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Topology optimisation is a computational design approach that generates high-performing, efficient structures uniquely suited to a design engineer’s goal. However, there exist two major obstacles to the accessibility, or ease of use, of topology optimisation: expensive computational costs and users’ binary decision between personal intuition and the algorithm’s result. Human-informed topology optimisation, or HiTop, presents an alternative approach to topology optimisation when a user lacks access to a high-performance computer or knowledge of code parameters. HiTop 2.0 prompts users to interactively identify a region of interest in the preliminary design and modify the size of the solid and/or void features. The novel contribution of this paper implements multi-phase minimum and maximum solid feature size controls in HiTop 2.0, and demonstrates 2D and 3D benchmark examples, including test cases that show how the user can interactively enhance issues related to eigenvalues, stress, and energy absorption, while solving the minimum compliance problem.

The available experimental results in the literature on the quasi-static bending and free flexural vibration of microcantilevers and nanocantilevers are used to calibrate the length scale parameter of the stress-driven nonlocal elasticity model. The Bernoulli–Euler theory is used to define the kinematic field. The closed form solution derived for the bending problem is used to calibrate the length scale parameter by fitting the load–displacement curves to the experimental results. For the vibration problem, the calibration is done using the least-squares curve fitting method for the natural frequencies. The stress-driven nonlocal theory can adequately capture the size-dependent experimental results. [ABSTRACT FROM AUTHOR]

This study explores the impact of assimilating radar radial velocity (RV) on the forecast of Super Typhoon Lekima (2019) using the Weather Research and Forecasting (WRF) model and three-dimensional variational (3DVAR) assimilation system with different background error length scales. The results of two single observation tests show that the smaller background error length scale is able to constrain the spread of radar observation information within a relatively reasonable range compared with the larger length scale. During the five data assimilation cycles, the position and structure of the near-land typhoon are found to be significantly affected by the setting of the background error length scale. With a reduced length scale, the WRF-3DVAR system could effectively assimilate the radar RV to produce more accurate analyses, resulting in an enhanced typhoon vortex with a dynamic and thermal balance. In the forecast fields, the experiment with a smaller length scale not only reduces the averaged track error for the 24-h forecasts to less than 20 km, but it also more accurately captures the evolutions of the typhoon vortex and rainband during typhoon landing. In addition, the spatial distribution and intensity of heavy precipitation are corrected. For the 24-h quantitative precipitation forecasts, the equitable threat scores of the experiment with a reduced length scale are greater than 0.4 for the threshold from 1 to 100 mm and not less than 0.2 until the threshold increases to 240 mm. The enhanced prediction performances are probably due to the improved TC analysis. [ABSTRACT FROM AUTHOR]

Adequate understanding of the flow structures in the proximity of an eroding riverbank is of great importance in protecting and controlling the bankline progression. An earlier initial field reconnaissance along the upper regime of the Hooghly River in India showed that the sediments comprise of clay-silt-sand mixture of different proportions. The present study experimentally investigates the temporal evolution of the coherent structures of turbulent flow as observed over the eroded bank topography in the Hooghly River for different compositions of the bank material in a laboratory flume. Artificial riverbanks have been fabricated using three different bank material composition in a laboratory flume to systematically understand the flow modulation inside the undercut region. The flow characterization is carried out to acquire sound knowledge on turbulence characteristics, such as mean velocity, intensity, shear stress, and power spectral density. It is evident from the turbulent coherent structure that the ejection events result in sediment pick up and the sweep events result in transportation of the sediment particles from the bank face. To quantify the eddy size, length scales are determined within the undercut region. Further, the anisotropy invariant maps are also computed to characterize the anisotropic structure of flow for different undercut roughness conditions. Results from the present study indicate that the near-wall streamwise integral length scales, turbulence intensity, and energy dissipation rate are significantly influenced by the different undercut morphologies. The Reynolds shear stress value appears to be strongly negative at the near-bank wall region and changes to positive at the mid-depth of the undercuts. The value of the integral length scale near the wall decreases with an enhancement of the roughness dimension. Furthermore, at the near-wall side bank region, the turbulence dissipation is found to be strongly dependent on the undercut morphology. [ABSTRACT FROM AUTHOR]

