Stress-deformation Relation Research Articles

A new method for rapidly capturing the strength and full nonlinear response of partially interacting steel–concrete composite beams

A semi-analytical procedure is presented for predicting the complete flexural response of partially interacting steel–concrete composite beams up to failure. The governing equation of the Euler–Bernoulli beam theory is solved wherein concrete, steel and the shear connectors joining the concrete slab to the steel beam are assumed to have nonlinear stress-deformation relationships. The adopted constitutive relationship for the connectors allows for partial or full composite action. The solution is applicable to beams and one-way slabs subjected to concentrated or uniform load and/or their combination. The governing equation is numerically solved by satisfying the equilibrium and compatibility requirements along the member. For the reinforced concrete part of the composite beam, a nonlinear moment–curvature relationship is developed that accounts for concrete nonlinearity in compression and for cracking and tension-stiffening in tension as well as for steel reinforcement nonlinearity. The steel profile is assumed to have a bilinear elasto–plastic strain-hardening moment–curvature relationship. Comparison of the proposed model results with the corresponding experimental load–deflection curves and interfacial shear–slip curves of several beams tested by others shows good agreement. The relative simplicity, efficiency and easy application of the present solution make it possible to accurately predict the failure load, interfacial slip and full nonlinear response of partially interacting composite beams.

Flow Modeling of a Non-Newtonian Viscous Fluid in Elastic-Wall Microchannels

The use of polymer microspheres is becoming increasingly widespread. Along with their most common applications, they are beginning to be used in the synthesis of photonic crystals, microstructure analysis and multiplexed diagnostics for disease control purposes. This paper presents a simple mathematical model that allows us to study the transport mechanisms involved in the deformation of an elastic microchannel under the flow stream of a power-law fluid. In particular, we analyze the momentum transfer to a non-Newtonian fluid (Polydimethylsiloxane, PDMS) due to the deformation of the elastic ceiling of a rectangular microchannel. Hooke’s law is used to represent the stress–deformation relationship of the PDMS channel ceiling. Stop-flow lithography is modeled, and the pressure exerted by the deformed PDMS ceiling on the fluid when the microchannel returns to its original form is taken into account. It is found that the response time of the elastic ceiling deformation increases with the channel width and length and decreases with the channel height independently of the power-law exponent of the injected fluid. However, an increase in the power-law exponent beyond unity causes an increase in the wall-deformation response time and the maximum deformation of the channel height compared to a Newtonian fluid.

A Numerical and Experimental Analysis of the Mechanical Behavior of the Aluminum Beverage Can with Internal Varnish Layers during Axial Load Force Testing.

This article presents a numerical and experimental investigation into the impact of can wall thickness and the internal varnish layer thickness on the results of an axial load force test. This study also shows the levels of thermal stresses that emerge after the drying of varnish coating, and how they affect the results of the axial load force test. This research involves the development of suitable numerical models and the experimental acquisition of stress-deformation relationships for the both can material, aluminum, and the varnish. The numerical simulation of the axial load force test has been verified through experimental tests, with a resulting difference of 8.9% between the two sets of results. The findings highlight that changes in the can wall thickness have a more pronounced effect on test outcomes compared to variations in the varnish thickness. Specifically, an increase in the can wall thickness from 90 µm to 100 µm results in a substantial 116 N increase in the force required for a can to collapse. Nevertheless, the presence of a 5 µm varnish layer also contributes measurably, increasing the can's collapse force by 21 N. These results offer valuable practical insights for manufacturers, enabling them to effectively optimize can strength characteristics.

Experimental and Numerical Study on Progressive Collapse of the Midcolumn in a Glulam Timber Frame Exposed to Fire

This paper presents experimental and numerical investigations on the progressive collapse behavior of a two-story glulam frame when the middle column is exposed to ISO834 standard fire. The stress characteristics, temperature distributions, and deformation relationship curves of the bottom middle column after a fire are obtained. The collapse resistance performance and the mechanism of the glulam structure under local fire are studied in combination with the failure mode of the frame. The experimental results show that the failure time of the middle column in fire is 16.8 min, and the time to determine the progressive collapse of the structure is 17.17 min when the rotation angle exceeds the limit. The structure can remain stable for some time after the failure of the fire column, but as the fire continues to spread, the structure will collapse further. In addition, a temperature-field analysis model with the grid element type of DC3D8 and a structural field analysis model with the grid element type of C3D8R was established by using ABAQUS software under the experimental conditions. The numerical simulation is carried out by means of thermal–mechanical sequential coupling. The numerical simulation results show that the temperature of the beam end rose to more than 300 °C in about 15 min although it was not directly exposed to fire. The temperature of the whole connector at the pillar foot in the bottom column reached more than 700 °C when the fire time reached 60 min. The numerical simulation results agree well with the experimental results. The analysis results show that, in the failure process of the fire column, the junction of beam ends begins to produce stretching force to form a catenary effect. As the fire time increases, the catenary effect eventually fails. The stretching force at the beam end rapidly decreases and withdraws from the work, causing the progressive collapse of the frame.

