Uniaxial Behavior Research Articles

Uniaxial behavior of circular ultra-high-performance fiber-reinforced concrete columns confined by spiral reinforcement

This paper presents results from the uniaxial tests of six large-scale ultra-high-performance fiber-reinforced concrete (UHPFRC) circular columns confined by spirals. The UHPFRC used in this study had 1.5% of hybrid micro-steel fibers (1.0% of 19.5 mm fibers and 0.5% of 16.3 mm fibers) in the mixture and had compressive strengths varying from 163 to 181 MPa. The effects of the volumetric ratio of spiral reinforcement, compressive strength of concrete, and presence of hybrid micro-steel fibers on the axial load responses, including post-peak deformability, were investigated. In addition, the ductility level reached by circular UHPFRC columns designed according to the minimum spiral reinforcement of current design provisions of the CSA A23.3-14 Standard and the ACI 318-14 Code were evaluated to investigate the applicability of these equations to UHPFRC columns. Test results showed that the combined effect of the minimum spiral reinforcement and steel fibers resulted in sufficient post-peak ductility of the UHPFRC columns. To investigate the efficiency of the shape of the confinement reinforcement, the test results of the circular UHPFRC columns confined by spirals were compared with those from equivalent-sized square UHPFRC columns confined by hoops. The test results demonstrate the superior performance of circular spirals for developing the ductile behavior of UHPFRC columns than the same volumetric ratio of rectilinear hoops. A design recommendation for spiral reinforcement that ensures the ductile behavior of UHPFRC columns in moderate seismic regions is proposed.

Uniaxial tensile creep of a cracked polypropylene fiber reinforced concrete

This paper details and discusses the results of an experimental campaign into the uniaxial tensile creep behavior of a polypropylene fiber reinforced concrete (PFRC). In total, 16 FRC prisms are cast and tested according to the European Standard EN 14651. Cylindrical cores, taken from the prisms, are notched and are then used to identify the uniaxial tensile behavior of the FRC. The notched cores are also precracked and subjected to creep loading to study the time-dependent crack width growth at two different load ratios, 30 and 45% of the residual capacity. The results show that the load ratio has an important influence on the long-term tensile behavior of cracked FRC. For the samples subjected to 45%, the crack width exceeds the precrack width within hours, while the average crack width for the 30% samples remained below the initial crack width after 180 days under load. Based on the creep rate and crack width recovery upon unloading, it is shown that the 45% loaded samples exhibited plastic deformations, attributed to the onset of plasticity in the fibers. Furthermore, in terms of structural design, it is found that the residual post-creep capacity of the 30% samples was similar to that of undamaged cores, but significant differences were found for the higher loaded samples. For these samples, the average tensile capacity of the post-creep cores is 40% lower than that of the undamaged specimens. A comparison with steel FRC yielded that the time-dependent crack widening is similar, contrary to results in literature.

A simple model for indentation creep

A simple model for indentation creep is developed that allows one to directly convert creep parameters measured in indentation tests to those observed in uniaxial tests through simple closed-form relationships. The model is based on the expansion of a spherical cavity in a power law creeping material modified to account for indentation loading in a manner similar to that developed by Johnson for elastic-plastic indentation (Johnson, 1970). Although only approximate in nature, the simple mathematical form of the new model makes it useful for general estimation purposes or in the development of other deformation models in which a simple closed-form expression for the indentation creep rate is desirable. Comparison to a more rigorous analysis which uses finite element simulation for numerical evaluation shows that the new model predicts uniaxial creep rates within a factor of 2.5, and usually much better than this, for materials creeping with stress exponents in the range 1 ≤ n ≤ 7. The predictive capabilities of the model are evaluated by comparing it to the more rigorous analysis and several sets of experimental data in which both the indentation and uniaxial creep behavior have been measured independently.