Topology optimisation is a computational design approach that generates high-performing, efficient structures uniquely suited to a design engineer's goal. However, there exist two major obstacles to the accessibility, or ease of use, of topology optimisation: expensive computational costs and users' binary decision between personal intuition and the algorithm's result. Humaninformed topology optimisation, or HiTop, presents an alternative approach to topology optimisation when a user lacks access to a high-performance computer or knowledge of code parameters. HiTop 2.0 prompts users to interactively identify a region of interest in the preliminary design and modify the size of the solid and/or void features. The novel contribution of this paper implements multi-phase minimum and maximum solid feature size controls in HiTop 2.0, and demonstrates 2D and 3D benchmark examples, including test cases that show how the user can interactively enhance issues related to eigenvalues, stress, and energy absorption, while solving the minimum compliance problem. [ABSTRACT FROM AUTHOR]

Aerodynamic noise in high frequency band related in the flows around A-pillar is easily propagated to the cabin of the automobile. Understanding of the characteristics of the aerodynamic noise is needed to reduce the cabin noise. Moreover, understanding of the separated shear layer which is strongly affected by the condition of the step hight H to the boundary layer thickness δ is needed. In this study, far field sound and correlation measurement of flow velocity are measured to investigate the characteristics of aerodynamic noise and the relationship between H/δ and turbulence length scale L as eddy scale for the forward facing step. As a result, multiple gradual peaks in more than 1 kHz are observed in the aerodynamic sound spectra. Furthermore, it is found that turbulence length scale L increases for increasing of H/δ and turbulence length scale to step height for H/δ = 2.0 is almost equal to that for H/δ = 3.0. Then, it is clear that spectral peaks are strongly affected by step height. And it is confirmed that H/δ = 2.0 and 3.0 are equal in turbulence length scale to the step height.

Stably stratified roughness sublayer flows are ubiquitous yet remain difficult to represent in models and to interpret using field experiments. Here, continuous high‐frequency potential temperature profiles from the forest floor up to 6.5 times the canopy height observed with distributed temperature sensing (DTS) are used to link eddy topology to roughness sublayer stability correction functions and coupling between air layers within and above the canopy. The experiments are conducted at two forest stands classified as hydrodynamically sparse and dense. Near‐continuous profiles of eddy sizes (length scales) and effective mixing lengths for heat are derived from the observed profiles using a novel conditional sampling approach. The approach utilizes potential temperature isoline fluctuations from a statically stable background state. The transport of potential temperature by an observed eddy is assumed to be conserved (adiabatic movement) and we assume that irreversible heat exchange between the eddy and the surrounding background occurs along the (vertical) periphery of the eddy. This assumption is analogous to Prandtl's mixing‐length concept, where momentum is transported rapidly vertically and then equilibrated with the local mean velocity gradient. A distinct dependence of the derived length scales on background stratification, height above ground, and canopy characteristics emerges from the observed profiles. Implications of these findings for (1) the failure of Monin–Obukhov similarity in the roughness sublayer and (2) above‐canopy flow coupling to the forest floor are examined. The findings have practical applications in terms of analysing similar DTS data sets with the proposed approach, modelling roughness sublayer flows, and interpreting nocturnal eddy covariance measurements above tall forested canopies. [ABSTRACT FROM AUTHOR]

