Optical methods for monitoring the infusion process in vacuum Assisted Resin Transfer Molding (VARTM)

In composite manufacturing, Vacuum Assisted Resin Transfer Molding (VARTM) is becoming increasingly important as a cost effective manufacturing method of structural composites. In this process the dry preform (reinforcement) is placed on a rigid tool and covered by a flexible blanket to form an airtight vacuum bag. Liquid resin is drawn under vacuum through the preform inside the vacuum bag. Modeling of these processes relies on good understanding of closely coupled phenomena. The resin flow depends on the preform permeability, which in turn depends on the local fluid pressure, the preform compaction behavior, and the membrane stresses in the vacuum bag. To understand the nature of these coupled phenomena it is necessary to measure the preform thickness and degree of preform saturation during the infusion and consolidation process. To monitor the thickness variation of the preform during resin infusion, a shadow moiré apparatus was designed and built. In a series of experiments full-field moiré fringe patterns were recorded and analyzed. A method for interpreting the moiré images for the process was developed using an averaging procedure to unwrap the phase images. The method is presented along with experimental data taken from the infusion of a glass preform with silicone oil. During the infusion, the liquid fills the spaces between fiber tows first before the interfiber spaces. The process and degree of saturation were monitored by measuring the light intensity of transmitted light by means of a digital camera. The refractive index of the fluid being infused (epoxy resin) closely matched that of the glass fibers in the preform. The attenuation of the light intensity can be regarded as a function of the amount of unsaturated fibers in the light path. The degree of fiber saturation can therefore be calculated directly from the measured light intensity attenuation. Experimental results showed excellent agreement with theoretical predictions. These two optical methods allow for a better physical understanding of the infusion process, and highlight areas where current models do not properly predict the infusion process behavior.