Influence of stress level on uniaxial ratcheting effect and ratcheting strain rate in austenitic stainless steel Z2CND18.12N

Uniaxial ratcheting behavior of Z2CND18.12N austenitic stainless steel used nuclear power plant piping material was studied. The results indicated that ratcheting strain increased with increasing of stress amplitude under the same mean stress and different stress amplitude, ratcheting strain increased with increasing of mean stress under the same stress amplitude and different mean stress. Based on least square method, a suitable method to arrest ratcheting by loading the materials was proposed, namely determined method of zero ratcheting strain rate. Zero ratcheting strain rate occur under specified mean stress and stress amplitudes. Moreover, three dimensional ratcheting boundary surface graph was established with stress amplitude, mean stress and ratcheting strain rate. This represents a graphical surface zone to study the ratcheting strain rates for various mean stress and stress amplitude combinations. The graph showed the ratcheting behavior under various combinations of mean and amplitude stresses. The graph was also expressed with the help of experimental results of certain sets of mean and stress amplitude conditions. Further, experimentation cost and time can be saved.

Behavior of hybrid-synthetic fiber reinforced cellular lightweight concrete under uniaxial tension – Experimental and analytical studies

This paper investigates the effect of hybrid-synthetic fiber reinforcement on uniaxial tension behavior of low strength cellular lightweight concrete (CLC). Low strength CLC material is increasingly used for structural and non-structural masonry applications. These masonry units of low compressive strength typically offer little resistance to tensile stresses under lateral loading resulting in the collapse of CLC walls. A unique experimental setup was developed to test CLC under uni-axial tension. Dog-bone CLC specimens of length 600 mm and 150 mm × 100 mm cross section (test region) were cast with different synthetic fiber dosages and tested under uni-axial tension. Digital Image Correlation (DIC) technique was used to understand the crack-bridging mechanisms of the fibers. Experimental surface strains and crack openings were inferred using DIC technique. Presence of fibers prevented the premature fracture and led to improved post-cracking stiffness and ductility. Restricted crack localization and improved ductility were also observed due to addition of fiber reinforcement. Analytical models were used to predict the behavior of fiber reinforced cellular lightweight concrete (FRCLC) in tension based on the matrix and fiber parameters. The predictions had a good correlation with the experimental results.

Evolution of structural mechanisms in thermoplastic polyimide (BTDA-DAH) from amorphous precursors as revealed by real-time uniaxial mechano-optical behavior

The real-time mechano-optical behavior of Polyimide 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) 1,6-diaminohexane (DAH) [PI(BTDA-DAH)] films during uniaxial deformation was investigated in the glassy and rubbery states as a function of temperature and stretch rate to understand the stretching characteristics of the polymer and elucidate the structural evolution. The study combined real-time spectral birefringence with true stress and true strain measurements and the data were further augmented by offline DSC and WAXD measurements. During deformation near glass transition temperature, initial photoelastic regime exhibiting very small slope is followed by regimes II and III. As the deformation temperature increases, the initial photoelastic regime disappears and gives way to temperature-independent regime I following classic stress-optical behavior. Regime I is followed by either a negative deviation from SOR into regime IIIa at lower deformation temperatures, or a positive deviation from SOR into regime II at higher deformation temperatures which eventually transitions into regime IIIc at large strains. The polymer was essentially amorphous in regime I. As the deformation increased, strain-induced crystallization occurred in regime II where crystallinity as high as 24% was achieved. The strain-induced crystals enhanced network connectivity and increased the rate of polymer chain orientation before transitioning to regime IIIc. Under conditions where regime II was not observed, a linear mechano-optical response in regime I extended to higher deformations before transitioning into regime IIIa where the highly oriented chains exhibited a “nematic-like” order with translational disorder along the chain directions. Based on the above observations a schematic for the structural evolution of PI(BTDA-DAH) during uniaxial deformation was proposed.

Predictive capabilities of various constitutive models for arterial tissue.

