Post-yield Region Research Articles

A novel methodology of predicting temperature and rate effects on tensile properties of viscoelastic materials

A novel methodology to predict the tensile properties of viscoelastic materials as a synergistic function of temperature and loading/displacement rate is presented in this paper. The model is based on the amount of energy dissipated during tensile loading, requiring less input data compared to existing models. The deformation activation energy was measured and accordingly an energy-based temperature shift factor was suggested. Additionally, to describe the material’s rate dependency, two new types of energy-based loading/displacement rate shift factors based on the Eyring and Cooperative models were derived. The shift factors were implemented into the theory of viscoelasticity to predict the material tensile behavior at different temperatures and loading/displacement rates. In addition, a new failure criterion was suggested to predict the mechanical strength of the tested materials through the definition of a critical energy level, which determined the maximum energy that a material would be able to dissipate before failure. The model was applied to two different viscoelastic materials; an epoxy adhesive, and an angle-ply glass/epoxy fiber-reinforced polymer (GFRP) composite, two common viscoelastic materials used in the structural and construction industries. To determine the accuracy of the model, the results obtained from the model were compared to the results obtained from experiments. It was observed that the suggested methodology well predicted the stress–strain curves of the tested materials in the linear region and yield stress up to the post-yield region and failure point. In addition, the strength surface of the studied materials, which showed the synchronous effect of temperature and loading/displacement rate on the ultimate tensile strength in a 3-dimensional representation, was introduced.

Choosing Conventional Concrete, Ultra-High Performance Concrete or Steel as Material of Construction: An Opinion

Ultra-High Performance Concrete (UHPC) is an emerging concrete material that has potential to become a popular choice of material for construction of structures. UHPC is defined as concrete with characteristic compressive strength of anywhere from 120-150 MPa and above combined with superior properties such as high durability, corrosion resistance, low water permeability among many others. However its high tensile strength and cracking behavior is what makes it unique and interesting. UHPC is reported to have 10 to 15 MPa of tensile strength and excellent post-crack behavior as compared to Conventional Concrete (CC). UHPC is reported to indicate strain-hardening behavior when subjected to post-cracking loads. This unique combination of high compressive strength with tensile strength and resilient cracking behavior reminds us of structural steel which is the other major structural building material with similar behavior. In view of these behavioral similarities, an effort has been made in this paper to compare and contrast the properties of UHPC with structural steel. The stress-stain and load-deflection behaviors of both materials are compared. Also a comparison is made between the two materials when subjected to loading beyond post-crack or post-yield regions. To make this study complete, a comparison of material behavior of UHPC, CC and structural steel with regards to their engineering properties, cost, environmental footprint, construction ease and design life is made. All these three materials are rated based on several parameters such as performance, economy, reusability, sustainability, among others. Finally, the order of choice of these three construction materials for various types of structures for typical scenarios is recommended.

Material extrusion of stainless-steel plate-lattice structure: Part shrinkage, microstructure, and mechanical performance

Additive manufacturing (AM) has been rapidly developed to fabricate metal lattice components owing to its capability in building complex geometries. However, the prevalent powder bed fusion-based technologies possess low manufacturing efficiency and high energy consumption that encumber their broad adoption. Here in this study, a cost-effective material extrusion-debinding-sintering technology using a commercial AM system was introduced to produce 17-4 PH stainless steel plate-lattice structures. Three types of plate-lattice structures were successfully fabricated in a minimum dimension of 0.4 mm. The anisotropic shrinkage of the plate-lattices has been calculated by measuring the dimensions of green, brown and final sintered parts in both top and side views. The body-centered cubic (BCC) plate-lattice exhibited equiaxed grains with an average grain size of 12.7 µm, and a few carbides were distributed on the surface. A compression test has been conducted to reveal the compressive stiffness and yield strength. Both BCC and face-centered cubic (FCC) plate-lattices exhibited higher stiffness than BCC-FCC type, and the FCC plate-lattice has the highest yield strength under compressive load. The FCC and BCC-FCC structures possessed higher ductility than BCC type due to the even deformation at the post-yield region. The successfully built metal plate-lattice structure evidences the great potential and possibility of material extrusion in fabricating low-cost metal components for a variety of industrial applications.

