Brian S. Hayes

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.

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.

Scaling complications of dual temperature cure resin prepreg systems in airplane part manufacture

Carbon and glass fabric epoxy prepreg with a highly accelerated curing system were compared and used to build a Boeing 737 flap hinge fairing production part. The prepreg materials, uncured and cured, were analyzed to determine physical and mechanical properties. Initial evaluations of the carbon prepreg indicated unique and desirable characteristics including dual temperature cure and excellent handling characteristics. Therefore, this material was selected for the use in the development of a production part. However, the part selected for manufacture, a Boeing 737 flap hinge fairing, required the use of both carbon and glass fabric prepregs. Therefore, the resin system, which was developed for carbon prepreg, had to also be impregnated into glass fabric. After cure of the part, it was found that areas on the part were scorched including the bagging material. This scorching led to an investigation of the exotherm produced by the accelerated resin in the cure of the carbon and glass prepreg materials. Two sets of test panels were made to simulate optimal and poor processing conditions for the glass and carbon fiber fabric prepregs. Thermocouples were placed in the layers of prepreg to track the temperature effects during the curing process. It was found that the glass fabric prepreg did not conduct the heat produced by the curing exotherm as well as the carbon material and therefore was the cause of material degradation or charring of the components in the composite system.

Cryogenic Microcracking of Carbon Fiber/Epoxy Composites: Influences of Fiber-Matrix Adhesion

The impact of fiber-matrix adhesion on the transverse microcracking of fiber reinforced polymeric materials thermally cycled at cryogenic temperatures was investigated using symmetric cross-ply carbon fiber/epoxy laminates containing fibers with different surface treatments. Past research explored the role of fiber-matrix adhesion in determining the room temperature properties of composite materials, but this work is original in that it examined how fiber-matrix adhesion affected the behavior of composite materials at cryogenic temperatures. Three fiber surfaces were used: Unsized but exposed to an oxidative surface treatment, epoxy sized, and surfactant sized. Modifications of the fiber surfaces changed the adhesion of the matrix to the fibers as determined by interlaminar shear strength and dynamic mechanical analysis. The extent of microcracking in the laminates exhibited a dependence on fiber-matrix adhesion, with high levels of adhesion corresponding to decreased microcracking.

Examination of interphase thermal property variance in glass fiber composites

In an effort to understand the implications of the fiber/matrix interphase on the performance of aerospace grade composite materials, a study was performed to examine the properties of the interphase region on the micro-scale. An epoxy resin was cured with several curing agent systems to evaluate the material based variations in experimental detection of glass transitions with scanning thermal microscopy (SThM). After an appropriate material was selected, glass fibers with different finishes were impregnated with an epoxy/amine resin system, and the properties of the interphase regions were examined using scanning thermal microscopy.

Influence of Particle Size Distribution of Preformed Rubber on the Structure and Properties of Composite Systems

Four different size distributions of preformed rubber particles were used to interlayer toughening carbon fiber composites. Prepregs were made using single pass impregnation with model epoxy resins containing the different preformed particle distributions at two concentrations. The particle distributions and average particle sizes were selected to investigate the effect on laminate structure and fracture toughness. It was found that the average particle size affected the interlayer formation in the cured laminate. A significant difference in the fracture toughness of the laminates was found due to a change in the particle size distribution, average particle size, concentration, and resultant laminate structures. In this study, it is shown that the particle size distribution must be taken into account when designing interlayer toughened composites so that mode I and mode II toughness can be optimized.

The effect of fabric tension and the number of impregnation rollers on woven fabric prepreg quality and cured laminates

The effects of fabric tension and number of impregnation rollers used during hot-melt prepreg processing were investigated as they relate to woven fabric prepreg and final composite part quality. Specifically, the processing parameters varied in a design of experiments (DOE) were applied fabric tension and the number of impregnation rollers, while maintaining the same impregnation force. Six experimental prepregs were characterized in terms of prepreg thickness, resin content, tack, and morphology. The results show that fabric tension had a large influence on the prepreg characteristics due to more cylindrical tow shapes and a ridged fiber bed. The number of impregnation rollers was found to affect only the characteristics of the prepregs manufactured with no tension. To investigate the effects of tension on cured composites, six ply laminates were made with each experimental prepreg. Two cure cycles, differing only in consolidation pressures, were used to examine the void content and morphology in the cured laminates. The laminates made with prepregs manufactured with high tension and cured only under vacuum had a greater void content than laminates made with prepregs manufactured with no tension. In both cure cycles, the tow shape in the laminates made from the prepreg manufactured with high tension remained in almost the same shape as in the uncured prepreg. The void content, however, was negligible for all laminates cured at higher consolidation pressure.

Variable Density Composite Systems Constructed by Metal Particle Modified Prepregs

Recently, new product designs have been suggested where it is necessary to vary the density of a composite part. In an earlier work, it was shown that the specific gravity of a cured laminate based on carbon fiber prepreg could be tailored to these applications by modifying the polymeric matrix with fine metal powders of various densities. This follow up study focused on altering the density of composites by changing the concentration of iron particles as a matrix modifier in prepreg materials. Experiments were performed to determine prepreg tack, laminate morphology and laminate fracture toughness. It was found that increasing the density of the materials with different concentrations of iron particles did not adversely effect toughness or interlaminar shear strength, although prepreg handling characteristics suffered slightly.

Influence of Polymer Specimen Structure on The Reproducibility of Micro-thermomechanical Transitions

Glass transitions of amorphous polystyrenes with low polydispersity were evaluated using the modulated Local Thermal Analysis mode of the TA Instruments 2990 µ TA and evaluating the thermomechanical signal. Transition temperature variance and fraction of transitions measured were compared for high molecular mass thermosetting materials and the melt of Nylon 6.6. The transition reproducibility was found to decrease as the molecular size of the polymer samples increased. Reproducibility also decreased for thermosetting materials when the experimental ramp rate was decreased. Heat transfer within the specimen was evaluated using finite element analysis, allowing scaling of microscale experimental results for comparison to bulk transitions.

SCALED ANALYSIS OF COMPOSITE ADHESIVE INTERPHASE PROPERTIES

High-performance film adhesives are often employed in aerospace composite structures to provide adequate adhesion between substrates. This study was performed to evaluate the fracture properties of a commercial film adhesive and a carbon fiber prepreg material in both co-cured and bonded applications. In addition, the fundamental reasons for macroscopic fracture properties were evaluated on the microscale using interphase analysis and surface analysis techniques. From this information, it was possible to examine the commingling of prepreg and adhesive resins in the bond line interphase region. The information presented in this study points to a critical aspect of material selection and utilization that is often not considered in composite design: resin compatibility. *Dedicated to Prof. Francisco J. Baltá Calleja on the occasion of his 65th birthday.