Resin Hydrostatic Pressure and Flow

High pressures are commonly used during autoclave processing to provide ply compaction and suppress void formation. Autoclave gas pressure is transferred to the laminate due to the pressure differential between the autoclave environment and the vacuum bag interior. Translation of the autoclave pressure to the resin depends on several factors, including the fiber content, laminate configuration, and the amount of bleeder used. 

An appreciation of the importance of hydrostatic resin pressure must be developed to understand void growth fully. Because of the load-carrying capability of the fiber bed in a composite layup, the hydrostatic resin pressure needed to suppress void formation and growth is typically only a fraction of the applied autoclave pressure. The hydrostatic resin pressure is critical because it is the pressure that helps to keep volatiles dissolved in solution. If the resin pressure drops below the volatile vapor pressure, then the volatiles will come out of solution and form voids. 

In the early stages of the cure cycle, the hydrostatic resin pressure should be equal to the applied autoclave pressure. As resin flow occurs, the resin pressure drops. If a laminate is severely overbled, then the resin pressure could drop low enough to allow void formation. Thus, the hydrostatic resin pressure is directly dependent on the amount of resin bleeding that occurs. As the amount of bleeding increases, the fiber volume increases, resulting in an increase in the load carrying capability of the fiber bed. 

The rate of bleeding is dependent on several factors, including the permeability of the fiber bed, both vertically and horizontally, and the viscosity of the liquid resin. The permeability of the fiber bed will depend on the weave of the fabric, the fiber diameter, and the fiber volume fraction. The resin viscosity is determined by the chemistry of the resin and the thermal profile of the cure cycle. The cure cycle greatly affects resin viscosity and the flow process, both directly through the pressure application and indirectly through the effect of the thermal profile on resin viscosity. 

Step 1 There is initially No Load on the system. The Liquid Hydrostatic Pressure and the Load Carried by the Fiber Bed is Zero. 

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