Three-dimensional coordinate systems define the location of zero-dimensional points. Higher dimensional spatial objects such as lines and planes become apparent from sets of adjacent points. Any point defined in three-dimensional space within a sedimentary basin could be simultaneously located on the surface of a sand grain and a bedding plane in a kilometer-scale fold, and structure at any other scale. Therefore, like many other aspects of geology and geophysics, structural geology is multiscale which is why filtering, smoothing, scale bars and the like are prerequisites for any kind of geological mapping and outcrop imagery. It is argued here that the association of points with higher-dimensional objects that form the spatial building blocks of geology singles out length scale as a geometrical parameter that is fundamental to the definition of an object. Properties such as curvature, porosity, color and so on can only be attributed after location and scale are defined. It is proposed that length scale be treated as a fourth spatial dimension in geological and geophysical applications, on an equal footing to the three spatial coordinate dimensions. The utility of adopting a four spatial dimension approach in geoscience is that it forces length scale to be explicitly specified in descriptions where it is routinely omitted, for example, measurements of structural orientation where a scale parameter greatly simplifies later mapmaking and model building. Taken together with geological time, it is convenient to define geological and geophysical structure in five spacetime dimensions. [ABSTRACT FROM AUTHOR]

This article reviews recent developments in small-scale mechanical property testing with some emphasis on intermediate (meso) length scales in complex microstructures and coated systems. The introduction summarizes size effects discovered from a century ago up to the recent explosion in micropillar testing that established many length scale effects in yielding and fracture. The bulk of the article deals with plasticity and fracture in polyphasic and microstructurally graded systems, including biomaterials, composites, and thermal protection systems, highlighting the use of in situ methods where mechanical tests are coupled to synchrotron X-ray scattering, electron backscattering, radiation damage, and digital image correlation. [ABSTRACT FROM AUTHOR]

This study explores the impact of assimilating radar radial velocity (RV) on the forecast of Super Typhoon Lekima (2019) using the Weather Research and Forecasting (WRF) model and three-dimensional variational (3DVAR) assimilation system with different background error length scales. The results of two single observation tests show that the smaller background error length scale is able to constrain the spread of radar observation information within a relatively reasonable range compared with the larger length scale. During the five data assimilation cycles, the position and structure of the near-land typhoon are found to be significantly affected by the setting of the background error length scale. With a reduced length scale, the WRF-3DVAR system could effectively assimilate the radar RV to produce more accurate analyses, resulting in an enhanced typhoon vortex with a dynamic and thermal balance. In the forecast fields, the experiment with a smaller length scale not only reduces the averaged track error for the 24-h forecasts to less than 20 km, but it also more accurately captures the evolutions of the typhoon vortex and rainband during typhoon landing. In addition, the spatial distribution and intensity of heavy precipitation are corrected. For the 24-h quantitative precipitation forecasts, the equitable threat scores of the experiment with a reduced length scale are greater than 0.4 for the threshold from 1 to 100 mm and not less than 0.2 until the threshold increases to 240 mm. The enhanced prediction performances are probably due to the improved TC analysis.

• A novel topology optimization approach is proposed for maximizing the band gap of acoustic waves at a specified central frequency. • Virtual temperature and minimum length scale techniques are utilized for ensuring air permeability. • Gradient-free algorithm is employed for solving the complicated optimization problem. The optimized designs are customized to different frequencies and air channel widths. • Experiments indicate optimized results exhibit exceptional wide band gap range. Designing phononic crystals (PnCs) to exhibit the widest attainable band gap, or to encompass a designated frequency range, is imperative for the realization of PnCs tailored to specific functionalities. However, achieving a wide band gap of acoustic waves centered around a specified frequency remains a significant challenge. The challenge arises from the fact that the optimization objective function may be enhanced by disconnected air regions. Nonetheless, these disconnected regions obstruct the circulation of air, rendering PnCs devoid of practical engineering utility. In this study, we propose a topology optimization methodology aimed at crafting air/solid PnCs that maximize the band gap of acoustic waves at a specified central frequency. To ensure adequate air permeability within the PnCs, the optimization model incorporates the virtual temperature technique in conjunction with the minimum length scale technique. The topology of the PnC unit cell is represented with a low number of design variables through the material-field series expansion, and the Kriging-based optimization algorithm is used to solve the complicated optimization problem. Diverse optimized configurations are presented, and experimental findings compellingly underscore the efficacy of the proposed optimization approach in the design of phononic band gap crystals geared towards acoustic waves at the specified central frequency. [Display omitted] [ABSTRACT FROM AUTHOR]

