Properties Of Constituent Materials Research Articles

A multiscale approach for virtual testing of highly aligned short carbon fiber composites

This paper presents an efficient multiscale approach for high fidelity virtual testing of highly aligned short fiber reinforced composites (SFRCs), based solely on constituent material and interface properties with no need of calibration data. The method is based on a hierarchical, bottom-up characterization of SFRCs and the link of mechanical behaviors of materials and microstructures from a lower scale to a higher scale. It starts with a microscopic unit cell model to estimate the transverse and shear properties of the aligned short fiber composites. Next, two types of mesoscale models with explicit consideration of fiber discontinuity and possible local fiber misalignments are employed to obtain effective properties of material domains with such morphologies. Such obtained meso-scale mechanical properties and damage behaviors are then integrated into the macroscale virtual laminar via the stochastically integrating of possible material defects. Results indicate that for perfectly aligned SFRCs the strength reduction is about 25% as compared to that of the continuously reinforced composites. However, misalignment of short fibers can cause a further strength reduction of 20–35% with the volume fraction of the misalign regions varying from 1% to 10%.

In situ Wear Study Reveals Role of Microstructure on Self-Sharpening Mechanism in Sea Urchin Teeth

Summary Animals' teeth have evolved to provide food procurement, mastication, and protection. These functions, directly linked to survival of many living animal species, require superior hardness and abrasion resistance in the animal's dentition system. Such resistance typically emerges from damage tolerance and sharpness preservation during the organism’s life span. An example is the sea urchin tooth, which through gradients in mechanical properties together with exploitation of microstructural features achieves such functionality. Using contact mechanics, dimensional analysis, and a novel in situ scanning electron microscopy experimental methodology, conditions for tooth deformation and wear, via a self-sharpening mechanism consisting in plate chipping, were imaged and quantified. Nonlinear finite-element modeling of the self-sharpening mechanism provided insight into the synergy between constituent material properties and tooth microstructural elements. The findings reported here should inspire the design of novel tools used in machining operations, e.g., cutting and grinding, as well as in mining and tunnel boring.

Microstructural topology optimization for minimizing the sound pressure level of structural-acoustic systems with multi-scale bounded hybrid uncertain parameters

Abstract This paper mainly provides a robust microstructural topology optimization method for minimizing the sound pressure level (SPL) of the structural-acoustic system with multi-scale bounded hybrid uncertainties. During the microstructural topology optimization process, both the uncertainty at macro-scale that comes from the physical parameters of the structural-acoustic system and the uncertainty existed in the constituent material properties of the microstructure are taken into account and handled by the bounded hybrid uncertain model, in which the uncertain parameters are treated as either bounded random variables or interval variables. The homogenization-based Gegenbauer polynomial expansion method (HGPEM) is introduced to calculate the sound pressure of the bounded hybrid uncertain structural-acoustic system. On the basis of the HGPEM, the microstructural topology optimization is conducted through the enhanced genetic algorithm (GA) with adaptive crossover and mutation probability. Two numerical examples are used to demonstrate the capabilities and efficiency of the presented robust microstructural topology optimization method. The results indicate that the robust optimum design considering multi-scale bounded hybrid uncertainties can achieve better performance than the deterministic optimum design.

Experimental Investigation on Behavior of High-Strength Light weight Concrete-Filled Steel Tube Strut Under Axial Compression

