James C. Seferis

Nanoclay reinforcement effects on the cryogenic microcracking of carbon fiber/epoxy composites

The matrices of carbon fiber/epoxy composites were modified with layered inorganic clays and a traditional filler to determine the effects of particle reinforcement, both micro and nano scale, on the response of these materials to cryogenic cycling. The mechanical properties of the laminates studied were not significantly altered through nanoclay modification of the matrix. The incorporation of nanoclay reinforcement in the proper concentration resulted in laminates with microcrack densities lower than those seen in the unmodified or macro-reinforced materials as a response to cryogenic cycling. Lower nanoclay concentrations resulted in a relatively insignificant reduction in microcracking and higher concentrations displayed a traditional filler effect.

Prediction of Process-Induced Residual Stresses in Thermoplastic Composites

A model to predict the macroscopic in-plane residual stress state of semi crystalline thermoplastic composite laminates induced by process cooling is presented. Heat transfer during processing is based upon an incremental transient formulation that consists of a finite difference heat transfer analysis coupled to the crystallization kinetics. Micromechanics is used to evaluate the instantaneous spatial variation of mechanical prop erties as a function of temperature and degree of crystallinity. Residual stresses are based upon an incremental laminate theory that includes temperature gradients, shrinkage due to crystallization and thermal contraction. Temperature dependent relaxation times are used to model first order viscoelastic effects. A parametric study is conducted to explore the sensitivity of residual stresses to process history. Input parameters varied include surface temperature history (cooling rate), the amount of shrinkage caused by crystallization and the relaxation time at the reference temperature. The model predictions were in good agreement with experimental residual stress measurements for unidirectional graphite (AS4) reinforced polyetheretherketone (PEEK) laminates.

Characterization and exposure of polyetheretherketone (PEEK) to fluid environments

Abstract The sorption and desorption behaviour in fluids is described for polyetheretherketone (PEEK) films of different crystallinity. Water, Skydrol, and methylene chloride were used as fluid environments at two temperatures, and the weight gain of the films as a function of time was recorded. Density, thermomechanical, and dynamic mechanical measurements were made for the films before and after the fluid exposure. The results confirmed the general solvent resistance of PEEK. However, exposure to methylene chloride produced two significant effects: plasticization and additional crystallization for incompletely crystallized films. Exposure to different fluids was shown to suppress the exhibited thermomechanical transitions of these films. For the incompletely and fully crystallized films a comparison was made for crystallinity values obtained by the differential scanning calorimetric and density gradient techniques. Finally,dynamic mechanical and sorption in methylene chloride data obtained for PEEK reinforced with continuous carbon fibres are provided for comparison to the neat polymer results.

Matrix and fiber influences on the cryogenic microcracking of carbon fiber/epoxy composites

Abstract Cryogenic cycling effects on symmetric carbon fiber/epoxy laminates were examined using model prepreg systems. The properties of the composite materials studied were altered through the introduction of variations in their structure and composition. The curing agent used, matrix backbone flexibility, toughening agents, and longitudinal coefficient of thermal expansion of the reinforcing fibers were changed to investigate their role in cryogenic microcracking. Examination of the laminates after cycling provided insight into the mechanism and origins of thermal stress-induced microcracking. Matrix properties and fiber tensile modulus were shown to have a significant impact on the response of the composite materials to cryogenic cycling. It was found in this study that higher glass transition temperatures of the laminates and the presence of toughening agents in the matrix decreased the microcracking propensity of these laminates. Higher tensile moduli and linear coefficients of thermal expansion of the fibers were found to increase the microcrack density in the laminates studied.

Effect of reinforcement and solvent content on moisture absorption in epoxy composite materials

The effect of fibrous reinforcement and solvent content on moisture uptake in composite laminate was investigated. Two materials using identical epoxy resin systems but different reinforcements—glass vs. carbon fibers—and of different solvent content—low vs. normal— were examined. Samples were characterized in terms of water absorption and desorption. Mechanical and thermal properties including flexural modulus, flexural strength, and glass transition temperature were measured. Results clearly show the contribution of the fiber reinforcement as well as solvent content on the water absorption rate and mechanical property changes. q 2000 Elsevier Science Ltd. All rights reserved.

Characterization of epoxy-bismaleimide network matrices

This study investigated intercrosslinked networks formed by co-curing two thermosets: a tetrafunctional epoxy/amine and a bismaleimide formulation. The mechanical properties and phase behaviour of the intercrosslinked networks were compared with the corresponding neat epoxy and bismaleimide systems. In addition, the reaction scheme of the epoxy-bismaleimide resin mixtures was investigated using differential scanning calorimetry. A homogeneous structure for the networks was suggested by the results of both dynamic mechanical analysis and scanning electron microscopy. In contrast to most epoxy systems modified with rubbers or thermoplastics, phase separation was not observed in these intercrosslinked epoxy-bismaleimide networks.

