Single Material Model Research Articles

Structural behavior of small-scale reinforced concrete secant pile wall

PurposeTo predict the real behavior of the full-scale model using a scale model, optimized simulation should be achieved. In reinforced concrete (RC) models, scaling can be substantially more critical than in single-material models because of multiple reasons such as insufficient bonding strength between small-diameter steel bars and concrete, and excessive aggregate size. Overall, there is a shortfall of laboratory and field-testing studies on the behavior of secant pile walls under lateral and axial loads. Accordingly, the purpose of this study is to investigate the validity and the performance of the 1/10th scaled RC secant pile wall under the influence of different types of loading.Design/methodology/approachThe structural performance of the examined models was evaluated using two types of tests: bending and axial compression. A self-compacting concrete mix was suggested, which provided the best concrete mix workability and appropriate compressive strength.FindingsUnder axial and bending loads, the failure modes were typical. Where the plain and reinforced concrete piles worked in tandem to support the load throughout the loading process, even when they failed. The experimental results were relatively consistent with some empirical equations for calculating the modulus of elasticity and critical buckling load. This confirmed the validity of the proposed model.Originality/valueAccording to the analysis and verification of experimental tests, the proposed 1/10th scaled RC secant pile model can be used for future laboratory purposes, especially in the field of geotechnical engineering.

Simulating brittle and ductile response of alumina ceramics under dynamic loading

Alumina ceramic is often used in armour systems. This material is known to have a brittle response under tensile loading, while a ductile response is found when sufficiently high pressures are reached. During projectile impact a ceramic material experiences both tensile loading and high pressures, hence fails in both a brittle and ductile way. Properly capturing the ceramic failure in a single material model remains challenging. A viscosity regularized Johnson-Holmquist-2 model has been used to simulate dynamic loading on alumina ceramic. The simulations show that the brittle and ductile nature of the material can not be captured simultaneously in the current material model. A new failure strain formulation is proposed where the behaviour under tensile and compressive loading can be controlled independently. This allows to properly capture both the brittle and ductile response of the material in a single constitutive framework, with a single set of model parameters.

Characterization of perfused and sectioned liver tissue in a full indentation cycle using a visco-hyperelastic model

Realistic modeling of biologic material is required for optimizing fidelity in computer-aided surgical training and assistance systems. The modeling of liver tissue has remained challenging due to its nonlinear viscoelastic properties and high hysteresis of the stress-strain relation. While prior studies have described the behavior of liver tissue during the loading status (in elongation, compression, or indentation tests) or unloading status (in stress relaxation or creep tests), a hysteresis curve with both loading and unloading processes was incompletely defined. We seek to use a single material model to characterize the mechanical properties of liver tissue in a full indentation cycle ex vivo perfused and then sectioned. Based on measurements taken from ex-vivo perfused porcine livers, we converted force-displacement curves to stress-strain curves and developed a visco-hyperelastic constitutive model to characterize the liver's mechanical behavior at different locations under various rates of indentation (1, 2, 5, 10, and 20 mm/s). The proposed model is a mixed visco-hyperelastic model with up to 6 coefficients. The normalized root mean square standard deviations of fitted curves are less than 5% and 10% in low (<0.05) and high strain (>0.3) conditions respectively.