This investigation was conducted for experimental and analytical study on the behavior of lightweight concrete-filled steel tube strut (short column) concentrically loaded in compression to failure. Sintagg (expanded clay aggregates) light-weight aggregate was used to prepare high-strength structural lightweight concrete of 40 N/mm2 grade. Sixty-four specimens were tested to failure to investigate the axial compressive behavior of lightweight concrete-filled tube strut (LWCFT). Different strut lengths, sectional sizes, and thickness were used to enumerate the influence of member geometry and constituent material properties on the structural behavior of CFT struts. Ultimate strength results were compared to current specifications governing the design of concrete-filled steel tube struts. This investigation provided a review of the current design rules for concrete-filled sections in Euro code 4 (EC4) and American standards (AISC). Consequently, deadweight of strut member was reduced by 25.30% as compared to normal concrete-filled steel tube strut. The buckling failure pattern for square CFT strut was observed as local buckling takes place, while for circular and rectangular CFT struts plastic buckling takes place. Also, it was observed that the experimental results were adequate to predict the current design specifications. Experimental results suggest that circular tubes offer substantial post-yield strength and stiffness, not available in most square or rectangular cross sections. The calculated confining strengths show that Euro Code-4 provides more realistic results than in case LWCFT strut under axial compression.

Machine learning aided stochastic structural free vibration analysis for functionally graded bar-type structures

Abstract This paper presents a machine learning aided stochastic free vibration analysis for functionally graded (FG) bar-type structures through finite element method (FEM). The considered system uncertainties including the constituent material properties, the dimensions of structural members, and the degree of the gradation of the FGM are incorporated. A novel kernel-based machine learning technique, namely the extended support vector regression (X-SVR), is presented to estimate the governing relationship between the uncertain system parameters and the structural natural frequencies. Subsequently, by applying the Monte-Carlo Simulation (MCS) through the established regression model, various types of statistical characteristics (i.e., mean, standard deviation, probability density function or PDF, and cumulative distribution function or CDF) of structural natural frequencies can be effectively established. Four numerical examples including test functions and practically stimulated engineering structures are thoroughly investigated herein to demonstrate the accuracy, applicability, and computational efficiency of the proposed approach.

Material properties of human lumbar intervertebral discs across strain rates

BACKGROUND CONTEXTThe use of finite element (FE) methods to study the biomechanics of the intervertebral disc (IVD) has increased over recent decades due to their ability to quantify internal stresses and strains throughout the tissue. Their accuracy is dependent upon realistic, strain-rate dependent material properties, which are challenging to acquire. PURPOSEThe aim of this study was to use the inverse FE technique to characterize the material properties of human lumbar IVDs across strain rates. STUDY DESIGNA human cadaveric experimental study coupled with an inverse finite element study. METHODSTo predict the structural response of the IVD accurately, the material response of the constituent structures was required. Therefore, compressive experiments were conducted on 16 lumbar IVDs (39±19 years) to obtain the structural response. An FE model of each of these experiments was developed and then run through an inverse FE algorithm to obtain subject-specific constituent material properties, such that the structural response was accurate. RESULTSExperimentally, a log-linear relationship between IVD stiffness and strain rate was observed. The material properties obtained through the subject-specific inverse FE optimization of the annulus fibrosus (AF) fiber and AF fiber ground matrix allowed a good match between the experimental and FE response. This resulted in a Young modulus of AF fibers (—MPa) to strain rate (ε˙, /s) relationship of YMAF=31.5ln(ε˙)+435.5, and the C10 parameter of the Neo-Hookean material model of the AF ground matrix was found to be strain-rate independent with an average value of 0.68 MPa. CONCLUSIONSThese material properties can be used to improve the accuracy, and therefore predictive ability of FE models of the spine that are used in a wide range of research areas and clinical applications. CLINICAL SIGNIFICANCEFinite element models can be used for many applications including investigating low back pain, spinal deformities, injury biomechanics, implant design, design of protective systems, and degenerative disc disease. The accurate material properties obtained in this study will improve the predictive ability, and therefore clinical significance of these models.

