Time-dependent Material Properties Research Articles

A fluidic device for the controlled formation and real-time monitoring of soft membranes self-assembled at liquid interfaces

Membrane materials formed at the interface between two liquids have found applications in a large variety of technologies, from sensors to drug-delivery and catalysis. However, studying the formation of these membranes in real-time presents considerable challenges, owing to the difficulty of prescribing the location and instant of formation of the membrane, the difficulty of observing time-dependent membrane shape and thickness, and the poor reproducibility of results obtained using conventional mixing procedures. Here we report a fluidic device that facilitates characterisation of the time-dependent thickness, morphology and mass transport properties of materials self-assembled at fluid-fluid interfaces. In the proposed device the membrane forms from the controlled coalescence of two liquid menisci in a linear open channel. The linear geometry and controlled mixing of the solutions facilitate real-time visualisation, manipulation and improve reproducibility. Because of its small dimensions, the device can be used in conjunction with standard microscopy methods and reduces the required volumes of potentially expensive reagents. As an example application to tissue engineering, we use the device to characterise interfacial membranes formed by supra-molecular self-assembly of peptide-amphiphiles with either an elastin-like-protein or hyaluronic acid. The device can be adapted to study self-assembling membranes for applications that extend beyond bioengineering.

The capability of three seperable finite-strain viscoelastic models to predict response of a filled rubber material

Three integral-based finite strain viscoelastic models under the assumption of time-strain separability have been investigated within this work. To this end, experimental investigation has been conducted on a carbon black filled rubber, a mixture of natural rubber and bromobutyl. The monotonic tests were performed to capture the long-term response of the material. Relaxation tests were intented to identify the time-dependent material properties, and completed with a dynamic mechanical analysis. Models under consideration are Christensen, Fosdick and Yu and Simo model implemented in the finite element solver Abaqus. Under the assumption of an homegeneous incompressible material with a Mooney-Rivlin elastic potential, the response of the three models is compared for uniaxial tension and simple shear motions in time and frequency domains with respect to strain-rate, frequency and static predeformation dependencies. The equilibrium and non-equilibrium stress is predicted in a limited range mainly related to the choice and identification of the hyperelastic model. For stress relaxation, the long-term response is asymptotically reached with good accuracy, while the hysteritic response is mainly overestimated for Christensen model, and better approximated for both Fosdick and Yu and Simo models. In the frequency domain, Fosdick and Yu model shows no dependence on the predeformation. Christensen model is likely adapted to unfilled rubbers, and is unable to predict the softening effect due to the static predeformation effect. Simo model shows better prediction capabilities with a minimal set of material parameters.

Differential axial shortening and its effects in high rise buildings with composite concrete filled tube columns

Use of concrete filled steel tube columns is becoming increasingly popular in high rise buildings due to their composite action, superior strength, seismic and fire resistance capacities and construction simplicity. These composite columns and the reinforced concrete (RC) lift core in the framing systems of high rise buildings are commonly coupled with reinforced concrete outrigger and belt systems to facilitate lateral load resistance. Axial shortening (AS) of the vertical structural components due to time dependent phenomena of basic creep, shrinkage and elastic deformation, is a common problem in concrete high rise construction. The creep and shrinkage of these composite columns develop more rapidly, but are comparatively lower in magnitude than for RC columns due to the concrete core having no direct exposure to the external environment. There is a need for a comprehensive understanding of the differential axial shortening (DAS) in concrete filled tube (CFT) buildings which will be different from that in a RC building. An appraisal of the DAS and accurate quantification of all the adverse effects that can occur in a building due to DAS, are required to facilitate a safe and efficient design. This paper develops and applies a comprehensive technique to evaluate the DAS in a high rise building with composite CFTs. This technique incorporates the effects of (i) construction sequence and concrete levelling (ii) stress relaxation of concrete due to the presence of the steel tube (iii) time dependent material properties and (iv) effects of belt and outrigger systems. The technique has been validated using experimental data and is then illustrated through its application to a 60 storey building with CFT columns. The DAS between the vertical members are evaluated and its effects on the structural components are studied. Finally, the technique is applied to a similar building with RC columns and the results compared with those from the CFT building. The technique developed in this paper and the new information on DAS generated will facilitate safer designs of buildings with composite CFT columns.