On the vibrations of the Electrorheological sandwich disk with composite face sheets considering pre and post-yield regions

Electrorheological (ER) is a type of material that can change its state from liquid to solid by being under an electrical field, bringing much attention from researchers. So, in this article, the vibration of a three-layered disk made of an ER core and two graphene nanoplatelets reinforced composite (GPLRC) face sheets is carried out. The shear strain and stress of ER layer were obtained in the pre- and post-yield state. First- and third-order shear deformation theories (FSDT and TSDT) are utilized to model the face sheets and core of the structure. By using HDQM – hyperbolic differential quadrature method – the results are acquired. By employing other published studies, the validation investigation is carried out. Lastly, the influence of such parameters as graphene nanoplatelets (GPLs) distribution, foundation parameters, open-angle of the disk, the thickness of different layers, inner in addition to the outer radius of the disk, and GPLs’ weight fraction on the vibrational behavior of the system is investigated.

Formation of free radicals in human hair under strain: Combined electron paramagnetic resonance (EPR) – Strain technique

In this work, the formation of free radicals in human hair and the evolution of the radical concentration under strain, using a combined electron paramagnetic resonance (EPR) – strain technique, has been investigated. The radicals formed in the hair as a result of homolytic bond cleavage in cystine residues of polypeptide chains were identified. Stability of the radicals formed in dry hair and in the presence of water were studied. The spin–strain curves for the grey human hair in dry state and in water are presented and compared with conventional hair stress vs. strain curves. The evolution of sulfur radical species has been found to only occur at strains of above ca. 25%, corresponding with the Post-Yield region of the stress vs. strain behaviour for hair; this indicates that the matrix of the hair in this region behaves like a highly crosslinked gel and helps to explain the reversibility in hair mechanical behaviour below the Post-Yield region.

Stability/instability of magnetorheological core sector structure for mechanical control braking system by the intelligent computer method

ABSTRACT Due to the amazing mechanical properties of the magnetorheological (MR) materials, which can quickly transit from fluid to solid and vice versa, these materials have attracted a lot of attention among designers. So, for the first time, a dynamic stability analysis of a sandwich sector plate made of an MR core and two graphene nanoplatelets reinforced composite (GPLRC) face sheets has been presented. Using the role of mixture and Halpin–Tsai model, material properties of the GPLRC layers have been modeled. For modeling the MR core, shear stress and strain properties of MR fluids have been characterized into pre-yield and post-yield regions. Via first-order shear deformation theory, compatibility equations between the layers, and Hamilton’s principle, motion equations of the structure have been achieved. A generalized differential quadrature method with Chebyshev polynomials greed points has been applied for obtaining the responses of structure. The verification section has been separated into two parts. The first and second ones are verification between the current outcomes with the results of the published report and the machine learning method based on deep neural networks, respectively. Consequently, the results show that some geometrical and physical parameters have an important role in the dynamic stability of the current structure.

Characteristic analysis under small-stroke and medium-high frequency of magneto-rheological damper with pressure controlled mechanism

Due to the high frequency hardening effect in applications of satellite and precision vibration isolation, it is an urgent need to analyze and improve the magnetorheological (MR) performance of MR damper under small-stroke and medium-high frequency excitations. In this paper, a compact MR damper with pressure controlled mechanism is proposed, analyzed and tested. Considering the inertial, pre-compression and compressibility of MR fluid, the dynamic physic model is established to analyze the output characteristics with pre-compressions under small-stroke and medium-high frequency excitations. And then, the critical model between the pre-yield region and post-yield region is obtained. The critical boundary between different excitations is derived and comparatively analyzed. Finally, the experiments with the proposed MR damper are carried out. Compared with theoretical results, the damping force and dynamic stiffness increase with the increase of the applied current and excitations. Besides these, the energy dissipated performance can be improved by the pre-compression. When the initial pressure increases to 5 MPa with the fixed frequency of 10 Hz, the energy dissipation is increased by 43.5%. So the pre-compression is an effective way to solve the high frequency hardening. Besides these, the derived critical model can predict the performance of MR damper at different excitations.

Experimental and modeling study of the viscoelastic-viscoplastic deformation behavior of amorphous polymers over a wide temperature range

The deformation behavior of amorphous polymers in the post-yield region is viscoelastic-viscoplastic (VE-VP) as it exhibits both viscoelastic and viscoplastic characteristics. However, a systematic experimental and modeling study for it is lacking. In this paper, the VE-VP deformation behavior of an amorphous polymer was experimentally investigated over a wide temperature range. It was found that some of the observed deformation behaviors cannot be reasonably explained by the common assumptions of the thermally-activated segmental motions and the formation of a topologically-entangled rubbery network. With the assumption that there is a glassy network formed from weak linkages in the polymer, the observed deformation behaviors over a wide temperature range can be well explained by considering the stretch and relaxation of the glassy network. The glassy network assumption was first proposed by one of our coauthors and employed for modeling the VE-VP deformation behavior of amorphous polymers at room temperature. In this paper, the glassy network assumption was further examined at different temperatures and employed to establish a constitutive model for simulating the VE-VP deformation behavior of amorphous polymers over a wide temperature range. The model was validated by corroboration with experiments. It was demonstrated that a constitutive model based on the glassy network assumption is capable of representing the loading, strain recovery, and stress relaxation behaviors simultaneously, in contrast to most of the existing constitutive models. Hence, this work not only proposes a reasonable physical picture for understanding but also provides a robust constitutive model for simulating the VE-VP deformation behavior of amorphous polymers and its temperature dependence.

Effects of anti-resorptive treatment on the material properties of individual canine trabeculae in cyclic tensile tests.

Osteoporosis is defined as a decrease of bone mass and strength, as well as an increase in fracture risk. It is conventionally treated with antiresorptive drugs, such as bisphosphonates (BPs) and selective estrogen receptor modulators (SERMs). Although both drug types successfully decrease the risk of bone fractures, their effect on bone mass and strength is different. For instance, BP treatment causes an increase of bone mass, stiffness and strength of whole bones, whereas SERM treatment causes only small (4%) increases of bone mass, but increased bone toughness. Such improved mechanical behavior of whole bones can be potentially related to the bone mass, bone structure or material changes. While bone mass and architecture have already been investigated previously, little is known about the mechanical behavior at the tissue/material level, especially of trabecular bone. As such, the goal of the work presented here was to fill this gap by performing cyclic tensile tests in a wet, close to physiologic environment of individual trabeculae retrieved from the vertebrae of beagle dogs treated with alendronate (a BP), raloxifene (a SERM) or without treatments. Identification of material properties was performed with a previously developed rheological model and of mechanical properties via fitting of envelope curves. Additionally, tissue mineral density (TMD) and microdamage formation were analyzed. Alendronate treatment resulted in a higher trabecular tissue stiffness and strength, associated with higher levels of TMD. In contrast, raloxifene treatment caused a higher trabecular toughness, pre-dominantly in the post-yield region. Microdamage formation during testing was not affected by either anti-resorptive treatment regimens. These findings highlight that the improved mechanical behavior of whole bones after anti-resorptive treatment is at least partly caused by improved material properties, with different mechanisms for alendronate and raloxifene. This study further shows the power of performing a mechanical characterization of trabecular bone at the level of individual trabeculae for better understanding of clinically relevant mechanical behavior of bone.

Field–Frequency-Dependent Non-linear Rheological Behavior of Magnetorheological Grease Under Large Amplitude Oscillatory Shear

This article investigates the influence of frequency on the field-dependent non-linear rheology of magnetorheological (MR) grease under large amplitude oscillatory shear (LAOS). First, the LAOS tests with different driving frequencies were conducted on MR grease at four magnetic fields, and the storage and loss moduli under the frequency of 0.1, 0.5, 1, and 5 Hz were compared to obtain an overall understanding of the frequency-dependent viscoelastic behavior of MR grease. Based on this, the three-dimensional (3D) Lissajous curves and decomposed stress curves under two typical frequencies were depicted to provide the non-linear elastic and viscous behavior. Finally, the elastic and viscous measures containing higher harmonics from Fourier transform (FT)-Chebyshev analysis were used to quantitatively interpret the influence of the frequency on the non-linear rheology of MR grease, namely, strain stiffening (softening) and shear thickening (thinning), under LAOS with different magnetic fields. It was found that, under the application of the magnetic field, the onset of the non-linear behavior of MR grease was frequency-dependent. However, when the shear strain amplitude increased in the post-yield region, the non-linear rheology of MRG-70 was not affected by the oscillatory frequency.

Photostability of TiO2-Coated Wool Fibers Exposed to Ultraviolet B, Ultraviolet A, and Visible Light Irradiation

Abstract This study is to investigate the role of the coating of TiO2 nanoparticles deposited on wool fibers against high-intensity ultraviolet B (UVB), ultraviolet A (UVA), and visible light irradiation. The properties of tensile and yellowness and whiteness indices of irradiated TiO2-coated wool fibers are measured. The changes of TiO2-coated wool fibers in optical property, thermal stability, surface morphology, composition, molecular structure, crystallinity, and orientation degree are characterized using diffuse reflectance spectroscopy, thermogravimetric analysis, scanning electronic microscopy, energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction techniques. Experimental results show that the tensile properties of anatase TiO2-coated wool fibers can be degraded under the high-intensity UVB, UVA, and visible light irradiation for a certain time, resulting in the loss of the postyield region of stress–strain curve for wool fibers. The coating of TiO2 nanoparticles makes a certain contribution to the tensile property, yellowness and whiteness indices, thermal stability, and surface morphology of wool fibers against high-intensity UVB, UVA, and visible light irradiation. The high-intensity UVB, UVA, and visible light can result in the photo-oxidation deterioration of the secondary structure of TiO2-coated wool fibers to a more or less degree. Meanwhile, the crystallinity and orientation degree of TiO2 coated wool fibers decrease too.

Post-Yield characteristics of electrorheological fluids in nonlinear vibration analysis of smart sandwich panels

The present study deals with the nonlinear free vibration analysis of sandwich plates containing electrorheological (ER) fluid as the core layer. Since yield stress of the ER fluid is relatively low, the fluid widely operates in the post-yield region in which the shear stress and shear strain relation is no longer linear. This study employs an equivalent linearized complex shear modulus to quantify the storage and loss moduli of the fluid in different levels of shear strain amplitude and electric field intensity. Also, Classical Plate Theory (CPT) along with von Kármán kinematic formulations are incorporated to extract governing equations of motion of the ER based sandwich plates, in a finite element formulation. The nonlinear governing equations of motion of the structure are solved using a displacement control strategy to evaluate the resonant frequencies and corresponding loss factors of the sandwich plate. Then, the primary attention is focused on the effect of post-yield characteristics of ER fluid on the nonlinear dynamic behavior of the ER based sandwich structure. Furthermore, the effect of applied electric field, boundary conditions and plate parameters such as core layer thickness on the natural frequencies and damping of the structure are comprehensively investigated.

Experimental observations and data analysis of a commercially grade aluminium alloy (6082T6) for large elastoplastic deformation

Large elastoplastic deformation generally occurs in sheet-metal forming processes during stretching, blanking, punching, stamping, bending, bending-unbending, reverse bending and deep drawing. In today’s world, due to very high competition and swift development of products in metal forming industries, there is a growing demand in high precision end products to be manufactured in short duration. It is very essential and important to have a knowledge of an in-depth understanding of material behaviour in all these metal forming steps. Therefore, to predict material behaviour, an accurate material modelling is required by considering the simultaneous effects of strain and strain rate. Classical flow rules are generally used to predict the material behaviour in the post yield range. Overall material deformation can be represented as the change in size, change in shape and their rates. A new generalised relation is developed to represent the material behaviour from elastic range to failure. This relation involves eight material parameters which includes two classical material constants like bulk modulus and shear modulus, two material viscosity terms and four in the form rule which relates elastic strain rate to plastic strain rate. All these material parameters can be obtained from uniaxial tension experiments. The material deformation is 3-D in nature and therefore classical yield criteria is avoided while formulating the relation. Instead, concept of activation energy is used. In the present work, experimental observations and data analysis on the commercial grade aluminium (6082T6) was discussed. The material behavior in the post yield region with new understanding of the material model was presented here for uni-axial loading case. Based on the experimental observations and the results obtained it was found that the both the elastic moduli and the viscosities remain constant throughout the large deformation.

