Microcellular PP-CaCO3 Nanocomposites: Relationship Between Nanocomposite Morphology and Foam Morphology

Polymeric nanocomposites exhibit high potential as a new material for carbon dioxide (CO2) foaming. In this paper, a polypropylene (PP)/nano-calcium carbonate (nano-CaCO3) composite was selected to investigate the relationship between nanocomposite morphology and foam morphology. Nanocomposites were prepared using a twin-screw extruder with screw including both shearing and mixing elements. Nanocomposites with different morphology via changing the nano-CaCO3 content were then foamed by using supercritical CO2 in a batch system. Effect of nano-CaCO3 content on the volume expansion ratio, and cell coalescence were studied.

Relaxation in Nanoscale Freestanding Gold Films Using MEMS

We present experimental results to describe the stress relaxation behavior of thin (125 nm) freestanding gold films at room temperature. The experiments were performed inside a field emission scanning microscope using a MEMS-based test bed which is only 3mm × 10mm in size. The effect of stress relaxation on the young’s modulus of gold thin films is observed. The thin film specimen used in the experiment is co-fabricated with the micromechanical loading device and hence eliminates problems of alignment and gripping. Freestanding thin films provide us with information about the mechanical behavior of thin films in the absence of substrate effects.

Dielectrostriction and Piezoresistance Response in Liquid Polymers

Variations of dielectric and resistive responses of a material with deformation are called dielectrostriction and piezoresistance respectively. Both phenomena have the same microscopic foundation — they involve the change of relative positions of polarizable or conductive species, leading to change in the material’s electric properties. Since both dielectrostriction and piezoresistance are determined by the pair distribution function of inclusions, these two phenomena are sensitive to a material’s microstructure, which renders them effective for monitoring liquid polymer and polymer suspension processing and mixing. In this study, a planar sensor is implemented to detect the dielectrostriction effect in shear flow of pure silicone elastomer and piezoresistance effect in silicone/graphite suspensions. In both measurements, the electric responses are found to be scaled with the flow-induced stresses, which constitute new approaches to study the rheological properties of bulk materials and suspensions and compliment each other for revealing the microstructure in various systems.

Elastic Properties of Composites Based on Tree Root Fibers

The in plane mechanical properties of composites based on fibres which shape is that of a tree root has been investigated. The number of fibrils as well as the angle between them have been shown to influence significantly the effective properties of the composite. It can be shown that there is an optimal number of fibrils and an optimal angle for which the stiffness of the material is maximum

Processing and Characterization of Hybrid Nanoparticle Infused Structural Fiber Composites

Effective conventional manufacturing techniques are required to integrate the nanomaterial configurations into material systems at a larger component and structural level to obtain the enhanced benefits offered by the material configurations at the nano length scale. A low cost manufacturing process based on vacuum assisted resin transfer molding (VARTM) is demonstrated for the effective processing of fiber composite laminates using modified epoxy resin systems dispersed with nano and sub-micron alumina oxide particles. The effect of alumina oxide particles on the thermo physical properties (glass transition temperature, etc), are studied via differential scanning calorimetry and thermal gravimetric analysis. Higher glass transition temperatures with the alumina oxide and other nano particulate systems provide an opportunity to use conventional resin systems in high temperature applications. Ultrasonic mixing is employed to uniformly disperse the particles into an epoxy resin system. The flow characteristics of the modified resin system are not significantly different than the neat resin system and allowed the use of traditional VARTM processes successfully. The details of the resin modification and current studies on particulate modification for better interfacial bond are discussed in this paper. Wear performance for reinforced plastics are also investigated in this paper. Composite laminates with S2 glass and modified resins are fabricated. The mechanical behavior of the fabricated composite laminates with the neat and modified resin system using different sized and loading of alumina oxide particles are presented and discussed.

Suppression of Edge Delamination Through Meso-Scale Structuring

We are developing a new class of fiber-reinforced polymer composite materials to facilitate imbedding multifunctional features and devices in material systems, and to manage interlaminar stresses at free edges and cut-outs. The idea is centered on introducing one more level of design space by composing plies with individual tiles possessing the same degrees of design freedom that are associated with individual plies. In this work, we have focused on tiling schemes that will allow blending of laminates (lay-ups), where a lay-up suitable for suppressing interlaminar stresses could be placed at necessary locations whereas another lay-up could be used for the main objective. This results in the introduction of matrix-rich tile-to-tile interface pockets in the blending region. Preliminary mechanical testing shows that uniaxially reinforced tiled composites attain stiffness levels near those of their traditional counterparts, yet with a potential degradation of strength. We used the finite element method to investigate the effects of resin-rich pocket size, the use of supporting continuous layers, tile size, and tile overlapping (interface stacking) schemes on stress distribution around interfaces in uniaxially reinforced tiled composites, with the aim to identify parameters controlling overall strength. We discovered that alignment of the resin-rich pockets through the thickness exacerbates stress-concentration and that outer continuous layers on the composite may help in better load transfer. As a first step in the application of this technique for the suppression of delamination at the free edges of holes in laminates, a bilaminate material was modeled, and the concept was shown to be effective in the suppression of edge delamination.