Material property testing for the refurbishment of a historic URM building in Yangon, Myanmar

The architecture of Yangon, Myanmar has been heavily influenced by British colonisation from the mid-1800s, with a large portion of the historic building stock being constructed of unreinforced masonry (URM). The city’s historic buildings have regularly been exposed to the ravages of war and natural disasters, as well as low levels of maintenance despite a rapidly growing population. One of the primary needs when assessing a building for refurbishment and/or retrofit is to characterise the constituent material properties. On-site material investigation was undertaken in a large historic building originally constructed in (circa) 1896, with multiple renovations and expansions made at different times. The original structure is comprised of unreinforced loadbearing Laterite stone and clay-brick masonry walls. In-situ testing and sample extraction were conducted at 14 discrete test locations throughout the building and consisted of: (i) bed joint shear tests in clay-brick masonry with 16 tests throughout the building; (ii) in-situ deformability (aka ‘flat-jack’) tests on the Laterite stone masonry and on the clay-brick masonry; (iii) laboratory compression tests and petrographic examinations of individual clay-bricks, Laterite stones and mortar samples. The findings from the materials characterisation are presented herein with the aim of serving as guidance for engineers assessing similar historic buildings in Myanmar and other Southeast Asian countries, where few characterisation studies on local, historic materials – particularly on Laterite stone masonry – have been publicized.

Thermomechanical nonlinear stability of pressure-loaded functionally graded carbon nanotube-reinforced composite doubly curved panels with tangentially restrained edges

Geometrically nonlinear response of doubly curved panels reinforced by carbon nanotubes exposed to thermal environments and subjected to uniform external pressure are presented in this paper. Carbon nanotubes are reinforced into isotropic matrix through uniform and functionally graded distributions. Material properties of constituents are assumed to be temperature dependent, and effective elastic moduli of carbon nanotube-reinforced composite are determined according to an extended rule of mixture. Basic equations for carbon nanotube-reinforced composite doubly curved panels are established within the framework of first-order shear deformation theory. Analytical solutions are assumed, and Galerkin method is used to derive closed-form expressions of nonlinear load–deflection relation. Separate and combined effects of carbon nanotube distribution and volume fraction, elasticity of in-plane constraint, elevated temperature, initial imperfection, geometrical ratios and stiffness of elastic foundations on the nonlinear stability of nanocomposite doubly curved panels are analyzed through numerical examples.

Comparative analysis of micromechanical models for the elastic composite laminae

This paper deals with the first step required for the analysis and design of composite materials and structures: estimation of the effective macromechanical properties according to the structure of composite, properties of constituent materials and their volume fractions. There exist many micromechanical models proposed in the literature to estimate these effective elastic properties. Each one of these models is based on hypotheses that are valid for certain types of composite structures. The present paper aims to highlight the main assumptions of these models and compare their predictions with a set of 188 experimental data, compiled from 25 references, assuming that just the constituents’ properties are available as input. The following nine major micromechanical models are evaluated: asymptotic homogenization with hexagonal unit cell; asymptotic homogenization with square unit cell; Bridging; Chamis; generalized self-consistent; Halpin-Tsai; modified Halpin-Tsai; Mori-Tanaka; and rule of mixture (ROM). Besides, a novel modified version of the rule of mixture allowing better agreement with the experimental data is also proposed. It is shown, in particular, that the newly proposed modified rule of mixture model provides the best correlation with the experimental data among the ROM-based models, while the asymptotic homogenization presents the best predictions among the elasticity-based models.

Robust design optimization of variable angle tow composite plates for maximum buckling load in the presence of uncertainties

A robust design optimization algorithm is proposed for variable angle tow composite structures in the presence of uncertainties in the constituent material properties and applied loads. The proposed algorithm uses a stochastic perturbation method to propagate these uncertainties through to the simulated structural response, measured in terms of buckling load. The expected value plus a selected number of standard deviations of the response in the form of a bi-criteria problem. To describe the curvilinear fibres, two types of fibre path function, namely linear- and nonlinear-variation formulae, are adopted to illustrate the proposed methodology. A comparison between the resulting robust designs and deterministic designs is made, and changes to the final designs of fibre tow paths arising from the inclusion of uncertainty are discussed. It is shown that the robust designs out-perform the deterministic designs under real-world situations that include uncertainties.