A data-driven reduced-order surrogate model for entire elastoplastic simulations applied to representative volume elements

This contribution discusses surrogate models that emulate the solution field(s) in the entire simulation domain. The surrogate uses the most characteristic modes of the solution field(s), in combination with neural networks to emulate the coefficients of each mode. This type of surrogate is well known to rapidly emulate flow simulations, but rather new for simulations of elastoplastic solids. The surrogate avoids the iterative process of constructing and solving the linearized governing equations of rate-independent elastoplasticity, as necessary for direct numerical simulations or (hyper-)reduced-order-models. Instead, the new plastic variables are computed only once per increment, resulting in substantial time savings. The surrogate uses a recurrent neural network to treat the path dependency of rate-independent elastoplasticity within the neural network itself. Because only a few of these surrogates have been developed for elastoplastic simulations, their potential and limitations are not yet well studied. The aim of this contribution is to shed more light on their numerical capabilities in the context of elastoplasticity. Although more widely applicable, the investigation focuses on a representative volume element, because these surrogates have the ability to both emulate the macroscale stress-deformation relation (which drives the multiscale simulation), as well as to recover all microstructural quantities within each representative volume element.

The Choice of the Most Appropriate Suture Threads for Pancreatic Anastomoses on the Basis of Their Mechanical Characteristics.

The choice of the most appropriate suture threads for pancreatic anastomoses may play an important role in reducing the incidence of post-operative pancreatic fistula (POPF). The literature on this topic is still not conclusive. The aim of this study was to analyze the mechanical characteristics of suture materials to find the best suture threads for pancreatic anastomoses. A single-axial electromagnetic actuation machine was used to obtain the stress-deformation relationship curves and to measure both the ultimate tensile strength (UTS) and the Young's modulus at the 0-3% deformation range (E0-3) of four different suture materials (Poliglecaprone 25, Polydioxanone, Polyglactin 910, and Polypropylene) at baseline and after incubation in saline solution, bile, and pancreatic juice for 1, 3, and 7 days. Polydioxanone and Polypropylene showed stable values of UTS and E0-3 in all conditions. Polyglactin 910 presented significant UTS and E0-3 variations between different time intervals in all types of liquids analyzed. Poliglecaprone 25 lost half of its strength in all biological liquids analyzed but maintained low E0-3 values, which could reduce the risk of lacerations of soft tissues. These results suggest that Polydioxanone and Poliglecaprone 25 could be the best suture materials to use for pancreatic anastomoses. In vivo experiments will be organized to obtain further confirmations of this in vitro evidence.

Elastoplastic analysis of the enhancing mechanism of tensile performance of ductile thin-walled circular tubes with internal support

Composite components with internal support, such as concrete-filled steel tubes (CFST), are widely applied in a variety of industries. Various experimental and numerical studies have revealed that internal supports could effectively enhance the tensile performance of ductile tubes. However, stress–deformation relations and strengthening mechanisms of thin-walled circular tubes under tension, (especially in plastic stages) have not been studied based on plastic increment theory. In this study, based on circumferential deformation coordination and generalized Hook’s law, the effects of internal support on yield strength and axial deformation of tubes were analyzed and the yield strength calculation equation of filled tubes was derived under true triaxial conditions. Based on plastic increment theory and isotropic linear hardening hypothesis, the differential relationship between the pressure increment of internal support and the axial plastic strain increment of steel tubes was established. The effects of elastic modulus, radius–thickness ratio and axial plastic strain on strength increase were also evaluated. Comparative analyze showed that the value of stress increase on filled tubes predicted by the proposed model was consistent with the results obtained from CFST tensile test and flexible support-filled thin-walled circular tube (FTWCT) test. In conclusion, a theoretical model was established for the first time for tensile strength enhancement mechanism of FTWCTs in plastic large-deformation stage, which was important in predicting the performance of ductile thin-walled structures under extreme loads.

