Heat and pressure
optimization of fiber
placement in terms of quality
of laminates
Background
Expected effect of different heat sources on
final quality
Conventional methods of fabrication of composite
structures have proven to be labor intensive and timeconsuming. As a solution to that, through automation
highly consistent quality and cost-effective manufacture of
composites can be achieved.
Laser beam:
• Laser beam delivers higher energy intensities to the
surface of the material so that production rate increases
• The heat flux is related to the laser power used by the
area on surface that is directly exposed to the laser
beam. The amount of energy absorbed or reflected by
the surface depends on the optical properties of the
material
• Undesirable Result: high temperatures on surface of
tape
• Solution for greater absorption: reduction of incidence
angle, use of an infrared preheater, use of wider laser
Traditional methods
They involve manual hand lay-up, tape laying and filament
winding techniques but not only is the material scrap rate
in those techniques high but also the repeatability of the
process is quite limited.
Breakthrough methods
The introduction of automation circumvents the previous
disadvantages. Automated Tape Laying (ATL) and
Automated Fiber Placement (ATP) are the breakthrough
technologies. ATL is a process employed to deliver wide
prepreg tape onto a surface whilst automatically removing
the ply backing . ATP utilizes a band of narrow prepreg
tows which are collimated on the head and then delivered
together.
Head design
Where the challenges are and their effect on
final quality of parts
• At the point when torches apply most → polymer
degradation.
• Before and after compaction roller → void growth
• Under the roller → Intimate contact (Dic), healing (Dh)
and consolidation.
Aerospace Engineering
Most important indicator of quality of the
parts : Degree of bonding
• Depends on Dic and Dh and thermal and pressure
history of the material. A coupled bonding model of Dh
and Dic is needed to determine the bond strength.
Temperature history background
Most dominant heat transfer model is a non-isothermal 1Dimensional steady state for thermoplastics.
Exampl
Experimental set-up of fiber placement
Why automated fiber placement?
• Its ability of cutting and restarting of fiber tows and of
reducing material wastage
• Its precise control of orientation of the fibers
• Its high degree of repeatability and lay–up rates
• Capability to manufacture large parts and handle high
areal weight materials
• Ability to combine several serial tasks into a single or
parallel operation and to manufacture complex shaped
parts
are aspects that contribute to the appeal of this new
technological fabrication process
Alternative transfer models:
• Isothermal Incompressible
• Isothermal Compressible
• Model where both the substrate and the head are
moving
For thermosets the model can be a 3-Dimensional.
For dry fibers models can be related to Darcy’s Law.
Pressure history background
Consolidation under the rollers is modeled as a squeeze
flow continuum, in which the rheological properties are
dependent on the temperature, fiber volume fraction, and
void content. The consolidation model also defines the
roller/substrate contact area required for the heat
transfer analysis.
Aim of the research
To optimise the heat and pressure parameters in order to
achieve better quality of the laminates and to establish a
cost-effective fabrication process
References:
Infrared heat source:
• Polymers have a high absorption of energy in the
infrared region of the spectrum in comparison to other
materials
• Short response time and durability in a manufacturing
environment
• Longest wavelength with high power output
• Energy required to heat the material depends on
emissivity ,temperature, surface condition and
wavelength
• At low surface speeds it works well
Methodology of research for final quality
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Testing of the performance of dry fibers, fibers with
binders,
thermosets
and
thermoplastics
and
characterisation of the mechanical properties of the
laminate.
Different heat sources will be tested in terms of the final
quality of the part such as laser beam, infrared,
ultrasonic and combinations of those.
Optimization of the roller may be needed.
Shear beam and optical assessment tests may be
conducted for extracting information about the void
content and thickness.
Doubled Cantilever Beam tests may be executed for
measuring the interlaminar bond quality.
Different fabrication processes of fiber placement will be
investigated such as Resin Transfer Molding and Tape
Placement .
Possible optimization directions derived from
literature
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Experimental set-up
• 8-32 fiber impregnated tows are stored in tensioners
and move through the rollers
• A force sensor measures the compaction force
• Tows are cooled at fiber processing head by a vortex
cooling tube
• Tows are heated by a heat source
• A compaction roller removes trapped air and voids
between the tows
• Process repeated till the desired thickness the
component is vacuum bagged in an autoclave
PhD Candidate: Maria Skandali
Department: ASM
Section: SI&C
Supervisor: S. Koussios
Promoter: B.Rinze
Start date: 16-12-2012
Funding: Tinkerbell scholarship
Cooperations:
Preheating of tool (expensive)
Dry placement at first plies with short duration between
the passes
More plies
Maximum F as possible
Use of a mechanical shutter to reflect IR away from
substrate if infrared heating is used
Use of an infrared preheater before the laser beam if
laser is used as a heater
Possible Personal approach
Schematic of idealized surface elements being deformed
to the dotted line with the application of pressure
Effect of voids on quality of the parts
• Through optical assessment the location, shape,
content and size of voids can be determined.
• Minimum void content →maximum properties.
• Different shape and size influence in-plane and out-of
plane properties.
• Shear and bending depend on void locations, size and
shape.
Given that there have been only 3.5 months since the
beginning of the research I will try to suggest a possible
approach of the project. With main goal the optimization of
the quality of the laminates I would suggest an analytical
modelling of the heat transfer from the source to the part
at the consolidation point of process and a comparison
with a finite element solution. Compaction roller and
pressure distribution are also factors that would affect the
model. Parametric studies and experimental testing of the
heat and pressure parameters should be included.
• 𝐽𝑂𝐻𝑁 𝑇𝐼𝐸𝑅𝑁𝐸𝑌1 & J. W. GILLESPIE, 𝐽𝑅2,∗ “Modeling of In Situ Strength Development for the Thermoplastic Composite Tow Placement Process “,pp1494
• Bijan Shirinzadeh et al., “Robotic fibre placement process planning and control”
• T. Aized &. Shirinzadeh,2009,” Robotic fiber placement process analysis and optimization using response surface method”