ASME IMECE2003 -43837 FORMATION OF MICROSCOPIC VOIDS IN RESIN TRANSFER MOLDED COMPOSITES

ABSTRACT Performance of composite materials usually suffers from process-induced defects such as dry spots or microscopic voids. While effects of void content in molded composites have been studied extensively, knowledge of void morphology and spatial distribution of voids in composites manufactured by resin transfer molding (RTM) remains limited. In this study, through-the-thickness void distribution for a diskshaped, E-glass/epoxy composite part manufactured by resin transfer molding is investigated. Microscopic image analysis is conducted through-the-thickness of a radial sample obtained from the molded composite disk. Voids are primarily found to concentrate within or adjacent to the fiber preforms. More than 93% of the voids are observed within the preform or in a so-called transition zone, next to a fibrous region. In addition, void content was found to fluctuate through-the-thickness of the composite. Variation up to 17% of the average void content of 2.15% is observed through-the-thicknesses of the eight layers studied. Microscopic analysis revealed that average size of voids near the mold surfaces is slightly larger than those located at the interior of the composite. In addition, average size of voids that are located within the fiber preform is observed to be smaller than those located in other regions of the composite. Finally, proximity to the surface is found to have no apparent effect on shape of voids within the composite

Effect of Preform Thickness and Volume Fraction on Injection Pressure and Mechanical Properties of Resin Transfer Molded Composites

An experimental study is performed to characterize the effect of the thickness of random preforms on injection pressure and mechanical properties of resin transfer molded (RTM) parts. Center-gated, disk-shaped parts are molded using two different chopped-strand glass fiber preforms. Both preforms have random microstructure but different planar densities (i.e., different uncompressed layer thicknesses). Tensile strength, short-beam shear strength, and elastic modulus are measured for parts molded with each preform type at three different fiber volume fractions of 6.84, 15.55, and 24.83%. Although mechanical properties are found to increase linearly with volume fraction, significant difference is not observed between disks containing thick and thin mats at equivalent fiber volume fraction.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline

Performance of Glass Woven Fabric Composites with Admicellar-Coated Thin Elastomeric Interphase

Adequate stress transfer between the inorganic reinforcement and surrounding polymeric matrix is essential for achieving enhanced structural integrity and extended lifetime performance of fiber-reinforced composites. The insertion of an elastomeric interlayer helps increase the stress-transfer capabilities across the fiber/matrix interface and considerably reduces crack initiation phenomena at the fiber ends. In this study, admicellar polymerization is used to modify the fiber/matrix interface in glass woven fabric composites by forming thickness-controlled poly(styrene-co-isoprene) coatings. These admicellar interphases have distinct characteristics (e.g., topology and surface coverage) depending on the surfactant/monomer (S/M) ratios used during the polymerization reaction. Overall, the admicellar coatings have a positive effect on the mechanical response of resin transfer molded (RTM), E-glass/epoxy parts. For instance, ultimate tensile strength (UTS) of composites with admicellar sizings improved 50 to 55% over the control desized samples. Interlaminar shear strength (ILSS) also showed increases ranging from 18 to 38% over the same control group. Interestingly, the flexural properties of these composites proved sensitive to the type of interphase formed for various admicellar polymerization conditions. Higher surface coverage and film connectedness in admicellar polymeric sizings are observed to enhance stress transfer at the interfacial region.Ye

Calorimetric and Rheological Measurements of Three Commercial Thermosetting Prepreg Epoxies

The cure kinetics of three different thermosetting resins are investigated using differential scanning calorimetry and oscillatory shear rheometry. For the latter, two different types of plates are used, smooth plates and grooved plates; the latter are used to improve sample–plate contact. In addition, oscillatory compression rheology is used; however, machine compliance prevents accurate measurements at high conversions. A fractional conversion is defined based on the maximum storage modulus achieved at a given temperature, and is compared to the fractional conversion calculated from enthalpy measurements. As expected, the rates of reaction derived from these fractional conversions are very different for calorimetry and rheometry. However, the rates of reaction using the two types of plates are identical, although the grooved plates give much more reproducible storage moduli. A number of previously used mathematical expressions are employed to fit the calorimetric and rheological data, and the activation energies calculated from these fits are compared.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline

Effect of Nanoclay Content on Void Morphology in Resin Transfer Molded Composites

Effects of nanoclay content on morphology and spatial distribution of voids in resin transfer molded nanoclay/E-glass/epoxy composite disks are investigated. Closite®25A nanoclay loads of 2, 5, and 10wt% are mixed by sonication with a low-viscosity epoxy resin prior to filling the mold cavity containing 13.6% E-glass preform by volume. A disk without nanoclay is also molded. Once the molded composites are cured, voids on radial composite samples are evaluated via microscopic image analysis. The addition of nanoclay is found to result in a significant increase in the apparent viscosity of the clay-epoxy mixture, thus increasing the molding pressure. Void occurrence is observed to increase considerably with increasing nanoclay content, from 2.1% in the composite without nanoclay to 5.1 and 8.3% in the composites molded with 5 and 10wt% nanoclay, respectively. However, the composite with 2wt% nanoclay yields the lowest void content of 0.7%. Voids are observed to be, in average, smaller after the addition of nanoclay at all nanoclay concentrations. Presence of nanoclay in the impregnating resin induces at least 60% reduction in voids located inside fiber tows, which are trapped by the fluid front motion during impregnation. Irregularly shaped voids are also observed to decrease with increasing nanoclay content. A nonuniform void content and morphology is observed radially, which seems to be affected by the flow kinematics as well as possible breakdown and filtration of clay clusters.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline

Filtration and Breakdown of Clay Clusters during Resin Transfer Molding of Nanoclay/Glass/Epoxy Composites

Dispersion of nanoclay clusters during resin transfer molding of nanoclay/glass/epoxy disks is investigated. In addition to a center-gated disk containing only 14% glass fibers, three nanocomposite disks are fabricated with the addition of 2, 5 or 10 wt% Cloisite® 25A nanoclay. The spatial distribution of nanoclay clusters along the radial axis of the nanocomposite disks are characterized at two length scales. Clusters larger than 1.5 μm are characterized by performing image analysis on the SEM micrographs whereas smaller nanoclay clusters are identified by wavelength dispersive spectrometry. Results obtained from image analysis indicate that nanoclay clusters are filtered out by as much as 50% in the flow direction by the glass fiber preforms. In addition, increasing nanoclay content led to higher filtration, suggesting that cluster formation is more prominent at higher nanoclay loadings. Cluster size distribution analyses revealed that the outer edges of the disks, on average, contain finer nanoclay particles. For instance, the outer edge of the nanocomposite with 2% clay contains 22% more small nanoclay clusters compared to center of the disk. Glass transition temperature, Tg, of four specimens obtained from each molded disks is characterized under oscillatory shear. Glass transition temperature of the samples are shown to increase with the nanoclay content, yielding a 40% higher Tg at 10% nanoclay loading compared to glass/epoxy composite without clay. Increasing glass transition temperature with increasing nanoclay content may be an indication of intercalation of nanoclay within the epoxy matrix.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline

Remote training as a common tool for the different professionals involved in the acute phase after terror attacks across Europe:Perspectives from an expert panel

The acute response after a terror attack may have a crucial impact on the physical and psychological wellbeing of the victims. Preparedness of the professionals involved in the acute response is a key element to ensure effective interventions, and can be improved through trainings. Today in Europe there is a recognized lack of inter-professional and international trainings, which are important, among others, to respond to the needs and the rights of victims affected by a terrorist attack in another country than their home country. In this paper we report the perspectives of an expert panel composed by different categories of professionals on the possible role of interprofessional trainings provided remotely. The experts discussed the pertinence of remote trainings for professionals involved in the acute response of a terror attack, and highlighted their Strengths, Weaknesses, Opportunities and Threats (SWOT analysis). We concluded that, while remote trainings cannot replace in-person trainings, they may be useful to share knowledge about the role and the organization of the different categories of professionals, thus potentially improving response coordination, and to easily share good practices across professionals and countries

Effects of Supersymmetric Threshold Corrections on High-Scale Flavor Textures

Integration of superpartners out of the spectrum induces potentially large contributions to Yukawa couplings. These corrections, the supersymmetric threshold corrections, therefore influence the CKM matrix prediction in a non-trivial way. We study effects of threshold corrections on high-scale flavor structures specified at the gauge coupling unification scale in supersymmetry. In our analysis, we first consider high-scale Yukawa textures which qualify phenomenologically viable at tree level, and find that they get completely disqualified after incorporating the threshold corrections. Next, we consider Yukawa couplings, such as those with five texture zeroes, which are incapable of explaining flavor-changing proceses. Incorporation of threshold corrections, however, makes them phenomenologically viable textures. Therefore, supersymmetric threshold corrections are found to leave observable impact on Yukawa couplings of quarks, and any confrontation of high-scale textures with experiments at the weak scale must take into account such corrections.Comment: 25 pages, submitted to JHE

Porosity Reduction in the High-Speed Processing of Glass-Fiber Composites by Resin Transfer Molding (RTM)

High-speed processing is essential to achieve lower production cost in the fabrication of fiber-reinforced composites with the current liquid molding practices. A major consequence of increasing the resin injection velocity is the formation of defects such as voids and dry regions that decrease the load-bearing capability of the composite. Void formation mechanisms and analytical predictions of the detrimental effect of porosity on the structural integrity of molded parts have been studied extensively. In contrast, knowledge of void removal strategies is very limited. In this investigation, various postfill pressure levels were applied to disk-shaped random-mat glass/epoxy parts molded at high volumetric flow rates as a method to reduce their voidage content. Quantitative image analysis over cross-sections cut from these composites revealed that significant changes in porosity concentration take place with the postfill pressure. For instance, overall void content dropped more than 70% with the application of a postfill pressure as low as 300 kPa. Other important void morphometry characteristics such as void shape, size, and spatial distribution could also be manipulated by this method. As the packing pressure increases, large voids gradually disappear, and at the same time, the small circular voids are mobilized towards radial locations near the vents. In addition to this spatial voidage gradient in the radial direction, voidage gradient also exists through the specimen thickness. It seems that higher front velocities promote the appearance of secondary flow phenomena inside the mold cavity (e.g. microfountain flow), which may explain why more voids tend to concentrate at the surface of the specimen irrespective of the postfill pressure level reached inside the mold.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline