Nonlinear Stress-strain Characteristics Research Articles

Machine Learning-Enabled Environmentally Adaptable Skin-Electronic Sensor for Human Gesture Recognition.

Stretchable sensors have been widely investigated and developed for the purpose of human motion detection, touch sensors, and healthcare monitoring, typically converting mechanical/structural deformation into electrical signals. The viscoelastic strain of stretchable materials often results in nonlinear stress-strain characteristics over a broad range of strains, consequently making the stretchable sensors at the body joints less accurate in predicting and recognizing human gestures. Accurate recognition of human gestures can be further deteriorated by environmental changes such as temperature and humidity. Here, we demonstrated an environment-adaptable high stress-strain linearity (up to ε = 150%) and high-durability (>100,000 cycles) stretchable sensor conformally laminated onto the body joints for human gesture recognition. The serpentine configuration of our ionic liquid-based stretchable film enabled us to construct broad data sets of mechanical strain and temperature changes for machine learning-based gesture recognition. Signal recognition and training of distinct strains and environmental stimuli using a machine learning-based algorithm analysis successfully measured and predicted the joint motion in a temperature-changing environment with an accuracy of 92.86% (R-squared). Therefore, we believe that our serpentine-shaped ion gel-based stretchable sensor harmonized with machine-learning analysis will be a significant achievement toward environmentally adaptive and multianalyte sensing applications. Our proposed machine learning-enabled multisensor system may enable the development of future electronic devices such as wearable electronics, soft robotics, electronic skin, and human-machine interaction systems.

The influence of aspect ratio on the determination of tearing energy in mode I fracture tests

The energy for tearing is a classical measure of fracture toughness for elastic materials at finite strains. The prominent approach to quantify the tearing energy utilizes a tensile test on edge-cut thin rectangular specimens with low height-to-width ratio to determine the critical stretch at which a crack propagates from the tip of the cut. The analysis of the experiment, proposed by Rivlin and Thomas (1953), relies on the assumption that a large portion of the test piece is in a state of plane strain, and that the change of the total elastic energy is equal to the energy release due to the formation of new crack surfaces. While these assumptions are well justified for test pieces with sufficiently low height-to-width ratio, limitations in the availability and homogeneity of various sample materials have enforced the use of specimens with higher height-to-width ratios. In order to analyse the applicability of the classical theory and the corresponding errors, we investigate in the present work the influence of the sample’s height-to-width ratio on the estimation of the tearing energy in mode I fracture tests. Exemplified with experiments and simulations for the elastomer Ecoflex 1:1, we show that reliable measurements can be obtained even for a quadratic sample geometry. For tough materials, however, significant overestimation of the tearing energy is expected already for a height-to-width ratio larger than 1/2. Surprisingly, in very brittle materials the tearing energy is vice-versa prone to be underestimated by up to 10% for ratios up to 1. The error also depends on the non-linear stress–strain characteristics, as illustrated by the use of different constitutive models. While a generally valid geometrical criterion cannot be defined based on the present results, our results suggest that the error hardly exceeds 10% for height-to-width ratios of up to 1/2.

Mechanical properties and nonlinear deformation description model of cement stone

The cement stone exhibits obvious plastic deformation performance under confining pressure, and a simple linear elastic model cannot describe the entire stress-strain process. In this work, the mechanical properties of oil well cement stone are tested and analyzed from different aspects by uniaxial compression test and triaxial stress test of typical cement stone. Based on obtaining the mechanical properties of cement stone and referring to the nonlinear elastic constitutive theory of concrete, an uncoupled secant bulk modulus and secant shear modulus total stress-strain description equation is constructed to describe the nonlinearity deformation properties of cement stone. The research results show that cement stone is a pressure-sensitive material, and its mechanical properties are greatly affected by confining pressure. The linear elastic constitutive model is a simplified and approximate description of the deformation process of cement stone. Like concrete, cement stone has functional relationships between the secant bulk modulus and octahedral normal strain, as well as the secant shear modulus and octahedral shear strain of different cement stones. Substituting the fitted function into the linear elastic constitutive model, the nonlinear deformation description model of cement stone can be obtained, which can effectively describe the nonlinear stress-strain characteristics of cement stone, especially in the case of confining pressure.

