Structural Design Point Of View Research Articles

Customized Pore Creation Strategies for Hyperelastic, Robust, Insulating Multifunctional MXene Aerogels for Microwave Absorption.

The construction of heterogeneous microstructure and the selection of multicomponents have turned into a research hotspot in developing ultralight, multifunctional, high-efficiency electromagnetic wave absorbing (EMA) materials. Although aerogels are promising materials to fulfill the above requirements, the increase in functional fillers inevitably leads to the deterioration of intrinsic properties. Tuning the electromagnetic properties from the structural design point of view remains a difficult challenge. Herein, we design customized pore creation strategies via introducing sacrificial templates to optimize the conductive path and construct the discontinuous dielectric medium, increasing dielectric loss and achieving efficient microwave absorption properties. A 3D porous composite (MEM) was crafted, which encapsulated an EVA/FeCoNi (EVA/MNPs) framework with Ti3C2Tx MXene coating by employing a direct heated cross-linking and immersion method. Controllable adjustment of the conductive network inside the porous structure and regulation of the dielectric character are achieved by porosity variation. Eventually, the MEM-5 with a porosity of 66.67% realizes RLmin of -39.2 dB (2.2 mm) and can cover the entire X band. Moreover, through off-axis electronic holography and the calculation of conduction loss and polarization loss, the dielectric property is deeply investigated, and the inner mechanism of optimization is pointed out. Thanks to the inherent characteristic of EVA and the porous structure, MEM-5 showed excellent thermal insulating and superior compressibility, which can maintain 60 °C on a 90-100 °C continuous heating stage and reached a maximum compressive strength of 60.12 kPa at 50% strain. Conceivably, this work provides a facile method for the fabrication of highly efficient microwave absorbers applied under complex conditions.

Experimental Investigation of Surface Waves Effect on a Ducted Twin Vertical Axis Tidal Turbine

The cost effective design of tidal turbines requires a good estimation of the loading cycles and their extrema that are related to the unsteady fluctuation of the current velocity. Apart from the ambient turbulence, the main source of velocity fluctuation is the presence of surface waves. In the present study, we analyse the effect of waves propagating against the current on the performance and the loads of a twin vertical axis tidal turbine by an experimental approach at a 1/20 scale. Overall, the results show little or no effect of the waves on the average power and loads compared to the conditions with current only, but a significant impact on their standard deviation that rises linearly with the amplitude of the waves. The drag, lift, and pitching moment show extended ranges up to 7.5 times higher and extreme values exceedance by 60 to 100% with irregular waves compared to the conditions without waves. That load and power fluctuation increase is totally due to the presence of waves as the coherence function between the rotor torque or the loads and the velocity spectra exceed 0.5 on the whole wave frequency bandwidth. The results also reveal a rotational sampling of the waves by the rotors that had also been observed on horizontal axis turbines. From a structural design point of view, the authors recommend conducting tests in irregular wave conditions as both the load ranges and extreme values are 1.5 to 2 times higher than those encountered with regular waves of the same significant height and period.

Systematic Review of Material and Structural Design in Interfacial Solar Evaporators for Clean Water Production

Rational and sustainable utilization of resources is critical for the continuous development of this society. Solar energy, as one of the renewables, shows great potential in replacing part of the traditional energy supplies since it is clean, abundant, and easily convertible to thermal, electrical, and biological energies. Using solar energy as the green driving force, interfacial solar evaporation is a promising way for clean water production to alleviate global water shortage, taking advantage of its high evaporation efficiency (more than 80%) and strong adaptability toward various water sources and fields. In recent years, various kinds of materials with diverse designs have been synthesized and applied in interfacial solar evaporation for clean water production. Herein, recent progress in interfacial solar evaporators for clean water production is systematically reviewed, based on the photothermal conversion mechanisms of solar absorbers, including carbonous, semiconductor‐based, and plasmatic ones. Furthermore, key design factors and strategies in interfacial solar evaporators are reviewed and discussed from material and structural design point of view, such as water transport, thermal management, latent heat for water vaporization, and salt accumulation. Finally, some perspectives related to resolving existing problems in the field are given.

An experimental investigation into the effects of front wall geometry on OWC performance for various levels of applied power take off dampings

Despite the achieved progress, exploiting wave energy in a competitive and efficient manner is still a grand challenge problem. This study aims to improve the performance of an Oscillating Water Column (OWC) by experimentally testing different streamlined front wall underlip geometries. Results show that the proposed geometries have an important bearing on OWC hydrodynamic performance provided that OWC does not operate in sloshing mode. Relative to rectangular front wall plate, underlip geometry with a circular cross-section, provided a maximal increase of 21.2% in capture width ratio via reducing vortex shedding and increasing conveyed amount of incident wave energy into the chamber. Observations indicate that the value of Power Take-off (PTO) damping has a crucial impact on the amount of acquired performance improvement. Therefore, applied PTO damping is a key parameter that has to be determined with care. All tested geometries failed to diminish the sloshing present in the chamber which highlights the fact that sloshing phenomenon is only associated with incident wave characteristics independent of wave-structure interaction. Introduced front wall shapes are also found significant from structural design point of view as they are beneficial in attenuating large vortex induced shear stresses that OWC front wall has to endure.