Aim of this study is to validate some constitutive models by assessing their capabilities in describing and predicting uniaxial and biaxial behavior of porcine aortic tissue. 14 samples from porcine aortas were used to perform 2 uniaxial and 5 biaxial tensile tests. Transversal strains were furthermore stored for uniaxial data. The experimental data were fitted by four constitutive models: Holzapfel-Gasser-Ogden model (HGO), model based on generalized structure tensor (GST), Four-Fiber-Family model (FFF) and Microfiber model. Fitting was performed to uniaxial and biaxial data sets separately and descriptive capabilities of the models were compared. Their predictive capabilities were assessed in two ways. Firstly each model was fitted to biaxial data and its accuracy (in term of R2 and NRMSE) in prediction of both uniaxial responses was evaluated. Then this procedure was performed conversely: each model was fitted to both uniaxial tests and its accuracy in prediction of 5 biaxial responses was observed. Descriptive capabilities of all models were excellent. In predicting uniaxial response from biaxial data, microfiber model was the most accurate while the other models showed also reasonable accuracy. Microfiber and FFF models were capable to reasonably predict biaxial responses from uniaxial data while HGO and GST models failed completely in this task. HGO and GST models are not capable to predict biaxial arterial wall behavior while FFF model is the most robust of the investigated constitutive models. Knowledge of transversal strains in uniaxial tests improves robustness of constitutive models.

Constitutive modelling on the whole-life uniaxial ratcheting behavior of sintered nano-scale silver paste at room and high temperatures

Uniaxial ratcheting and fatigue behaviors of low-temperature sintered nano-scale silver paste at both room and high temperatures were studied in the authors' previous work. The deterioration of sintered silver pastes under cyclic loading which manifest itself as decrease in elastic modulus is addressed by introducing a damage variable as represented by θ function. Then, the damage variable is coupled into a visco-plastic constitutive model, based on the Ohno-Wang and Armstrong-Frederick (OW-AF) nonlinear kinematic hardening rule. In addition, the temperature-dependent ratcheting behavior are also captured by incorporating the Arrhenius power-law model into the flow rule. The whole-life ratcheting behaviors of low-temperature sintered nano-scale silver paste are simulated and are in well agreement with experimental data.

A new constitutive bulk material model to predict the uniaxial tensile nonlinear behavior of fiber metal laminates

In this investigation, a constitutive material model to predict elastic–plastic behavior of fiber metal laminates is introduced. The constants of the model can be obtained from the geometry and mechanical properties of the sublayers. This model can significantly reduce the computational efforts and central processing unit time by ignoring the contact between the fiber metal laminate layers. The ability of the model to predict plastic behavior of material makes it applicable to different metallic layers. Mechanical properties of each sublayer are obtained from tensile tests. The results of finite element analysis of the fiber metal laminate specimens using layered and bulk models revealed that the influence of glue was ignorable. The proposed model was validated by performing tensile tests on fiber metal laminate grades I and II and also on low and high metal volume fraction.

Microstructural deformation in fatigued nanotwinned copper alloys

In this study, the uniaxial tension-tension fatigue behavior of fully nanotwinned magnetron sputtered Cu-6wt%Al, Cu-2wt%Al, and Cu-10 wt%Ni is presented. These alloys have average twin thicknesses ranging from 4 to 8 nm, average grain widths from 90 to 180 nm, and tensile strengths from 1 to 1.5 GPa. In the high cycle regime (103 to 107 cycles), the nanotwinned alloys exhibit fatigue strengths ranging from 210 to 370 MPa, which is higher than previously observed in nanotwinned Cu (fatigue strengths between 80 and 200 MPa). Fatigue strengths are normalized by tensile strength for Cu alloys with different microstructures to study the correlation between tensile and fatigue properties. Post-mortem analysis of the materials reveals a newly observed deformation mechanism, where localized detwinning leads to intergranular fracture between columnar grains. Overall, materials displaying detwinning as a deformation mechanism show lower normalized fatigue strengths in comparison to materials that deform with slip band like behavior.

