Tow Impregnation Model and Void Formation Mechanisms during RTM

Abstract

A simple analytical model has been developed for the impregnation of fiber bundles where the macroscopic flow is parallel to the fiber axis. Based on the study of the contribution of the axial and transverse flow mechanisms inside tows, this model shows that the main tow impregnation process is transverse to the fiber axis. A criterion has been established to indicate when the axial flow can be neglected to simplify the tow impregnation model. This case represents the majority of situations in the RTM of woven fabrics and the model predicts that the flow front in tows has a pointed meniscus shape whose length depends on the effective permeability of the large pores formed between the tows, transverse permeability of tows and thickness. A new boundary condition at the unsaturated tow surface is proposed. It conveys the interactions between the flows occurring inside and outside axial tows during a constant pressure driven impregnation. Two air entrapment mechanisms have been observed. Although the source of these void formation processes is the local difference in fiber arrangement, the dependence of the amount of formed bubbles on the impregnation front length has been clearly identified. In addition, a few means for voids mobilization or bubbles dividing have been investigated.

Optimization of injection flow rate to minimize micro/macro-voids formation in resin transfer molded composites

“…Experimental as well as numerical analyses were also performed to identify and describe the mechanisms of void formation [8,9]. Bréard et al [4] presented microscopic observations of composite specimens characterized by image analysis to study their porosity and void content.…”

“…The optimal of Ca* minimizes the total void content. [8,9], the optimal impregnation velocity ðv opt imp Þ can be calculated as follows:…”

Analytic characterization of the permeability of dual-scale fibrous porous media

“…Although this issue of dual-scale flow behavior has existed for some time and there have been subsequent models that have addressed this issue [2,6,[27][28][29][30][31][32][33][34], only recently have simulations developed capabilities to account for fiber tow saturation separately, in addition to predicting the movement of the macroscopic flow front [17,35,36]. However, such simulations require one to provide a fiber tow permeability value in addition the bulk permeability value for the preform under consideration.…”

Experimental investigation of high fiber tow count fabric unsaturation during RTM

“…Different indirect experimental observations proved the existence of the unsaturation phenomenon. The microvoid formation processes has been studied [3][4][5] and this is the most critical consequence of unsaturation since voids can dramatically reduce mechanical properties of composites. As bubbles may be evacuated from the mold before resin cures, microvoid content cannot be directly linked to an unsaturation degree.…”