3D thickness effects around notch and crack tip stress/strain fields

Notches and cracks are usually approximately modeled as two-dimensional problems using solutions from plane elasticity to quantify localized stress/strain concentration effects around their tips. However, they may be associated with high gradients that can severely restrict local Poisson-induced transversal strains and cause important 3D stress fields around those tips. Fatigue crack initiation and growth, plastic zone sizes and shapes, and localized constraint effects that affect toughness are typical problems associated with such 3D effects, which may lead to non-conservative damage and life predictions if neglected. To quantify how important they can be, first finite element techniques are used to simulate thickness and notch-tip radius effects in the fields around such tips, and to evaluate their importance from the structural design point of view. Then, versatile sub-modeling techniques are used to study similar effects along the fronts of short and long cracks, and a stepwise re-meshing routine is used to show how an initially straight crack must slightly curve its front during its propagation by fatigue, due to the unavoidable 3D effects that always surround real crack tips.

3D effects around notch and crack tips

Notches and cracks are usually treated as two-dimensional problems in most structural design and analysis applications, employing 2D limit solutions from plane elasticity theories to evaluate highly localized stress/strain concentration effects around their tips. However, such effects are also associated to high stress gradients, which cause three-dimensional stress/strain fields around those tips that can severely restrict local Poisson-induced transversal strains. Modeling of fatigue crack initiation and propagation, estimation of plastic zone sizes and shapes, and localized constraint effects are typical problems affected by such 3D effects, which may lead to non-conservative damage and life predictions if neglected. To quantify how important they can be in practical applications, first traditional finite element techniques are used to simulate effects of component thickness and notch tip radius in the stress and strain fields along and around such tips, and to evaluate their importance from the structural design point of view. Then, versatile sub-modeling techniques are used to study similar effects along the fronts of short and long cracks. Finally, a stepwise remeshing routine is used to show how an initially straight crack must slightly curve its front during its propagation by fatigue, due to the unavoidable 3D effects that always surround real crack tips.

The effect of distortion on the buckling strength of stiffened panels

This paper predicts the behaviour of stiffened plates with different distortion levels in order to address a rational structural design procedure, as pre-existing and fabrication-related (like weld-induced) initial geometrical distortion is of great importance from a structural design point of view. The range of scantlings, the distortion types and levels were chosen based on panels used at BVT Surface Fleet Ltd., where Type 45 destroyers were under construction. An analytical relation is presented based on Perry's column approach to establish the variation of buckling strength against the geometrical distortion. A parametric form of non-linear finite element (FE) analysis using ABAQUS was carried out under axial loading condition to predict the behaviour and the buckling strength. The effect of residual stress is not considered in this study. A new strength parameter is proposed to represent buckling strength, which takes into account the inelastic post-buckling behaviour of the structure. The results from FE analysis are plotted in non-dimensional terms and some important conclusions are arrived at.

Design and optimization of thermo-mechanical reliability in wafer level packaging

In this paper, a variety of wafer level packaging (WLP) structures, including both fan-in and fan-out WLPs, are investigated for solder joint thermo-mechanical reliability performance, from a structural design point of view. The effects of redistribution layer (RDL), bump structural design/material selection, polymer-cored ball application, and PCB design/material selection are studied. The investigation focuses on four different WLP technologies: standard WLP (ball on I/O WLP), ball on polymer WLP without under bump metallurgy (UBM) layer, ball on polymer WLP with UBM layer, and encapsulated copper post WLP. Ball on I/O WLP, in which solder balls are directly attached to the metal pads on silicon wafer, is used as a benchmark for the analysis. 3-D finite element modeling is performed to investigate the effects of WLP structures, UBM layer, polymer film material properties (in ball on polymer WLP), and encapsulated epoxy material properties (in copper post WLP). Both ball on polymer and copper post WLPs have shown great reliability improvement in thermal cycling. For ball on polymer WLP structures, polymer film between silicon and solder balls creates a ‘cushion’ effect to reduce the stresses in solder joints. Such cushion effect can be achieved either by an extremely compliant film or a ‘hard’ film with a large coefficient of thermal expansion. Encapsulated copper post WLP shows the best thermo-mechanical performance among the four WLP structures. Furthermore, for a fan-out WLP, it has been found that the critical solder balls are the outermost solder balls under die-area, where the maximum thermal mismatch takes place. In a fan-out WLP package, chip size, other than package size, determines the limit of solder joint reliability. This paper also discusses the polymer-cored solder ball applications to enhance thermo-mechanical reliability of solder joints. Finally, both experimental and finite element analysis have demonstrated that making corner balls non-electrically connected can greatly improve the WLP thermo-mechanical reliability.

