Jandro L. Abot

Processing of clay/epoxy nanocomposites by shear mixing

Abstract A three-roll mill was used to disperse/exfoliate the clay nanoparticles in an epoxy matrix. The compounding process was carried out with varying clay contents (1–10 wt.%). The technique was found highly efficient and environmentally friendly in achieving high levels of exfoliation and dispersion within a short period of time.

Failure Modes of Composite Sandwich Beams

An investigation was conducted of failure modes and criteria for their occurrence in composite sandwich beams. The initiation of the various failure modes depends on the material properties of the constituents (facings and core), geometric dimensions and type of loading. The beams were made of unidirectional carbon/epoxy facings and aluminum honeycomb and PVC closed-cell foam cores. The constituent materials were fully characterized and in the case of the foam core, failure envelopes were developed for general two-dimensional states of stress. Sandwich beams were loaded under bending moment and shear and failure modes were observed and compared with analytical predictions. The failure modes investigated are face sheet compressive failure, adhesive bond failure, indentation failure, core failure andfacing wrinkling.

Fabrication, testing and analysis of composite sandwich beams

The objective of this work was to determine experimentally the flexural behavior of composite sandwich beams and compare the results with predictions of theoretical models. Sandwich beams were fabricated by bonding unidirectional carbon/epoxy face sheets (laminates) to aluminum honeycomb cores with an adhesive film. All constituent materials (composite laminates, adhesive and core) were characterized independently. Special techniques were developed to prevent premature failures under the loading pins and to ensure failure in the test section. Sandwich beams were tested under four-point and three-point bending. Strains to failure in the face sheets were recorded with strain gages, and beam deflections, and strains in the honeycomb core were recorded by using moire techniques. The beam face sheets exhibited a softening non-linearity on the compression side and a stiffening non-linearity on the tension side. Experimental results were in good agreement with predictions from simple models which assume the face sheets to behave like membranes, neglecting the contribution of the honeycomb core, and accounting for the non-linear behavior of the face sheets.

In-plane mechanical, thermal and viscoelastic properties of a satin fabric carbon/epoxy composite

The objective of this study was to determine the in-plane mechanical, viscoelastic and thermal properties of a satin fabric composite. This study was performed on a composite laminate material consisting of a satin weave carbon fabric impregnated with an amine cured epoxy resin. The in-plane quasi-static behavior including the failure modes under tension, compression and shear was analyzed and all mechanical properties including elastic moduli and strengths were determined. The viscoelastic properties including the glass transition temperature were determined as well as the coefficients of thermal expansion. These measured properties for the fabric composites were also correlated with their corresponding ones for the unidirectional composite with the same fiber and matrix.

Through-Thickness Mechanical Characterization of Woven Fabric Composites

The objective of this study was to investigate the interlaminar orthrough-thickness deformation and failure of woven fabric composites subjected toquasi-static loading and develop testing methods and specimens that would provideboth elastic properties and strengths for moderately thick composites. This study wasperformed on a thick composite consisting of a satin weave carbon fabricimpregnated with an amine-cured epoxy resin. All through-thickness mechanicalproperties under compressive, tensile, and shear loading with their correspondingfailure mechanisms were determined. These properties were also correlated with thecorresponding ones for a unidirectional composite having the same fiber and matrix.

Nonlinear behavior of composite sandwich beams in three-point bending

The load-deflection behavior of a composite sandwich beam in three-point bending was investigated. The beam was made of unidirectional carbon/epoxy facings and a polyvinyl chloride closed-cell foam core. The load-deflection curves were plotted up to the point of failure initiation. They consist of an initial linear part followed by a nonlinear portion. A nonlinear mechanics of materials analysis that accounts for the combined effect of the nonlinear behavior of the facings and core materials (material nonlinearity) and the large deflections of the beam (geometric nonlinearity) was developed. The theoretical predictions were in good agreement with the experimental results. It was found that the effect of material nonlinearity on the deflection of the beam is more pronounced for shear-dominated core failures in the case of short span lengths. It is due to the nonlinear shear stress-strain behavior of the core. For long span lengths, the observed nonlinearity is small and is attributed to the combined effect of the facings nonlinear stress-strain behavior and the large deflections of the beam.

Hygroscopic Behavior of Woven Fabric Carbon-Epoxy Composites

The hygroscopic behavior of a woven fabric carbon-epoxy composite and its effect on the viscoelastic properties and glass transition temperature was investigated. The mechanical and thermal properties of the material had been previously fully determined. An experimental study was conducted at full immersion in water and at a specific temperature condition. The moisture absorption process was found to be reversible with a low-saturation moisture uptake. The absorption through-the-thickness was determined to be lower than in the in-plane directions. The coefficients of moisture expansion or hygroelastic coefficients were determined and found to be similar in the warp and fill directions and much lower than through-the-thickness direction. The results were correlated to the behavior of a unidirectional composite with the same fiber and matrix. The viscoelastic properties were not affected during the process but the plasticization was very pronounced.

Processing of Clay/Epoxy Nanocomposites with a Three-Roll Mill Machine

Abstract : In the present study, a three-roll mill machine was used to disperse/exfoliate the nanoclay particles in an epoxy matrix. The compounding process was carried out with varying mixing time and concentrations of clay particles (1 to 10 wt%). It was found that the longer the mixing time, the higher the degree of intercalation. Mechanical properties, XRD and TEM were used to characterize the nanocomposites. Elastic modulus was found to increase with increasing clay content, however, the tensile strength was not found to vary accordingly. Compared to conventional direct and solution mixing techniques, the compounding of clay/epoxy nanocomposites by a three roll mill was found to be highly efficient in achieving higher levels of intercalation/exfoliation in a short period of time and also environmentally friendly.

Contact Law for Composite Sandwich Beams

In sandwich structures with soft foam core materials, the relationship between load and deflection under the load, also called contact law or indentation, plays a very important role. This work presents a combined experimental and analytical study of the contact behavior of sandwich beams with polyvinyl chloride (PVC) foam cores. The mechanical properties of the sandwich constituents were fully determined and sandwich beams, facesheet-foam and foam blocks were tested under quasi-static loading. These experimental results were used to model the load-deflection curve that includes a linear range followed by a nonlinear portion. The linear model was derived from the Winkler foundation theory and the nonlinear one was obtained by fitting experiments.

Deformation and Failure of Composite Sandwich Structures

An investigation was conducted of failure modes and criteria for their occurrence in composite sandwich columns and beams. The initiation of the various failure modes depends on the material properties of the constituents (facings and core), geometric dimensions and type of loading. The loading type or condition determines the state of stress throughout the sandwich structure, which controls the location and mode of failure. The appropriate failure criteria at any point of the structure account for the biaxiality or triaxiality of the state of stress. The specimens were made of unidirectional carbon-epoxy facings and aluminum honeycomb and PVC closed-cell foam cores. The constituent materials were fully characterized and, in the case of the foam core, failure envelopes were developed for general two-dimensional states of stress. Sandwich specimens were loaded under bending moment, shear and axial loading and failure modes were observed and compared with analytical predictions. The failure modes investigated are face sheet compressive failure, adhesive bond failure, indentation failure, core failure and facing wrinkling. The transition from one failure mode to another for varying loading or state of stress was discussed. Experimental results are compared with analytical predictions.