Physical modeling and analysis of site liquefaction subjected to biaxial dynamic excitations

The paper presents a series of centrifuge tests of level sites consisting of a granular soil deposit, with and without an embedded structure subjected to various biaxial and uniaxial base excitations. The tests were conducted at RPI NEES centrifuge facility to assess the dynamic response characteristics of level deposits and soil-structure interaction (SSI) under multidimensional conditions. Synthetic sinusoidal waves were used as base excitations to test soil models under biaxial and uniaxial shaking. A dense array of accelerometers was used to monitor the deposit response along with pore water pressure transducers. One uniaxial test and two biaxial shaking tests were conducted on three similar soil models, to study the effect of multidirectional shaking on the generation of soil liquefaction for dense deposits. The observed acceleration and pore pressure are used along with non-parametric identification procedures to evaluate the corresponding dynamic shear stress–strain histories. The measured results along with the obtained time histories are used to shed light on the mechanisms of liquefaction occurring through the stratum under multiaxial shaking. It also shows the effect of soil relative density on soil behavior when subjected to biaxial shaking. The soil–structure interaction test showed significant differences in soil behavior at different locations beneath the footing when subjected to biaxial shaking. This difference is associated with coupling in stress–strain in both experiments compared to the more known uniaxial soil behavior.

Design, Development, Analysis and Comparison of Instrumented Irradiation Capsule for Online Determination of Uniaxial Creep Behavior in Structural Specimen Out-of-Pile Test Results with Analytical Results in FBTR

The main intent of this work is to develop an instrumented capsule for online determination of uniaxial creep behavior of SS 316L structural specimen. The irradiation capsule has three different zones located one over the other. In the bottom zone of the capsule, the specimen is located, with one end fixed to the bottom portion of capsule and the other end connected to a central tube through a connecting plug. In the top zone a linear variable differential transducer (LVDT) is placed and its core is passed through the central tube. The end point of the LVDT core touches the plug connected to the specimen to measure the elongation of the structural specimen. In this capsule setup, bellows are used to apply a load on the structural specimen with the use of pressurized argon gas. With the application of pressure, the bottom bellow will expand and the top bellow will get compressed. During this expansion of the bottom bellow, tensile load will be applied on the specimen, and the elongation of the structural specimen will be measured by the movement of the core of the LVDT. This paper discusses the details of the design, assembly of an out-of-pile version of instrumented capsule and its experimental results are compared with literature results. The creep experiments have been carried out at three different temperatures and three different stresses (269 MPa at 450 °C, 287 MPa at 500 °C and 306 MPa at 550 °C). Initially the capsule is filled with pre-pressurized argon gas at 6 MPa at room temperature. We have observed that the force/load due to argon gas pressure is the major component for the tensile loading of the specimen and 95% of the total load acts on the SS 316L specimen and remaining 5% load only acts on the bellow. Design concept of instrumented capsule for uniaxial creep measurement has been validated.

Uniaxial ratcheting behaviour of 304L stainless steel and ER308L weld joints

In this study, a series of uniaxial strain cycling and ratcheting tests were conducted on 304L stainless steel and weld metal made with ER308L stainless steel welding wire. The ratcheting strain and fatigue life of the materials were measured at different loading levels and their cyclic behaviours are discussed. Microstructural evolution during cyclic loading was investigated by electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The experimental results show that the cyclic behaviours of the base metal and the weld metal are different. The cyclic behaviour of the base metal includes primary hardening, slight softening, and significant secondary hardening, while the weld metal exhibits a short hardening stage for several early cycles and later exhibits cyclic softening until failure. The evolution of dislocations during strain cycling and ratcheting deformation is discussed and the cyclic and ratcheting behaviours of the two materials are explained qualitatively. The final ratcheting strain of the materials depends greatly on the mean stress and stress amplitude. However, the fatigue life variation in the two materials is different. The base metal exhibited minimum fatigue life when a mean stress of 30MPa was loaded; at constant mean stress, the fatigue life decreases when the stress amplitude is increased. In the case of the weld metal, the fatigue life decreases with increasing mean stress and stress amplitude.

Graphene Foam: Uniaxial Tension Behavior and Fracture Mode Based on a Mesoscopic Model.