Breakage Strength of Wood Sawdust Pellets: Measurements and Modelling

Wood pellets are an important source of renewable energy. Their mechanical strength is a crucial property. In this study, the tensile strength of pellets made from oak, pine, and birch sawdust with moisture contents of 8% and 20% compacted at 60 and 120 MPa was determined in a diametral compression test. The highest tensile strength was noted for oak and the lowest for birch pellets. For all materials, the tensile strength was the highest for a moisture content of 8% and 120 MPa. All pellets exhibited a ductile breakage mode characterised by a smooth and round stress–deformation relationship without any sudden drops. Discrete element method (DEM) simulations were performed to check for the possibility of numerical reproduction of pelletisation of the sawdust and then of the pellet deformation in the diametral compression test. The pellet breakage process was successfully simulated using the DEM implemented with the bonded particle model. The simulations reproduced the results of laboratory testing well and provided deeper insight into particle–particle bonding mechanisms. Cracks were initiated close to the centre of the pellet and, as the deformation progressed, they further developed in the direction of loading.

Localised failure of geomaterials: how to extract localisation band behaviour from macro test data

The formulation and calibration of constitutive models for geomaterials require material behaviour from experiments under a wide range of triaxial loading conditions. However, failure of geomaterials usually involves localisation of deformation that leads to very strong inhomogeneous behaviour. Therefore, the experimentally measured macro (specimen) behaviour is a mix between very different responses inside and outside the localisation zone and thus should not be used as a true representation of the material responses. This paper proposes a theoretical framework that provides links between mechanical responses inside and outside the localisation band, alongside their contributions toward the overall behaviour of a specimen undergoing localised deformation. This meso–macro connection allows the quantification of behaviour inside the localisation band, which is the main source of material inelasticity, from experimentally measured specimen behaviour. Correlation between the thickness of the localisation band and its behaviour is shown, bounded by a unique stress–deformation relationship describing the behaviour of an idealised zero-thickness localisation band.

Mechanism of Structural–Slip Rockbursts in Civil Tunnels: An Experimental Investigation

Structural–slip rockbursts are frequently faced in civil tunnels, which pose huge threats to the safety of personnel and equipment in tunnels. In this study, the core features of structural–slip rockbursts were summarized qualitatively based on the many structural–slip rockburst cases encountered in civil tunnels. Direct shear tests on splitting granite joints were performed to reproduce structural–slip rockbursts in the laboratory, and stress–deformation relationship, sound waves, acoustic emissions (AEs), ejected fragments and failure characteristics of the structural planes were investigated to comprehensively make clear the mechanisms of structural–slip rockbursts. Two generation modes of structural–slip rockbursts were revealed in the experiments. One occurs in the early stage of structural plane slip and shows stress drops resulted from the brittle rupture of asperities on structural surfaces, and the other is that a part of ruptured asperities are gradually pulverized into powder shapes and the powdery debris impels the movement mode of structural planes suddenly from sliding friction to rolling friction, which lead to sudden stress drops and cause the rockbursts. Further, the normal stress effect on the rockburst intensity was discussed. It was found that the larger the normal stress, the greater the intensity of rockbursts. Moreover, the characteristics of rockburst intensities in the tests were analyzed by means of the sound waveforms, and it was induced that the rockburst intensity presents alternating circulation laws. The findings from this study may be valuable for understanding the structural–slip mechanism in civil tunnels.

Numerical analysis of coupled liquid water, vapor, stress and heat transport in unsaturated freezing soil

Most existing empirical formulas and freezing models to describe the process of frost heave are established in saturated soil. However, most frozen soils in situ are unsaturated and vapor phase plays a critical role in the overall water, energy and stress balance. On the basis of existing frost heave models of saturated freezing soil, a mathematical model of coupled water, vapor, stress and heat movement is established here. Void ratio is an important variable adopted in the numerical simulation, which is related to variation of degree of saturation and used to describe the pore pressure distribution. In the proposed model, void ratio is also adopted as a criterion for the ice lens initiation and to establish the stress-deformation relation. The model is validated by comparisons of simulated results with reference model of saturated freezing soil. Seven cases are computed to study the coupled process in unsaturated freezing soil according to different initial and boundary conditions, including various degrees of saturation, temperature gradients and overburden pressures. The result shows that the impacts of different cases on frost heave properties and interactions among liquid water, vapor, temperature and stress fields. Finally, a representative unsaturated freezing case is adopted to investigate the temperature profiles, water and vapor content profiles and pore pressure profiles, respectively.

A new inverse method for estimation of in vivo mechanical properties of the aortic wall

The aortic wall is always loaded in vivo, which makes it challenging to estimate the material parameters of its nonlinear, anisotropic constitutive equation from in vivo image data. Previous approaches largely relied on either computationally expensive finite element models or simplifications of the geometry or material models. In this study, we investigated a new inverse method based on aortic wall stress computation. This approach consists of the following two steps: (1) computing an “almost true” stress field from the in vivo geometries and loading conditions, (2) building an objective function based on the “almost true” stress fields, constitutive equations and deformation relations, and estimating the material parameters by minimizing the objective function. The method was validated through numerical experiments by using the in vivo data from four ascending aortic aneurysm (AsAA) patients. The results demonstrated that the method is computationally efficient. This novel approach may facilitate the personalized biomechanical analysis of aortic tissues in clinical applications, such as in the rupture risk analysis of ascending aortic aneurysms.

USE OF DISC SPRINGS IN A PELLET FUEL MACHINE FOR PRESSURE REGULATION AND PREVENTION OF DAMAGE

The use of biomass fuel pellets is becoming widespread as a renewable and environment-friendly energy. Pellet fuels are produced in various pellet machine types. Pellet machines encounter problems such as pressure irregularities and choke at the initial start for different kinds of biomass feedstock. In this study, disc springs are integrated into a vertical axis pellet machine for a pressure regulation and design optimization. Force-deformation and stress-deformation relations of disc springs are investigated using analytical and finite element methods. Pelletizing pressures were calculated based on disc spring force values using the Hertzian stress formula. Utilized disc springs ensured the pressure regulation, production efficiency increase and damage prevention on the die-roller mechanism.

Serviceability design for geosynthetic reinforced column supported embankments

In recent years, geosynthetic reinforced column supported embankments (GRCSEs) have become an increasingly popular design solution for road and rail infrastructure constructed over soft soil sites. However, the serviceability behaviour and deformation that often govern the suitability of their design is not well understood. This is due, in part, to the difficulties in describing the arching stress development in the load transfer platform (LTP). This paper highlights the need for coupled arching stress-deformation models to describe accurately serviceability behaviour. This approach contrasts the widely adopted two-step design approach, which uses limit-equilibrium models that de-couple the arching stress-deformation relationship to describe ultimate limit state behaviour. Using an analytical example, an arching stress/deformation model and an empirical relationship (developed by others) relating base LTP settlement to surface settlement, the relationship between serviceability behaviour and soft soil parameters is highlighted and the conditions leading to progressive collapse in GRCSEs are described. The approach presented provides a means to predict serviceability behaviour, and at the same time, raises questions about the long-term performance and the manner in which acceptable performance has been achieved in the short-term in several field case studies. In particular, those constructed at, or near, a minimum embankment height.

Cohesive zone modelling of nucleation, growth and coalesce of cavities

The stress-deformation relation i.e. cohesive law representing the fracture process in an almost incompressible adhesive tape is measured using the double cantilever beam specimen. As in many ductile materials, the fracture process of the tape involves nucleation, growth and coalesce of cavities. This process is studied carefully by exploiting the transparency of the used materials and the inherent stability of the specimen configuration. Utilising the path independence of the J-integral, the cohesive law is measured. The law is compared to the results of butt-joint tests. The law contains two stress peaks—the first is associated with nucleation of cavities at a stress level conforming to predictions of void nucleation in rubber elasticity. The second stress peak is associated with fracture of stretched walls between fully-grown cavities. After this second peak, a macroscopic crack is formed. The tape suffers at this stage an engineering strain of about 800%. A numerical analysis with the determined cohesive law recreates the global specimen behaviour.