Foundation Differential Settlement Included Time-dependent Elevation Control for Super Tall Structures

Due to the time-dependent properties of materials, structures, and loads, accurate time-dependent effects analysis and precise construction controls are very significant for rational analysis and design and saving project cost. Elevation control is an important part of the time-dependent construction control in supertall structures. Since supertall structures have numerous floors, heavy loads, long construction times, demanding processes, and are typically located in the soft coastal soil areas, both the time-dependent features of superstructure and settlement are very obvious. By using the time-dependent coupling effect analysis method, this paper compares Shanghai Tower's vertical deformation calculation and elevation control scheme, considering foundation differential settlement. The results show that the foundation differential settlement cannot be ignored in vertical deformation calculations and elevation control for supertall structures. The impact of foundation differential settlement for elevation compensation and pre-adjustment length can be divided into direct and indirect effects. Meanwhile, in the engineering practice of elevation control for supertall structures, it is recommended to adopt the multi-level elevation control method with relative elevation control and design elevation control, without considering the overall settlement in the construction process.

Prediction of Time-dependent Lateral Movement Induced by Differential Shortening in Tall Buildings Using Construction Stage Analysis

High-rise buildings move during construction due to time-dependent material properties of concrete (creep and shrinkage), construction sequences, and structural shapes. The building movements, including vertical and horizontal displacements, result from the sum of axial and lateral deformation of vertical members at each level. In addition to the vertical shortenings, the lateral movement induced by differential shortening can have adverse effects on the construction tolerance and serviceability of non-structural elements such as elevators and curtain walls. In this study a construction stage analysis method is developed to predict lateral movement induced by shortening, including the effect of creep and shrinkage. The algorithm of construction stage analysis is combined with the FE analysis program. It is then applied to predict lateral movement of a 58-story reinforced concrete building that was constructed in Kuala Lumpur, Malaysia. Gravity induced lateral movement of this building is predicted by the construction stage analysis. A field three-dimensional laser scanning survey is carried out to verify the prediction results, and satisfactory agreement is obtained.

A clinically applicable non-invasive method to quantitatively assess the visco-hyperelastic properties of human heel pad, implications for assessing the risk of mechanical trauma

IntroductionPathological conditions such as diabetic foot and plantar heel pain are associated with changes in the mechanical properties of plantar soft tissue. However, the causes and implications of these changes are not yet fully understood. This is mainly because accurate assessment of the mechanical properties of plantar soft tissue in the clinic remains extremely challenging. PurposeTo develop a clinically viable non-invasive method of assessing the mechanical properties of the heel pad. Furthermore the effect of non-linear mechanical behaviour of the heel pad on its ability to uniformly distribute foot-ground contact loads in light of the effect of overloading is also investigated. MethodsAn automated custom device for ultrasound indentation was developed along with custom algorithms for the automated subject-specific modeling of heel pad. Non-time-dependent and time-dependent material properties were inverse engineered from results from quasi-static indentation and stress relaxation test respectively. The validity of the calculated coefficients was assessed for five healthy participants. The implications of altered mechanical properties on the heel pad's ability to uniformly distribute plantar loading were also investigated in a parametric analysis. ResultsThe subject-specific heel pad models with coefficients calculated based on quasi-static indentation and stress relaxation were able to accurately simulate dynamic indentation. Average error in the predicted forces for maximum deformation was only 6.6±4.0%. When the inverse engineered coefficients were used to simulate the first instance of heel strike the error in terms of peak plantar pressure was 27%. The parametric analysis indicated that the heel pad's ability to uniformly distribute plantar loads is influenced both by its overall deformability and by its stress-strain behaviour. When overall deformability stays constant, changes in stress/strain behaviour leading to a more “linear” mechanical behaviour appear to improve the heel pad's ability to uniformly distribute plantar loading. ConclusionsThe developed technique can accurately assess the visco-hyperelastic behaviour of heel pad. It was observed that specific change in stress-strain behaviour can enhance/weaken the heel pad's ability to uniformly distribute plantar loading that will increase/decrease the risk for overloading and trauma.

