Large Strain Behavior Research Articles

Geogrid-confined pervious geopolymer concrete piles with FRP-PVC-confined concrete core: Analytical models

The geogrid-confined pervious geopolymer concrete pile (GPGCP) with fibre reinforced polymer (FRP)-polyvinyl chloride (PVC)-confined concrete core (FPCC) is a new form of composite piles proposed in a previous study, which consists of a circular geogrid outer tube, a FPCC, and pervious geopolymer concrete (PGC) filled in between. For FPCC, the normal geopolymer concrete (NGC) was used to fill the FRP-PVC tubes. In this study, new analytical models are proposed to simulate the axial compressive behaviour of GPGCPs without and with FPCC. According to the characteristics of the mechanical behaviour of GPGCPs, analytical models are proposed which can predict the large axial strain behaviour. The unconfined concrete model proposed by the European Standards, and an existing popular FRP-confined concrete model are combined here. It was found that the analytical results are in good agreement with the experimental results. Based on the analytical models, the influences of different parameters on the axial compressive behaviour of GPGCPs have been investigated through parametric analyses.

Combined effects of temperature and humidity on the mechanical properties of polyurethane foams

The effects of temperature and environmental moisture on the viscoelastic behavior of polyurethane foams were investigated both theoretically and experimentally. It was shown that the effect of the environmental parameters can be explained in terms of a variation of the free volume of the solid fraction of the foams, thus allowing the use of the superposition principle to predict their influence on the viscoelastic behavior of the materials. Dynamic mechanical analyses were performed to measure the dependence on frequency, temperature, and relative humidity of the complex modulus of two different polyurethane foams, differing in terms of their glass transition temperature. The time-temperature-humidity superposition principle was proved to be applicable for the tested materials. Next, the relaxation spectra and their dependence on the relative humidity were adopted to assess its effects on the large strain behavior of the foams.The effects of temperature and environmental moisture on the viscoelastic behavior of polyurethane foams were investigated both theoretically and experimentally. It was shown that the effect of the environmental parameters can be explained in terms of a variation of the free volume of the solid fraction of the foams, thus allowing the use of the superposition principle to predict their influence on the viscoelastic behavior of the materials. Dynamic mechanical analyses were performed to measure the dependence on frequency, temperature, and relative humidity of the complex modulus of two different polyurethane foams, differing in terms of their glass transition temperature. The time-temperature-humidity superposition principle was proved to be applicable for the tested materials. Next, the relaxation spectra and their dependence on the relative humidity were adopted to assess its effects on the large strain behavior of the foams.

Rate-dependent constitutive modeling of brain tissue.

In this paper, the dynamic behavior of bovine brain tissue, measured from a set of in vitro experiments, is investigated and represented through a nonlinear viscoelastic constitutive model. The brain samples were tested by employing unconfined compression tests at three different deformation rates of 10, 100, and 1000mm/s. The tissue exhibited a significant rate-dependent behavior with different compression speeds. Based onthe parallel rheological framework approach, a nonlinear viscoelastic model that captures the key aspects of the rate dependency in large-strain behavior was introduced. The proposed model was numerically calibrated to the tissue test data from three different deformation rates. The determined material parameters provided an excellent constitutive representation of tissue response in comparison with the test results. The obtained material parameters were employed in finite element simulations of tissue under compression loadings and successfully verified by the experimental results, thus demonstrating the computational compatibility of the proposed material model. The results of this paper provide groundwork in developing a characterization framework for large-strain and rate-dependent behavior of brain tissue at moderate to high strain rates which is of thehighest importance in biomechanical analysis of the traumatic brain injury.

The effect of shape memory alloys on the ductility of exterior reinforced concrete beam-column joints using the damage plasticity model

Using shape memory alloys (SMA) bars can significantly enhance the ductility of exterior reinforced concrete joints, where they can replace the conventional steel reinforcement. This research focuses on studying the effect of using SMA on the ductility capacity of exterior reinforced concrete beam-column joints at different column axial load levels. Finite element analysis was carried out and compared with the experimental results from the literature for verification purposes, and both were compared with theoretical solutions. The results show that the use of SMA bars can improve the ductility of reinforced concrete joints noticeably, without losing load capacity. The finite element method was successful in capturing the large strain and superelastic behavior of SMA bars.