Effect of Structural Bonding Patterns on Mechanical Characteristics of Clay Brick Masonry under Different Loadings Using Digital Image Correlation Technique

This paper assesses the effect of two different structural bonding patterns on the mechanical characteristics of clay brick masonry assemblages under compression, flexure, and shear loading conditions. A 2D digital image correlation technique (DIC) was used to appraise the nonlinear compressive stress-strain characteristics, orthotropic properties, damage, and failure mechanisms of masonry prisms. The experimental investigation ascertained that full-brick thick masonry exhibited reasonably higher capacity and deformation characteristics than half-brick thick masonry under compression, flexure, and diagonal compression loadings, except in bed joint shear strength. The compressive strength and corresponding strain of full-brick thick Flemish bond prisms were higher and had more deformable (maximum strain recorded 2.4 times) behavior than half-brick thick masonry prisms. The failure mechanics of the masonry was quantitatively established by strain analysis obtained from DIC. The rupture modulus of Flemish bond masonry walls was four times higher, and its failure (a combination of tensile splitting of bricks and bed joint failure) was gradual compared to stack bond masonry. The diagonal compression strength of full-brick thick masonry wallettes was about 1.3 times as large as running bond wallettes, as the bond arrangement resisted diagonal cracks propagation. The applicability of the available idealized compressive stress-strain analytical models was evaluated, and a few improvements were recommended. The research study assessed that due attention needs to be given to structural bonding patterns in evaluating the mechanical characteristics of masonry required in the design and analytical modelling of masonry constructions. DIC technique can be conveniently used to capture the nonlinear strain characteristics and can be handy in detecting the damage early.

Web Crippling Behaviour of Cold-Formed Carbon Steel, Stainless Steel, and Aluminium Lipped Channel Sections with Web Openings

This paper reviews the research advancements and design practices related to the structural web crippling response of lipped Cold-Formed (CF) carbon steel, stainless steel, and aluminium channels with web perforations. The web crippling response differs among each material based on the non-linear stress-strain characteristics and degree of strain hardening. Therefore, the reduction in the web crippling strength of web-perforated CF channel sections made of different materials may not be equivalent. The research activities surrounding the web crippling response of CF channels with and without web openings were reviewed initially. Despite the limited design provisions given in the international specifications for the web crippling design of lipped CF channels with web openings, web crippling studies conducted across the world have developed suitable design equations in the form of reduction factors. Past research studies have substantially captured the web crippling response of carbon steel channels with web openings while that of stainless steel and aluminium are limited, as identified in this paper. Lastly, numerical models were developed for simulating the web crippling behaviour of lipped CF carbon steel, stainless steel, and aluminium channels with web opening and validated with past experimental data, with a view for developing unified design guidelines.

DSM design of CFS columns subject to distortional buckling at elevated temperatures

Cold-formed steel (CFS) columns are subject to distortional buckling unlike hot-rolled steel columns. Many experimental and numerical investigations have been conducted on the distortional buckling behaviour and capacity of CFS columns at ambient and elevated temperatures. However, constitutive laws of CFS at ambient and elevated temperatures are different with varying levels of nonlinear stress-strain characteristics. Thus, the ultimate distortional buckling capacities predicted by the elevated temperature design rules can be unconservative when the ambient temperature design equations are used with elevated temperature mechanical properties. However, no study has fully incorporated the effects of nonlinear stress-strain characteristics of CFS at elevated temperatures. Therefore, this research study investigated the effects of nonlinear elevated temperature stress-strain characteristics on the distortional buckling capacities of CFS columns using three parameters, namely, nonlinearity, strain hardening and yield strength to Young's modulus ratio, through a numerical parametric study. A comparison of ultimate capacities from the parametric study with those predicted by the current AS/NZS 4600 direct strength method (DSM) showed that the current design equations cannot accurately capture the effects of nonlinear elevated temperature stress-strain characteristics. Consequently, modified DSM equations were proposed to accurately predict the distortional buckling capacities of CFS columns at elevated temperatures.

A Nonlinear Constitutive Model for Disintegrated Carbonaceous Mudstone Based on Logarithmic Functions

To study the mechanical characteristics of the disintegrated carbonaceous mudstone (DCM), consolidated drained triaxial tests were conducted on the DCM with three degrees of compaction (i.e., 90%, 93%, and 96%). Then, the nonlinear constitutive model suitable for the DCM was established based on test results using a logarithmic function. The stress-strain characteristics of the DCM were analyzed. The results revealed that the axial strain of the DCM was positively correlated with the deviatoric stress and lateral strain. The slopes of deviatoric stress-axial strain curves decreased with the increase of axial strain and so did the slopes of the axial strain-volumetric strain curves. The strength of the DCM increased with the increase of the confining pressure and the degree of compaction. In addition, the axial strain induced by dilatancy was also positively correlated with the degree of compaction and the confining pressure. Furthermore, under triaxial loading conditions, the relationship between the stress and strain of the DCM can be expressed by a logarithmic function; based on this, a nonlinear constitutive model with ten material parameters was derived. In addition, the results of numerical tests using the model showed similar stress-strain characteristics of the DCM comparing with the triaxial tests. Hence, it indicated that the nonlinear constitutive model based on the logarithmic function can reflect the nonlinear stress-strain characteristics of the DCM.

Coupled nonlinear stress and electric field numerical simulation for all-solid-state lithium-ion batteries

An accurate analysis of ion transportation in an all-solid-state battery is crucial to improve and estimate performance. For an all-solid-state battery using sulfide solid electrolytes, coupling the stress and electric field simulations is required because the battery is pressurized, this affects the void space, which then affects ion transportation. In this paper, we propose a new method to analyze the ionic conductivity in a pressurized all-solid-state battery by coupling nonlinear stress analysis and electric field analysis. The nonlinear stress analysis can estimate the stress distribution of solid electrolyte and it was found that nonlinear stress-strain characteristics that originate from the voids diminished with increased pressure. The numerical results based on a model-electrode were nearly identical to the experimental results. • Numerical simulation method with nonlinear FEM and FVM are proposed. • Nonlinear mechanical characteristics of SE are simulated with nonlinear FEM. • The numerical results are validated with EIS and X-ray CT measurements. • The numerical and experimental results are in good agreement.

Beyond Constant Curvature: A New Mechanics Model for Unidirectional Notched-Tube Continuum Wrists.

This paper presents a new mechanics model for unidirectional notched-tube continuum wrists, a class of mechanisms frequently used to implement distal steering in needle-sized surgical robotic instruments. Existing kinematic models available for these devices are based on the simplifying assumption that, during actuation, all the notches undergo the same amount of deflection, so that the shape of a wrist can be approximated by an arc of constant curvature. This approach is analytically attractive, but, as we show in this paper, it can sometimes fail to provide good tracking accuracy. In this article, we provide a new model that relaxes the assumption above, and we report experimental evidence showing its superior accuracy. We model wrist deflection using Castigliano's second theorem, with the addition of a capstan friction term that accounts for frictional losses on the actuation tendon. Because notched-tube wrists are typically made of Nickel-Titanium (Nitinol), which has nonlinear stress-strain characteristics, we use a technique to obtain a local linearized approximation of the material modulus, suitable for use in the deflection model. The result of our modeling is a system of nonlinear equations that can be solved numerically to predict the wrist configuration based on the applied actuation force. Experimental results on physical specimens show that this improved model provides a more accurate estimate of wrist kinematics than prior models assuming constant curvature bending.

Design of cold-formed steel columns subject to local buckling at elevated temperatures

Cold-formed steel (CFS) is one of the popular materials in low and mid-rise building construction. However, CFS member capacities reduce at elevated temperatures as their yield strength and Young's modulus decrease with increasing temperature. In addition, CFS exhibits nonlinear stress-strain characteristics at elevated temperatures, which is significantly different to the ambient temperature stress-strain characteristics. Although several studies have been conducted on the elevated temperature local buckling capacities of CFS columns, they have not considered the effects of nonlinear stress-strain characteristics such as the nonlinearity between proportional limit stress and yield strength, varying yield-strength to Young's modulus ratio and strain hardening between yield and ultimate strengths. Thus, this research study investigated their effects in detail, and proposed modified design equations for the elevated temperature local buckling capacities of CFS columns based on the effective width and direct strength methods given in AS/NZS 4600, since the current design equations give unsafe predictions in many cases. The modified design equations incorporating the effects of elevated temperature nonlinear stress-strain characteristics were developed using the local buckling capacities of CFS columns obtained from 740 finite element models of lipped channel sections. A simplified design method is proposed to determine the compression capacities of commonly used Australian open CFS sections at elevated temperatures.