Nonlinear vibration analysis of circular/annular/sector sandwich panels incorporating magnetorheological fluid operating in the post-yield region

This study offers a comprehensive analysis on the nonlinear vibratory behavior of circular, annular, and sector sandwich panels containing magnetorheological (MR) fluid as the core layer. Due to the large deformation experienced by the sandwich structures, the MR fluid operates in the post-yield region, in which the shear strain and shear stress are nonlinearly dependent. The post-yield characteristics of the MR fluid are quantified using the experimental results available in the literature. The present study also employs the experimental results presented in the literature on the dynamic characteristics of a MR based sandwich circular plate to demonstrate accuracy of the results. To identify governing equations of motion of the structures, a finite element approach based on von Karman formulations is employed. Moreover, displacement control strategy is used to solve the extracted nonlinear equations and evaluate dynamic characteristics of the MR based circular/annular/sector sandwich plates in terms of resonant frequencies and loss factors. This study highlights the effect of post-yield behavior of the MR fluid in the nonlinear vibration attenuation of MR sandwich structures, under different levels of magnetic field. Also, the effects of maximum deformation and magnetic field on the functionality of the MR fluid are comprehensively investigated.

Effect of bolt inclination angle on shear behavior of bolted joints under CNL and CNS conditions

Rock bolts are widely used in rock engineering projects to improve the shear capacity of the jointed rock mass. The bolt inclination angle with respect to the shear plane has a remarkable influence on the bolting performance. In this study, a new artificial molding method based on 3D scanning and printing technology was first proposed to prepare bolted joints with an inclined bolt. Then, the effects of the bolt inclination angle and boundary conditions on the shear behavior and failure characteristic of bolted joints were addressed by conducting direct shear tests under both CNL and CNS conditions. Results indicated that rock bolt could significantly improve the shear behavior of rock joints, especially in the post-yield deformation region. With the increase of bolt inclination angle, both the maximum shear stress and the maximum friction coefficient increased first and then decreased, while the maximum normal displacement decreased monotonously. Compared with CNL conditions, the maximum shear stress was larger, whereas the maximum normal displacement and friction coefficient were smaller under the CNS conditions. Furthermore, more asperity damage was observed under the CNS conditions due to the increased normal stress on the shear plane.

Post-yield Damping Ratio Estimation Studies for Masonry-Infilled Reinforced Concrete Frames

Damping ratio is an important and central parameter for the dynamic analysis of structures. The present study focuses on the estimation of the equivalent damping ratio in a post-yield region for RC frames, both with and without infills. One-story one-bay RC frames casted non-integrally with fly ash (FA) bricks and autoclaved aerated concrete (AAC) blocks infills are considered and are tested under monotonic lateral half-cycle loading. Post-yield behavior parameters, displacement ductility and maximum drift ratio, are used to estimate the equivalent damping ratio at each cycle of lateral loading, and empirical relationships are established. Secant stiffness and stiffness degradation for RC frames are also extracted to determine the contribution and effectiveness of type of infills. Free vibration tests on RC frame specimens are conducted subsequent to each cycle of monotonic lateral loading, and the damping ratio is evaluated. It is found that the equivalent damping ratio substantially increases for RC frames with the onset of damage in the post-yield region. The major advantage of the infilled RC frame is a drastic reduction in the maximum drift ratio. However, types of infill are found to yield equivalent damping ratio at par, suggesting similar energy dissipation capabilities. RC frame infilled with AAC block is recommended due to its higher initial as well as secant stiffness vis- $$\grave{a}$$ -vis FA brick-infilled RC frames and higher stiffness degradation that allows it to behave similar to RC frame without infill.

A finite strain thermodynamically-based constitutive modeling and analysis of viscoelastic-viscoplastic deformation behavior of glassy polymers

