Optical measurement of voids in situ during infusion of carbon reinforcements

Abstract

Liquid composite molding (LCM) is growing in importance as an alternative to traditional prepreg-autoclave methods for manufacture high-performance composites. The most significant roadblock to industry’s implementation of LCM is the usually higher void content compared with prepreg processing. One tool for reducing void levels in LCM involves optimization of flow velocity, which requires models to be developed to describe void formation at a given velocity. To help solve this problem, the following research illustrates the first known method for optical void measurement in situ during infusion in a carbon fiber reinforcement. Similar to previous studies on glass fiber, this work utilizes fluorescent dye and a digital camera to produce sufficient contrast and resolution for image analysis. Visible bubbles are photographed against the opaque carbon fiber background. An automated method of image analysis is outlined, which was used to analyze 230 images for three different flow orientations of a single fabric, producing the highest amount of experimental data seen so far on in situ void measurement. The resulting data identifies a minimum velocity threshold for minimal macro-void formation. The resultant void characterization framework will better enable optimization of LCM processing for high-performance composites based on carbon reinforcements.

Capillary Effects in Fiber Reinforced Polymer Composite Processing: A Review

“…Void content as a function of Ca has been characterized experimentally (Leclerc and Ruiz, 2008) and numerically (Devalve and Pitchumani, 2013) and is known to follow a "V-shaped" trend as sketched in Figure 2. However the range of Ca opt is known to differ for different matrix/fabric systems and as reported by Park (Park and Lee, 2011) and by Michaud (Michaud, 2016), typical values range in the order of 10-3 and in recent studies of Caglar et al (Caglar et al, 2019) found a value of 1.4•10 −3 , Lebel et al (LeBel et al, 2019) a value of 1.5•10 −3 and Lystrup et al (Lystrup et al, 2021) a value of 0.6•10 −3 . The soil science version of the capillary number (Ca*), as written in Eq.…”

“…Further contrast enhancement could be achieved by the addition of fluorophores into the probe liquid. This resulted in strong contrast enhancement, e.g., for visualization of intratow flow (Lebel et al, 2013;LeBel et al, 2014), even enabling the use of optical methods in opaque carbon fibrous preforms (Lystrup et al, 2021). The addition of dyes may however induce changes in resin properties, e.g., wettability, and thereby may affect the apparent Ca or Ca*, making the resulting observations not directly representative of pure resin systems.…”

Optical Detection of Void Formation Mechanisms during Impregnation of Composites by UV-Reactive Resin Systems

“…Detailed information about the microscopic dual-scale flow and associated pore formation becomes possible when the state of the impregnation can be imaged holistically in a cavity. The state of the art in investigating void formation and transport are optical methods [27][28][29]. Furthermore, ultrasonic measurements, as well as X-ray and micro-CT examinations, were conducted in several renowned studies [1].…”