnation of complex structures costly tools
are necessary if using standard techniques as e.g. RTM impregnation. Especially during the early
phase of structure development with a lot of geometric changes, impregnation techniques without
expensive tool costs are advantageous. Under this aspects a new membrane assisted impregnation
technique was developed. In the following figure 9 the principles of the new technique are shown
schematically.
resin flow promotor
resin conduit
microporous membrane
evacuation chamber
infiltration/reaction chamber
vacuum resin
Fig. 9 Principles of Membrane Assisted Resin Infusion
Mechanical properties of 3D-braids
The great complexity and variety of possible moving pattern in 3D-braiding makes it almost impossible
to describe the mechanical properties of the composites reinforced with such braids homogeneous
and complete. In general 3D-braid reinforced composites show minor stiffness and strength
120
100
80
60
40
20
0
25 33
83
103 100 102
85 96
87
89
90
84
80
82
62 70
56
37
67
66
61
62
57
46
65
67
55
57
59
21 20 20 32
Weight specific Energy Absorption [kJ/kg]
of 10x10 mm2 Profiles
56
59
53
50 61
58 65
65
20 24
Non crimp fabric, stitched (65kJ/kg)
3D-braid, aramid / carbon, 34° (63kJ/kg)
Prepreg, unidirectional (58 kJ/kg)
Non crimp fabric, unstitched (25 kJ/kg)
3D-braid, carbon / carbon, 48° (97kJ/kg)
3D-braid, aramid / micro rods, 30° (84 kJ/kg)
3D-braid
Non crimp fabric (2,5D)
stitched
UD-Prepreg (1D)
Non crimp fabric (2D)
unstitched
Fig. 10: Weight specific energy absorption of 3D-braid reinforced composites
and comparison with non 3D-fibre structures
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properties compared with unidirectional prepreg or non crimp-fabric constructions. This depends of
course on the differences in the structure of the fibre reinforcement, the perfectly straight oriented
fibres in prepregs or non-crimp fabrics and the interlaced fibre structure of the 3D-braids. Due to
the process even the standing threads are inserted in the braid with more or less waviness and not
straight as expected. Exactly this fibre orientation on the other hand is responsible for a high damage
tolerance and an excellent energy absorption behaviour of such composite structures.
In the above figure 10 values of the specific energy absorption, measured in crash tests, are listed
for different 3D-braided fibre structures and compared with fibre reinforced materials out of prepregs
and non-crimp fabrics (with and without stitching). Maximum energy absorption values up to
about 100J/g are reached with carbon fibre 3D-braids (braiding angle 48°, Tenax HTA 5331, 12K).
A comparison with metals, which reach values in the range of 10 to 30 J/g, demonstrate impressively
the potential of 3D-braid reinforced composites in the field of crash absorbing structures.
Costs of 3D-braids
To evaluate and analyse the costs of 3D-Rotary braiding structures, a lot of investigations have been
done. The whole production process, including time for preparation, was analysed for instance by
REFA-methods. Weak spots regarding the stability of the process, the sequence of the different
production steps and the handling in general were shown and because of this the consortium was
able to find solutions for optimisation with respect to control systems, automation and simplifying
the handling. So the efficiency for example of the production of a T-profile could be increased from
40% to 65% and the productivity for about 50%. Due to this, the price of such a profile could be
less than 1/3 of the price when starting this project.
Conclusion
The deficits of 3D-rotary braiding concerning fibre damage and pre-braiding at the beginning of the
project were largely solved by machine and bobbin carrier modifications and the integration of
braiding needles. For the very complex task to develop optimised bobbin set-ups and moving patterns
a CAE concept inclusively the suitable software tools was developed. In this way a virtual
design of braids is possible. The whole evaluation concept for 3D-braids was completed by an FEM
simulation tool. In this way the braiding process which delivers results on the generated fibre
structures (orientation and position of each single yarn in 3D-space) can be simulated and the prediction
of the mechanical properties of the final composites is possible.
For the impregnation of the fibre preforms a membrane assisted resin infusion process with a high
potential for saving tool costs was developed and tested. The mechanical properties of the 3D-braid
reinforced composites suffer from reduced strength and stiffness properties compared with conventional
straight fibre reinforcements, but guarantee on the other hand high damage tolerance, structural
integrity and an excellent specific energy absorption behaviour. Measured values of up to 100
J/g demonstrate the potential of this property compared with metals, which are in the range of 10 -
30 J/g. The application of 3D-braiding at the moment limited by the small cross-section areas which
are realisable with the actual prototype machine.
Based on the experience with this machine a new industrial standard 3D-braiding machine was
designed and manufactured during this project. With an increased number of horngears (144 instead
of 100), each single motor driven and an integrated multi-sensoric control system the productivity
of the process should clearly be increased and the field of applications extended. However further
developments are necessary, especially in the automation of still time consuming processes as bobbin
set-up and -change should be realised in the near future to establish this powerful textile technique
in the field of composites.
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