Advanced composites using non-autoclave processes: manufacturing and characterization

The objective of the present study is to develop non-autoclave processes to manufacture high performance composites for aerospace applications. In Paper 1, vacuum assisted resin transfer molding (VARTM) process was developed for elevated temperature composites. Use of VARTM process for fabricating high temperature resins presents unique challenges such as high porosity and low fiber volume contents. Two different vacuum bagging methods: Seeman Composite Resin Infusion Molding Process (SCRIMP) and Double Vacuum Bagging Infusion (DVBI) process were evaluated. Flow simulation tool was used to predict key flow parameters needed for the successful infusion. In Paper 2, honeycomb sandwich panels were manufactured using commercially available film adhesive and modified VARTM process. The resin incursion into the core openings is a major challenge for applying VARTM process to open cell core sandwich composites. Panels manufactured using the developed process did not show any resin accumulation in the core. The mechanical performance of the manufactured sandwich composites was evaluated. Results indicate that the VARTM process can be successfully used to manufacture honeycomb composite sandwich structures using currently available barrier adhesive films. In Paper 3, a new generation vacuum-bag-only cure out-of-autoclave (OOA) manufacturing process was studied. Physical and mechanical performance of the composites was evaluated. The influence of size, lay-up configuration, thickness and their interactions on the impact behavior of the composites was studied using Design of Experiments (DoE) --Abstract, page iv

Manufacturing of high performance composites using vacuum assisted resin transfer molding

The objective of this research is to establish and enhance the existing Vacuum Assisted Resin Transfer Molding (VARTM) process to manufacture high performance composite parts usable in the aerospace industry. Flat, hat, pi and multi stiffened panels have been manufactured by the VARTM process using AS4-5HS and AS4-8HS carbon fabric, and SI-ZG-5A low viscosity epoxy resin. Silicone-based reusable molds have been designed and developed to manufacture stiffened panels. Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) experiments of the SI-ZG-5A resin system were conducted to determine the storage modulus, loss modulus, and the glass transition temperature. The density and fiber volume fraction of manufactured graphite/epoxy panels have been determined. A three dimensional mathematical model has been developed for flow simulation, and is implemented in the ABAQUS finite element package code. It is used to predict the resin flow front during the infusion process and to optimize the flow parameters. The flow simulation model was validated with experimental findings available in literature. Tensile tests on coupons were performed to determine the elastic constants required for finite element structural analysis. Hat-stiffened panels have been tested under transverse loading and the results were validated by the finite element simulation. A flow monitoring, experimental setup is being developed at UMR. Resin infusion of a flat plate was monitored experimentally using both the video camera and infrared camera --Abstract, page iii

Low Velocity Impact of Composites Manufactured Using Out-Of-Autoclave Process

Autoclaves have been commonly used to manufacture high performance composites for aerospace applications. However, high capital and tooling costs make these composites very expensive. Vacuum-bag-only cure out-of-autoclave (OOA) composite manufacturing process is potentially a lower-cost alternative to autoclave manufacturing. The OOA process does not require the positive pressure of an autoclave but still produces high quality composite parts. In the present study, high performance carbon/epoxy (MTM45-1/CF2412 carbon fabric) composite laminates have been manufactured using the OOA process. The low velocity impact response of the manufactured panels has been evaluated. Series of experiments based on Design of Experiments and Analysis of Variance (ANOVA) were designed and conducted to study the effect of varying the size of the test panel, lay-up configuration and thickness on the impact behavior of composites. Energy absorbed, peak force, contact duration, and maximum displacement were evaluated. Composites manufactured using OOA process had less than 0.25% void content. Impact energy versus time, contact force versus time, and contact force versus displacement plots were presented. Results showed that the amount of energy absorbed by the composites is significantly influenced by size, lay-up, thickness and the two-way interactions among the parameters. The peak force, contact duration and maximum displacement are mainly influenced by size, thickness and the interactions between size and thickness

Hybrid Composites Using Out-Of-Autoclave Process for Aerospace Sub-Structures

Carbon/epoxy composite parts are replacing traditional aluminum aerospace components. However, replacing aluminum parts with composites will require bonding/riveting of the composite parts to the aluminum structure. The coefficient of thermal expansion of carbon/epoxy composites is significantly lower than aluminum and directly bonding carbon/epoxy composites to aluminum will result is large residual thermal stresses which can lead to failure. To overcome this issue, glass/epoxy composite laminates are introduced in between the aluminum and carbon/epoxy to bridge the mismatch in thermal expansion. In the present study, hybrid carbon-glass/epoxy composite laminates have been fabricated using the Out-of-Autoclave (OOA) manufacturing process. OOA is a oven/vacuum bagging process in which prepregs are laid-up and vacuum bagged. The prepregs are then cured in an oven. The OOA process does not require external pressure which is typical to the traditional autoclave molding process and produces high quality composite parts. The hybrid panels are bonded to an aluminum substrate. The response of these panels to mechanical and thermal loads is studied. The test results obtained are compared with finite element simulation. The input material properties for the simulation are determined experimentally. The simulation results are in good agreement with experimental values. Results indicate that hybrid composites exhibit significant increase in failure strains and have lower thermal strains as compared to the carbon/epoxy composites

