Resin pressure is one of the most important parameters in manufacturing composites during autoclave process. It not only greatly influences resin flow behavior, but also has effects on void formation and elimination. Online monitoring resin pressure can provide an important guidance for the optimization of the processing parameters and the control of the quality of composites. In this study, a resin pressure online measuring system for autoclave process was established based on the principle of pressure transfer in liquid, and the size of the measuring probe of the system was optimized to increase the accuracy of measured resin pressure. The results indicate that the accuracy and the dynamic response of the system can meet the requirements of resin pressure measurement during autoclave process. Furthermore, by means of this proposed resin pressure measuring system and the measurements of compaction properties of the fabric stacks, the resin pressures inside carbon fiber fabric/epoxy resin and glass fiber fabric/epoxy resin prepreg stacks during autoclave process were investigated, especially for the zero-bleeding process which is prevailing for aircraft composite structures. It is demonstrated that during zero-bleeding process, the resin pressures, which conform to the spring and piston model, uniformly distribute along throughthickness and in-plane directions. In addition, the resin pressure profile is significantly influenced by the fiber volume fraction of the prepregs, indicating that fiber content of prepreg should be optimized for achieving free defects and uniform fiber distribution. POLYM. COM-POS., 32:314-323, 2011. ª
Nowadays, zero-bleeding process and out-of-autoclave process are prevailing for aircraft composite structures. For these cases, the prepreg property such as air permeability becomes a crucial element for entrapped air venting off from prepreg stack during cure process. Therefore, the measurement of air permeability is important for the understanding and optimization of autoclave and vacuum-bag processes to obtain void-free composite parts. In this paper, an air permeation measuring system was established to test in-plane and through-thickness air permeabilities of prepreg stack, and the effects of compacting pressure and temperature on the air permeability were investigated. Furthermore, the influences of air permeability of prepreg stack on the void characteristics inside the cured laminates processed by autoclave and vacuum-bag processes were analyzed. The results indicate that the proposed air permeation measuring systems can quantitatively measure the in-plane and through-thickness gas permeabilities of prepreg stack. The compacting pressure and temperature have important effects on air permeability. For the same prepreg system, the in-plane air permeability is two orders of magnitudes higher than the through-thickness one. Finally, void defects are sensitive to the air permeability for vacuum-bag process, especially in the case of thick laminates. It provides an effective way to evaluate the quality of the prepreg for control of void defect and an important guideline for prepreg manufacturers.
Resin pressure greatly influences resin flow, void defect, and fiber distribution inside the laminates, especially inside the composite with complex geometry. Thus, obtaining a fundamental understanding on the resin pressure inside laminates with complex shapes can provide an important guidance to control these qualities. This article is mainly focused on the resin pressure inside the composite laminates with tapered thickness and its effect on the compaction of fiber bed. The resin pressure variations inside carbon fiber fabric/epoxy resin prepreg stacks during zero-bleeding and bleeding processes were investigated by means of an on-line monitoring method. It is demonstrated that significant differences are found between the two cases. The resin pressure inside tapered stack under zero-bleeding condition evenly distributes throughout the whole cycle and leads to even distribution of fiber content. In the case of bleeding condition, the application of bleeder materials results in through-thickness and in-plane resin pressure gradients, which provide driving forces for resin flow, and it directly results in different level of fiber compaction at different regions. These results provide a basic understanding on the resin pressure characteristic inside tapered laminate during autoclave process.
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