Ex Vivo Regional Mechanical Characterization of Porcine Pulmonary Arteries.

Regional mechanical characterization of pulmonary arteries can be useful in the development of computational models of pulmonary arterial mechanics. We performed a biomechanical and microstructural characterization study of porcine pulmonary arteries, inclusive of the main, left, and right pulmonary arteries (MPA, LPA, and RPA, respectively). The specimens were initially stored at -20°C and allowed to thaw for 12-24 hours prior to testing. Each artery was further subdivided into proximal, middle, and distal regions, leading to ten location-based experimental groups. Planar equibiaxial tensile testing was performed to evaluate the mechanical behavior of the specimens, from which we calculated the stress at the maximum strain (S 55), tensile modulus (TM), anisotropy index (AI), and strain energy in terms of area under the stress-strain curve (AUC). Histological quantification was performed to evaluate the area fraction of elastin and collagen content, intima-media thickness (IMT), and adventitial thickness (AT). The constitutive material behavior of each group was represented by a five-constant Holzapfel-Gasser-Ogden model. The specimens exhibited non-linear stress-strain characteristics across all groups. The MPA exhibited the highest mean wall stress and TM in the longitudinal and circumferential directions, while the bifurcation region yielded the highest values of AI and AUC. All regions revealed a higher stiffness in the longitudinal direction compared to the circumferential direction, suggesting a degree of anisotropy that is believed to be within the margin of experimental uncertainty. Collagen content was found to be the highest in the MPA and decreased significantly at the bifurcation, LPA and RPA. Elastin content did not yield such significant differences amongst the ten groups. The MPA had the highest IMT, which decreased concomitantly to the distal LPA and RPA. No significant differences were found in the AT amongst the ten groups. The mechanical properties of porcine pulmonary arteries exhibit strong regional dissimilarities, which can be used to inform future studies of high fidelity finite element models.

One-way cyclic deformation behavior of natural soft clay under continuous principal stress rotation

The design of traffic infrastructure foundations formations has traditionally been empirically rather than analytically based, and for the advanced methods currently in use, input parameters are typically determined from cyclic triaxial testing. In actual engineering, the stresses acting on soil elements induced by passing wheels are not as simple as cyclic triaxial loading. Due to real traffic loads involve the change of the principal stress direction in the ground, cyclic loading induced by traffic loading are typically one-way cyclic loading involved continuous principal stress rotation. Testing in a hollow cylinder apparatus (HCA) can impose the rotations in principal stress direction likely to be experienced by a soil element in the field, and may therefore be preferable to triaxial testing. The soil mechanics problem is one of understanding how soils respond to cyclic loading and applying this knowledge to foundation design. Non-linear stress–strain characteristics are a particular feature of the problem and have to be catered for design and evaluation. In this paper, a series of normally consolidated undrained HCA tests are performed to investigate the one-way cyclic deformation behavior of natural soft clay from Wenzhou, China. The principal stress axis continuously rotates while holding the deviator stress at a constant level. The tests results show that, the pore pressure, stress–strain hysteretic loop, dynamic modulus of the tested samples are significantly dependent on cyclic stress ratios and initial effective confining pressures. The value of degradation index becomes larger as the cyclic stress ratio and initial effective confining pressure increase. The threshold limit of the tested clay samples in terms of cyclic stress ratio can be considered as 0.255.