A unified simulation for effects of gellan polymer concentrations on large strain elastic behaviors of gellan gels

PurposeA direct and unified approach is proposed toward simultaneously simulating large strain elastic behaviors of gellan gels with different gellan polymer concentrations. The purpose of this paper is to construct an elastic potential with certain parameters of direct physical meanings, based on well-designed invariants of Hencky’s logarithmic strain.Design/methodology/approachFor each given value of the concentration, the values of the parameters incorporated may be determined in the sense of achieving accurate agreement with large strain uniaxial extension and compression data. By means of a new interpolating technique, each parameter as a function of the concentration is then obtained from a given set of parameter values for certain concentration values.FindingsThen, the effects of gellan polymer concentrations on large strain elastic behaviors of gellan gels are studied in demonstrating how each parameter relies on the concentration. Plane-strain (simple shear) responses are also presented for gellan gels with different polymer concentrations.Originality/valueA direct, unified approach was proposed toward achieving a simultaneous simulation of large elastic strain behaviors of gellan gels for different gellan polymer concentrations. Each parameter incorporated in the proposed elastic potential will be derived as a function of the polymer concentration in an explicit form, in the very sense of simultaneously simulating large strain data for different concentrations.

Sex- and age-specific mechanical properties of liver tissue under dynamic loading conditions

The liver is the most commonly injured abdominal organ following either blunt or penetrating impact. Current mechanical properties available in the literature are typically only measured at low strain rates, low strains, or use linear viscoelastic models. There is also a dearth of high-rate, large strain, viscoelastic data available for liver tissue which are required to model the deformation of the liver during high-rate impacts. Furthermore, the issue of whether mouse liver's mechanical properties are sex-dependent has not been addressed previously. Here, we present the first in vitro sex- and age-controlled mechanical characterisation of mixed-strain (C57BL and wild-type) mouse liver tissue at a localised length scale using large-deformation and high strain rate micro-indentation. We also investigated the effects of age on the mechanical properties of liver tissue. Force-relaxation experiments were performed on both male and female mouse livers up to 35% strain at 10/s and allowed to relax for 1s. The neo-Hookean based quasi-linear viscoelastic model was fitted to the experimental data to determine the large-strain behaviour of the tissue. A comprehensive statistical analysis was performed to determine whether any significant differences existed for (i) the short-term shear moduli and (ii) long-term shear moduli between 10 weeks-old male and female mouse livers, and (iii) the short-term and (iv) long-term shear moduli for 6, 10, and 56 weeks-old mouse livers. No significant differences were found between the mechanical properties in the sex groups. The 56 weeks-old liver tissue was found to be significantly stiffer than the 6 weeks-old liver tissue, but not the 10 weeks-old.

Nonsolvent-assisted surface modification of silica by silane and antioxidant for rubber reinforcement

Abstract To improve the reinforcing efficiency of silica (SiO2) nanoparticles for rubber and make the treatment procedure easy to commercialize in industrial level, antioxidant 2, 2′-methylenebis (6-tert-butyl-4-methyl-phenol) (2246) and silane were utilized to modify SiO2 via a nonsolvent-assisted process. Antioxidant 2246 was chemically grafted on the surface of SiO2 with the aid of silane coupling agent and performed better than free antioxidant in protecting rubber from thermo-oxidative aging due to the elimination the so-called “blooming” defects. Moreover, the antioxidant and silane modified SiO2 (SiO2-s-2246) was much better than modified SiO2 (m-SiO2) only by silane and unmodified SiO2 in dispersion and compatibility with styrene-butadiene rubber (SBR) based on the results of electron microscope observations, contact angle and bound rubber. Therefore, SBR/SiO2-s-2246 composites exhibited higher mechanical strength than SBR/m-SiO2 or SBR/SiO2 composites. For instance, SBR/SiO2-s-2246 composites with better elasticity exhibited an increase of 48% in tensile strength compared with SBR/m-SiO2 composites. The reinforcing mechanism of SiO2-s-2246 and m-SiO2 was also investigated based on the large strain behavior and swelling Gibbs free energy of SBR composites, which may be instructive for the rational filler surface modification.

Material model parameter estimation with genetic algorithm optimization method and modeling of strain and temperature dependent behavior of epoxy resin with cooperative-VBO model

A constitutive model is proposed for modeling rate and temperature dependent stress-strain behavior of amorphous polymers. The model developed here is a modification of cooperative-viscoplasticity theory based on overstress (VBO) model. The modifications are performed to capture the temperature dependent behaviors below and through the glass transition temperature (Tg) of amorphous polymers as well as rate dependency from quasi static to high strain rate loading. In the proposed constitutive model, adiabatic heating due to high strain rate deformations is included to capture the high strain rate behaviors. The temperature dependent behavior below, around and through Tg will be captured with the modifications performed on the viscoplastic strain rate. The model has been shown to be predictive of compression behavior of DER 353 epoxy at quasi and high strain rates and a temperature range from room to Tg. The genetic algorithm (GA) parameter optimization procedure has been proposed to determine material parameters of the models. The model parameters are determined using GA optimization method. The newly modified model is able to capture the mechanical behavior below and through Tg and also all of the features of the large strain behavior, including the initial modulus, yield stress, strain softening and strain hardening.

Mechanical behaviors and equivalent configuration of a polyurea under wide strain rate range

The tension tests are systematically conducted on a polyurea by Instron and split Hopkinson tension bar apparatuses for strain rate ranging from 0.0001 s−1 to–3000 s−1, to investigate its mechanical behaviors. It is found that the polyurea presents a high rate-dependent, and the stress strain curves share a similar mechanical trend and consistent strain hardening coefficient trend under wide loading strain rates. The interrupted experiments are designed, and the results demonstrate that the polyurea is sensitive to temperature, which induces distinct softening influence on flow stress. However, the strain hardening coefficient is sharing the same trends and little influenced by the strain rate and adiabatic heating. The equivalent mechanical configuration is investigated by using the two-dimension unit cell model to simulate the polyurea mechanical characteristics. The systematical simulation results facilitate to outline its equivalent configuration: the soft phase dispersing in the hard network matrix. Furthermore, the matrix play a major role in the overall mechanical properties of polyurea. The configuration is further evidenced by the deduced data from reference. Finally, a simple analytical model is proposed with two components, and well capable of describing the mechanical characteristics including large strain behavior, strain rate effect and induced thermal softening effect.

CPTU correlations for Norwegian clays: an update

Geotechnical design in clay areas in Norway is mainly based on piezocone (CPTU) tests results. Strength and stiffness parameters are usually derived from CPTU parameters and empirical correlations. In order to improve geotechnical design practice (e.g. more cost-effective solutions) and to reduce risks related to the occurrence of catastrophic events (e.g. landslides, excavation failure) the Norwegian Geotechnical Institute (NGI) has recently updated its block sample database and worked on updating CPTU correlations for clays. This paper provides a short overview of NGI's block sample database consisting of 61 block samples data points collected from 17 Norwegian clay sites. Multiple regression analyses were used to evaluate possible correlations among CPTU parameters (e.g. excess pore pressure, Δu, net cone resistance, qnet, and effective cone resistance, qe), undrained shear strength (suC) and basic clay properties (e.g. overconsolidation ratio, OCR, plasticity, sensitivity). The target was to establish correlations characterized by low uncertainty. The most reliable assessment of undrained strength was obtained when using the Stress History and Normalized Soil Engineering Properties, SHANSEP, framework associated with the best estimate OCR profile extrapolated from the CPTU measurements. This well reflects the strong relation that suC has with OCR. Despite the high quality of the samples, high scatter was observed for some of the equations that compare cone factors and basic soil parameters. In addition to the natural variability of soil properties, other possible reason to justify the scatter is that even though the accuracy of CPTU probes has improved over the past decades, especially in terms of the ability to measure low values, the results can vary among the different manufacturers. Furthermore there may be several other soil parameters than the peak undrained strength that impacts the cone resistance, for instance stiffness and large strain behavior. Such factors can affect the correlation results.

Numerical study of large strain behavior of OFHC copper: The role of latent hardening

Two mean-field crystal plasticity models of the Taylor model and the Affine model have been employed to study the large strain behaviors of OFHC copper under uniaxial compression, uniaxial tension, plane strain compression, simple shear and fixed-end thin-walled tubular torsion. The effect of latent-hardening on the predictability of the models is assessed through comparing the simulated results with the corresponding experiments. Both the stress strain curves at macroscale and the deformed textures associated with microscopic deformation mechanisms are investigated. From the assessment, Affine model with isotropic latent hardening effect gives the overall best performance among all the models and latent hardening factors examined.

Large-strain tensile behaviour of geomembranes with defects using 3D digital image correlation

A series of uniaxial tensile tests was conducted to investigate the large-strain tensile behaviour of high-density polyethylene (HDPE) geomembranes with defects. The experimental optical technique of 3D digital image correlation (DIC) was used to measure the full-field displacements and strains of the geomembranes during the tensile process. The differences between the average axial elongations and the local strains of intact geomembranes were discussed based on the strain measurement results for different tensile phases. For geomembranes with defects, large local strains near the defects sharply increased with increasing average axial elongations and the influence ranges of different types of defects in the geomembranes exhibited small differences after large tensile deformations. Compared with an intact geomembrane, defects significantly changed the local tensile behaviour of a geomembrane.