In elastic and plastic deformation of glassy polymers and with the increase of deformation, the material would exhibit elastic, viscoelastic, and viscoelastic-viscoplastic (VE-VP) responses in sequence. These deformation behaviors should be constitutively modeled in such a way to accurately and efficiently simulate many important physical behaviors and phenomena involved in polymer deformation and processing. The elastic and viscoelastic deformation behavior in the pre-yield region can be well represented by the existing constitutive models. However, the VE-VP response in the post-yield region, such as stress relaxation and strain recovery behaviors, cannot be well modeled yet. In this research, a series of uniaxial compression tests of the stress relaxation and the loading-unloading-recovery behaviors of glassy polymer were firstly carried out. The experimental phenomena show strain-rate-dependent characteristics that cannot be explained by the thermally-activated viscosity/viscoplasticity theory. Therefore, it is proposed that a glassy polymer can be treated as a glassy network linked by the secondary bonds and entanglements; while the glassy network is locally yielded and elastically distorted in the pre-yield region, giving rise to a glassy network resistance (GNR) which accounts for the strain-rate-dependent characteristics. A finite strain thermodynamically-based constitutive model was then proposed to incorporate the effects of the GNR. Finally, the newly proposed model was calibrated and its predictive capability was elaborated with the comparison of experiments as well as two widely reported models. The predicted results demonstrate that the GNR is critical in controlling the post-yield stress relaxation and strain recovery. In addition, the nonlinear evolution of the GNR plays a key role in controlling the nonlinear pre-peak hardening and the nonlinear unloading curve. This research thus not only provides a new constitutive theory for modeling of the VE-VP deformation behavior of glassy polymers, but also advances the understanding of the mechanical responses of glassy polymers in deformation process.

Modelling Magnetorheological Dampers in Preyield and Postyield Regions

The use of magnetorheological dampers has rapidly spread to many engineering applications, especially those related to transportation and civil engineering. The problem arises upon modelling their highly nonlinear behaviour: in spite of the huge number of apparently accurate models in the literature, most fail when considering the overall magnetomechanical behaviour. In this study, a brief but broad review of different magnetorheological damper models has been carried out, which includes characterisation, modelling, and comparison. Unlike many other studies, the analyses cover the behaviour from preyield to postyield regions of the MR fluid. The performance of the different models has been assessed by means of numerous experimental tests and by means of simulations in a simple and straightforward semiactive control case study. The results obtained prove that most models usually fail in predicting accurate low‐velocity behaviour (before iron chains yield) and, as a result, may lead to bad estimations when used in control schemes due to modelling errors and chattering.

Cohesive zone modeling of the autoclave pressure effect on the delamination behavior of composite laminates

Current manufacturing processes using resin transfer molding or low-pressure prepreg curing may result in different defects and interfacial properties. The effect of autoclave pressure on the delamination behavior of T800/X850 composite laminates is explored. Cohesive zone model was used to model the delamination of unidirectional composite laminates under short-beam bending. Composites with various interlaminar properties were manufactured using autoclave under cure pressure from 0 MPa to 0.6 MPa. Cohesive zone model was validated using the material parameters of the composite cured under 0.6 MPa. The effect of cohesive zone model parameters including cohesive strength, mode I fracture toughness ([Formula: see text]), and mode II fracture toughness ([Formula: see text]) on the delamination behavior and load–displacement response was investigated. Parametric study shows that interlaminar cohesive strength and mode II fracture toughness dominated the initiation of yield and post-yield region, respectively. The correlation between autoclave pressure and mode II fracture toughness was predicted, which is mainly affected by void content.

Combining specimen-specific finite-element models and optimization in cortical-bone material characterization improves prediction accuracy in three-point bending tests

Although the beam theory is widely used for calculating material parameters in three-point bending test, it cannot accurately describe the biomechanical properties of specimens after the yield. Hence, we propose a finite element (FE) based optimization method to obtain accurate bone material parameters from three-point bending test. We tested 80 machined bovine cortical bone specimens at both longitudinal and transverse directions using three-point bending. We then adopted the beam theory and the FE-based optimization method combined with specimen-specific FE models to derive the material parameters of cortical bone. We compared data obtained using these two methods and further evaluated two groups of parameters with three-point bending simulations. Our data indicated that the FE models with material properties from the FE-based optimization method showed best agreements with experimental data for the entire force-displacement responses, including the post-yield region. Using the beam theory, the yield stresses derived from 0.0058% strain offset for the longitudinal specimen and 0.0052% strain offset for the transverse specimen are closer to those derived from the FE-based optimization method, compared to yield stresses calculated without strain offset. In brief, we conclude that the optimization FE method is more appropriate than the traditional beam theory in identifying the material parameters of cortical bone for improving prediction accuracy in three-point bending mode. Given that the beam theory remains as a popular method because of its efficiency, we further provided correction functions to adjust parameters calculated from the beam theory for accurate FE simulation.