Resin pressures

If we assume that the body force is only due to gravity and use the definition r = —prL +1 where pr is the isotropic resin pressure, / is the unit tensor, and r is the deviatoric stress as well as assuming a constant density and define a new pressure Pr —pr + prgh, Equation 5.21 simplifies to,... 

Figure 6.5 Equilibrium void stability map for a typical epoxy resin system. Curves indicate stable void equilibrium states for liquid-resin pressures indicated. Growth takes place above the lines and dissolution occurs below the lines for any given resin pressure...

Hinrichs [1] has shown that resin pressures can drop to about 103-117 kPa (15-17 psig) even though autoclave pressures of up to 586 kPa (85 psig) are used. This means that void growth will occur for sufficiently high water contents and the problem of transporting the voids out of the laminate is extremely important. 

The pressures inside and outside of the void are effectively equal until the resin viscosity becomes so high that viscous effects become important. As the resin proceeds toward solidification, the pressure in the void can rise significantly above the resin pressure. Surface tension effects are also negligible for voids larger than 100 pm. 

Because the actual pressure profile in the resin is as yet unknown, it is assumed that the void experiences a resin pressure of 0.1 atm during Stages 1 and 2, which then increases to 5.78 atm during stages 3-5. The resin never actually experiences the total autoclave pressure, so 5.78 atm represents an upper bound. 

The real question in light of Figure 6.10 is what exactly is the resin pressure throughout the product part Is it equal to the gas pressure above the bag in the autoclave process, or the entry resin pressure in the RTM process If not, how can the actual resin pressure be reliably predicted ... 

In overcoming the shortcomings of the earlier models, Dave et al. [21,22] proposed a comprehensive three-dimensional consolidation and resin flow model that can be used to predict the following parameters during cure (1) the resin pressure and velocity profiles inside the composite as a function of position and time, (2) the consolidation profile of the laminate as a function of position and time, and (3) resin content profile as a function of position and time. 

The Dave model considers a force balance on a porous medium (the fiber bed). The total force from the autoclave pressure acting on the medium is countered by both the force due to the springlike behavior of the fiber network and the hydrostatic force due to the liquid resin pressure within the porous fiber bed. Borrowing from consolidation theories developed for the compaction of soils [23,24], the Dave model describes one-dimensional consolidation... 

Figure 6.11 depicts the numerical solutions for the time dependence of the resin pressure profile in the vertical direction for one-dimensional flow in the vertical direction (corresponding to an edge-dammed laminate). The laminate is 1.4-in thick (z direction) and is a unidirectional lay-up. Values for the specific permeability in the z direction, (kz in.2), the... 

Figure 6.11 Resin pressure profiles in the laminate thickness direction (vertical) in a 1.4-in. thick unidirectional graphite-epoxy laminate for one-dimensional flow (edge-dammed) under conditions indicated in the figure...

This model can also provide resin pressure gradients, resin flow rates, consolidation profiles, and, when combined with the void model, void profiles at any point in the laminate. 

Similar kinds of resin pressure calculations must also be made for other processes to examine the potential for void stability and growth. This has become a subject of considerable interest [26-30] as quality is sought in RTM parts. 

Figure 6.12 Profiles of typical resin pressures monitored by the laminate, tool, and bleeder transducers. The profiles of time-temperature and the corresponding time—Pmjn for 35% and 85% initial relative humidity exposure of the prepreg—are also shown. (Source From [25])...

A pressure-temperature stability map can be constructed as a function of humidity exposure, which identifies the resin pressure values for each temperature below which void growth is possible and above which voids cannot grow but rather tend to collapse via dissolution. 

A generalized three-dimensional resin flow model has been developed that employs soil mechanics consolidation theory to predict profiles of resin pressure, resin flow velocity, laminate consolidation, and resin content in a curing laminate. 

The numerical solutions necessary to solve the practical three-dimensional problems agree well with the closed-form analytical solutions for simpler one- and two-dimensional cases with constant material properties. The resin pressure gradient in the thickness (vertical) direction for a well-dammed laminate (no horizontal flow) is nonlinear. 

The resin pressure is almost never equal to the autoclave pressure. If the resin pressure drops due to resin flow, then it may become less than the minimum pressure necessary to prevent void stability and growth. In order to produce quality void-free laminates consistently, accurate resin pressure predictive software is a necessity. 

The second ramp portion of this cure cycle is critical from a void nucleation and growth standpoint. During this ramp, the temperature is high, the resin pressure can be near its minimum, and the volatile vapor pressure is high and rising with temperature. These are the ideal conditions for void formation and growth. 

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. 

Resin flow models are capable of determining the flow of resin through a porous medium (prepreg and bleeder), accounting for both vertical and horizontal flow. Flow models treat a number of variables, including fiber compaction, resin viscosity, resin pressure, number and orientation of plies, ply drop-off effects, and part size and shape. An important flow model output is the resin hydrostatic pressure, which is critical for determining void formation and growth. 

To investigate the potential pressure gradients that exist within a laminate during autoclave processing, miniature pressure transducers (Fig. 10.5), which are capable of measuring the hydrostatic resin pressure, were embedded at multiple locations within several laminates to study the effects of vertical and horizontal pressure gradients [9]. 

The pressure curves also illustrate the horizontal flow process. The resin pressure initially approaches the applied autoclave pressure and then decreases as bleeding occurs. The opposite occurs in the bleeder. The applied vacuum is measured initially, and the pressure increases as resin begins to fill the bleeder. Note that the horizontal pressure gradient is very small for a majority of the laminate but becomes large near the edges. 

Resin content results confirmed the resin pressure results, showing a large resin content gradient (Fig. 10.7) existed at the edge of the laminate. The figure also illustrates that resin bled further into the bleeder when a large gap was used between the laminate edges and the dams. 

Rigid caul plates are typically constructed of thick metal or composite materials. Thick caul plates are used on very complex part applications or cocured parts where dimensional control is critical. Many rigid caul plates result in a matched die configuration similar to compression or resin transfer molding. Parts processed in this manner are extremely challenging because resin pressure is much more dependant on tool accuracy and the difference in thermal expansion between the tool and the part. Tool accuracy is critical to ensure no pinch points are encountered that would inhibit a tool from forming to the net shape of the part. 

Void formation and growth in addition curing composite laminates is primarily due to entrapped volatiles. Higher temperatures result in higher volatile pressures. Void growth will potentially occur if the void pressure (i.e., the volatile vapor pressure) exceeds the actual pressure on the resin (i.e., the hydrostatic resin pressure) while the resin is a liquid (Fig. 10.9). The prevailing relationship, therefore, is ... 

Figure 10.15 Vacuum only can create negative resin pressure...

As a result, an iterative solution technique is required. Initial values for the fiber volume fraction ly and resin pressure are assumed and compared with calculated values found by solution of Equations 13.9-13.13. This iterative process is described in detail in Reference [11]. 

C.A.Thomas, USP 2742672 (1956) CA 50, 11018-20 (1956) (Comp proplnt prepd by coating each grain of finely divided Amm picrate-Na nitrate with liq thermosetting resin, pressure molding curing) 5)G.S.Sutherland "The Mechanism of Combustion of an Ammonium Perchlorate Polyester Resin Composite Propellant , Princeton Univ, New Jersey (1956), 233 6)A.J.Zaehringer, "Solid Propellant... 

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