Mechanical behaviour of tissue mimicking breast phantom materials

The mechanical properties of soft tissue have long been of interest in biomedical research and applications, and increasingly for breast cancer imaging. In this paper, the mechanical properties of three different materials used to emulate the mechanical behaviour of real breast tissue are measured: agar, gelatine, and silicone, to assess their suitability for use in phantoms in systems assessing tissue mechanics for diagnostics. Two widely recognised measurement procedures are used. Quasi-static uniaxial compression was performed under a strain rate of 0.5 mm min−1 up to 15% strain with preloads of 0.05, 0.1, and 0.2 N, was used to measure the elastic moduli. Dynamic testing over a frequency range of 0.1–50 Hz for agar and 0.1–100 Hz for gelatine and silicone with the same preload was used to measure the storage moduli. Elastic and storage moduli were (5–81 kPa, 17–85 kPa, 5–112 kPa) and (3–128 kPa, 10–109 kPa, 5–73 kPa) for agar, gelatine, and silicone, respectively at the three preloads. All materials can be cast into arbitrary shapes and are suitable for tissue-mimicking phantoms. Silicone was the most consistent across the different preloads and frequencies, and can provide a range of stiffness ratios of adipose to tumour tissue that match experimentally reported values. More specifically, silicone samples for skin, adipose and tumour tissues show nonlinear stress–strain characteristics at 3 preloads characterised using hyperelastic parameters by fitting Neo-Hookean, Mooney Rivlin and Ogden models. Silicone also does not contain water, so environmental influences do not affect its mechanical properties as much as the other materials, and is thus more durable for consistent re-use. Finally, breast shaped mimicking silicone phantoms were fabricated for in vitro trials of a digital image elasto tomography breast cancer screening system assessing changes in mechanical properties.

Evaluation of Nonlinear Material Properties of Fly Ash Brick Masonry under Compression and Shear

This paper reports nonlinear stress-strain characteristics and failure mechanisms of fly ash brick masonry and its constituents (fly ash brick units and mortar) obtained under compression and shear. Based on the experimental results, simple relations were proposed to evaluate the modulus of elasticity of fly ash brick masonry prisms in terms of the compressive strength of masonry prisms. Shear strength and shear modulus of masonry were evaluated by testing two different sizes of masonry wallettes, and a parametric study was carried out to evaluate the effect of the compressive strength of brick units, mortar cubes, and masonry prisms on the shear strength of masonry wallettes. The compressive strength of masonry prisms and fly ash brick units was found to be less than that of the mortar cubes. Fly ash bricks exhibited a very weak and soft nature in comparison with that of burnt clay bricks.

Characterization of biomechanical properties of agar based tissue mimicking phantoms for ultrasound stiffness imaging techniques

Pathological changes of the body have been observed to change the mechanical properties of soft tissue types which can be imaged by ultrasound elastography. Though initial clinical results using ultrasound elastography in detection of tumors are promising, quantification of signal to noise ratio, resolution and strain image patterns are the best achieved under a controlled study using phantoms with similar biomechanical properties of normal and abnormal tissues. The purpose of this work is to characterize the biomechanical properties of agar based tissue mimicking phantoms by varying the agar concentration from 1.7 to 6.6% by weight and identify the optimum property to be used in classification of cancerous tissues. We performed quasi-static uniaxial compression test under a strain rate of 0.5mm/min up to 15% strain and measured Young's modulus of phantom samples which are from 50kPa to 450kPa. Phantoms show nonlinear stress–strain characteristics at finite strain which were characterized using hyperelastic parameters by fitting Neo-Hookean, Mooney Rivlin, Ogden and Veronda Westmann models. We also investigated viscoelastic parameters of the samples by conducting oscillatory shear rheometry at various precompression levels (2–5%). Loss modulus values are always less than storage modulus which represents the behavior of soft tissues. The increase in agar concentration increases the shear modulus of the samples as well as decreases the linear viscoelastic region. The results suggest that dynamic shear modul are more promising than linear and nonlinear elastic modul in differentiation of various classes of abnormal tissues.

Study on the Small-Strain Behaviors of Unsaturated Soils

Non-linear stress-strain characteristics and strain dependency of stiffness of saturated soils at small strain level have been reported by many researchers. However, the non-linear stiffness of unsaturated soils is not fully studied. The aim of this study is to clarify main factors affecting the non-linear stiffness of unsaturated soil at small strain range for a precise prediction of various ground deformations. Experiments were conducted with both sand and cohesive soil, by using a triaxial apparatus developed for small strain measurement. As the important factors for mechanical behaviour of unsaturated soils under small strain range, the effects of suction and mean net stress, which introduce the decrease of void ratio and degree of saturation, were discussed in the study.

