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

Process Induced Defects in Liquid Molding Processes of Composites

Liquid Composite Molding (LCM) processes are cost efficient manufacturing alternatives to traditional autoclave technology for producing near-net shape structural composite parts. However, process induced defects often limit wider usage of LCM in structural applications. Thorough knowledge of these defects, as well as their formation mechanisms and prevention techniques is essential in developing improved LCM processes. In this article, process induced defects in liquid molding processes of composites, categorized into preform, flow induced and cure induced defects, are reviewed. Preform defects are further presented as fiber misalignment and fiber undulation (waviness and wrinkling). The respective causes, detrimental effects, and possible prevention methods of these defects are presented. Thereafter, flow induced defects are classified as voids and dry spots. Dry spot formation mechanisms in LCM processes and available prevention techniques are summarized. In addition, void formation mechanisms, adverse effects on composite properties, and removal techniques are presented. Cure induced defects include microcracks, void growth and geometrical distortions (warpage and spring-in). Each of these defects are discussed along with their underlying causes as well as their control and reduction schemes.Ye

Rapid Microwave Polymerization of Porous Nanocomposites with Piezoresistive Sensing Function

In this paper, polydimethylsiloxane (PDMS) and multi-walled carbon nanotube (MWCNT) nanocomposites with piezoresistive sensing function were fabricated using microwave irradiation. The effects of precuring time on the mechanical and electrical properties of nanocomposites were investigated. The increased viscosity and possible nanofiller re-agglomeration during the precuring process caused decreased microwave absorption, resulting in extended curing times, and decreased porosity and electrical conductivity in the cured nanocomposites. The porosity generated during the microwave-curing process was investigated with a scanning electron microscope (SEM) and density measurements. Increased loadings of MWCNTs resulted in shortened curing times and an increased number of small well-dispersed closed-cell pores. The mechanical properties of the synthesized nanocomposites including stress–strain behaviors and Young’s Modulus were examined. Experimental results demonstrated that the synthesized nanocomposites with 2.5 wt. % MWCNTs achieved the highest piezoresistive sensitivity with an average gauge factor of 7.9 at 10% applied strain. The piezoresistive responses of these nanocomposites were characterized under compressive loads at various maximum strains, loading rates, and under viscoelastic stress relaxation conditions. The 2.5 wt. % nanocomposite was successfully used in an application as a skin-attachable compression sensor for human motion detection including squeezing a golf ball.This research received no external funding and The APC was funded by University Libraries Open Access fund. Open Access fees paid for in whole or in part by the University of Oklahoma Libraries.Ye

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

Prediction of moisture saturation levels for vinylester composite laminates : a data-driven approach for predicting the behavior of composite materials

Presented at the 34th International Conference of the Polymer Processing Society, May 24, 2018.This paper introduces a comprehensive, data-driven method to predict the properties of composite materials, such as thermo-mechanical properties, moisture saturation level, durability, or other such important behavior. The approach is based on applying data mining techniques to the collective knowledge in the materials field. In this article, first, a comprehensive database is compiled from published research articles. Second, the Random Forests algorithm is used to build a predictive model that explains the investigated material response based on a wide variety of material and process variables (of different data types). This advanced statistical learning approach has the potential to drastically enhance the design of composite materials by selecting appropriate constituents and process parameters in order to optimize the response for a specific application. This method is demonstrated by predicting the moisture saturation level for vinylester-based composite laminates. Using 90% of the available published data available as the training dataset, the Random Forests algorithm is used to develop a regression model for the moisture saturation level. Variables considered by the model include the manufacturing process, the fiber type and architecture, the fiber and void contents, the matrix filler type and content, as well as the conditioning environment and temperature. On this training data, the model proved to be a good fit with a prediction accuracy of R^2(training)=94.96%. When used to predict the moisture saturation level for the remaining unseen 10% of the compiled data, the model exhibited a prediction accuracy of R^2(test)=85.28%. Furthermore, the Random Forests model allows the assessment of the impact of the different variables on the moisture saturation level. The fiber type is found to be the most important determinant on the moisture saturation level in vinylester composite laminates.YesPeer reviewed for the proceedings of the 34t

Accurate characterization of moisture absorption in polymeric materials

The importance of using the exact solution of the hindered diffusion model is demonstrated on experimental data from a nanoclay/epoxy composite.Ye

Funnel-Shaped Floating Vessel Oil Skimmer with Joule Heating Sorption Functionality