Structural analyses for equatorial and lower ports of the ITER vacuum vessel

To investigate the structural integrity of the ITER vacuum vessel (VV) and ports, the structural analyses of the regular equatorial and the lower remote handling (RH) ports have been performed. The advanced design of the equatorial regular port adopting a pure friction type flange has been recommended as a reference design by the ITER International Organization. The structural integrity of the equatorial port flange, sealing unit, and connecting duct has been reviewed by conducting nonlinear finite element analyses. The advanced design of the regular equatorial port flange with proper pretension is acceptable in the structural design point of view. From the local analyses for a connecting duct and a sealing unit, it has been found that the stresses are less than the allowable values. The structural analyses of the lower RH port have been also performed to verify the capability for supporting the VV. Since high local stress occurs at the gusset and supporting block, the case study for the lower port has been conducted to mitigate the stress concentration and to modify the component design. The strength of the lower RH port structures can be improved by the design modification of poloidal and toroidal gusset.

Steel Fiber-Reinforced Self-Compacting Concrete: Experimental Research and Numerical Simulation

Over the last few decades, the astonishing developments of superplasticizers technology allowed great achievements in the conception of concrete mixes exhibiting self-compacting ability. Since the 1980s, some methodologies have been proposed to achieve self-compacting requirements in fresh concrete mixes, based on the evaluation of the flowing properties of these mixes. There still persist, however, some doubts about the most appropriate strategy to define the optimum composition of a self-compacting concrete (SCC) mix, based on a required performance. The behavior of SCC as a structural material can be improved if adequate steel fiber reinforcement is added to SCC mix composition. In fact, the fiber-reinforcement mechanisms can convert the brittle behavior of this cement-based material into a pseudoductile behavior up to a crack width that is acceptable under the structural design point of view. Fiber addition, however, increases the complexity of the mix design process, due to the strong perturbation effect that steel fibers cause on fresh concrete flow. In the present work, a mix design method is proposed to develop cost effective and high performance steel fiber-reinforced self-compacting concrete (SFRSCC). The material properties of the developed SFRSCC are assessed as well as its potentiality as a structural material, carrying out punching and flexural tests on panel prototypes. A material nonlinear analysis is carried out, aiming to address the possibility of calibrating the constitutive model parameters by obtaining, with an inverse analysis, the fracture parameters using force-deflection relationships recorded in simpler laboratory tests, such as the three-point notched beam bending test. The contribution of steel fibers for punching resistance is also discussed.

Structural analysis for an upper port of the ITER vacuum vessel

The international thermonuclear experimental reactor (ITER) vacuum vessel (VV) upper port has a trapezoidal/rectangular cross-section and consists of a port stub, a stub extension and a port extension with a connecting duct. To investigate the structural integrity and to increase the structural reliability of the VV and ports, the structural analyses of the upper port structure have been performed. Two advanced designs from the ITER international team have been deeply analyzed. To verify the strength of the reinforcing ribs for the connecting duct and of the fastening/sealing units, the local analyses utilizing the sub-modeling technique have been performed. The upper port flange based on the original design could withstand design loads, but the flange surface could be detached under the design condition. The advanced design using large superbolts is highly acceptable in the structural design point of view. The deflection of the plug for the other advanced design with a removable flange is relatively large since the stiffness of a port and a plug is reduced by the nut groove and the removable flange.

Rational Design of DNA Nanoarchitectures

DNA has many physical and chemical properties that make it a powerful material for molecular constructions at the nanometer length scale. In particular, its ability to form duplexes and other secondary structures through predictable nucleotide-sequence-directed hybridization allows for the design of programmable structural motifs which can self-assemble to form large supramolecular arrays, scaffolds, and even mechanical and logical nanodevices. Despite the large variety of structural motifs used as building blocks in the programmed assembly of supramolecular DNA nanoarchitectures, the various modules share underlying principles in terms of the design of their hierarchical configuration and the implemented nucleotide sequences. This Review is intended to provide an overview of this fascinating and rapidly growing field of research from the structural design point of view.

