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Porosity in composite media using ultrasonic waves is characterized. The derivation of local approximations to the Kramers-Kronig relations are presented and it is shown that they may also be applicable to systems that could conceivably exhibit considerable dispersion such as composite laminates containing porosity.
Two complementary ultrasonic techniques for characterizing porosity in fiber-reinforced composite laminates are evaluated. Five uniaxial graphite-fiber/epoxy-matrix composites having a range of 1 to 8 percent volume fraction of solid glass inclusions to model porosity were investigated. In one technique, signal loss was measured in transmission mode and slope of attenuation, obtained from the first order coefficient of a two-parameter polynomial fit about the center frequency of the useful bandwidth, was used as the ultrasonic parameter to characterize the porosity. The results of these transmission mode measurements displayed a good correlation between the volume fraction of porosity and the slope of attenuation. Integrated polar backscatter was used as a second ultrasonic parameter for the characterization of the porosity in these samples. A single transducer insonified the samples and measured the resulting backscatter at a polar angle of 30 deg with respect to the normal of the sample surface with the azimuthal angles centered at 0 deg with respect to the fiber orientation (i.e., along the fibers). Integrated polar backscatter also displayed good correlation with the volume fraction of porosity.
This work aims to compute and analyze the mechanical properties of 3D woven composite materials that share the same yarn structure but differ in their matrix porosity and yarn's fiber volume fraction. Assuming linear elastic behavior, NASA's PuMA software was used to characterize the different composites from their constituents' data, averaging their elastic properties in three main directions to obtain their effective orthotropic mechanical properties. This was performed in three steps: firstly, the porous matrix phase was analyzed at the micro-scale; then, these results were used, along with the mechanical properties of the fibers, to model the yarns at the meso-scale; finally, results from the constituents were homogenized and used to model the mechanical behavior of the unit cell at the macro-scale. The 3D woven composite materials selected for this study are made of a mixture of phenolic resin with porosity values between 0 and 0.6, and infiltrated carbon fiber yarns with fiber volume fractions between 0.5 and 0.8. Results from this study show the effect of different matrix porosities and fiber volume fractions on the macro-scale mechanical properties, and were validated through theoretical and semi-empirical expressions found in the literature.
Production of highly porous composite materials by traditional materials processing is limited by difficult processing techniques. This work investigates the use of self propagating high temperature (combustion) synthesis (SHS) to create porous tricalcium phosphate (Ca3(PO4)2), TiB-Ti, and NiTi in low and microgravity. Combustion synthesis provides the ability to use set processing parameters to engineer the required porous structure suitable for bone repair or replacement. The processing parameters include green density, particle size, gasifying agents, composition, and gravity. The advantage of the TiB-Ti system is the high level of porosity achieved together with a modulus that can be controlled by both composition (TiB-Ti) and porosity. At the same time, NiTi exhibits shape memory properties. SHS of biomaterials allows the engineering of required porosity coupled with resorbtion properties and specific mechanical properties into the composite materials to allow for a better biomaterial.
Neutron radiation exposure is one of the main challenges faced during space missions. There is a critical need for advanced lightweight radiation shielding materials. Two-dimensional (2D) boron nitride nanoplatelets (BNNP) are excellent candidates for polymer matrix nanofillers due to their superior neutron shielding and thermal and mechanical properties. Furthermore, the 2D material anisotropic behavior unlocks the potential for composite property tailoring. This study fabricated ultra- lightweight lamellar BNNP foams (density 0.05 g cm –3 and 97.5% porous) via freeze-drying processing. The neutron shielding effectiveness or mass absorption coefficient of the BNNP foams with walls perpendicular to the direction of the radiation source was 14.47 cm 2 g –1 , while that of the foam with parallel configuration was only 8.51 cm 2 g –1 . The orientation-dependent neutron radiation shielding properties were modeled using the Beer-Lambert law for porous composite materials. The BNNP foam in this study has the potential to benefit advanced tailorable radiation shielding technologies for future aerospace missions.
The feasibility of using porous composite materials (Kevlar, Doweave, and Leno Weave) as lightweight, efficient laminar flow control (LFC) surface materials is compared to the metallic 319L stainless Dynapore surfaces and electron beam drilled composite surfaces. Areas investigated include: (1) selection of the LFC-suitable surface materials, structural materials, and fabrication techniques for the LFC aircraft skins; (2) aerodynamic static air flow test results in terms of pressure drop through the LFC panel and the corresponding effective porosity; (3) structural design definition and analyses of the panels, and (4) contamination effects on static drop and effective porosity. Conclusions are presented and discussed.