General cohesive zone model for prediction of interfacial stresses induced by intermediate flexural crack of FRP-plated RC beams

In this paper, a general model is developed to predict the distribution of interfacial shear and normal stresses induced by intermediate flexural crack of FRP-plated reinforced concrete beam. For that, a new theoretical model based on the bi-linear cohesive zone model for intermediate crack-induced debonding is established, with the unique feature of unifying debonding initiation and growth. The stress deformation relationship is generally referred to as bond-slip law since the deformation of the interface is mainly the relative displacement (slip) between the FRP plate and the reinforced concrete beam (RC beam). Adherent shear deformations have been included by assuming a parabolic shear stress through the thickness of the adherents, verifying the cubic variation of the longitudinal displacement function, whereas all existing solutions neglect this effect. To obtain interfacial normal stress, the adhesive layer is assumed in linearly elastic stage in normal direction. This assumption is adopted in this study, because the FRP plate and RC beam are in contact during the whole debonding process. A complementary experimental study is made in order to show the debonding process according to mechanical and geometrical characteristics of adhesive and FRP plate. Analytical results obtained through the closed-form of interface stresses are in good agreement with those given by numerical and experimental models. Finally, parametric studies are carried out to demonstrate the effect of the mechanical properties and thickness variations of FRP, concrete and adhesive on interface debonding.

Creep response of intermediate flexural cracking behavior of reinforced concrete beam strengthened with an externally bonded FRP plate

In this study, a creep response model for the long-term behavior of interfacial shear stresses induced by intermediate flexural crack of FRP-plated RC beams is developed. A theoretical model based on the bi-linear cohesive zone model for intermediate crack-induced debonding is established, with the unique feature of unifying debonding initiation and growth with time increments. The creep behavior of the RC beam, adhesive layer and FRP plate have been included by considering the time-dependent mechanical properties. The time-dependent stress-deformation relationship caused by creep is referred to as bond-slip law. Consequently, a new interfacial law which combines time-dependent mechanical properties of all retrofitted beam constituents is proposed by considering constant the total energy during the creep response with intermediate crack. Obtained results are in good agreement with those given in literature. A parametric study is carried out to demonstrate the effect of the mechanical properties and thickness variations of FRP, concrete and adhesive on interface debonding. Indeed, the creep behavior, activates the debonding process in long term. Size of the softening zone is reached quickly and the bearing capacity of the retrofitted beam will be relatively affected by the time increase.

Homogenization of long fiber reinforced composites including fiber bending effects

This paper presents a homogenization method, which accounts for intrinsic size effects related to the fiber diameter in long fiber reinforced composite materials with two independent constitutive models for the matrix and fiber materials. A new choice of internal kinematic variables allows to maintain the kinematics of the two material phases independent from the assumed constitutive models, so that stress–deformation relationships, can be expressed in the framework of hyper-elasticity and hyper-elastoplasticity for the fiber and the matrix materials respectively. The bending stiffness of the reinforcing fibers is captured by higher order strain terms, resulting in an accurate representation of the micro-mechanical behavior of the composite. Numerical examples show that the accuracy of the proposed model is very close to a non-homogenized finite-element model with an explicit discretization of the matrix and the fibers.

Special Issue on Sustainable Civil Infrastructures: Innovative Technologies and Materials

Special Issue on Sustainable Civil Infrastructures: Innovative Technologies and Materials

Dielectrophoresis-Mediated Electrodeformation as a Means of Determining Individual Platelet Stiffness.

Platelets, essential for hemostasis, are easily activated via biochemical and mechanical stimuli. Cell stiffness is a vital parameter modulating the mechano-transduction of exogenous mechanical stimuli. While methods exist to measure cell stiffness, no ready method exists for measuring platelet stiffness that is both minimally-contacting, imparting minimal exogenous force and non-activating. We developed a minimal-contact methodology capable of trapping and measuring the stiffness of individual platelets utilizing dielectrophoresis (DEP)-mediated electrodeformation. Parametric studies demonstrate a non-uniform electric field in the MHz frequency range (0.2-20 MHz) is required for generating effective DEP forces on platelets, suspended in isotonic buffer with conductivity ~100-200 μS/cm. A nano-Newton DEP force (0.125-4.5 nN) was demonstrated to be essential for platelet electrodeformation, which could be generated with an electric field with strength of 1.5-9 V/μm. Young's moduli of platelets were calculated using a Maxwell stress tensor model and stress-deformation relationship. Platelet stiffness was determined to be in the range of 3.5 ± 1.4 and 8.5 ± 1.5 kPa for resting and 0.4% paraformaldehyde-treated cells, respectively. The developed methodology fills a gap in approaches of measuring individual platelet stiffness, free of inadvertent platelet activation, which will facilitate further studies of mechanisms involved in mechanically-mediated platelet activation.