Printing conductive plastics

Researchers at Loughborough University, UK, have been looking at how 3D printed conductive plastic materials respond to strain, and how this should influence design of printed electronic components like sensors. 3D printing initially started as a rapid prototyping technique that was only capable of printing single material models using non-functional thermoplastics. Recently, both research and industry has focused on printing multiple materials with functional electronics. Industry has released 3D printers that purely focus on interconnect, such as Voxel8 and Nano-dimension, which are a modern take on printed circuit board manufacturing. Currently, research is focusing on the 3D printing of passive electronic components, such as sensors. This progression has mirrored the rise in 2D printed electronics where development of interconnect led to printed passive components and then to fully functional printed devices. 3D printing is a sustainable, efficient manufacturing process with little waste. The ability to fully print a multi-material, three-dimensional and functional object gives the opportunity for electronic engineers and researchers to rapidly iterate and revolutionise the design of electronic systems. Jack McGhee with the 3D printer used to print conductive plastics at Loughborough University Printed electronic sensor made of conductive plastic To date, 3D printable conductive plastics tend to be the commonly-used thermoplastics polylactic acid and acrylonitrile butadiene styrene filaments, which have been mixed with conductive carbon. The electronic and mechanical characteristics of these materials are important for 3D printing and additive manufacturing to initiate a new industrial revolution. Material extrusion-based 3D printing uses layer-by-layer deposition. The effect of this manufacturing method on the electrical properties of the plastic to create reproducible applications needs to be better understood. In this Issue of Electronics Letters, Jack McGhee and colleagues at Loughborough take three sets of 3D printable conductive plastics and measure changes in resistance of the material in response to an applied strain. This was repeated for all three plastics in order to quantify the effect of the three-dimensionality of the material with different layer thicknesses. Altering the layer thickness showed consistent, repeatable and reproducible strain-sensing measurements until a specific material thickness was reached. At a certain thickness, the mechanical strength of the material increases the resistance upon bending. However, the manufacturing process creates a solid structure from tightly packed threads where upon bending threads move closer together to increase conductivity. This combination of factors creates erratic behaviour in the material. Analysis of this enabled the authors to create guidance for the design and fabrication of functional, reliable and repeatable 3D printed electronic sensors. A force sensing resistor platform that outputs the measurements to an LCD display was printed in this Letter to demonstrate an application for the work. For the additive manufacturing of reproducible devices and applications, how the manufacturing process affects the materials and the material properties needs to be understood. This Letter provides insight into how the manufacturing method, material extrusion 3D printing, and material structure fabricated from the technique, affects the bulk properties of a printed functional device. The results show how the layer-by-layer nature of the printing process can affect the properties of strain sensing devices made using conductive plastics. In the short term, the characterisations reported in this Letter can inform other researchers working on 3D printed electronics. Also, any person with a 3D printer can follow the guidance for the design and fabrication for 3D-printed strain sensors, which can be used with low-cost, low-profile and easily accessible parts, rather than with bulky signal amplification equipment. In the longer term, if 3D functional devices are to be mass produced, the manufacturers need to know a set of design rules to follow. This paper contributes to a larger set of knowledge that manufacturers will need to know to print devices. In general, McGhee and colleagues work with 2D printed electronics and ink formulations with new materials. Since this work, the authors have mainly focused on formulating conductive inks from transparent conducting oxides. “This paper was largely an exploratory study into the potential of 3D printed electronics and we are planning on using the insight to start introducing inks for 2D printed electronics into 3D printed objects” explains McGhee. Over the next 10 years, McGhee believes it would be interesting to see a conductive plastic made with metallic conductive filler, such as silver rather than graphite. McGhee says “seeing how the properties of this material compares to printed silver ink interconnect could enable new avenues of 3D printed electronics to be opened.” The general direction of 3D printed electronics currently suggests functional materials will be printed as inks and pastes in combination with non-functional thermoplastics.

Harmonic balanced Jiles-Atherton hysteresis implementation for finite element simulation

PurposeThe high calculation effort for accurate material loss simulation prevents its observation in most magnetic devices. This paper aims at reducing this effort for time periodic applications and so for the steady state of such devices.Design/methodology/approachThe vectorized Jiles-Atherton hysteresis model is chosen for the accurate material losses calculation. It is transformed in the frequency domain and coupled with a harmonic balanced finite element solver. The beneficial Jacobian matrix of the material model in the frequency domain is assembled based on Fourier transforms of the Jacobian matrix in the time domain. A three-phase transformer is simulated to verify this method and to examine the multi-harmonic coupling.FindingsA fast method to calculate the linearization of non-trivial material models in the frequency domain is shown. The inter-harmonic coupling is moderate, and so, a separated harmonic balanced solver is favored. The additional calculation effort compared to a saturation material model without losses is low. The overall calculation time is much lower than a time-dependent simulation.Research limitations/implicationsA moderate working point is chosen, so highly saturated materials may lead to a worse coupling. A single material model is evaluated. Researchers are encouraged to evaluate the suggested method on different material models. Frequency domain approaches should be in favor for all kinds of periodic steady-state applications.Practical implicationsBecause of the reduced calculation effort, the simulation of accurate material losses becomes reasonable. This leads to a more accurate development of magnetic devices.Originality/valueThis paper proposes a new efficient method to calculate complex material models like the Jiles-Atherton hysteresis and their Jacobian matrices in the frequency domain.