Performance of natural rubber and silicone-based magnetorheological elastomers under large-strain combined axial and shear loading

This study presents an experimental investigation on large-strain behavior of natural rubber– and silicone-based magnetorheological elastomers within a larger scope of structural vibration mitigation due to wind, traffic and seismic events. Magnetorheological elastomer samples with different weight percentages of iron particles, additives, and elastomer matrix were fabricated. The microstructures of specimens were examined, and their mechanical properties were investigated by a unique electromagnetic double-lap shear experimental setup capable of applying simultaneous compression and shear loads. The experimental results demonstrated that the isotropic natural rubber–based magnetorheological elastomers exhibit about 30% magnetorheological effect under large strains, while they achieve a higher magnetorheological effect under the combined axial and shear loading. The magnetorheological effect was 92% and 33% for 10% and 100% shear strains when 100 psi axial stress was applied. A natural rubber–based magnetorheological elastomer was further investigated applying dynamic cyclic load with and without compression load for different strains, frequencies, and magnetic field intensities. It was observed that for higher frequency, magnetorheological effect was reduced. Magnetorheological effects were 73% and 29% for 0.1 and 10 Hz frequencies, respectively, under 100 psi axial stress at 150% shear strain. The result of this study suggests that isotropic natural rubber–based magnetorheological elastomers may be suitable for high-demand-force applications, and in particular, in civil structures.

A parametric study of FE modeling of die-less clinching of AA7075 aluminum sheets

Die-less clinching process is a relatively new process for joining two sheet materials and aspects of this process are being optimized for many sheet materials. Die-less clinching tool optimizing by conducting many experimental trials with different tool design can be quite expensive and time consuming. Tool optimization via numerical simulations can be an effective and economical alternative provided material and other characteristics during clinching are adequately captured by the models. Also, there are several material, process and simulation parameters that can affect the simulation accuracy. In the present work a finite element (FE) modeling investigation of the influence of different numerical, material and tool stiffness conditions on die-less clinch characteristics is carried out with a goal towards clarifying and optimizing die-less clinching simulations for improved and efficient predictions. The numerical parameters include element size and aspect ratio, adaptive re-meshing criterion, tool stiffness and material parameters (such as non-linear work hardening behavior and friction between mating tool-sheet surfaces respectively). Adaptive re-meshing technique is required to avoid excessive element distortion as the strain is very large in die-less clinching. A mass scaling factor of less than 150 is suggested for acceptable accuracy from the model. Extended-Voce constitutive material model has been shown to accurately capture the large strain behavior of AA7075-O sheet for die-less clinching simulations. Inter-sheet as well as punch-sheet friction are shown to primarily influence geometric interlock.

Crystal Plasticity Modelling of Large Strain Behaviour of HCP Materials

In this paper, we first introduce various polycrystal plasticity models including the Visco-Plastic Self-Consistent (VPSC) model, the Elastic-Plastic Self-Consistent (EPSC) model, and the Elastic Visco-Plastic Self-Consistent (EVPSC) model, as well as twinning models including the Predominant Twin Reorientation (PTR) model and the Twinning and De-Twinning (TDT) model, with emphasizing on characteristic differences between these models. Then, we demonstrate that the EVPSC-TDT model is able to capture key features of large strain behaviour of HCP materials under various deformation processes including uniaxial tension and compression, simple shear, and torsion.

A new crystal plasticity framework to simulate the large strain behaviour of aluminum alloys at warm temperatures

To improve metal formability, high temperature forming has become a desired manufacturing process. Phenomenological plasticity models are widely used in this application, however lack good predictive capability concerning microstructure evolution during forming. Many crystal plasticity hardening models have been developed to predict deformation phenomena of metals during high temperature forming; however, few have thermodynamic self-hardening formulations based on deformation mechanisms. This work presents a crystal plasticity based analysis with a Taylor polycrystal averaging scheme of warm forming employing a new microstructure and dislocation based strain hardening model to simulate deformation behaviour. The hardening model is derived from energy balance between dislocation storage, dislocation accumulation, and dislocation recovery, based on remobilization of immobile dislocations, due to thermal activation. The constitutive formulation is extended to include alloying effects due to solute strengthening of Mg. The material hardening properties of AA5754 are characterized for a range of temperatures at constant strain-rates. A formulation for the kinematics of dynamic strain aging is presented and employed for room-temperature simulations. The hardening characterization is then used to predict stress-strain behaviour of AA5182 for similar conditions. The model shows excellent predictability of experimental results. An analysis on the microstructural connection between temperature and stress-strain response is presented.