Short- and long-term analyses of shear lag in RC box girders considering axial equilibrium

An analytical method considering axial equilibrium is proposed for the short- and long-term analyses of shear lag effect in reinforced concrete (RC) box girders. The axial equilibrium of box girders is taken into account by using an additional generalized displacement, referred to as the longitudinal displacement of the web. Three independent shear lag functions are introduced to describe different shear lag intensities of the top, bottom, and cantilever plates. The time-dependent material properties of the concrete are simulated by the age-adjusted effective modulus method (AEMM), while the reinforcement is assumed to behave in a linear-elastic fashion. The differential equations are derived based on the longitudinal displacement of the web, the vertical displacement of the cross section, and the shear lag functions of the flanges. The time-dependent expressions of the generalized displacements are then deduced for box girders subjected to uniformly distributed loads. The accuracy of the proposed method is validated against the finite element results regarding the short- and long-term responses of a simply-supported RC box girder. Furthermore, creep analyses considering and neglecting shrinkage are performed to quantify the time effects on the long-term behavior of a continuous RC box girder. The results show that the proposed method can well evaluate both the short- and long-term behavior of box girders, and that concrete shrinkage has a considerable impact on the concrete stresses and internal forces, while concrete creep can remarkably affect the long-term deflections.

Transient heat transfer analysis using Galerkin finite element method for reinforced concrete slab exposed to high elevated temperature

Fire loading causes a critical collapse of RC (Reinforced Concrete) Structures since the embedded steels inside are relative week against high elevated temperature. Several numerical frameworks for fire resistance have been proposed, however they have limitations such as unstable convergence and long calculation period. In the work, 2-D nonlinear FE technique is proposed using Galerkin method for RC structures under fire loading. Closed-form element stiffness with a triangular element is adopted and verified with fire test on three RC slabs with different fire loading conditions. Several simulations are also performed considering fire loading conditions, water contents, and cover depth. The proposed numerical technique can handle time-dependent fire loading, convection, radiation, and material properties. The proposed technique can be improved through early-aged concrete behavior like moisture transport which varies with external temperature.

The Effect of Residual Stress on Coupling Power Loss of VCSEL Modulus

Abstract In this article, the effect of residual stress on active region misalignment of laser light sources for vertical cavity surface emitting laser (VCSEL) modulus has been studied. The post-weld-shift variation of active region introduced from the residual stress distribution variation for different VCSEL solder joints, i.e. tin-silver (Sn/3.5Ag) and tin-lead (Sn/37Pb) solder joints, are simulated by employing the thermal-elastic-plastic finite element model of MARC package. The ball grid solidification in reflow process and the creep deformation in the acceleration aging were simulated and analyze. The time and temperature dependent material properties of solders are employed. Numerical results indicate that the post-weld-shift introduced from residual stress in the solidification process are significant, and are also the key reasons to reduce the coupling efficiency in VCSEL packaging. The non-sequential components models in commercial Zemax optical package were used to estimate the optical coupling power loss between the active region and the fiber tip in VSCEL modulus. The results show that the proposed post-weld-shift model is feasible to analyze and to improve the solder joint design in the VCSEL packaging.

Thermally Activated Deformation Behavior of ufg-Au: Environmental Issues During Long-Term and High-Temperature Nanoindentation Testing.

For testing time-dependent material properties by nanoindentation, in particular for long-term creep or relaxation experiments, thermal drift influences on the displacement signal are of prime concern. To address this at room and elevated temperatures, we tested fused quartz at various contact depths at room temperature and ultra-fine grained (ufg) Au at various temperatures. We found that the raw data for fused quartz are strongly affected by thermal drift, but corrected by use of dynamic stiffness measurements all the datasets collapse. The situation for the ufg Au shows again that the data are only useful with drift correction, but with this applied it turns out that there is a significant change of elastic and plastic properties when exceeding 200°C, which is also reflected by an increasing strain rate sensitivity.