We describe a new mechanically strong lightweight porous composite material obtained by encapsulating the skeletal framework of amine-modified silica aerogels with polyurea. The conformal polymer coating preserves the mesoporous structure of the underlying silica framework and the thermal conductivity remains low at 0.041 plus or minus 0.001 W m(sup -1 K(sup -1). The potential of the new cross-linked silica aerogels for load-carrying applications was determined through characterization of their mechanical behavior under compression, three-point bending, and dynamic mechanical analysis (DMA). A primary glass transition temperature of 130 C was identified through DMA. At room temperature, results indicate a hyperfoam behavior where in compression cross-linked aerogels are linearly elastic under small strains (less than 4%) and then exhibit yield behavior (until 40% strain), followed by densification and inelastic hardening. At room temperature the compressive Young's modulus and the Poisson's ratio were determined to be 129 plus or minus 8 MPa and 0.18, respectively, while the strain at ultimate failure is 77% and the average specific compressive stress at ultimate failure is 3.89 x 10(exp 5) N m kg(sup -1). The specific flexural strength is 2.16 x 10(exp 4) N m kg(sup -1). Effects on the compressive behavior of strain rate and low temperature were also evaluated.
Results of an experimental investigation are presented in which the use of porous and microporous honeycomb composite materials is evaluated as an alternate to perforated solid plates for boundary-layer bleed in supersonic aircraft inlets. The terms "porous" and "microporous," respectively, refer to bleed orifice diameters roughly equal to and much less than the displacement thickness of the approach boundary-layer. A Baseline porous solid plate, two porous honeycomb, and three microporous honeycomb configurations are evaluated. The performance of the plates is characterized by the flow coefficient and relative change in boundary-layer profile parameters across the bleed region. The tests were conducted at Mach numbers of 1.27 and 1.98. The results show the porous honeycomb is not as efficient at removing mass compared to the baseline. The microporous plates were about equal to the baseline with one plate demonstrating a significantly higher efficiency. The microporous plates produced significantly fuller boundary-layer profiles downstream of the bleed region for a given mass flow removal rate than either the baseline or the porous honeycomb plates.
We have investigated visible photoluminescence (PL) from thin porous Si(1-x)Ge(x) alloy layers prepared by stain etching of molecular-beam-epitaxy-grown material. Seven samples with nominal Ge fraction x varying from 0.04 to 0.41 were studied at room temperature and 80 K. Samples of bulk stain etched Si and Ge were also investigated. The composition of the porous material was determined using X-ray photoemission spectroscopy and Rutherford backscattering techniques to be considerably more Ge-rich than the starting epitaxial layers. While the luminescence intensity drops significantly with the increasing Ge fraction, we observe no significant variation in the PL wavelength at room temperature. This is clearly in contradiction to the popular model based on quantum confinement in crystalline silicon which predicts that the PL energy should follow the bandgap variation of the starting material. However, our data are consistent with small active units containing only a few Si atoms that are responsible for the light emission. Such units are present in many compounds proposed in the literature as the cause of the visible PL in porous Si.
Composite molecular-sieve material has pore structure designed specifically for preferential adsorption of organic molecules for sizes ranging from 3 to 6 angstrom. Design based on principle that contaminant molecules become strongly bound to surface of adsorbent when size of contaminant molecules is nearly same as that of pores in adsorbent. Material used to remove small organic contaminant molecules from vacuum systems or from enclosed gaseous environments like closed-loop life-support systems.
Combustion Synthesis (self-Propagating high-temperature synthesis-(SHS)) of porous Ti-TiB(x), composite materials has been studied with respect to the sensitivity to the SHS reaction parameters of stoichiometry, green density, gasifying agents, ambient pressure, diluents and gravity. The main objective of this research program is to engineer the required porosity and mechanical properties into the composite materials to meet the requirements of a consumer, such as for the application of bone replacement materials. Gravity serves to restrict the gas expansion and the liquid movement during SHS reaction. As a result, gravitational forces affect the microstructure and properties of the SHS products. Reacting these SHS systems in low gravity in the KC-135 aircraft has extended the ability to form porous products. This paper will emphasize the effects of gravity (low g, 1g and 2g) on the SHS reaction process, and the microstructure and properties of the porous composite. Some of biomedical results are also discussed.