Thermal Modeling of Inductor and Transformer Windings Including Litz Wire

Design optimization of magnetic components, which considers both electrical and thermal performance, can help in designing power converters with a high power density. This paper proposes an approximate analytical model for thermal resistance of inductor and transformer windings, including litz wire, which only requires knowledge of thermal properties of constituent materials and geometric dimensions. The model is based on regular square and hexagonal packing of insulated wires and is derived by integration of infinitesimal thermal resistances along a specified heat flow path. The model closely agrees with finite-element analysis (FEA) and can be used in place of time-consuming FEA as part of larger thermal network models for magnetic components. The model is also experimentally validated using a toroidal inductor with a litz-wire winding; the model has less than 12% error compared to the experimental measurement. Application of the model for practical windings, including litz-wire windings, is also discussed. The model can be used during the design process of magnetic components of a switching power converter to evaluate the temperature and potential hot spots. The model provides closed-form results, hence fast computation times, and so can be used in design optimization procedures.

Modeling and reliability of insert in composite pyramidal lattice truss core sandwich panels

Valid configuration of inserts is commonly used for assembly in sandwich structures. Inherently, various uncertainty results in variability in mechanical characteristics of inserts, and need to be addressed adequately. This study explores the reliability of inserts in composite pyramidal lattice truss core sandwich panels. Finite element method is utilized to analyze the sources and sensitivity of uncertainty in mechanical properties of constituent materials and geometry of sub-components. To quantify the reliability of the insert structure in the presence of uncertainty, machine learning technology is used to investigate the uncertainty propagation by a statistical description. Failure probability is observed and the insert structure shows outstanding stability under pull-out loading. The thickness of face-sheets and the properties of the bonding interface play large roles in stiffness and strength, respectively. Suitable process controls should be implemented to maintain the both critical parameters at a high and stable level.

Thermal postbuckling behavior of CNT-reinforced composite sandwich plate models resting on elastic foundations with tangentially restrained edges and temperature-dependent properties

Buckling and postbuckling behaviors of sandwich plates reinforced by single-walled carbon nanotube (CNT), rested on elastic foundations and subjected to uniform temperature rise, are investigated in this article. CNT is embedded into matrix phase through uniform or functionally graded distributions. The properties of constituent materials are assumed to be temperature-dependent, and effective properties of nanocomposite are determined by extended rule of mixture. Two models of sandwich plates with face sheets and core layer reinforced by CNTs are presented. Formulations are based on the first-order shear deformation theory taking geometrical nonlinearity, initial geometrical imperfection, plate-foundation interaction, and elasticity of tangential edge constraints into consideration. Analytical solutions of deflection and stress function are assumed, and Galerkin method is applied to derive nonlinear temperature–deflection relation from which buckling temperatures and thermal postbuckling paths are obtained through an iteration algorithm. Numerical examples show the effects of CNT volume fraction, distribution patterns, in-plane edge constraint, elastic foundations, geometrical ratios, initial imperfection, and temperature dependence of properties on thermal postbuckling behavior of nanocomposite sandwich plates. The most important finding is that sandwich plate constructed from CNT-poor nanocomposite core layer and thin homogeneous face sheets with partially movable edges bring the best capacities of thermal buckling resistance and postbuckling load carrying.

Heat transfer analysis of a high-power and large-capacity thermal battery and investigation of effective thermal model

We perform heat transfer analysis of a thermal battery module for a high-power and large-capacity thermal battery system based on a detailed thermal model as well as an effective thermal model. Firstly, the transient thermal behavior of a small scale thermal battery module consisting of 13 stacks of unit cells is investigated by the detailed thermal model that considers every components. We show that the net effect of Joule heating (exothermic) and chemical reaction heat (endothermic) occurring in the unit cell result in different degree of cooling down the thermal battery depending on the operating condition. It is also found that anisotropic heat conduction due to the stacked geometry of the unit cell causes temperature non-uniformity within the thermal battery module. Secondly, we also develop an effective thermal model based on the properly averaged thermophysical properties of constituent materials of unit cells as well as the homogenized heat sources. The effective thermal model is validated by comparing it with the detailed thermal model for the small scale thermal battery module consisting of 13 stacks of unit cells, and it is further extended to the large scale thermal battery module consisting of 54 stacks of unit cells.

