Effects of Stabilizing Binder on the Formability, Microstructure, and Mechanical Performance of Wet Compression Molded Unidirectional Non-Crimp Fabric Composites

Abstract

Wet Compression Molding (WCM) with highly reactive resins is a manufacturing process capable of high-volume production that has recently gained interest in the automotive industry as an alternative to traditional methods for producing structural components. These components are subject to high loads and may experience impact loads during service; therefore, to achieve the desired mechanical properties and performance requirements, the structural components may require several layers and a significant amount of resin. For typical WCM processes utilizing molds with deep cavities, resin management can be challenging as the fabric stack may drape prematurely due to the mass of the resin; however, the use of binder-stabilized fabric can overcome this problem by enhancing the fabric bending stiffness. While the influence of stabilizing binder on the permeability of various fabrics and the flow characteristics of different resins has been previously studied, its impact on void formation and mechanical performance is less understood. This study focuses on the effects of stabilizing binders on the intra-ply draping mechanisms of wet, unidirectional, non-crimp fabric (UD-NCF), as well as on the microstructure and mechanical performance of the UD-NCF composite fabricated via WCM, which comprises PX35-UD300 carbon fiber fabric and EPIKOTE resin 06150 snap cure epoxy resin, through physical experiments. The objectives of this study are to investigate the influence of the stabilizing binder on the formability of infiltrated carbon-fiber UD-NCF (including membrane behaviour, bending, and compaction), to examine its effects on the microstructure and mechanical performance of UD-NCF composites manufactured via WCM, and to assess the impact of the stabilizing binder on the energy-absorption performance.

For Objective 1, the UD-NCF carbon fiber was characterized through a series of physical experiments, including membrane, bending, and compaction tests. An infiltrated bias-extension test setup was used to analyze the membrane mechanism, a rheometer bending test setup was employed to examine the bending mechanism, and a punch-to-plate setup was utilized to study the compaction mechanism. The fabric infiltration was found to influence the membrane and bending behaviors by reducing the friction between the carbon fiber and the stitching yarns, which consequently decreased the membrane stiffness and the bending stiffness up to 30%. However, impregnation was found to have no significant impact on the compaction response due to the low friction of the carbon fibers. In contrast to fabric impregnation, the pre-activation of the stabilizing binder was found to affect all three draping mechanisms by increasing fiber/fiber and fiber/yarn friction, thereby increasing membrane stiffness by up to 100% and bending stiffness by up to 50%.

For Objective 2, flat UD-NCF composite panels were fabricated by WCM to examine how the stabilizing binder and its state, as well as the vacuum application to the mold and its duration, influence the formation of voids and mechanical properties. It was observed that the use of binder-stabilized fabrics decreased the void content of WCM parts by up to 70%, likely due to reduced relative layer movement and lower air entrapment. The void size decreases further when a vacuum is applied to the mold for more than 20 seconds, which partially removes air inside the mold. This reduction in void size leads to an increase in interlaminar shear strength. Additionally, applying a vacuum enhances preform compaction, resulting in more consistent panel thickness and a higher fiber volume fraction (FVF).

For Objective 3, UD-NCF composite hat channels were fabricated by WCM to examine the influence of the stabilizing binder and vacuum application on energy absorption during axial crush experiments. The use of binder-stabilized fabric and vacuum increased energy absorption; however, this increase was not statistically significant, possibly due to the high FVF of the components. The WCM hat channels achieved energy absorption levels comparable to those of similar hat channels comprising the same constituents and ply stacking sequence and fabricated by high-pressure resin transfer molding (HP-RTM) in a previous study. The WCM hat channels also showed a similar brittle fracture failure mode to the HP-RTM hat channels.

The main results of this investigation include a new dataset on the viscous draping mechanisms of the binder-stabilized UD-NCF. Additionally, mechanical tests provided strong evidence of the influence of the stabilizing binder on the mechanical performance of the UD-NCF composites. These results indicate the feasibility of using the WCM process as an alternative to HP-RTM in the manufacturing of structural components.