Stiffness optimization of multi-material composite structure under dependent load

Aiming at the characteristics that gravity load changes with the disappearance of unit in the topology optimization iteration, the optimization model of multi-material stiffness is established considering the dependent load. The structural stiffness optimization is applied to the multi-material composite structure. The sensitivity of the mathematical formula is derived, and the complete optimization iteration process is given. Base on multi-material theory, the relationship between the multi-material model and the single material model is discussed. Two kinds of models are used to analyze the topology optimization of the structure which contain two kinds of materials and three kinds of materials, respectively. Then, the optimization results of the two models are compared, which are consistent with the laws of mechanical properties of materials. The obtained structure is similar to the actual engineering structure. It demonstrates the method is feasible to solve the topology optimization of a single material or a variety of materials.

Modeling shockwaves and impact phenomena with Eulerian peridynamics

Most previous development of the peridynamic theory has assumed a Lagrangian formulation, in which the material model refers to an undeformed reference configuration. In the present work, an Eulerian form of material modeling is developed, in which bond forces depend only on the positions of material points in the deformed configuration. The formulation is consistent with the thermodynamic form of the peridynamic model and is derivable from a suitable expression for the free energy of a material. It is shown that the resulting formulation of peridynamic material models can be used to simulate strong shock waves and fluid response in which very large deformations make the Lagrangian form unsuitable. The Eulerian capability is demonstrated in numerical simulations of ejecta from a wavy free surface on a metal subjected to strong shock wave loading. The Eulerian and Lagrangian contributions to bond force can be combined in a single material model, allowing strength and fracture under tensile or shear loading to be modeled consistently with high compressive stresses. This capability is demonstrated in numerical simulation of bird strike against an aircraft, in which both tensile fracture and high pressure response are important.

Characterizing and Simulating Needle Insertion Forces for Percutaneous Renal Access.

Percutaneous needle access is a critical step of performing percutaneous nephrolithotomy. Development of a synthetic model that accurately represents the forces encountered while gaining percutaneous renal access, allows for high ease of use and prevention of negative skill transfer. The objectives of this study were to define the needle insertion forces used during percutaneous renal access and to develop a multilayer synthetic physical simulator model based on human tissue data that is compatible with the SimPORTAL fluoro-less C-arm trainer (CAT) camera system. Needle insertion data were collected using in situ fresh human cadaveric tissue within 72 hours of death. Ultrasound guidance was used to place percutaneous reference needles into the kidney, and axial force vs displacement data was collected using a custom-built force measurement device. A novel multilayer model that includes several types of synthetic materials for simulation of distinct tissue layers was developed based on the human tissue reference data. The multilayer prototype model and an existing single material model were subsequently tested using the same needle insertion protocol and the results were compared with human tissue data. Average maximum forces for needle puncture into skin ranged from 2.75 to 2.80 N for human tissue and from 4.53 to 4.19 N for simulated human tissue. The overall slope for all force vs displacement data was 0.35 to 0.59 N/cm for human tissue. The overall slope was comparatively lower for the multilayer model (0.17 N/cm) and was highest in the existing single material model (5.08-9.79 N/cm). We have defined the forces for percutaneous renal access using fresh human cadaveric tissue and designed a multilayer synthetic simulator model that can be utilized for training percutaneous needle access to the renal collecting system using the CAT camera system.