The development and optimization of composite materials designed for thermal protection of NASA’s spacecraft require understanding their physical response to high-enthalpy environments. To predict their macro-scale properties and behavior, high-fidelity 3D simulations are performed at the microscale on realistic representations of these composites. The digital microstructures are generated either synthetically or through X-ray micro-computed tomography reconstructions. One of the main challenges in the prediction of the structural response of heatshield materials is the computation of the effective elasticity of the fibrous composite, as well as the understanding of the deformation and stresses generated at the microscale. These are driven by the fiber layout within the microstructure and the distribution of the infused matrix. In this effort, the micro-mechanical linear elastic behavior of fibrous ablators is modeled using a numerical method based on the Multi-Point Stress Approximation (MPSA) finite volume scheme, a generalization of the more commonly used Multi-Point Flux Approximation (MPFA) that was presented at the 10th Ablation Workshop. To predict the behavior of fibrous and woven architectures, algorithms that compute the local fiber orientation are used. The implementation of the MPSA was verified using analytical solutions, engineering test cases, and compared against legacy Finite Element Analysis (FEA) software. The stress analysis models were then applied to real geometries used by NASA in thermal protection systems such as fibrous preforms and woven materials and the results were compared to experimental data.
Novel processing techniques, such as self-propagating high temperature synthesis (SHS), have the capability to rapidly produce advanced porous materials that are difficult to fabricate by other methods. This processing technique is also capable of near net shape synthesis, while variable gravity allows the manipulation of the structure and composition of the material. The creation of porous tricalcium phosphate (TCP) is advantageous in the biomaterials field, since it is both a biocompatible material and an osteoconductive material. Porous tricalcium phosphate produced via SHS is an excellent candidate for bone scaffold material in the bone regeneration process. The porosity allows for great vascularization and ingrowth of tissue. Titanium Carbide is a nonstoichiometric biocompatible material that can be incorporated into a TiC-Ti composite system using combustion synthesis. The TiC-Ti composite exhibits a wide range of mechanical and chemical properties. Both of these material systems (TCP and TiC-Ti) can be used to advantage in designing novel bone replacement materials. Gravity plays an important role in both the pore structure and the chemical uniformity of these composite systems and offers considerable potential in advanced bone engineering.
New process produces magnetic microspheres with controllable sizes, compositions, and properties for use in medical diagnostic tests, biological research, and chemical processes. Paramagnetic microspheres also made with process. Porous plastic microspheres prepared by polymerization of monomer in diluent by cross-linking agent. When diluent removed, it leaves tiny pores throughout polymerized spheres. Size and distribution of pores determined by amount and type of diluent and cross-linking agent.
A suitable interphase material for non-oxide ceramic-matrix composites must be resistant to oxidation. This means it must exhibit a slow rate of oxidation, and its oxidation product must be such as to ensure that the system survives oxidation when it does occur. Because the current benchmark interphase materials, carbon and boron nitride, lack these qualities, a porous fiber coating was developed to satisfy both the mechanical and oxidative requirements of an interphase for the SiC/SiC and SiC/Si2N4 composites that are of interest to NASA. This report presents the interphase microstructure achieved and the resulting characteristics of fiber push-out from a matrix of reaction-bonded silicon nitride (RBSN), both as-fabricated and after substantial annealing and oxidation treatments.
An improved method for markedly increasing the bond strength between a rigid, porous refractory material and non-rigid substrate by densifying the face of the rigid material opposing the substrate is discussed. Densification is accomplished by wetting the refractory material and then impregnating it with a composite slurry having a particle size to fill voids of the porous material.
A three-dimensional, porous anode material suitable for use in a lithium-ion cell. The three-dimensional, porous anode material includes active anode particles embedded within a carbon matrix. The porous structure of this novel anode material allows for the expansion and contraction of the anode without the anode delaminating or breaking apart, thus improving the life-cycle of the lithium-ion cell. An example of this three-dimensional porous anode material is a porous silicon-carbon composite formed using a bi-continuous micro-emulsion (BME) template.
Impact tests were performed on a series of ceramic plate specimens. Monolithic (unreinforced) and composite specimens with various fiber layups were tested to determine the effect that the fiber reinforcement has on impact damage initiation and dynamic response of the ceramic materials. Results show that a porous surface layer of Si3N4 on the composite specimens can enhance the energy absorbing capability of the composite specimens. The addition of SiC fiber reinforcement to the RBSN matrix material is also shown to significantly change the mode of failure and reduce the extent of damage due to impact.