Phase structure and toughening mechanism of a thermoplastic-modified aryl dicyanate

Abstract A high temperature thermosetting aryl dicyanate resin was modified with thermoplastic polysulfone and/or poly(ether imide) at various compositions. Differential scanning calorimetry was used to characterize the phase behaviour, while the morphologies and continuities of the thermoplastic/dicyanate networks were investigated by using dynamic mechanical analysis, scanning electron microscopy, fracture toughness testing, and solvent sorption studies. The phase structure was found to change from a thermosetting continuity to a co-continuity of both thermoplastic and thermosetting domains. The mechanical and physical properties of the modified polycyanates exhibited a step change at the shift of the phase structure.

Investigation of thermal degradation behavior of polymeric composites: prediction of thermal cycling effect from isothermal data

Abstract Composite degradation was investigated by measuring weight loss using quantitative methods. Weight loss experiments were performed on carbon fiber/epoxy laminates under both isothermal and dynamic conditions using various fiber directions and lay-up sequences in the laminate specimens. Weight loss rates were specimen geometry dependent, implying an anisotropic degradation behavior. A model was used to describe the sample weight loss at isothermal temperatures. The model was also used in combination with time–temperature superposition theory to predict the effect of thermal cycling on weight loss to understand the long-term properties of composites. It was found that the model successfully quantified the thermal degradation behavior observed under thermal cycling conditions.

The role of the polymeric matrix in the processing and structural properties of composite materials

Introductory Remarks.- The Role of the Matrix in Fibrous Composite Structures.- Composites in Commercial Aircraft.- Current Material Research Needs in Aircraft Structures.- Chemical and Environmental Effects.- Characterization of the Matrix Glass Transition in Carbon-Epoxy Laminates Using the CSD Test Geometry.- Network Structure Description and Analysis of Amine-Cured Epoxy Matrices.- The Time-Temperature-Transformation (TTT) State Diagram and Cure.- Control of Composite Cure Processes.- A Study of the Thermo-Oxidative Process and Stability of Graphite and Glass/PMR Polyimide Composites.- Rubber Modified Matrices.- A Preliminary Study of Composite Reaction Injection Molding.- Characterization of High Performance Composite Matrices.- Environmental Aging of Epoxy Composites.- Factors Affecting the Development of New Matrix Resins for Advanced Composites.- A Stress Transfer Model for the Deformation and Failure of Polymeric Matrices under Swelling Conditions.- Short Fiber Reinforcement Effects.- Constitutive Relationships for Sheet Molding Materials.- Effects of Matrix Characteristics in the Processing of Short Fiber Composites.- Time Dependent Properties of Injection Moulded Composites.- Viscoelastic Properties of Particulate Reinforced Matrices.- On Feasibility of Accelerated Creep Measurements in Some Polymeric Materials.- Fatigue Crack Propagation in Short-Glass-Fiber-Reinforced Nylon 66: Effect of Frequency.- Creep and Fracture Initiation in Fiber Reinforced Plastics.- Effect of Fiber Aspect Ratio on Ultimate Properties of Short-Fiber Composites.- An Optical Technique for Measuring Fiber Orientation in Short Fiber Composites.- Low Cost Energy Storage Flywheels from Structural Sheet Molding Compound.- Reinforced Polyester Structural Foam.- Dimensional Stability of Reinforced Matrices.- Interfacial Effects.- Definition of Interphase in Composites.- In Situ Analysis of the Interface.- Internal Stresses in Fibre Reinforced Plastics.- Stress and Strength Analysis in and Around Composite Inclusions in Polymer Matrices.- Continuous Fiber Reinforcements and Design.- Design of Continuous Fiber Composite Structures.- Compression Fatigue Behavior of Notched Composite Laminates.- Consideration on the Fatigue Damage of Specimens Used for Composite Critical Components Qualification.- Unresolved Stress Analysis Problems in Kevlar Composite Pressure Vessels.- Delamination in Graphite-Epoxy.- Industry-Academia and International Collaborative Research Efforts.- Finalized Researches on Polymeric Materials: Sub-project on Polymeric Materials of the Fine and Secondary Chemistry Research Program of the Italian National Research Council.- Industry-University Cooperation in Research.- Author Index.- Participants.

Interlayer toughening of resin transfer molding composites

In previous research it was shown that through using preformed elastomer particle modified tackifier/binder, materials interlayered RTM composite structures could be manufactured. These interlayers resulted in excellent toughness improvements of traditionally brittle RTM materials. In this work, the effects of tackifier application and composition were investigated by modifying spray and powder epoxy tackifiers with polyamide 6 particles. The spray tackifier provided 30% improvements in Mode II interlaminar fracture toughness, and slight increases in the interlaminar shear strength without reducing the thermal properties. The powder tackifier showed a slightly lower performance increase due to a less homogeneous laminate structure.