Baseline-free damage identification of metallic sandwich panels with truss core based on vibration characteristics

A baseline-free damage identification method is proposed to identify damages in metallic sandwich panels with truss core in the article. The method is based on flexibility matrix and gapped smoothing method, with damage index defined DIm. The weight coefficient m is introduced to consider the effect of damages on both low-order modes and high-order modes. Numerical simulations and experiments are conducted to evaluate the present method. Besides, damage index DIm* is also defined by processing DIm with Teager energy operator, and comparisons between DIm and DIm* are also carried out. Results show that the proposed method is effective in detecting single damage and multiple damages of the same or different extent. The weight coefficient m plays a very important role in identification of multiple damages of different styles. When comparing with DIm*, it is found that the present index DIm is better at suppressing the singularity caused by contact nodes and detecting of multiple damages which contain small or slight damages.</p

Applications, Manufacturing and Thermal Characteristics of Micro-Lattice Structures: Current State of the Art

Micro-lattice structures are emerging as multi-functional devices. They possess excellent heat transfer capabilities, energy absorption abilities, vibration control abilities, etc. The higher surface area to volume ratio of micro-lattice structures makes them suitable for heat transfer applications where compact and lightweight heat transfer mechanism is necessary such as in case of space and transportation. The heat transfer and mechanical load-bearing properties of micro-lattice structures can be tailored by altering several parameters such as the lattice strut angle, node-to-node spacing, the diameter of the strut, etc. In this paper, micro-lattice structures, their manufacturing methods, applications are reviewed and a passive heat transfer mechanism consisting of micro-lattice heat pipe is proposed for the battery thermal management system in electric vehicles

Design and modeling of a periodic single-phase sandwich panel for acoustic insulation applications

Sandwich and composite panels are widely adopted in acoustic applications due to their sound insulation properties that overcome mass-law-based partitions in medium–high frequency regions. A key aspect in the design procedure of acoustic panels is the control of the resonance-dominated region of the sound transmission loss (STL) curve. Within that frequency range, such systems usually show acoustic weakness and poor insulation performances with respect to standard single-layer solutions. In the present contribution, we want to highlight an innovative approach to the sandwich partition concept. A novel single-phase sandwich panel is realized by adopting a periodic repetition of a properly designed unit cell. The resulting internal truss structure is self-sustained, and its mechanical stiffness can be tuned to maximize the STL in the resonance-dominated region. A set of parametric analyses is reported to show how the topology of the unit cell affects the noise reduction properties of the panel. Experimental validation is performed on a nylon 3D-printed prototype. The proposed panel is then integrated with some locally resonant elements that can be adopted to further improve the low-frequency STL of the solution. Industrial and production considerations are also taken into account during the design process to make the solution industrially valid with a circular economy focus

Truss core sandwich panels with compacted aerogel insulation

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references.Silica aerogels are well known for their low thermal conductivity, approximately 15 mW/m-K. Their low relative density (typically less than 5%) reduces conduction through the solid and their small pore size, typically less than one hundred nanometers, on the order of the mean free path of air, reduces conduction through air as well as convection and radiation. As they are exceptionally fragile and brittle (flexural strength is typically 0.03-0.08 MPa), they are often used in a granular form in thermal insulation, with some increase in their thermal conductivity, to about 24 mW/in-K, from the air between the granules. Here, we describe a technique for compacting granular silica aerogel that reduces the thermal conductivity to 13 mW/m-K, roughly half that of the uncompacted aerogel. We also report on the development of a truss-core sandwich panel filled with compacted aerogel granules, designed to provide both mechanical support and thermal insulation to facilitate the practical commercialization of aerogel insulation, particularly in energy efficient building applications. Mechanical and thermal properties of the sandwich panel prototype were measured and compared with theoretical models available in literature. The models give a good description of the properties of aerogel-filled truss-core sandwich panels.by Kevin Chen.S.M

Review of phononic crystals and acoustic metamaterials

As a new type of acoustic functional material, phononic crystal has great research value and application environment. It is a periodic structure of two or more elastic materials, which are derived from photonic crystals. The main research work on phononic crystals focuses on the two band gap formation mechanisms of Bragg scattering and local resonance, and some new methods of vibration reduction and noise reduction can be obtained by studying its banding mechanism. Similarly, a "metamaterial" has been proposed for the ability to achieve new vibration reduction and noise reduction, which is a composite structure or material with physical properties not available in natural materials. By analysing the acoustic metamaterials of various structures, in this work we can understand how to achieve vibration reduction and noise reduction under the local resonance mechanism

Flow and thermal transport in additively manufactured metal lattices based on novel unit-cell topologies

The emergence of metal Additive Manufacturing (AM) over the last two decades has opened venues to mitigate the challenges associated with stochastic open-cell metal foams manufactured through the traditional foaming process. Regular lattices with user-defined unit cell topologies have been reported to exhibit better mechanical properties in comparison to metal foams which extend their applicability to multifunctional heat exchangers subjected to both thermal and mechanical loads. The current study aims at investigating the thermal-hydraulic characteristics of promising novel unit cell topologies realizable through AM technologies. Experimental investigation was conducted on four different topologies, viz (a) Octet, (b) Face-diagonal (FD) cube, (c) Tetrakaidecahedron, and (d) Cube, printed in single-cell thick sandwich type configuration in 420 stainless steel via Binder Jetting technology at same intended porosity. The effective thermal conductivity of the samples was found to be strongly dependent on the lattice porosity, however, no significant dependence on the unit-cell topology was demonstrated. Face-diagonal cube lattice exhibited the highest heat transfer coefficient and pressure drop, and consequently provided the lowest thermal-hydraulic performance. A procedure to incorporate the manufacturing-induced random roughness effects in the samples during numerical modelling is introduced. The numerical simulations were conducted on samples exhibiting the roughness profiles having statistically same mean roughness as the additively manufactured coupons and the results were compared to that obtained from the intended smooth-profiled CAD models that were fed into the printing machines. The analysis showed that inclusion of roughness effects in computational models can significantly improve the thermal performance predictions. Through this study, we demonstrate that additively manufactured ordered lattices exhibit superior thermal transport characteristics and future developmental efforts would require extensive experimentations to characterize their thermal and flow performance as well as local surface quality and AM-induced defect recognition. Experimental findings would also need to be supported by computational efforts where configurations which closely mimic the real AM parts could be modeled. A combined experimental-numerical framework is recommended for advancements in metal additive manufacturing-enabled enhanced heat transfer concepts

Structural performance of aerogel composite panels

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 169-177).Aerogels are well known as exceptional thermal insulators. Thermal conductivities of 9 to 10 mW/m.K have been achieved at atmospheric pressure, and a moderate vacuum (between 1/3 and 1/10 of an atmosphere) can lower this number even further, to around 5 mW/m.K. However aerogels for insulation purposes are not widespread on the market. One of the major shortcomings of aerogels that prevent them from being more widely used is their extreme mechanical weakness and brittleness. In this thesis, we characterize and explain these low mechanical properties. We then propose a composite structure for a rigid aerogel panel, a sandwich panel with a truss core filled with monolithic aerogel. Mechanical and thermal properties are derived and partially tested for the truss cores. Several designs are studied and mechanical properties are derived in order to compare these designs. Some criteria for an efficient designs are given. Finally, we describe a first attempt to manufacture one of these composite structures on a small scale.by Thomas Goutierre.S.M