Direct initiation of one-dimensional Chapman–Jouguet detonations in a framework of reactive Riemann problems has been numerically studied with a detailed chemical reaction mechanism in a stoichiometric hydrogen/oxygen mixture diluted by 70% argon. The reactive Riemann problem, with reactants of high temperature and high pressure to the left of the diaphragm as an energy source with a finite length scale, is applied to initiate detonations downstream. Based on the length scales and initial thermodynamic parameters of the energy source, three different regimes of detonation initiation were found, namely supercritical, critical, and subcritical regimes, which is in accord with the classical blast initiation. The initiation process is essentially an inert shock tube problem nonlinearly coupled with a constant-volume explosion. When the auto-ignition delay time of the constant-volume explosion is large as compared to that behind the leading shock, the mechanism can be identified as an incident shock initiation because the explosion far behind the leading shock cannot affect it. This is essentially the same as the experiments of detonation initiation in a shock tube. Conversely, when the auto-ignition delay time of the constant-volume explosion is small as compared to that behind the leading shock, the initiation process is highly influenced by the heat release of chemical reaction occurring near the rear boundary. A pressure pulse is generated at the tail of the modified expansion fan because of localized maximum reaction rate and eventually evolves into a shock to interact with the leading shock to enhance it. The subsequent detonation downstream is also a result of the initiation of the enhanced leading shock. The numerical results also suggest that the critical energy scaled by the product of the initial pressure and the length scale of the energy source keeps constant with the change in length scale, but can be highly influenced by the initial temperature of the energy source. The scaled critical energy is found to be approximately between 1 and 2 for a wide temperature range above the auto-ignition temperature. [ABSTRACT FROM AUTHOR]

A benchmark study of small size plate structures exposed to permanent environmental interactions is presented. Micromechanically-motivated model is introduced for thin crystalline microplates under in-plane and out-of-plane excitations. Lower-order strain gradient plasticity (SGP) model is developed based on Chen-Wang Hardening law incorporating the influence of effective length scale. Since hardening parameter is a fundamental factor in physical descriptions of material, obtained results demonstrate the effect of length scale on the effective stresses due to in-plane and out-of- plane excitations. While substrate – supported foundation is included, deflection and stress-strain curves are compared as well as the effect of variable tangent module. [ABSTRACT FROM AUTHOR]

When using the robust topology optimization formulation in the density framework, the minimum size of the solid and void phases must be imposed implicitly through the parameters that define the density filter and the smoothed Heaviside projection. Finding these parameters can be time consuming and cumbersome, hindering a general code implementation of the robust formulation. Motivated by this issue, in this article, we provide analytical expressions that explicitly relate the minimum length scale and the parameters that define it. The expressions are validated on a density-based framework. To facilitate the reproduction of results, MATLAB codes are provided. [ABSTRACT FROM AUTHOR]

In the presented work, a nonlocal gradient enhanced damage model for concrete is proposed with a stress decomposition, to account for shear induced damage. The nonlocal model is an extension of the recently proposed local plasticity damage model by the authors, which can handle directional dependency of damage, pure shear and biaxial damage, damage activation/deactivation and microcracks opening/closure. The gradient enhanced approach is utilized for the extension of the local model. Due to the distinct behavior of concrete in tension, compression and shear, three length scales (tension, compression and shear) are incorporated, depending on local damage variables. The model is implemented in Abaqus UEL-UMAT subroutine with eight node quadrilateral user defined element, having five degrees of freedom u x , u y , eq ̲ + , eq ̲ - , eq ̲ s at corner nodes and two degrees of freedom at internal nodes u x , u y . Five examples of mixed crack mode and mode-I cracking are modeled to show the performance of the model. [ABSTRACT FROM AUTHOR]

