Improving the Quality of Laminates in Liquid Composite Molding Using Magnetic Compaction: Experiments and Process Model

Despite the extensive use of liquid composite molding (LCM) processes such as wet lay-up vacuum bagging (WLVB) and vacuum assisted resin transfer molding (VARTM) in composite manufacturing, they have two major drawbacks. First, the fiber volume fraction of the composite parts made by LCM is lower than those made under an elevated pressure using either autoclave or hot press, leading to lower mechanical properties. Second, the process induced defects in LCM parts are quite high, which may significantly reduce the mechanical performance and environmental durability of composites. The focus of this dissertation is to tackle the important problems encountered with WLVB and VARTM to improve the quality of molded parts. The first part of this dissertation introduces a novel technique, magnet assisted composite manufacturing (MACM), to improve the quality of WLVB laminates. In this technique, the composite lay-up is sandwiched between a magnetic tool plate and a set of Neodymium-Iron-Boron (NdFeB) permanent magnets during cure. The details and effectiveness of MACM process are investigated by fabricating of E-glass/epoxy composite laminates with and without magnetic pressure and comparing their void content and morphology, fiber volume fraction, and mechanical properties. The results clearly show that the quality of composite laminates is significantly improved in the presence of magnetic consolidation pressure, where the fiber volume fraction increases by more than 50% to almost 30% and process-induced voids decrease to less than 3%. As a result, the flexural strength and modulus of the parts are enhanced by approximately 60% and 50% to ~245 MPa and ~10 GPa, respectively. The second part of the dissertation extends the application of the MACM technique to fabricate high-quality VARTM laminates. In VARTM, unlike the WLVB process, the preform impregnation takes place under vacuum, which results in different mechanisms of void formation and different ranges of fiber content. Thus, enhancing VARTM is quite different than enhancing the WLVB process which provides the motivation to investigate the effectiveness of utilizing MACM in VARTM. In this regard, thin (i.e. 6-ply), as well as moderately thick (i.e. 12- and 18-ply) E-glass/epoxy laminates are fabricated by applying MACM either before or after infusion. The results prove the effectiveness of MACM in fabricating high-quality VARTM laminates where a fiber volume fraction of more than 50% and void content of less than 1% is achieved. In addition, a transient magnetic consolidation model is developed, predicting the final thickness and fiber volume fraction of the VARTM/MACM parts. The third part of this dissertation introduces a novel technique of compacting dry fibrous reinforcement to control the resin flow rate, thus eliminating the void formation in VARTM parts. In this technique, the fibrous preform is compacted by either stationary or moving magnets prior to resin infusion. As a result, the pore size between the fabric layers and permeability are reduced, and the filling rate of resin into preform decreases. The results show that in the absence of magnetic pressure, the void content could be up to 5.7%, much higher than 0.1-0.8% voids in the laminates made by 0.2 MPa magnetic compaction. In addition, moving magnets with a smaller footprint over a larger vacuum bag surface is a feasible approach to apply compaction pressure on medium to large parts, thus dramatically decreasing their void content to below 1%

EFFECT OF AUTOCLAVE CURE PRESSURE ON MECHANICAL PROPERTIES AND VOID CHARACTERISTICS OF COMPOSITE LAMINATES

International audienceAutoclave curing is a commonly used fabrication process for high-performance structural composite laminates used in aerospace industry. During the manufacturing, a variety of process parameters such as the temperature and the pressure in the autoclave influence the formation of voids throughout the laminate. In particular, the magnitude of autoclave pressure determines the final fiber volume fraction, overall void content, and mechanical properties, including flexural strength and modulus. In this study, a number of composite laminates made of IM7/EX-1522, a carbon fiber reinforced epoxy prepreg, are produced by autoclave curing. The influence of different pressures on flexural properties of composite laminate is examined. In addition, void volume fraction as well as shape and size distributions of voids are presented. The experimental results have shown that increasing consolidation pressure during cure alone may not increase all the mechanical properties. Flexural modulus is found to be higher at higher consolidation pressure which is attributed to the higher fiber volume fraction. Unlike the flexural modulus, the flexural strength is significantly affected by the location, size, and shape of the voids. If the magnitude of cure pressure is not chosen properly, elongated voids may form at the fiber-matrix and could lead to considerable reduction of interfacial strength of the composites