The Effects of Bony Structure Simplification Methods on the Biomechanics of Tibiofemoral Joint in Series of Flexion Angles

To investigate the effects of bony structure simplification methods on the biomechanics of tibiofemoral joint under compression and torsion effects in series of flexion angles, the MRI images of the normal human knee at flexion angles of 0°/25°/60°/80° were developed through SONATA MAESTRO 1.5T, then through the technology of threshold segmentation and registration assembly, bone tissue distinguished models and single material models were built based on these images. The results show that: (1) at the flexion angles of 0°/60°/80°, the maximum equivalent stress on femur and femoral cartilage significantly were slightly higher than single material models, only at 25 °, the maximum equivalent stress of the femoral cartilage in single material model was obviously larger; (2) difference of maximum equivalent stress on tibia and tibial cartilage between two kinds of models was not significant, and stress increased with the increase of flexion angle, only at 80 °, the stress on tibia of bone tissue distinguished models reduced; (3) The skeletal load was borne mainly by the cortical bone.

Finite Element Modeling of a Baseball

A baseball consists of a spherical cork core that is wrapped with two layers of rubber, three layers of wool, one layer of cotton, cement glue and one layer of cowhide. The respective nonlinear and time-dependent material properties of these layers make it challenging to capture the behavior of a baseball accurately over a range of impact speeds. As a result of this challenge, finite element models (FEMs) of the baseball typically treat the baseball as a single viscoelastic material where the respective material properties have been homogenized. These single-material models do not necessarily possess the same hyperelastic and viscoeleastic qualities as a real baseball, and these missing behaviors limit the applicability of these baseball models to simulate a bat-ball collision over a very narrow range of impact speeds. In the current work, a layer-by-layer approach is used to create a robust baseball FEM. The pill, which is a cork core wrapped in two layers of rubber, were subjected to bounce and compression testing at a wide range of impact speeds and compression rates. These data are used to tune the pill properties and ensure close agreement between experimental and FEM results. The remaining layers are added individually until the complete baseball is modeled and agrees closely with experimental results over a wide range of impact speeds. Possible applications for a new and improved baseball FEM include accurate simulations of baseball testing and bat-ball collisions. The improved ball model can also be used to predict performance of future bat designs not yet manufactured.

Prediction of shear strength of reinforced concrete beams by nonlinear finite element analysis

This paper describes the results of a nonlinear finite element analysis to predict the shear strength of rectangular reinforced concrete beams using a single material model for concrete. Over one hundred beams were analysed covering all important parameters governing shear strength. The results of the analysis enable one to predict ‘good’ lower bound to the experimental values. Unlike many investigations, emphasis was placed not only on good prediction of the ultimate shear strength, but also on the fact that the predicted mode of failure corresponded closely with the observed mode of failure. It is hoped that the investigation will encourage wide spread use of nonlinear analysis to predict the performance of unconventional structures.

Influence of weld material properties on fracture mechanics parameters in welds analysed by FEM calculations

Results of elastic-plastic FEM calculations on various plane strain models of a CT-100 specimen with the fusion line of welds of different weld material properties in the ligament, and of single material with properties of the weld components (i.e. base and weld material and the heat affected zone) are compared and discussed in the light of fracture mechanics considerations. Regarding the cleavage fracture regime, considerably higher crack opening stresses are obtained in the welded specimen with the high strength weld material than in that with low strength weld material, as soon as small-scale yielding occurs. This means that in the cleavage fracture regime reduction of yield strength of the weld material will decrease the temperature at which cleavage fracture will still occur. Conservative evaluation of the crack opening stresses of the welded joint can be achieved by using the welding simulated material properties. The J- integral can be calculated from the load versus load-line displacement curve, for the models with a weld also. The crack opening profiles for both welds are asymmetric to the ligament. Larger displacements occur on the weld material side than on the side of the heat affected zone. The relationship between crack tip opening displacement and load-line displacement is independent of whether a single-material model or a model with a weld is considered. A linear dependence of the crack tip opening displacement on the J- integral also exists for the models with a weld. General yield occurs on the base-material side for the weld with high strength weld material, and symmetrically to the ligament for that with low strength weld material.