Evaluation of stress intensity factor under pure shear stress in orthotropic material

AbstractVarious types of composite materials are currently being developed and used for automobiles, airplanes, ships and other structures in response to required service conditions which are getting increasingly more severe. Of growing importance under such circumstances is the study of stress analysis and fracture mechanics for these composite material structures. Particularly, the primary concern in design of structures and machines should be the initiation of cracks due to excessive deformation, delamination in material or other material defects. In evaluating safety, it is indispensable from the structural design point of view that K value should be known by an analysis conducted in advance. In this study, stress intensity factor (mode II) under a pure shear stress was obtained using the photoelastic method and caustic method and applying an isotropic material and orthotropic material (copper fibre epoxy composite (CFEC) developed by the authors), each containing the crack. Results were compared with theoretical values. As a result, this method was found useful and the effect of the direction of the primary axis of this material on the stress intensity factor was clarified.

Study on Fracture of Orthotropic Material for Cracks under Pure Shear Stress.

Of growing importance under sever circumstances is the study on stress analysis and fracture mechanics for composite material structures. Particularly, the primary concern in design of structures and machines should be the initiation of cracks due to excessive deformation, exfoliation in material or other material defects. In evaluating safety, it is indispensable from the structural design point of view that the k value should be made known by an analysis conducted in advance. In this study, the stress intensity factor (Mode II) under a pure shear stress was obtained using the photoelastic method and caustic method, applying an isotropic material and an orthotropic material (copper fiber epoxy composite (C.F.E.C.) developed by the authors), each containing a crack, to the fracture mechanics. Results were compared with theoretical values. As a result, this method was found useful and the effect of the direction of the primary axis of this material on the stress intensity factor was clarified.

Thermal and structural design issues of breeding blankets for testing in the Next European Torus (NET)

A review of the breeding blankets under study in Europe for testing in the Next European Torus is presented. In many concepts, the breeder modules are enclosed in boxes whose side walls in front of the plasma act as the first wall of the machine. Various types of breeder modules are investigated, involving both liquid and solid breeders, namely: - Pb 17Li liquid breeder concepts, the coolant being either water or Pb 17Li itself; - solid (ceramic) breeder concepts, the coolant being in all cases helium. The various ceramic concepts differ in the breeder/coolant arrangement (breeder-out-of-tube and breeder-in-tube), the orientation of the coolant tubes (poloidal or toroidal) and the breeder geometry (rods, plates or pebble bed). For each of these concepts the main design features are shown and the thermomechanical problems are discussed. The problems related to a coolant tube rupture are in many cases the most severe from the structural design point of view. The first wall box enclosing the breeder modules appears to be a weak secondary containment barrier. The liquid breeder-water cooled concept looks manageable from the thermal and structural design point of view. In the case of the self-cooled liquid breeder concept, the main problems are related to the magnetohydrodynamic effects. Solutions are envisaged to overcome these difficulties. In the case of ceramic breeders, the use of plates implies small dimensions in order to limit the thermal stresses and a poor exploitation of the permitted temperature operation window. Solutions involving rods associated with a multipass cooling scheme or pebble bed enable achievement of better thermomechanical conditions and, therefore, are preferred in the current investigations. However, they lead to design complications and require experimental verification which is in progress at the European laboratories.

Significant structural design features of commercial-sized liquid-metal fast breeder reactor (LMFBR) plants

From a structural design point of view, there are many significant differences between liquid-metal fast breeder reactor (LMFBR) plants and light water reactor (LWR) plants especially in the containment area, and the area containing the nuclear steam supply system (NSSS). This paper highlights the structural design features of a commercial-sized LMFBR plant. While the external loads are the same, the internal loads, due to characteristic differences in the LMFBR and LWR NSSS systems, have significant structural implications. Primarily, they lead to the low pressures and elevated temperatures in the LMFBRs in contrast to the high pressures and lower temperatures in the LWRs. Also, there are differences in the design basis accident scenarios. These and other differences are addressed.

Dual optimum aerodynamic design for a conventional windmill

The theoretical possibility of designing the blading of a conventional windmill so as to match the optimum loading for maximum output at two different operating conditions (i.e., at two different values of the ratio of tip speed to wind speed) is investigated using the vortex blade element theory. The theory of optimum loading is developed, including the effect of profile drag. For the range of parameters considered, it is shown that the effect of profile drag on blade loading is quite small, although the effect on efficiency is significant. The simpler theory, neglecting profile drag, and first presented by Glauert, then is used to design three sets of blading which match the Glauert loading conditions at two conditions, tip speed ratios of 4 and 5. The first design is for fixed blading and the other two involve pitch changes of 1.5° and 2° between the two conditions, the latter two showing a moderate degree of taper and being more desirable from a structural design point of view. It is concluded that the possibility of designing blading having high efficiency over a considerable range of operating conditions exists and can be used.