Intralaminar Reinforcement for Biomimetic Toughening of Bismaleimide Composites Using Nanostructured Materials

Many conventional composite materials are composed of multiple layers of continuous fiber reinforced resin produced by lamination of b-staged prepreg and subsequent cure. These materials exhibit very high strength and stiffness in the plane, dominated by the properties of the fibers. The Achilles heel of such composites is the interlaminar strength, which is dependent on the strength of the unreinforced resin, often leading to failure by delamination under load. Current methods for increasing the interlaminar shear strength of composites consist of inserting translaminar reinforcement fibers through the entire thickness of a laminated composite, such as z-pin technology developed by Foster-Miller [1]. While effective, this technique adds several processing steps, including ultrasonic insertion of the z-pins into the laminate, subsequently causing a significant cost increase to laminated composites. Described in this paper is a process utilizing single-walled carbon nanotubes (SWNTs) and vapor grown carbon nanofibers as reinforcing elements promoting interlaminar shear strength and toughness in carbon fiber/bismaleimide (BMI) resin composites. The resulting composites mimic the natural reinforcing mechanism utilized in insect cuticles. Three different methods of increasing the affinity of these carbon nanofillers for the BMI matrix were explored. The mechanical properties of these composites were assessed using end notch flexure testing. The results indicated that including nanofiller at the laminae interface could increase the interlaminar shear strength of carbon fiber/BMI composites by up to 58%. SEM micrographs revealed that the nanofiller successfully bridged the laminae of the composite, thus biomimicking the insect cuticle. Composite fabrication techniques developed on this program would have a wide variety of applications in space and aerospace structures including leading and trailing edges of aircraft wings.

Laser Scanning Vibrometer Studies of Electro-Active Papers

The paper presents the initial results on the performance of cellulose-based Electro-Active Papers (EAPap) as actuators. The electro-active papers depend on ion migration and dipole moment for the actuation. EAPap is fabricated by depositing gold electrodes on the top and bottom surfaces of cellophane sheets. Small sheet specimens measuring approximately 1 cm × 3 cm, suspended from the 1 cm width were tested in this study. The characteristics of EAPap are evaluated by applying electric fields at different exciting frequencies and humidity levels. The dynamic responses of the specimens were measured using a scanning Laser vibrometer. As expected, different plate modes are excited at different frequencies. Wide variations in the performance of the specimens were seen bat different levels of humidity. The results indicate the potential of EAPap for low power and lightweight actuation.

Damage Progression Identification in Woven Composites With Fiber Bragg Grating Sensors

In this study, measurements from low-impact velocity experiments including embedded and surface mounted optical fiber Bragg grating (FBG) sensors were used to obtain detailed information pertaining to damage progression in two-dimensional laminate woven composites. The woven composites were subjected to multiple strikes at 2m/s until perforation occurred, and the impactor position and acceleration were monitored throughout each event. From these measurements, we obtained dissipated energies and contact forces. The FBG sensors were embedded and surface mounted at different critical locations near penetration-induced damaged regions. These FBG sensors were used to obtain initial residual strains and axial and transverse strains that correspond to matrix cracking and delamination. The transmission and reflection spectra were continuously monitored throughout the loading cycles. They were used, in combination with the peak contact forces, to delineate repeatable sensor responses corresponding to material failure. From the FBG spectra, fiber and matrix damage were separated by an analysis based on signal intensity, the presence of cladding modes, and the behavior of individual Bragg peaks as a function of evolving and repeated impact loads. This provided an independent feedback on the integrity of the Bragg gratings. A comparison by number of impact strikes and dissipated energies corresponding to material perforation indicates that embedding these sensors did not affect the integrity of the woven systems and that these measurements can provide accurate failure strains.

Charged Droplets Emitted by Standing Capillary Waves in a Liquid Film: Part 1 — Theoretical Considerations

This paper presents a method of producing charged droplets using standing capillary waves in a thin liquid film on a vibrating surface. Capillary waves with wavelengths on the order of microns are set to reach a critical stable condition and an electric field is applied to extract charged droplets from the crest of unstable waves. This method is more efficient than the method based on stable Taylor cones working in the cone-jet mode in producing a large quantity of uniformly charged droplets. Theoretical analyses on the droplet diameter, charge-to-mass ratio and the total current emitted from capillary waves are also presented.