This article belongs to the Special Issue Artificial Intelligence Enhanced Design of Polymer Materials and Manufacturing.Floating vessel-type oil collecting devices based on sorbent materials present potential solutions to oil spill cleanup that require a massive amount of sorbent material and manual labor. Additionally, continuous oil extraction from these devices presents opportunities for highly energy-efficient oil skimmers that use gravity as the oil/water separation mechanism. Herein, a sorbent-based oil skimmer (SOS) is developed with a novel funnel-shaped sorbent and vessel design for efficient and continuous extraction of various oils from the water surface. A carbon black (CB) embedded polydimethylsiloxane (PDMS) sponge material is characterized and used as the sorbent in the SOS. The nanocomposite sponge formulation is optimized for high reusability, hydrophobicity, and rapid oil absorption. Joule heating functionality of the sponge is also explored to rapidly absorb highly viscous oils that are a significant challenge for oil spill cleanup. The optimized sponge material with the highest porosity and 15 wt% CB loading is tested in the SOS for large-scale oil spill extraction tests and shows effective cleaning of oil spilled on the water surface. The SOS demonstrates a high maximum extraction rate of 200 mL/min for gasoline and maintains a high extraction rate performance upon reuse when the sponge funnel is cleaned and dried.Open Access fees paid for in whole or in part by the University of Oklahoma LibrariesYe

Effect of Injection Rate and Post-Fill Cure Pressure on Properties of Resin Transfer Molded Disks

The effects of flow rate andpost-fill cure pressure, i.e., packing pressure, on the mechanical properties of resin transfer molded disks are experimentally investigated. An experimental molding setup is constructed to fabricate fiber-reinforced, center-gated, disk-shaped composite parts. Disks are molded at different flow rates and packing pressures in order to observe the effects of these parameters on the mechanical properties andvoidcontent of the final parts. Specimens are cut from three different locations in the molded disks for testing. Specimens from the first two locations are tensile testedto obtain strength and stiffness properties, and the third location is usedfor microscopic analysis to determine void content and void properties. Increased injection rate is found to reduce both the strength and stiffness of the molded parts due to more voids induced by the faster moving fluidfront. Packing pressure is also foundto have a significant effect on specimen properties. At higher packing pressures fewer voids and improved strength andstiffness are observed. Mechanical properties are correlatedwith total void fraction for disks molded at different packing pressures. Exponential decrease in both tensile strength andelastic modulus is observedwith increasing voidfraction. Doubling the voidvolume fraction from 0.35 to 0.72% results in a 15% decrease in strength and a 14% decrease in stiffness. The results demonstrate that selection of suitable injection rate and addition of packing pressure to resin transfer molding (RTM) process can improve mechanical properties of molded parts considerably.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline

Modeling and Experimental Validation of Anomalous Moisture Absorption of Micro and Nanocomposite Laminates

Susceptibility of polymeric composites to moisture has been well known for several decades. Most high performance epoxy or bismaleimide (BMI) resins and their fiber-reinforced composites may absorb up to 5 wt% moisture which could lead to 10-30% reduction in various mechanical properties, including flexural strength, stiffness, impact resistance, and interlaminar shear strength (ILSS). In particular, fiber-matrix interface and process-induced defects such as microvoids often act as moisture storage sites, thus increasing the maximum intake level. It has been common practice to use a Fickian model to characterize the diffusion of moisture into polymeric composites. However, in several high-performance and mission critical applications, more sophisticated models accounting for the edge effects, anisotropy of absorption, molecular interactions, and interfacial storage are required to fully describe the long- and short-term moisture absorption dynamics. In this article, a model that combines the classical Fickian behavior and diffusion hindrance due to molecular bonding is used to explain anomalous absorption. The hindered diffusion model (HDM) is shown to predict both short-term Fickian and long-term anomalous, non-Fickian absorption behavior often observed in structural composites. The total amount of absorption is shown to be the sum of bound and unbound liquids, which are coupled through a differential diffusion and a temporal storage model. The accuracy of the model predictions is discussed by comparing the model predictions with the experimentally measured mass gain of graphite/epoxy laminates and clay/epoxy nanocomposites. It is shown that the anomalous moisture absorption dynamics observed in these laminates can be accurately predicted by the hindered diffusion model.YesPolymer Processing Society Asia/Australia Conference PPS-2016, October 11-14, 2016, Chengdu, Chin

Synthesis, Characterization, and Modeling of Aligned ZnO Nanowire-Enhanced Carbon-Fiber-Reinforced Composites

This paper presents the synthesis, characterization, and multiscale modeling of hybrid composites with enhanced interfacial properties consisting of aligned zinc oxide (ZnO) nanowires and continuous carbon fibers. The atomic layer deposition method was employed to uniformly synthesize nanoscale ZnO seeds on carbon fibers. Vertically aligned ZnO nanowires were grown from the deposited nanoscale seeds using the low-temperature hydrothermal method. Morphology and chemical compositions of ZnO nanowires were characterized to evaluate the quality of synthesized ZnO nanowires in hybrid fiber-reinforced composites. Single fiber fragmentation tests reveal that the interfacial shear strength (IFSS) in epoxy composites improved by 286%. Additionally, a multiscale modeling framework was developed to investigate the IFSS of hybrid composites with radially aligned ZnO nanowires. The cohesive zone model (CZM) was implemented to model the interface between fiber and matrix. The damage behavior of fiber was simulated using the ABAQUS user subroutine to define a material’s mechanical behavior (UMAT). Both experimental and analytical results indicate that the hierarchical carbon fibers enhanced by aligned ZnO nanowires are effective in improving the key mechanical properties of hybrid fiber-reinforced composites.Ye