Behaviour and design of cold-formed steel beams subject to lateral–torsional buckling at elevated temperatures

Cold-formed steel beams are increasingly used as floor joists and bearers in buildings and often their behaviour and moment capacities are influenced by lateral–torsional buckling. With increasing usage of cold-formed steel beams their fire safety design has become an important issue. Fire design rules are commonly based on past research on hot-rolled steel beams. Hence a detailed parametric study was undertaken using validated finite element models to investigate the lateral–torsional buckling behaviour of simply supported cold-formed steel lipped channel beams subjected to uniform bending at uniform elevated temperatures. The moment capacity results were compared with the predictions from the available ambient temperature and fire design rules and suitable recommendations were made. European fire design rules were found to be over-conservative while the ambient temperature design rules could not be used based on single buckling curve. Hence a new design method was proposed that includes the important non-linear stress–strain characteristics observed for cold-formed steels at elevated temperatures. Comparison with numerical moment capacities demonstrated the accuracy of the new design method. This paper presents the details of the parametric study, comparisons with current design rules and the new design rules proposed in this research for lateral–torsional buckling of cold-formed steel lipped channel beams at elevated temperatures.

Nonlinear behaviour of steel–concrete composite bridges: finite element modelling and experimental verification

To ensure public safety, existing bridges are often evaluated for their load-bearing capacity. Realistic evaluation must take into account the actual nonlinear stress–strain characteristics of the ...

Nonlinear bend stiffener analysis using a simple formulation and finite element method

Flexible marine risers are commonly used in deepwater floating systems. Bend stiffeners are designed to protect flexible risers against excessive bending at the connection with the hull. The structure is usually analyzed as a cantilever beam subjected to an inclined point load. As deflections are large and the bend stiffener material exhibits nonlinear stress-strain characteristics, geometric and material nonlinearities are important considerations. A new approach has been developed to solve this nonlinear problem. Its main advantage is its simplicity; in fact the present method can be easily implemented on a spreadsheet. Finite element analysis using ABAQUS is performed to validate the method. Solid elements are used for the bend stiffener and flexible pipe. To simulate the near inextensibility of flexible risers, a simple and original idea of using truss elements is proposed. Through a set of validation studies, the present method is found to be in a good agreement with the finite element analysis. Further, parametric studies are performed by using both methods to identify the key parameters and phenomena that are most critical in design. The most important finding is that the common practice of neglecting the internal steel sleeve in the bend stiffener analysis is non-conservative and therefore needs to be reassessed.

Transverse cable stiffness and mechanical losses associated with load cycles in ITERNb3Sn and NbTi CICCs

The flexible nature of the cable bundles in the sizeable cable-in-conduit-conductorsfor ITER containing more than a thousand strands, in combinationwith a void fraction of around 30%, gives scope for significant cablecompression and strand deflection. In particular, the transverse stiffness of theNb3Sn type of cabled superconductors, being subjected to large electromagnetic forces, is criticalfor their long-term performance considering the impact of the strain variation on thetransport properties. What is more, the compression of the cable bundle under load andthe permanent deformation and relaxation in time or that associated with quenches, havean effect on the cooling and pressure drop along the turns of the windings and are valuableto account for in large magnets such as for ITER. The electromagnetic AC losses of ITERNb3Sn and NbTi CICCs, related to changing magnetic field and in this manner importantfor their stability, were broadly studied and reported but the associatedmechanical losses have received less attention so far. The lifetime characteristicsin terms of cable compression, changes in transverse stiffness and mechanicallosses are experimentally determined on several prototype ITER NbTi andNb3Sn conductors in the Twente press and a summary of the results is given. The nonlinearstress–strain characteristics of the cable bundle and its moderate time-dependent naturecan be considered as a viscoelastic–plastic phenomenon. The evolution of the stiffness andthe mechanical loss depends on the peak load, void fraction, strand type and strand coatingand changes with the number of load cycles. The dissipated heat from mechanical energy isnot a critical issue for ITER magnet operation but is not negligible, in particular in thecase of NbTi conductors.