In vivo measurements and numerical analysis of the biomechanical characteristics of the human periodontal ligament

The periodontal ligament is a complex tissue with respect to its biomechanical behaviour. It is important to understand the mechanical behaviour of the periodontal ligament during physiological loading in healthy patients as well as during the movement of the tooth in orthodontic treatment or in patients with periodontal disease, as these might affect the mechanical properties of the periodontal ligament (PDL). Up to now, only a limited amount of in vivo data is available concerning this issue.The aim of this study has been to determine the time dependent material properties of the PDL in an experimental in vivo study, using a novel device that is able to measure tooth displacement intraorally. Using the intraoral loading device, tooth deflections at various velocities were realised in vivo on human teeth. The in vivo investigations were performed on the upper left central incisors of five volunteers aged 21–33 years with healthy periodontal tissue. A deflection, applied at the centre of the crown, was linearly increased from 0 to 0.15mm in a loading period of between 0.1 and 5.0s. Individual numerical models were developed based on the experimental results to simulate the relationship between the applied force and tooth displacement. The numerical force/displacement curves were fitted to the experimental ones to obtain the material properties of the human PDL. For the shortest loading time of 0.1s, the experimentally determined forces were between 7.0 and 16.2N. The numerically calculated Young's modulus varied between 0.9MPa (5.0s) and 1.2MPa (0.1s). By considering the experimentally and numerically obtained force curves, forces decreased with increasing loading time. The experimental data gained in this study can be used for the further development and verification of a multiphasic constitutive law of the PDL.

Investigation of Stress in MEMS Sensor Device Due to Hygroscopic and Viscoelastic Behavior of Molding Compound

The stresses due to moisture saturation on microelectromechanical systems (MEMS) sensor devices after exposure to temperature cycling have been addressed. Moisture-, temperature-, and time-dependent material property of molding compounds for the MEMS devices were characterized. To determine the coefficient of hygroscopic swelling of a molding compound and diffusivity (D) of water in the molding compound, dimensional change and weight loss of moisture saturated samples at various temperatures were monitored by the digital image correlation method combined with a weight scale. To obtain the viscoelastic property of the molding compound, a series of stress relaxation tests was performed using dynamic mechanical analysis (DMA). To explain the moisture-induced viscoelastic behavior, a simple assumption was introduced based on the temperature of glass transition point (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ) shift from the DMA result. The experimental data were utilized in numerical simulations to estimate the temperature- and moisture-induced stress on MEMS sensor devices subjected to temperature cycles.

Non-linear material modeling of fiber-reinforced polymers based on coupled viscoelasticity–viscoplasticity with anisotropic continuous damage mechanics

A meso-scale material modeling of the fiber-reinforced polymer composites is presented. This model is based on coupled anisotropic viscoelasticity–viscoplasticity with anisotropic continuous damage mechanics. The constitutive equations are derived for three-dimensional statement and integrated implicitly by using return-mapping algorithm. Viscoelasticity model implies time dependent material properties. Viscoplasticity model is based on modified Hoffman criterion with combination of the Perzyna model. Anisotropic damage model is based on extended three-dimensional damage model. Developed material model is implemented in ABAQUS/Standard and is applied for modeling glass and carbon fiber laminate composite plates with various stacking sequences. Obtained results are in high agreement with already published experimental data.