Most materials are mechanically heterogeneous on a certain length scale. In many applications, this heterogeneity is crucial for the material’s function, and exploiting mechanical heterogeneity could lead to new materials with interesting features, which require accurate understanding of the local mechanical properties. Generally used techniques to probe local mechanics in mechanically heterogeneous materials include indentation and atomic force microscopy. However, these techniques probe stresses at a region of finite size, so that experiments on a mechanically heterogeneous material lead to blurring or convolution of the measured stress signal. In this study, finite element method simulations are performed to find the length scale over which this mechanical blurring occurs. This length is shown to be a function of the probe size and indentation depth, and independent of the elastic modulus variations in the heterogeneous material, for both 1D and 2D modulus profiles. Making use of these findings, we then propose two deconvolution methods to approximate the actual modulus profile from the apparent, blurred measurements, paving the way for an accurate determination of the local mechanical properties of heterogeneous materials.

Present study investigates a comparative study of lower and higher-order strain gradient plasticity (SGP) theories involving the size-dependent micromechanically flexural behaviors of crystalline thin plates. The investigation includes the Mechanism-Based and the Chen–Wang SGP models established on the Taylor dislocation hardening by evoking the statistically stored dislocations and geometrically necessary dislocations. In addition, these models are conjugated with a multiple plastic work-hardening law proposed for the microstructural applications of the SGP. An analytical approach based on energy minimizing method is used for obtaining deflection values in terms of the length scale, exponent of the work-hardening and the tangential module. The obtained results indicate a meaningful dependence of the deflections to the length scale, plastic work hardening and other parameters as well. [ABSTRACT FROM AUTHOR]

Local scour in a sediment bed varies depending on the type, size and shape of hydraulic structures, and the properties of approach flow and sediment particles. Published studies are hitherto confined primarily to empirical experiments based on particular types of hydraulic structure and there is almost no comparison of scour amongst different types of hydraulic structures. This study aims to provide an attempt to unify different types of clear‐water scour based on a new length scale, which is proposed to characterize the size of the large‐scale flow structure that governs scouring processes. The new length scale serves as a generalized hydraulic radius, which is applicable for all types of clear‐water scour related to junction flows at bridge piers and abutments, and wall jets including culverts. The analysis indicates that when normalized with the new length scale, the equilibrium scour depths can be described with a unified function of the densimetric Froude number, regardless of different scour types. In addition, this study also shows that the proposed function varies with the scour efficiency, in a fashion resembling the bedload function in the range of low to high transport regimes. This finding, which is founded on solid physical grounds, reveals that different kinds of local scour phenomena can be interpreted in unison rather than based on the individual type of hydraulic structures. Key Points: A new length scale is proposed for describing the size of the large‐scale flow structure associated with local scourDifferent types of clear‐water scour can be compared in a consistent approach based on the new length scaleThe scour depth normalized by the new length scale varies in a fashion that is comparable to the bedload function [ABSTRACT FROM AUTHOR]

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This study presents a numerical framework to model the effect of surface tension on a flexoelectric energy harvesting system with liquid inclusions. The equivalent Young's modulus and Poisson's ratio of the liquid inclusions embedded in flexoelectric composites considering the surface tension are derived for the first time. An extended isogeometric analysis (XIGA) based on non-uniform rational B-splines (NURBS) is developed to model and solve the electromechanical response of weak discontinuities in flexoelectric composites. The interface between liquid and solid materials is implicitly represented by the level set method. Several numerical examples are presented as linear dielectric solids embedded with liquid inclusions under mechanical compression and different electric circuit configurations, including square matrixes and truncated pyramids. The simulation results indicate that a significant enhancement in the electromechanical coupling coefficient can be obtained by considering the effect of surface tension in open- and closed-circuit configurations. The maximum electric potential and energy conversion are higher in the open-circuit configuration. In addition, the XIGA simulation shows that the energy conversion of the flexoelectric devices increases as the size of the structural system and Young's modulus of the flexoelectric material decrease. The maximum energy conversion capability of the flexoelectric composite, including the effect of surface tension, is enhanced by 32.4% with a central inclusion and 57% or higher with random inclusions. Finally, the influence of the length scale on the flexoelectricity is analyzed. [ABSTRACT FROM AUTHOR]

