Center for Composite Materials - University of Delaware

Research Summary

Gas Transportation in Void Reduction of Thermoplastic Composites via Oven Vacuum Bag (OVB) Processing

Authors: D.Zhang (PhDMSEG), J.W. Gillespie Jr

INTRODUCTION AND MOTIVATION

High performance thermoplastic composites – Increasingly used as structural materials
• Enhanced impact resistance
• Recyclability
• New forming, assembly and joining methods

OVB processing for thermoplastic composites – A potential cost effective alternative of autoclave
• Vacuum bag based, no positive pressure applied
• Low cost

Void reduction – Major concern to provide equivalent properties of autoclave parts
• Severely degrade the mechanical properties
• Vv<1% required for aerospace applications

• Void dynamics and consolidation mechanisms are not fully understood!

Rod-like voids sealed in the prepreg – gas escaping through diffusion – Diffusion time for thick laminates will be extremely long with diffusion mechanism only

Gas escaping through interlayer region may provide another way for volatile removal.

OBJECTIVES

• Investigate the permeability of the interlayer gaps
• Investigate the characteristic volatile removal time for gas flow through interlayer region and diffusion mechanism.
• Investigate how the time changes as the intimate contact develops

IN-PLANE AIR PERMEABILITY OF TP PREPREG STACKS - EXPERIMENTAL SET UP

In-plane Air Permeability of TP Prepreg Stacks – Experimental Set-up
• 1D Darcy’s Flow Set-up
• INSTRON applies uniformly 1atm pressure to maintain even contact of layers
• Reliable and repeatable results
-Number of layers
-Sample size (W, L)
• Permeability with various stacking angles

In-plane Air Permeability of TP Prepreg Stacks
• 3 or 4 layers
• 2 inch ~4 inch along flow direction
• 8 inch ~12 inch in width direction

PERMEABILITY PARAMETER WITH VARIOUS STACKING ANGLES

• Permeability along the fiber direction is about 10 X of that along the transverse direction

AIR PERMEABILITY AT A CERTAIN LEVEL OF INTIMATE CONTACT

• Modeling method – FEA
-Polyflow for intimate contact developing
-Fluent for Air flow through channels
--Laminar flow
--Pressure drop: 1atm
--Isothermal
--Sample length: 500mm
• Surface geometries of Intermediate levels of intimate contact → permeability prediction

Initial permeability
Mass flow rate: 5.55E-08 Kg/s
Flow rate: 4.54E-08 m3/s
Pe: 4.15E-17 m3

Partial intimate contact
Sample width:621.08um
Contact length: 487.71
Dic = 78.5%
Mass flow rate: 2.79E-08 Kg/s
Flow rate: 2.32E-08 m3/s
Pe: 2.08E-17 m3

CHARACTERISTIC TIME

Air Flow through permeable interface
• Initial void gas pressure is Patm
• Steady state pressure dropping: at each time increment, Pv = Papply +surface tension(ignored)
• Total volume of gas increases at T>298 K

Pure gas diffusion
• Gas diffuse from the center of the thickness
• Symmetric diffusing to both tape surfaces
• Fick’s second law
• To evacuate 99.9% of the void gas

CHARACTERISTIC TIME - DIFFERENT LEVEL OF PERMEABILITY

Panel In-plane dimension L = 1m
• As permeability decreasing, air escaping time through interface getting closer to diffusion time
• Both mechanisms may dominate for air escaping in thin laminates

COMPARISON OF CHARACTERISTIC TIME

• Panel In-plane dimension L = 1m
• Isothermal condition, Pe = 1E-15 m3
• Gas diffusing through 1 layer and flowing out through interlayer region before full intimate contact could reduce significant time for void reduction

SUMMARY AND ONGOING WORK

In-plane Air permeability of prepreg stacks were measured
• Fiber angles affect the permeability
• Along the fiber direction ~1E-15 m3 (1E-9m2 assuming interface region h =100mm)
• Across fiber direction ~ 1 E-16 m3
• Measurement with conditioned surfaces
• Analytic model to predict permeability of off-axis fiber angles

FEA can help with intimate contact modeling with real tape surface, and also predict the interface permeability at different level of intimate contact
• Characteristic tape surface is needed
• Contact of two tape surface could be modeled

Air escaping time mainly dominated by diffusion; could be effectively reduced by diffusing through 1 layer and flow out through permeable interlayer region – controlling of intimate contact
• Transient condition for estimation of time escaping

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
• John Tierney
• John Thiravong
• Funding
• EADS

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