Integrating durability-based service-life predictions with environmental impact assessments of natural fiber–reinforced composite materials

As concern about resource conservation has grown, research efforts have increased to develop materials out of rapidly renewable constituents, to assess their life cycle environmental impacts, and to predict their service-life performance. Assessing time-dependent material property deterioration, often a concern for polymers and their composites, is essential to evaluating the viability of novel materials to serve as lower environmental impact replacements for conventional materials. However, research in methods to combine environmental impacts from production and material deterioration is limited. In this research, a durability-based service-life model was used to assess and incorporate composite deterioration into life cycle environmental impact analyses. The inclusion of composite deterioration under differing temperature and moisture conditions in these analyses typically resulted in higher volumes of material needed to serve the desired function compared to volumes needed to satisfy initial design requirements, leading to a change in environmental impact. While this concept falls in line with the classic definition of material efficiency, namely improving mass yield for materials, the results of the life cycle impact assessment showed with certain process modifications lower environmental impacts could be achieved even in cases where more material was being used. These findings indicate that design decisions must account for application-specific requirements and consider environmental impacts concurrently with material deterioration.

Computer modelling of structures with account of the construction stages and the time dependent material properties

Numerical studies are performed on computer models taking into account the stages of construction and time dependent material properties defined in two forms. A 2D model of three storey two spans frame is created. The first form deals with material defined in the usual design practice way - without taking into account the time dependent properties of the concrete. The second form creep and shrinkage of the concrete are taken into account. Displacements and internal forces in specific elements and sections are reported. The influence of the time dependent material properties on the displacement and the internal forces in the main structural elements is tracked down. The results corresponding to the two forms of material definition are compared together as well as with the results obtained by the usual design calculations. Conclusions on the influence of the concrete creep and shrinkage during the construction towards structural behaviour are made.

Theory of Time-Temperature-Stress Equivalent Principle Based on Schapery Equation and Its Application on Granite

The time-temperature-stress equivalent (TTSE) principle refers to the phenomenon that the time-dependent mechanical properties of materials rely on the variations of temperature and stress level. Thus, it is reasonable to predict the long time material mechanical properties based on their relationships with rising temperatures and stress levels. According to the single-integral nonlinear constitutive equation proposed by Schapery, a general expression of the TTSE principle for nonlinear viscoelastic creep property is deduced. The specific expression for time-temperature-stress shift factor is presented by assuming quadratic polynomial form of the Doolittle formula for the viscosity as a function of free volume. Creep curves of granite at different temperatures and stress levels are introduced, and the validation of applying time-temperature-stress principle to predict the long time creep property of granite is provided. The master curves are generated through vertical shift modification and horizontal shift equivalence of all creep curves, and corresponding coefficients in the shift equations are determined by curve fittings. The results indicate the modified time-temperature-stress equivalent relation is appropriate to predict the long time creep property of granite. DOI: http://dx.doi.org/10.5755/j01.ms.20.4.6294

Stress analysis of thermal barrier coating system subjected to out-of-phase thermo-mechanical loadings considering roughness and porosity effect

This paper presents the out-of-phase thermo-mechanical stress analysis of thermal barrier coating (TBC) system in real working conditions used as thermal barrier in diesel engine cylinder heads. The coating system in this research comprises 350μm zirconium oxide top coat (TC) and 150μm metallic bond coat (BC). These layers were deposited on the substrate, aluminum A356 alloy, by the aid of air plasma spray (APS) method. Afterwards, the specimen was subjected to thermo-mechanical fatigue (TMF) loadings. Based on the experimental conditions, FE simulations were performed by both time-independent and time-dependent substrate material properties in ABAQUS software. Simulation results related to heat transfer analysis demonstrate only about 10.5% comparative error compared to experimental results. Moreover, defining time-dependent properties, which were obtained from two-layer visco-plastic model, yields results with 15% less comparative error in comparison to the results based on time-independent material properties. In addition, the effects of roughness and porosity in coating layers and substrate were studied on three different models by the aid of a scanning electron microscopy image. Obtained results based on real geometry illustrate that consideration of porosity in TC layer has an effective role in the stress distribution of this layer. However, BC layer stress distribution is much more dependent on interface morphology.

