Void growth

Tensile stresses stimulate void growth, and subsequent loss in local strength of the material, hence simulating spall in ductile materials. 

Kardos,J. L., Dudukovic, M. P., Dave,R. Void Growth and Resin Transport During Processing of Thermosetting — Matrix Composits. Vol. 80, pp. 101 — 123. 

M. Dadfamia, B. P. Somerday, P. Sofronis, and I. M. Robertson, On the Small Scale Character of the Stress and Hydrogen Concentration Fields at The Tip of an Axial Crack in Steel Pipeline Effect of Hydrogen-Induced Softening on Void Growth, Int. J. Mater. Res., to appear (2008). 

In addition to the cavitation process related to the presence of a dispersed phase, the formation of voids in the plastic zone has been observed to occur also in the matrix phase. Kinloch and Huang stated that the plastic void growth succeeding cavitation also contributes to energy absorption and might become as important as the shear banding, especially at fairly elevated temperatures [161]. 

The model framework for describing the void problem is schematically shown in Figure 6.3. It is, of course, a part of the complete description of the entire processing sequence and, as such, depends on the same material properties and process parameters. It is therefore intimately tied to both kinetics and viscosity models, of which there are many [3]. It is convenient to consider three phases of the void model void formation and stability at equilibrium, void growth or dissolution via diffusion, and void transport. 

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. 

In the preceding discussion we considered equilibrium void stability however, actual processing conditions involve changing temperature and pressure with time. Whereas equilibrium calculations provide bounds on void growth, it is the time-dependent growth process that is most important from a product quality viewpoint. 

In developing the void growth model, the following simplifying assumptions were made ... 

In the development of the computer code for the void growth model, the following input relationships are provided in the program. These could, of course, be modified to account for different cure cycles or different material systems. The values used in this study are provided as default options in the code. 

If zero initial void diameter is assumed, then no void growth occurs while Oat = Q,. The time increment from the start of the cure cycle to the moment when Qat — Coo is denoted as tBEGIN ar d is given by ... 

Figure 6.6 shows the effect of the processing cycle on the void diameter for pure water and air-water voids of 0.1 cm initial diameter under the specified cycle conditions. It was assumed that the air—water void initially consists of pure air, even though there is likely to be a small but finite water partial pressure. This plot can be divided into the various stages of void growth and dissolution and interpreted as follows. 

Figure 6.6 Progression of void growth during the cycle of Figure 6.2 for both pure water and air-water voids. Note the change when pressure is applied at the end of Stage 2...

For a prepreg equilibrated with moisture at a particular relative humidity, in order to prevent the potential for pure water void growth by diffusion at all times and temperatures during the curing cycle, the pressure at all points of the prepreg must satisfy the following inequality ... 

Equation 6.32 was derived from the requirement that void growth by diffusion at any temperature cannot occur if the pressure within the void is greater than the saturated vapor pressure at that temperature (i.e., if Csat > C ). 

A model and attendant computer code have been constructed to describe time-dependent void growth and stability for any processing cure cycle. Although the analysis is approximate, it does account for the moving void-resin boundary layer and its effect on the concentration profile and diffusion of water in the resin. It was found that for the duration of the cure cycle it makes little difference whether the initial small voids contain pure water or mixtures of air and water. 

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. 

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. 

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