Mesoscale studies on the effect of aggregate shape idealisation in concrete

Mechanical properties of concrete, such as strength, stiffness and its damage characteristics, are found to depend significantly on the properties of constituent materials. However, this dependency is rarely taken into consideration in the prediction of material response to mechanical loads. Instead, the most generally sought alternative for prediction of material response involves conducting laboratory tests. A mesoscale model that incorporates the properties of the mesostructure and its influence on mechanical response is adopted. Mesoscale models of concrete contain coarse aggregates, cement mortar and, optionally, an interfacial transition zone. An aggregate shape idealisation scheme is determined numerically to generate a circular aggregate mesoscale model by carrying out numerical simulations under uniaxial compression and tensile conditions. The numerical procedure followed in the present mesoscale simulations is validated with the available experimental data. The circular aggregate model is generated by idealising the actual irregular aggregate to a circular one using: (a) an area-equivalence approach and (b) a circumference-equivalence approach. To assess the effective idealisation scheme under static loading conditions, aspects such as strength, stiffness and damage distribution are considered. Simulation results show that the circumference-based model provides higher compressive strength and stiffness and lower tensile strength than the area-based model.

Quantifying thermal boundary conductance of 2D–3D interfaces

Heat dissipation in next-generation electronics based on two-dimensional (2D) materials is a critical issue in their development and implementation. A potential bottleneck for heat removal in 2D-based devices is the thermal pathway from the 2D layer into its supporting substrate. The choice of substrate, its composition and structure, can strongly impact the thermal boundary conductance (TBC). Here we investigate the temperature-dependent TBC of 42 interfaces formed between a group of six 2D materials and seven crystalline and amorphous substrates. We use first-principles density functional perturbation theory to calculate the full phonon dispersion of the 2D layers and substrates and then input them into our model for phonon transport across the 2D–3D interface. Our results show that the TBC depends on the overlap between the vibrational frequencies and can be varied by nearly two orders of magnitude, from as low as ∼0.6 MW · m K−1 (h-BN on diamond) to ∼40 MW · m K−1 (h-BN on SiO2), for the same 2D layer by changing the substrate material. We find that amorphous materials significantly boost the TBC relative to their crystalline counterparts, assuming the two interfaces have the same adhesion, owing to the low-frequency Boson peak feature in their vibrational density of states (vDOS). For crystalline substrates, we further correlate constituent material properties with the calculated TBCs and find that the TBC strongly depends on a combination of the speed of sound, Debye temperature, and density of the substrate as well as the bandwidth of the flexural branch in the 2D material. We conclude that softer substrates with sharp low-frequency features in their vDOS, such as amorphous materials, polymers, and nanoparticles, could have higher TBC, leading to a trade-off between TBC and the thermal conductivity of the substrate.

Production origins and matrix constituents of spiculate pottery in Florida, USA: Defining ubiquitous St Johns ware by LA-ICP-MS and XRD

Fine-grained “chalky” pottery containing microscopic sponge spicules is commonly recovered from archaeological sites throughout peninsular Florida, but many questions remain about its composition and origins. It is identified by different names, but most are associated with the St. Johns Type series. While it has been commonly assumed to originate in the St. Johns River drainage for which it is named, the prevalence of pottery with these characteristics in other locations has presented the likelihood of independent production in multiple places. In this study, we conducted LA-ICP-MS and XRD analysis of spiculate pottery from three Woodland period (ca. 1000 BCE to 1000 CE) sites, along with comparative clay samples, in order to characterize the raw materials and determine the geographic scope of production. Our results support the theory that this ware was independently produced across peninsular Florida. We further evaluate the hypothesis that this pottery was made with common wetland muck, through consideration of the material properties of muck constituents. This project emphasizes the importance of an ecosystem framework for understanding the long history of spiculate pottery production and its geographic spread within Florida.