This paper performs a combined numerical and experimental study to explore the role of minimum length scale constraints in multi-scale topology optimisation. Multi-scale topology optimisation is generally performed without considering the actual unit cell size, while an arbitrary value considerably smaller than the part is selected afterwards. However, this procedure would be problematic if including geometric constraints, e.g. minimum length scale constraints, since geometric constraints cannot be applied without knowing the unit cell dimensions. To address this issue, unit cell size should be defined beforehand, and guidelines will be provided in this work through a thorough numerical exploration, i.e. compliance minimisation multi-scale topology optimisation with different unit cell sizes and a consistent minimum length scale limit will be performed. The numerical results indicate that selecting the unit cell size considerably smaller than the part and larger than the length scale limit would be recommended. Then, experiments are conducted to explore the effect of minimum length scale limit on the stiffness and strength of the multi-scale design. It is observed that increasing the minimum length scale limit would reduce the structural mechanical performance in both aspects.

In this article, the wave finite element method (WFEM) is developed for analyzing the wave propagation of one-dimensional micropolar elastic beams at small strain. The micropolar WFEM (MWFEM) equations are derived without formulating the differential equations of motions, with introducing the additional microrotational degree of freedom. The solutions are unconditionally stable. The length scale effects on transverse and rotational stiffness of the micropolar beam along with the study of the combination of high and low-frequency waves are studied in this paper. The proposed equations are examined using the classical and micropolar numerical examples. Excellent agreements are achieved between the proposed equations and other numerical solutions available in the literature. [ABSTRACT FROM AUTHOR]

A common challenge in computer experiments and related fields is to efficiently explore the input space using a small number of samples, that is, the experimental design problem. Much of the recent focus in the computer experiment literature, where modeling is often via Gaussian process (GP) surrogates, has been on space-filling designs, via maximin distance, Latin hypercube, etc. However, it is easy to demonstrate empirically that such designs disappoint when the model hyperparameterization is unknown, and must be estimated from data observed at the chosen design sites. This is true even when the performance metric is prediction-based, or when the target of interest is inherently or eventually sequential in nature, such as in blackbox (Bayesian) optimization. Here we expose such inefficiencies, showing that in many cases a purely random design is superior to higher-powered alternatives. We then propose a family of new schemes by reverse engineering the qualities of the random designs which give the best estimates of GP length scales. Specifically, we study the distribution of pairwise distances between design elements, and develop a numerical scheme to optimize those distances for a given sample size and dimension. We illustrate how our distance-based designs, and their hybrids with more conventional space-filling schemes, outperform in both static (one-shot design) and sequential settings. [ABSTRACT FROM AUTHOR]

In this paper, the vibrational behaviors of circular and annular sandwich micro-plates coupled with two piezoelectric layers are analytically studied. For defining the equations of motion, the Hamilton's principle and the classical plate theory are used. In the micro-scale size, the properties of the material depend on dimensions, and therefore the modified couple stress theory is utilized to consider this effect. The electric potential of piezoelectric layers is obtained by satisfaction of the Maxwell equation. The natural frequencies for various boundary conditions in two kinds of open and close circuit electric conditions are calculated, and the effects of length scale and electrical field are studied. The results show that the natural frequencies for open piezoelectric micro-plates are larger than the closed ones. [ABSTRACT FROM AUTHOR]