Failure Pressure Prediction of Composite Cylinders for Hydrogen Storage Using Thermo-Mechanical Analysis and Neural Network

Safe installation and operation of high-pressure composite cylinders for hydrogen storage are of primary concern. It is unavoidable for the cylinders to experience temperature variation and significant thermal input during service. The maximum failure pressure that the cylinder can sustain is affected due to the dependence of composite material properties on temperature and complexity of cylinder design. Most of the analysis reported for high-pressure composite cylinders is based on simplifying assumptions and does not account for complexities like thermo-mechanical behavior and temperature dependent material properties. In the present work, a comprehensive finite element simulation tool for the design of hydrogen storage cylinder system is developed. The structural response of the cylinder is analyzed using laminated shell theory accounting for transverse shear deformation and geometric nonlinearity. A composite failure model is used to evaluate the failure pressure under various thermo-mechanical loadings. A back-propagation neural network (NNk) model is developed to predict the maximum failure pressure using the analysis results. The failure pressures predicted from NNk model are compared with those from test cases. The developed NNk model is capable of predicting the failure pressure for any given loading condition

Manufacturing and Impact Characterization of Soy-Based Polyurethane Pultruded Composites

Composites have several advantages such as high corrosion resistance, high strength to weight ratio, and lower maintenance costs over conventional materials. the major cost drivers for composites are raw materials and manufacturing process. Automated manufacturing processes like pultrusion and low cost raw materials can significantly lower the cost of composites. Polyurethane (PU) resin systems are commonly used in the pultrusion industry as they have higher performance characteristics and manufacturing feasibility when compared to conventional resin systems such as polyester and vinyl ester. Manufacturing cost can be further decreased by the use of bio-based materials such as soy-based resin systems. in the present work, solid pultruded panels have been manufactured using the base PU and two soy-based PU resin systems. Pultruded panels were subjected to low velocity impact testing. Soy-based PU resin systems showed comparable properties to that of the base PU resin system and is a viable alternative to the conventional petroleum-based PU. POLYM. COMPOS

Manufacturing and Performance Evaluation of Soy-Based Nanocomposites

Epoxy-clay nanocomposites were synthesized using a soy-based epoxy resin, which was prepared by the process of transesterfication and epoxidation of regular food grade soybean oil. Nanoclay was dispersed into the soy-based epoxy resin using a high shear mixer and sonication. Tensile testing of the nanocomposites showed that the nanoclay improved the modulus and the strength by 625% and 340%, respectively. Exfoliation of the nanoclay was investigated by X-ray diffraction. The influence of the montmorillonite clay upon the curing efficiency of the epoxy-anhydride resin system was studied as a function of the clay concentration using differential scanning calorimetry. Rheological test were also conducted to find the suitability of using the soy-based epoxy clay nanocomposites toward common composite manufacturing applications. The soy-based nanocomposites hold great promise as environmentally friendly and low cost materials for structural applications

Synthesis and Performance Evaluation of Soybased Aliphatic Polyurethane Nanocomposites for Pultrusion

Polyurethane (PU) resin systems are generally characterized as aromatic and aliphatic. Aliphatic PU has lower mechanical properties than the aromatic resin system due to its chemical structure. The objective of the present work is to improve the mechanical properties of aliphatic resin system by exfoliating silicate nano particles. A pultrudable, soy-based, polyol-isocyanate aliphatic resin has been used as the base system. Nanocomposites were synthesized using the base resin and modified montmorillonite (MMT) clay. Modification of Na-MMT was confirmed by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy analyses. Increase in the basal spacing of the nanoclay was observed by wide angle X-ray diffraction. The curing mechanism of soy-based PU resin with 1wt% of triethanol amine/methyl iodide exchanged MMT clay was studied using differential scanning calorimetry as well as FTIR. Tensile testing of the nanocomposites showed improved modulus when compared to the neat aliphatic resin system. Soy-based nanocomposites hold great promise as environmentally friendly and low cost materials for structural and automotive applications

Evaluation of Honeycomb Composite Sandwich Structures Manufactured Using Vartm Process

In spite of numerous advantages of open-cell core sandwich composites, the applications have been limited due to the problems involved in manufacturing using low cost processes. Resin accumulation in the core is a major challenge in the fabrication of honeycomb sandwich panels using resin infusion techniques. Foam-filled cores and polymer film barriers are some of the methods used in the literature to address this issue. However, these techniques will increase the weight of the sandwich composites. In the present work, honeycomb sandwich panels were manufactured using commercially available film adhesive and modified vacuum assisted resin transfer molding (VARTM) process. The resin incursion into the core openings was investigated. No accumulation of resin was observed in the core. Flatwise tension, flatwise/edgewise compression, and three-point bending tests were conducted to evaluate the mechanical performance of the sandwich composites. The performance of sandwich panels during a low velocity impact event was also evaluated. Results indicate that the VARTM process can be successfully used to manufacture honeycomb composite sandwich structures using currently available barrier adhesive films