Fabricating High-Quality Structural Composite Laminates and Tailoring Their Surface Microstructure by Applying Magnetic Pressure and Magnetic Field

Advanced fiber reinforced polymer (FRP) composites are widely used in structural applications due to their high specific strength and stiffness. Generally, high-quality, advanced FRP composites are produced using an autoclave. However, the disadvantages associated with autoclave curing include high initial capital investment and operating costs. As a result, there is an increasing interest in the development of out-of-autoclave (OOA) techniques to produce composite parts with comparable properties to those obtained in an autoclave, but at a lower cost. In the first part of this dissertation, a novel technique, magnet assisted composite manufacturing (MACM), is developed to produce high-quality FRP composite laminates out of an autoclave. In this technique, high-temperature permanent magnets are utilized to apply sufficiently high consolidation pressure during cure of laminates. To establish MACM as a viable OOA method for producing structural composite laminates, the microstructure and properties of laminates fabricated by MACM are compared with those cured in an autoclave. The high flexural properties, fiber volume fraction of over 60% with less than 1% voids of the laminates fabricated by MACM revealed the potential of this process to be used as a lower-cost alternative to autoclave cure, without diminishing the quality of the part. Despite the favorable mechanical properties of FRP composite materials, their non-mechanical properties, e.g., thermal or electrical conductivities still is a major concern, limiting their application. It is well-known that incorporation of a third phase into structural composite laminates is quite effective in improving multiple non-mechanical properties. The second part of the dissertation involves the development of a novel cascaded suspension deposition method to introduce well-dispersed short fibers into the molded laminates, allowing the control of the surface properties of the resulting laminates. Towards this goal, the three-phase composite laminates are fabricated first by depositing short nickel coated carbon (NiC) fibers on a glass fabric surface by the proposed method and then followed by vacuum infusion. To demonstrate the effectiveness of this technique, the microstructure morphology of the deposited fabric and the resulting composite is quantitatively characterized. The findings suggest that with this technique, short fibers are distributed on the laminate surface with a uniform fiber volume fraction and excellent dispersion with random orientation. Controlling the orientation and alignment of the third phase in three-phase composite laminates is an effective approach for tailoring the anisotropy, and thus improving the functionalities of structural composites. The third part of the dissertation introduces a new magnetic‐field assisted composite processing method to induce alignment of short fibers on the surface of three-phase composite laminates. For this purpose, magnetic field generated by permanent magnets are utilized to align NiC fibers in three ways: (i) during deposition of fibers by cascaded suspension deposition, (ii) on the deposited fabric after mold filling in VARTM, and (iii) on the deposited fabric during deposition, mold filling, and after mold filling. The degree of alignment and the required field strength are evaluated as a function of NiC fiber length and nickel coating thickness. The results suggest that with the proposed method, it is possible to obtain an anisotropic distribution of fibers while maintaining the uniform fiber volume fraction and good dispersion throughout the surface of the laminate. Thus, the proposed method allows tailoring surface anisotropy, thus improving the required properties in the desired direction for high-performance and other applications of FRP composites

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