• The proposed model can be applied to cohesive fracture. • The length scale has a negligible influence on the load-displacement curve. • General softening law in cohesive zone models can be considered. • The boundedness condition of the phase-field can be considered easily. Phase-field models have become popular to simulate cohesive failure problems because of their capability of predicting crack initiation and propagation without additional criteria. In this paper, a new phase-field damage model coupled with general softening laws for cohesive fracture is proposed based on the unified phase-field theory. The commonly used quadratic geometric function in the classical phase-field model is implemented in the proposed model. The modified degradation function related to the failure strength and length scale is used to obtain the length scale insensitive model. Based on the analytical solution of a 1-D case, general softening laws in cohesive zone models can be considered. Parameters in the degradation function can be calibrated according to different softening curves and material properties. Numerical examples show that the results obtained by the proposed model have a good agreement with experimental results and the length scale has a negligible influence on the load-displacement curves in most cases, which cannot be observed in classical phase-field model. [ABSTRACT FROM AUTHOR]

Modelling a fin-and-tube heat exchanger as porous media based on Volume Averaging Theory (VAT), specific geometry can be accounted for in such a way that the details of the original structure can be replaced by their averaged counterparts and the VAT based governing equations can be solved for a wide range of heat exchanger designs. To complete the VAT based model, proper closure is needed, which is related to a local friction factor and a heat transfer coefficient of a Representative Elementary Volume (REV). The present paper describes an effort to model a fin-and-tube heat exchanger based on VAT and obtain closure for the model. Experiment data and correlations for the air side characteristics of fin-and-tube heat exchangers from the published literature were collected and rescaled using the ‘porous media’ length scale suggested by VAT. The results were surprisingly good, collapsing all the data onto a single curve for friction factor and Nusselt number, respectively. It was shown that using the ‘porous media’ length scale is very beneficial in collapsing complex data yielding simple heat transfer and friction factor correlations and that by proper scaling, closure is a function of the porous media, which further generalizes macro scale porous media equations. The current work is a step closer to our final goal, which is to develop a universal fast running computational tool for multiple-parameter optimization of heat exchangers.

The appraisal is strongly focussed on challenges associated with the nuclear sector, however these are representative of what is generally encountered by a range of engineering applications. Ensuring structural integrity of key nuclear plant components is essential for both safe and economic operation. Structural integrity assessments require knowledge of the mechanical and physical properties of materials, together with an understanding of mechanisms that can limit the overall operating life. With improved mechanistic understanding comes the ability to develop predictive models of the service life of components. Such models often require parameters which can be provided only by characterisation of processes occurring in situ over a range of scales, with the sub-micrometre-scale being particularly important, but also challenging. This appraisal reviews the techniques currently available to characterise microstructural features at the nanometre to micrometre length-scale that can be used to elucidate mechanisms that lead to the early stages of environmentally-assisted crack formation and subsequent growth. Following an appraisal of the techniques and their application, there is a short discussion and consideration for future opportunities.

This paper presents a compilation of a large database consisting of 588 culvert scour experiments covering a wide combination of different culvert shapes, culvert outlet conditions, and sediment properties. The analysis shows that the maximum scour depth, when normalized with the hydraulic radius, can be expressed reasonably as a linear function of the particle densimetric Froude number for both full-flowing and non-full-flowing outlet conditions. In addition, the sediment nonuniformity effect is also considered by choosing d84 (rather than d50) as the representative grain diameter in the dimensional analysis, which improves the result of the data correlation. [ABSTRACT FROM AUTHOR]

Cauchy homogenisation methods are based on the assumption that materials show size independent behaviour. For example, the effective bending modulus of a beam is constant for all cross-sectional areas and is equal to the tensile modulus. However, as demonstrated by numerous authors, the stiffness of heterogenous materials under non uniform loading (e.g. bending) can be influenced by size effects, when the microstructural size scale is comparable to the macroscopic one. In other words, an effective elastic modulus can underestimate or overestimate the bending stiffness depending on the specimen size. In this work, the effect of the thickness dimension on the effective bending modulus of composite plies is investigated. The composite is represented by cylindrical fibres uniformly distributed in a uniform matrix. The cases of fibres stiffer and more compliant than the matrix are considered. The results show that the effective bending modulus depends on fibre volume fraction ratio, mismatch of fibre/matrix material properties and the number of fibres through the thickness. The implications for the analysis of bending of thin ply composites are discussed. [ABSTRACT FROM AUTHOR]

