Center for Composite Materials - University of Delaware
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Research Summary

Interlayer Air Permeability of Thermoplastic Prepreg Stacks during Oven Vacuum Bag (OVB) Processing

Authors: Danning Zhang (Ph.D MSEG), Dirk Heider, John W. Gillespie, Jr

Research Goal

• Investigation the evolution of interlayer permeability during OVB processing
• Effect of lay-up configurations
• Effect of temperature
• Effect of dwell time

Thermoplastic Composites and Oven Vacuum Bag (OVB) Processing

• High performance thermoplastic composites (TPCs) as primary structural materials
• OVB processing - potential cost effective alternative of autoclave
• Void reduction if the major concern for OVB processing
• Void air removal is the key for void consolidation

Microstructure of Thermoplastic Prepreg

• Sealed and Statistically Distributed Voids
• Rough Surfaces and Permeable Interlayer Gaps

Proposed Mechanism for Thick Laminates

• Single layer void air diffusion followed by air flow through interlaye gaps
• Interlayer permeability is very important
• Interlayer permeability can reduce with the development of intimate contact between layers

In-plane Air Permeability of TP Prepreg Stacks – Experimental Set-up

• 1D Darcy’s Flow Set-up
• Various stacking angles
• Permeability thickness product:

Directionally Dependent In-plane Air Permeability

• Significant higher permeability for 0º/ – one layer along measured 1D flow direction
• Minimum at 90º/90º -both layer transverse to 1D flow direction
• 0º/0º, 0º/90º, 90º/90º for processing cycle study

Process Temperature for Permeability Measurement

• Based on the phases of PEEK resin with increasing temperature
• Below the melting temperature of 340ºC where intimate contact develops quickly
• Temperature ramp: 140 ºC, 170 ºC, 240 ºC, 300 ºC.
• Temperature dwell: 240 ºC 10 min, 30 min, 60 min, 8 hrs

Khx Results – Temperature Ramp

• No significant reduction for T < Tg
• Significant reduction for UD lay-up
• Only slight reduction for 0º/90º

Khx Results – Temperature Ramp

• 60% reduction for 0º/0º after 10 min
• 50% reduction for 90º/90º after 1hr
• <20% reduction for 0º/90º after 8 hrs

Surfaces from 0º/0º Interlayer Samples

• Number of peaks and maximum peak height reduced after Tg

RMS Roughness and Average Height between Tape Surface and a Flat Surface - 0º/0º


• Reductions of Rq and h agree with the measured reduction of permeability

RMS Roughness and Average Height between Tape Surface and a Flat Surface - 0º/90º


• h plateaus at temperature after Tg and 240ºC dwell as permeability

Surfaces from 0º/90º Interlayer Samples

Conclusions


Summary
• Permeability during processing
o Significant reduction for unidirectional lay-ups
o Negligible reduction for 0º/90º lay-up
• Permeability may reduce with lower surface roughness
• Interaction between layers affects the reduction of permeability
• Fiber-fiber contacts limits the contact between layers – high interlayer volume for air transport

Acknowledgements

• Faculty Advisor: Prof. J. W. Gillespie, Jr.
• Committee Members: Prof. Suresh. G. Advani, Prof. Dirk. Heider, Prof. Michael Mackay
• CCM Researchers and Technician: Pavel Simacek, Steven Sauerbrunn, John Tierney, John Thiravong
• CCM Students: Thomas Cender, Min-young Yun
• Funding: EADS

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