Heat and pressure optimization of fiber

placement in terms of quality of laminates

BackgroundConventional methods of fabrication of composite structures have proven to be labor intensive and time-consuming. As a solution to that, through automation highly consistent quality and cost-effective manufacture of composites can be achieved.

Traditional methodsThey 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 methodsThe 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.

Expected effect of different heat sources on final quality

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

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

• 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

• 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

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.

Exampl

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

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

Aim of the researchTo optimise the heat and pressure parameters in order to achieve better quality of the laminates and to establish a cost-effective fabrication process

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.

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 1-Dimensional steady state for thermoplastics.

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.

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.

Aerodynamic drag coefficient for a sphere

PhD Candidate: Maria SkandaliDepartment: ASMSection: SI&CSupervisor: S. KoussiosPromoter: B.RinzeStart date: 16-12-2012Funding: Tinkerbell scholarshipCooperations:

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Experimental set-up of fiber placement

Head design

References:• & J. W. GILLESPIE, “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”