Numerical Simulation of Air Entrapment During Preform Impregnation in LCM Processes Caleb DeValve and Ranga Pitchumani Advanced Materials and Technologies Laboratory Department of Mechanical Engineering Virginia Tech Blacksburg, Virginia 24061-0238 cdevalve@vt.edu • http://www.me.vt.edu/amtl • (540) 231-2461 Presented at SAMPE 2011 • Long Beach, CA • May 23, 2011

Introduction ď ą General LCM Process: 1. Preform is laid up in the mold 2. Resin is forced into the mold

1

2

4

3

and through the preform, exiting through ports on the opposite side of the mold 3. Resin is cured within the mold around the fibrous preform 4. Finished composite product is removed from the mold for use ď One challenge to processing is the entrapment of voids (intra-tow) and dry spots (macroscopic, inter-tow) in the preform, resulting in defective parts. ď A model for predicting air void entrapment and dry spot formation would be beneficial for forecasting limits on the appropriate processing conditions to design the process for voidfree mold filling. GOALS OF PRESENT WORK: To conduct numerical modeling of the transient resin infiltration of a three-dimensional preform weave architecture, and to derive design guidelines for effective processing

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Model Geometry

FABRIC ARCHITECTURE: Two different plain weave fabric architectures were modeled: (1) Owens Corning WR10/3010 and (2) Rovcloth 2454 Fiber bundle cross-sections are approximately lenticular in shape Weave is contoured as a sine function Repeating unit cells along the direction of the infiltrating resin flow Nesting effects (multiple layers of preform weave) between adjacent fabric layers are considered Geometry is studied by defining thin slices of the stacked fabric layers in the direction of fluid flow

Owens Corning WR10/3010

Rovcloth 2454

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Model Formulation Resin flow is Newtonian, incompressible, and laminar Capillary forces at the resin-air interface ARE considered Fiber bundle is modeled as porous media Existing analytical models for the permeabilityvolume fraction relationships for staggered packing were used to project a permeability tensor onto the major Cartesian directions throughout the sinusoidal weave geometry in the numerical model. 1. Continuity: 2. Momentum:

3. Volume of fluid:

u 0 t T u uu p u u Fsur Fpor t u 0 t

Simulations were solved using ANSYS FLUENT on a mesh of ~10k elements Simulation of ~20 s of flow required 12-36 hours on a Computing Cluster utilizing Dell PowerEdge Rack Servers with Dual Quad-Core 2.93 GHz Processors

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Example Simulation Movies Owens Corning WR10/3010 (resin velocity = 0.25 cm/s): Fiber Bundle Volume Fraction = 0.8 Fiber Bundle Volume Fraction = 0.5

Rovcloth 2454 (fiber bundle volume fraction: 0.57): Resin Velocity = 2.0 cm/s Resin Velocity = 0.4 cm/s

Note that the air is represented by the empty shading and the resin is represented by the green shading.

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Void Content Map ď ą Resin (gray area) infiltration through the various two-dimensional cross-sections of the plain weave fiber preform with vf = 0.5 and uin = 0.25 cm/s at various time steps. The local void content of entrapped air, , along with the volume-averaged void content, avg, is given at each time step for each cross-section. Space Variation

Increase in Time

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Pressure Contours ď ą Pressure [Pa] contours at t = 1 s in the edge two-dimensional plane 0.25

Inlet Plane Resin Velocity, cm/s

2.00

0.5

Fiber Bundle Volume Fraction

0.8

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Experimental Validation and Parametric Study The numerical simulations were applied using the experimental conditions reported by Leclerc et al. 2008 and compared with the physically measured void content reported in this study, where good agreement was seen with the numerical simulation results. The minimum void content is reached at a Capillary number of approximately 0.005, where both the macro- and micro- scale void contents are minimized The drop in void content as the Capillary number exceeds approximately 0.03 is due to the increased pressure gradients within the flow field as a result of the increased velocity of the resin (a characteristic also noted by Schell et at. 2007)

Variable

Ca

V

μ = resin viscosity

V = velocity σ = surface tension

Parameter Values

Inlet Plane Resin Velocity, uin [cm/s]

0.25

0.50

1.00

2.00

Fiber Bundle Volume Fraction, vf

0.5

0.6

0.7

0.8

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Air Entrapment Evolution ď ą Void content in the edge two-dimensional plane for uin = (a) 0.25 cm/s, (b) 0.50 cm/s, (c) 1.00 cm/s, and (d) 2.00 cm/s

ď ą Void content in each of the twodimensional planes for the parameter combinations of (a) vf = 0.5 and uin = 0.25 cm/s, (b) vf = 0.5 and uin = 2 cm/s, (c) vf = 0.8 and uin = 0.25 cm/s, and (d) vf = 0.8 and uin = 2 cm/s

Large spatial variation

Increasing vf Increasing vf

Increasing vf

Increasing vf

Negligible spatial variation

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Void Content and Processing Metrics Increasing vf

(a) Maximum final void content found in the two-dimensional slices

(b) Volume-averaged final void content considering each of the two-dimensional slices.

Increasing vf

Increasing vf

(c) Excess flow time, tex, defined as the time between the resin initially reaching the exit plane of the unit cell and the time at which the void content approaches a steady-state condition.

Increasing vf

Increasing vf

(d) Volume of excess resin, Vex, wasted during the excess flow time necessary for resin infiltration into the plain weave fiber preform. Advanced Materials and Technologies Laboratory

Process Optimization Optimization trends using the normalized values of Vex multiplied by the normalized values of the final maximum void content, presented as a function of the inlet plane resin velocity for each of the fiber bundle volume fractions studied.

maxima

uin,opt ≈ 1.5 [cm/s] uin,min ≈ 2.6 [cm/s]

max

≈ 5.25

(a) Optimum inlet plane resin velocity for various fiber bundle volume fractions and the corresponding maximum void content to the respective flow condition. (b) Minimum inlet plane resin velocity required to achieved various final maximum void contents for different fiber bundle volume fractions. Advanced Materials and Technologies Laboratory

Conclusions A transient simulation of resin flow in a plain weave architecture was studied accounting for the dual scale resin flow, nesting of adjacent preform layers, and capillary forces at the resin-air interface. The modeling provides for analysis of void formation in LCM. Optimal design guidelines based on quantitative metrics of void formation

and excess resin flow for complete saturation were presented.

Acknowledgements

The work was funded in part by the U.S. National Science Foundation through Grant No. CBET-0934008 and a GAANN Fellowship to Mr. DeValve from the US Department of Education through Award No. P200A060289 Advanced Materials and Technologies Laboratory