Intensified dewetting of ultrathin polystyrene (PS) films induced by immersing in a homogeneous mixture of good solvent and water was previously reported to push the limits of dewetting to the sub-100 nm scale. Here, we systematically analyse the role of dewetting mixture composition, i.e. solvent to water ratio, on the length scales of instability. The effect of the solvent concentration in the dewetting mixture on the instability wavelength (λ ) and droplet diameter (d) are determined. The instability wavelength (λ ) for 50 nm thick PS film was found to be decreasing from nearly 17 to 7 μ m as the solvent concentration is decreased from 95 to 35% in the dewetting mixture. This is significantly smaller than the instability wavelength in air, which is nearly 50 μ m for the same PS film. The solubility of PS in the dewetting mixture is also examined and the mechanism of observed variation in the length scale is proposed. [ABSTRACT FROM AUTHOR]

The maximum size constraint restricts the amount of material within a test region in each point of the design domain, leading to a highly constrained problem. In this work, the local constraints are gathered into a single one using aggregation functions. The challenge of this task is presented in detail, as well as the proposed strategy to address it. The latter is validated on different test problems as the compliance minimization, the minimum thermal compliance, and the compliant mechanism design. These are implemented in the MATLAB software for 2D design domains. As final validation, a 3D compliance minimization problem is also shown. The study includes two well-known aggregation functions, p-mean and p-norm. The comparison of these functions allows a deeper understanding about their behavior. For example, it is shown that they are strongly dependent on the distribution and amount of data. In addition, a new test region is proposed for the maximum size constraint which, in 2D, is a ring instead of a circle around the element under analysis. This slightly change reduces the introduction of holes in the optimized designs, which can contribute to improve manufacturability of maximum size–constrained components. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel topology optimization framework is proposed for designing all-angle negative refraction phononic crystals. • Virtual temperature and minimum length scale techniques are utilized in design problem. • Gradient-free algorithm is employed for solving the complicated optimization problem. • All-angle negative refractions at different frequencies are guaranteed with optimized designs. All-angle negative refraction (AANR) is an effective way to achieve negative refraction in phononic crystals (PnCs). It is difficult to achieve AANR using a traditional heuristic design. In this work, we propose a novel topology optimization framework for optimizing the material distribution of air/solid PnCs to achieve AANR at a specified frequency. The optimization goal is to minimize the curvature at a specific point of the equal-frequency contours (EFCs), and the Kriging-based material-field series expansion (KG-MFSE) algorithm is adopted to solve the complicated optimization problem. To ensure the connectivity of the air domain inside the unit cell, the virtual temperature technique is employed, and the filtering and threshold projection are used to control the minimum length scale of the air domain. Numerical examples show that the proposed optimization framework is effective for the material design of AANR PnCs, and several optimized design samples were fabricated with additive manufacturing. The corresponding negative refraction is verified through numerical simulation and experiment. [ABSTRACT FROM AUTHOR]

Co-deformation behavior is a key issue for plasticity stability of heterogeneous nanolayered materials. Using indentation testing, we investigated the plastic deformation behavior of a Cu–Au nanolayered composite (NLC) with an individual layer thickness of 50 nm. We found that the softer Au layer exhibited less deformability than the harder Cu layer in the initial stage of deformation, while this abnormal phenomenon started to gradually disappear as the constituent layers were severely thinned down to thicknesses less than about 10 nm. The basic mechanism for the observed co-deformation behavior at different length scales of the Cu–Au NLC will be discussed.