Manufacturing silk/epoxy composite laminates : challenges and opportunities

Presented at the 34th International Conference of the Polymer Processing Society, May 24, 2018.Application of natural fibers in polymer composites has been gaining popularity in several industries pursuing environmentally friendly products. Among the natural fibers with proven potential applications, silk fibers have recently received considerable attention from researchers. Silk fibers provide higher mechanical properties compared to other commonly used natural fibers such as sisal, jute, and hemp. Silk may also exhibit comparable specific mechanical properties to glass fibers. However, silk composite laminates are rarely used in commercial products due to a number of fabrication challenges. This paper investigates such challenges for silk/epoxy laminates, especially issues related to manufacturing and preform architecture. First, challenges arising from preform architecture (i.e., random and woven preforms) are presented. Unlike glass fibers for which random mats are easier to manipulate, handling random silk preform proves to be more challenging, particularly compared to woven silk fabrics. The random silk/epoxy laminates show higher thickness variation and lower compaction, yielding lower fiber content. Second, fabrication of laminates by vacuum bag/wet lay-up and vacuum assisted resin transfer molding (VARTM) processes are presented. VARTM is found to be more appropriate for silk/epoxy laminate fabrication, as it allows a uniform impregnation of the silk preform, yielding higher part quality and limited void formation. Moreover, applying 0.21 MPa (30 psi) external pressure to the VARTM laminates allows to increase the fiber content of both random and woven silk/epoxy laminates from ~17 and ~30% to ~21 and ~33%, respectively. In contrast, wetting of silk preform during wet lay-up process, which is operator dependent, is difficult to achieve; and the produced laminates have high void content. Furthermore, SEM images show a weak silk/epoxy adhesion in laminates fabricated without external pressure. Finally, the mechanical performance of these laminates is assessed. The woven silk/epoxy laminates fabricated by pressurized VARTM exhibits the highest improvement in the specific flexural strength and modulus over pristine epoxy with 30 and 65% increase, respectively.YesPeer reviewed for the Proceedings of the 34th International Conference of the Polymer Processing Society, Taipei, Taiwan, May 21st-25th 2018

Fabrication of High Quality, Large Wet Lay-Up/Vacuum Bag Laminates by Sliding a Magnetic Tool

This study presents a novel method to fabricate high-quality, large composite parts which can be used in a wet lay-up/vacuum bag (WLVB) process. The new method utilizes a commercial lifting magnet, which is commonly used for transporting ferrous plates, to apply a magnetic consolidation pressure on the WLVB composite lay-up. The pressure is applied on a large area of the laminate by slowly sliding the magnet over the vacuum bag surface, which leads to an improved laminate quality. When further improvement is desirable, multiple passes of the magnet can be performed, where each pass successively compacts the lay-up. To explore the feasibility of implementing this technique, random mat and plain weave glass/epoxy laminates were fabricated, and their properties compared to conventional WLVB laminates. The effects of the number of moving passes of the lifting magnet on the laminate microstructure and properties are also investigated. As a result of multiple passes, the fiber volume fraction in random mat and plain weave laminates increases to 34% and 53%, representing 80% and 16% improvements, respectively. In addition, the void volume fraction reduces almost by 60% to a very low level of 0.7% and 1.1%, respectively. Consequently, the flexural properties considerably enhance by 20–81%, which demonstrates the potential of the proposed method to produce WLVB parts with substantially higher quality. It is also shown that there exists an optimal number of passes, depending on the fabric type where additional passes induce new voids as a result of excessive resin removal

Silk as a Natural Reinforcement: Processing and Properties of Silk/Epoxy Composite Laminates

With growing environmental awareness, natural fibers have recently received significant interest as reinforcement in polymer composites. Among natural fibers, silk can potentially be a natural alternative to glass fibers, as it possesses comparable specific mechanical properties. In order to investigate the processability and properties of silk reinforced composites, vacuum assisted resin transfer molding (VARTM) was used to manufacture composite laminates reinforced with woven silk preforms. Specific mechanical properties of silk/epoxy laminates were found to be anisotropic and comparable to those of glass/epoxy. Silk composites even exhibited a 23% improvement of specific flexural strength along the principal weave direction over the glass/epoxy laminate. Applying 300 kPa external pressure after resin infusion was found to improve the silk/epoxy interface, leading to a discernible increase in breaking energy and interlaminar shear strength. Moreover, the effect of fabric moisture on the laminate properties was investigated. Unlike glass mats, silk fabric was found to be prone to moisture absorption from the environment. Moisture presence in silk fabric prior to laminate fabrication yielded slower fill times and reduced mechanical properties. On average, 10% fabric moisture induced a 25% and 20% reduction in specific flexural strength and modulus, respectively