Mechanical Behavior of a Low-Cost Ti–6Al–4V Alloy

Mechanical compression tests were performed on an economical Ti–6Al–4V alloy over a range of strain-rates and temperatures. Low rate experiments (0.001–0.1/s) were performed with a servo-hydraulic load frame and high rate experiments (1000-80,000/s) were performed with the Kolsky bar (Split Hopkinson pressure bar). Emphasis is placed on the large strain, high-rate, and high temperature behavior of the material in an effort to develop a predictive capability for adiabatic shear bands. Quasi-isothermal experiments were performed with the Kolsky bar to determine the large strain response at elevated rates, and bars with small diameters (1.59 mm and 794 µm, instrumented optically) were used to study the response at the higher strain-rates. Experiments were also conducted at temperatures ranging from 81 to 673 K. Two constitutive models are used to represent the data. The first is the Zerilli-Armstrong recovery strain model and the second is a modified Johnson–Cook model which uses the recovery strain term from the Zerilli–Armstrong model. In both cases, the recovery strain feature is critical for capturing the instability that precedes localization.

The Combined Effect of Particle Size Distribution and Relative Density on the Large Strain Behavior of Sandy Soils

Sandy soils were investigated by means of a new ring shear device to understand the combined effect of particle size distribution and relative density on the soils’ deformation characteristics at large strains. The soils were sheared in undrained condition to at least 10 m to observe their responses before and after the attainment of steady state. Two main responses—purely contractive and partially contractive—were identified. Loose specimens were purely contractive and mobilized peak strength at small strains before undergoing rapid loss of shear resistance until steady state strength was achieved. On the contrary, dense soils exhibited partially contractive behavior by first contracting and then dilating as shearing progressed. Analyzed shear resistance-pore pressure relationships indicated that changes in relative density induced corresponding changes in dilation characteristics. The shear behavior of the soils at critical density demarcated the behavior of the loose from that of the dense soils. At densities below the critical, the specimens exhibited purely contractive behavior, and above the critical, the specimens’ behavior was defined by three distinct stages of deformation of phase transformation, peak and steady states. A comparison of the friction angle at phase transformation, steady state and critical state in all the soils revealed only little difference. At a given normal stress, the friction angle at phase transformation was equal to the friction angle at steady state in specimens at critical density. This study shows that the large strain behavior soils can offer valuable insight on the mechanism of rapid, long-travel slope movements that are often catastrophic.

A new multi-scale framework to incorporate microstructure evolution in phenomenological plasticity: Theory, explicit finite element formulation, implementation and validation

Representation of anisotropy is a key factor in the predictive capabilities of finite element simulations of metal alloys. Micromechanics-based computational models, such as crystal plasticity, have served as a powerful tool over phenomenological-based models for their ability to predict initial and evolution of anisotropy. However, these micromechanics-based models are often sacrificed for much faster phenomenological models that do not capture microstructure evolution. In this paper, a new multi-scale framework is presented to incorporate microstructure evolution into phenomenological plasticity. Crystal plasticity is used to calibrate yield functions and microstructural evolution in a phenomenological manner using only a uniaxial tensile response and an electron backscatter diffraction (EBSD) map. Both single crystal and polycrystal mechanical properties are calibrated and predicted by the new framework. The constitutive framework is implemented into the commercial finite element software, LS-DYNA, to simulate the large strain behaviour of the extruded aluminum alloy (AA) 6063-T6. The simulations, which are compared to corresponding experiments, highlight the importance of capturing microstructural evolution in the simulation of large deformation in lab-scale components.

Deformation mechanisms of thermoplastic elastomers: Stress-strain behavior and constitutive modeling

This work addresses the large strain behaviors of thermoplastic polyurethanes (TPUs) spanning a range of fractions of hard and soft contents in both experiment and theoretical modeling. The key mechanical features involve a combination of elasticity and inelasticity, and are quantified experimentally under a broad variety of loading scenarios. A finite deformation constitutive model is then presented to capture the main features of the stress-strain data, which are strongly dependent on fractions of hard and soft contents. The stress-strain behavior of these TPUs is characterized by highly nonlinear rate-dependent hyperelastic-viscoplasticity, in which substantial energy dissipation is accompanied by shape recovery as well as softening. Agreement between the model and the experimental data for the representative TPUs provides physical insight into the underlying deformation mechanisms in this important class of soft materials that exhibit both elastomeric and plastomeric characteristics.