State-of-the-art review of fabrication, application, and mechanical properties of functionally graded porous nanocomposite materials

Functionally graded porous (FGP) nanocomposites are the most promising materials among the manufacturing and materials sector due to their adjustable physical, mechanical, and operational properties for distinctive engineering applications for maximized efficiency. Therefore, investigating the underlying physical and materialistic phenomena of such materials is vital. This research was conducted to analyze the preparation, fabrication, applications, and elastic properties of functionally graded materials (FGMs). The research investigated for both porous and nonporous synthesis, preparation, and manufacturing methods for ceramics, metallic, and polymeric nanocomposites in the first section, which is followed by deep research of the development of elastic properties of the above-mentioned materials. Main nano-reinforcing agents used in FGMs to improve elastic properties were found to be graphene platelets, carbon nanotubes, and carbon nanofibers. In addition, research studied the impact of nano-reinforcing agent on the elastic properties of the FGMs. Shape, size, composition, and distribution of nano-reinforcing agents were analyzed and classified. Furthermore, the research concentrated on modeling of FGP nanocomposites. Extensive mathematical, numerical, and computational modeling were analyzed and classified for different engineering analysis types including buckling, thermal, vibrational, thermoelasticity, static, and dynamic bending. Finally, manufacturing and design methods regarding different materials were summarized. The most common results found in this study are that the addition of reinforcement units to any type of porous and nonporous nanocomposites significantly increases materialistic and material properties. To extend, compressive and tensile stresses, buckling, vibrational, elastic, acoustical, energy absorption, and stress distribution endurance are considerably enhanced when reinforcing is applied to porous and nonporous nanocomposite assemblies. Ultimately, the review concluded that the parameters such as shape, size, composition, and distribution of the reinforcing units are vital in terms of determining the final mechanical and materialistic properties of nanocomposites.

Active Acoustic Metamaterials With Programmable Densities Using an H-∞ Controller

Various types of acoustic metamaterials have been developed to control the flow of acoustical energy through these materials. Most of these metamaterials are passive in nature with pre-tuned and fixed material properties. In this paper, the emphasis is placed on the development of a class of one-dimensional acoustic metamaterials with programmable densities in order to enable the control the acoustic wave propagation in these media. With such unique capabilities, the proposed active acoustic metamaterials (AAMM) can be utilized to physically realize, for example, acoustic cloaks, wave shifters and focusers, tunable acoustic absorbers and reflectors, as well as non-reciprocal acoustic media. The theoretical analysis of this class of AAMM with programmable effective dynamical densities is presented for an array of cavities separated by piezoelectric boundaries. These boundaries provide means for controlling the stiffness of the individual cavity and, in turn, its dynamical densities. In this regard, a disturbance rejection strategy is considered which is based on an H-∞ robust controller. The time and frequency response characteristics of a unit cell of the AAMM are investigated for various parameters of the controller in an attempt to optimize the performance characteristics. Extension of this study to include active control capabilities of the bulk modulus of the metamaterials would enable the development of wide classes of AAMM that are only limited by our imagination.

Development and Testing of a Kilohertz Solar-to-Acoustic Energy Converter

A thermal-to-acoustic energy converter (TAC) was developed and tested to produce sound waves in the kilohertz range directly from solar energy. The converter consisted of a glass window and a small amount of steel wool in the shape of a disk sealed in an aluminum housing. A Fresnel lens and a chopper wheel with 60 holes in it were employed to generate a pulsed sunbeam of approximately 200 sun intensity as the heat source of the TAC. Various designs and techniques were tested to improve the sound amplitude and signal-to-noise ratio of the converter at high frequencies. Reduction in air volume, better cooling, and improvement in air tightness were found to be effective in enhancing the sound amplitude. A shockproof mount commonly used in radio studios to reduce microphone vibration was essential in noise reduction for the TAC at high chopper wheel rotations. The sound amplitude was found to rapidly decrease with the increase in pulse frequency of the sunbeam at low frequencies. The relationship between the decibel value and frequency of the generated sound waves was changed to linear for sunbeam frequencies above 1 kHz. This is the frequency at which the penetration of surface temperature fluctuations into the aluminum housing becomes comparable with the aluminum housing thickness. At a given frequency, the sound amplitude increased almost exponentially with the increase in solar flux intensity. To the best of our knowledge, the 3 kHz sound frequency measured in our experiments is by far the highest frequency produced by a solar-to-acoustical energy converter.

Qualification of Duct Resonator Array for Noise Reduction in Offshore Installations

A compressor noise reducing device was investigated in connection with internal qualification of the technology for use in offshore installations in the Norwegian Continental Shelf. The acoustic performance of two duct resonator arrays applied to the inlet and discharge pipes as spool pieces was studied by means of sound insertion loss measurements. This array configuration minimizes the acoustical energy propagating to the inlet/discharge pipes and can be an optimal solution in modification cases where it is not feasible to mount the device inside the compressor. Laboratory verification measurements demonstrated that the duct resonator array reduces 14–18 dB the noise level at designed frequency (2–3 kHz). Correlation between insertion loss and effective length and a comparative study of the arrays installed in series or separated 90° with a bend were additionally performed. The investigation showed that this technology allows further flexibility of implementation often required in piping layouts with limited space.

Development and Application of Piezoelectric Materials for Ultrasound Generation and Detection

The piezoelectric effect and its converse are the primary means used in biomedical ultrasound for converting acoustical energy into electrical energy and vice versa. Piezoelectricity has found many bioengineering applications ranging from ultrasound imaging and therapeutics, to piezoelectric surgery and microelectromechanical systems, and to biomedical implants with associated energy harvesting. Because of its fundamental importance to the proper functioning of most medical ultrasound systems, it is important to gain a general understanding of the effect, the history of its development and from this, an appreciation of its limitations and advantages in the generation and detection of ultrasound. This article describes the historical evolvement associated with its use in relation to most medical ultrasound applications and is intended to serve as an introduction for non-expert readers.

Acoustic Sensitivity Analysis Using the Distributed Source Energy Boundary Point Method

The determination of the sensitivity of the acoustical characteristics of vibrating systems with respect to the variation of the design parameters can provide a method to low-noise design of mechanical structure objectively and quantitatively. Using the Distributed source energy boundary point method, the expressions of the change of the acoustical energy density with respect to design variable is presented in this paper. The Distributed source energy boundary point method is a speedy and precise method which can avoid the complex computing of the singularity integral in EBEM. The correctness and availability is validated by the numerical simulation.