Construction stages analyses using time dependent material properties of concrete arch dams

Abstract. This paper presents the effects of the construction stages using time dependent material properties on the structural behaviour of concrete arch dams. For this purpose, a double curvature Type-5 arch dam suggested in ‘‘Arch Dams’’ symposium in England in 1968 is selected as a numerical example. Finite element models of Type-5 arch dam are modelled using SAP2000 program. Geometric nonlinearity is taken into consideration in the construction stage analysis using P-Delta plus large displacement criterion. In addition, the time dependent material strength variations and geometric variations are included in the analysis. Elasticity modulus, creep and shrinkage are computed for different stages of the construction process. In the construction stage analyses, a total of 64 construction stages are included. Each stage has generally 6000 m 3 concrete volume. Total duration is taken into account as 1280 days. Maximum total step and maximum iteration for each step are selected as 200 and 50, respectively. The structural behaviour of the arch dam at different construction stages has been examined. Two different finite element analyses cases are performed. In the first case, construction stages using time dependent material properties are considered. In the second case, only linear static analysis (not considered construction stages) is taken into account. Variation of the displacements and stresses are obtained from the both analyses. It is highlighted that construction stage analysis using time dependent material strength variations and geometric variations has an important effect on the structural behaviour of arch dams. The maximum longitudinal, transverse and vertical displacements obtained from construction stages and static analyses are 1.35 mm and 0 mm; -8.44 and 6.68 mm; -4.00 and -9.90 mm, respectively. In addition, vertical displacements increase from the base to crest of the dam for both analyses. The maximum S11, S22 and S33 stresses are obtained as 1.60MPa and 2.84MPa; 1.39MPa and 2.43MPa; 0.60MPa and 0.50MPa, respectively. The differences between maximum longitudinal, transverse, and vertical stresses obtained from construction stage and static analyses are 78%, 75%, and %17, respectively. On the other hand, there is averagely 12% difference between minimum stresses for all three directions.

Determination of structural behavior of Bosporus suspension bridge considering construction stages and different soil conditions

In this paper, it is aimed to determine the structural behavior of suspension bridges considering construction stages and different soil conditions. Bosporus Suspension Bridge connecting the Europe and Asia in Istanbul is selected as an example. Finite element model of the bridge is constituted using SAP2000 program considering existing drawings. Geometric nonlinearities are taken into consideration in the analysis using P-Delta large displacement criterion. The time dependent material strength of steel and concrete and geometric variations is included in the analysis. Time dependent material properties are considered as compressive strength, aging, shrinkage and creep for concrete, and relaxation for steel. To emphases the soil condition effect on the structural behavior of suspension bridges, each of hard, medium and soft soils are considered in the analysis. The structural behavior of the bridge at different construction stages and different soil conditions has been examined. Two different finite element analyses with and without construction stages are carried out and results are compared with each other. At the end of the analyses, variation of the displacement and internal forces such as bending moment, axial forces and shear forces for bridge deck and towers are given in detail. Also, displacement and stresses for bridge foundation are given with detail. It can be seen from the analyses that there are some differences between both analyses (with and without construction stages) and the results obtained from the construction stages are bigger. It can be stated that the analysis without construction stages cannot give the reliable solutions. In addition, soil condition have effect on the structural behavior of the bridge. So, it is thought that construction stage analysis using time dependent material properties, geometric nonlinearity and soil conditions effects should be considered in order to obtain more realistic structural behavior of suspension bridges.

A new viscoelastic model based on generalized method of cells for fiber-reinforced composites

A semi-analytical model is constructed to investigate the relaxation properties of polymer composites consisting of linearly viscoelastic matrices and transversely isotropic elastic fibers. A representative unit cell is subjected to some prescribed loadings to value the time-dependent properties of composite materials. The model is derived to describe the anisotropic viscoelastic properties of composite materials as functions of matrix and fiber properties. The present numerical algorithm is used to predict the viscoelastic properties of a graphite/epoxy composite and the results are compared with the finite element analysis of the micromechanical model. Very good agreement between numerical algorithm and finite element analysis results is illustrated for the viscoelastic properties of anisotropic composite materials.