Tensile behavior of ultra-high performance concrete: Analytical model and experimental validation

An analytical model is proposed to predict the tensile behavior of ultra-high performance concrete (UHPC) considering the pullout behavior of single fiber in matrix. The fiber pullout behavior is modeled from the work mechanisms of stress transfer at the matrix-fiber interface. The effect of fiber orientation, fiber snubbing, and matrix spalling on the tensile responses of UHPC is investigated by a parametric study, and a fiber reinforcement efficiency function α is derived to describe the combined effect. The fiber characteristics, interfacial bond strength, and fiber reinforcement efficiency show significant influence on the tensile strength and ascending branch of tensile stress-crack width curves, while some other model coefficients affect the shape of curves. The accuracy of the proposed analytical model is validated with the experimental direct tension test of UHPC, and good agreements are achieved between the analytical and experimental tensile responses. An underestimated tensile strength is observed when the prediction model is utilized with a lower volume fraction of fiber due to less group effect. The analytical model proposed can be used to effectively predict the tensile behavior of UHPC once their basic constituent material properties and single fiber pullout data are known and to better design the UHPC in tension.

Characterization of pervious concrete exposed to high levels of evaporation

When fresh concrete is exposed to high levels of water by evaporation in structures with large ratios of surface/thickness these are prone to be altered in their constituent material properties, generating a decreased or poorer performance than they were conceived. The study shows the effects of high evaporation levels of exposure on pervious concretes, plain and reinforced with polymeric fibers exposed to extreme drying. By the continuous monitoring of the measured parameters, it is possible to establish the material behavior from the initial stage up to a stable behavior. Measurements of evaporation, vertical and horizontal strain and temperature served as parameters to support the theoretical framework of the behavior of the material by drying. The results of evaporation rates resulted very low if compared with non-pervious concretes and seem to be related with the corresponded strain values. Vertical and horizontal strains resulted similar between concretes and with very low values, where cracking was inhibited. The fiber addition resulted without any benefit in the strain reduction. An important factor found in the relationship between water loss and paste binder is the risk that w/b ratios arrive to critical values, where there is the possibility the minimum hydration kinetics cannot be completed, observed particularly in lower w/b concretes.

Natural frequency of earthen dams at different induced strain levels

A method is proposed to estimate the degradation of the first natural frequency of vibration of earthen dams with increase in strain levels induced due to seismic events. A synthesised wave, referred to as the ‘sum of sines’, generated by the superposition of sinusoidal waves with frequencies ranging from 0.01 Hz to 25 Hz, is scaled to different peak accelerations and used to simulate seismic excitations at the base of the dam. The natural frequency is then determined by studying the response of the structure in the frequency domain. For this study, four dams with varying complexities of geometry, constituent material properties, and known natural frequencies were at first selected from previously published literature to establish the validity of the proposed method for determining natural frequencies of the dams at small strain levels. Plane strain models of these structures were constructed and analysed, using a commercially available finite element method-based software that is capable of performing time-history analyses. Results from the analyses indicate a good agreement between the natural frequencies predicted using the proposed method and the frequency values reported in the literature for the corresponding structures at small strain levels. The method was later used to determine the strain-dependent natural frequency of a hydraulic-fill dam in North Texas. Twenty-one different earthquake conditions, with different peak ground accelerations, frequency contents, and mean periods, were used to thoroughly validate the applicability of the developed methodology. Numerical analyses indicate that the strain-dependent variation of the first natural frequency follows a similar trend as that obtained using the ‘sum of sines’ excitation when the dam is subjected to widely different earthquake conditions.