Because of the combined advantages of both porous materials and two-dimensional (2D) graphene sheets, superior mechanical properties of three-dimensional (3D) graphene foams have received much attention from material scientists and energy engineers. Here, a 2D mesoscopic graphene model (Modell. Simul. Mater. Sci. Eng. 2011, 19, 054003), was expanded into a 3D bonded graphene foam system by utilizing physical cross-links and van der Waals forces acting among different mesoscopic graphene flakes by considering the debonding behavior, to evaluate the uniaxial tension behavior and fracture mode based on in situ SEM tensile testing (Carbon 2015, 85, 299). We reasonably reproduced a multipeak stress-strain relationship including its obvious yielding plateau and a ductile fracture mode near 45° plane from the tensile direction including the corresponding fracture morphology. Then, a power scaling law of tensile elastic modulus with mass density and an anisotropic strain-dependent Poisson's ratio were both deduced. The mesoscopic physical mechanism of tensile deformation was clearly revealed through the local stress state and evolution of mesostructure. The fracture feature of bonded graphene foam and its thermodynamic state were directly navigated to the tearing pattern of mesoscopic graphene flakes. This study provides an effective way to understand the mesoscopic physical nature of 3D graphene foams, and hence it may contribute to the multiscale computations of micro/meso/macromechanical performances and optimal design of advanced graphene-foam-based materials.

The creep of alloy 617 at 700 °C: Material properties, measurement of strain and comparison between finite element analysis and digital image correlation

Future generations of power plants, such as the Ultra-Super-Critical (USC) power plants, are being designed to be operated at more extreme pressures and temperatures in order to achieve higher efficiency. One candidate material for components is Inconel alloy grade 617, a nickel based superalloy, which is expected to possess better creep resistance in comparison to other types of alloys (Bhadeshia and Honeycombe, 2011; Evans and Wilshire, 1993). At present there is little available data or information about the behaviour of this material at the temperature of interest (700 °C) and hence there is a need to evaluate its properties under these conditions.This paper details experimentation on Alloy 617 to evaluate its uniaxial behaviour under tension and creep at 700 °C, using the results obtained to develop a creep damage model based on power law creep in conjunction with the Cocks–Ashby void growth approach Cocks and Ashby (1982) for creep in a multiaxial stress state. Finite Element (FE) simulations are compared to experimental results obtained by Digital Image Correlation (DIC), which is used in order to validate the effectiveness of a power law creep damage model. Results made using a novel electrical strain sensor using ACPD principles supplement this work to draw comparisons between the response of the sensor and the strain field experienced by the specimen.

An analysis-based model for axially loaded circular CFST columns

Concrete-filled-steel-tube (CFST) columns have been increasingly adopted in many modern structures due to the beneficial composite action between steel tube and core concrete. Previous experimental and theoretical studies have proved this composite action can be further improved by providing external confinement in the form of steel rings, tie bars, spirals or jackets. A theoretical model developed by the authors previously based on (1) an accurate hoop strain equation; (2) an actively confined concrete model; (3) a three-dimensional steel model; (4) Interaction of core concrete, steel tube and external confinement was adopted in this paper to conduct a parametric study, which aims at investigating the effects of steel tube yield strength, concrete compressive strength, steel ratio and external confinement on the uni-axial behaviour of CFST columns. From the results in parametric study, two sets of critical steel ratios based on two different levels of ductility at different steel yield and concrete compressive strengths have been obtained. In addition, the maximum column strength obtained from the model and previous experimental studies was compared with the design strength calculated using different design codes. New design equations that can predict the axial strength more satisfactorily have also been proposed.

Three-dimensional constitutive modelling of arbitrarily orientated timber based on continuum damage mechanics

An orthotropic constitutive model is developed in order to capture the complex response of unidirectional fibrous materials with arbitrary orientation under a three-dimensional (3D) stress state. Different modes of failure and their associated criteria are derived via decomposition of a single-surface yield function in the fibre and matrix directions. The uniaxial behaviour of each stress component is approximated by a multi-linear model, and the concepts of continuum damage mechanics are utilised to model the degradation of various components under a general 3D stress state. The proposed model is implemented as a user material subroutine in ABAQUS®, and it is utilised to analyse the non-linear mechanical response of bulk or laminated timber. The crack band model and the viscous regularisation techniques are employed to alleviate the spurious mesh sensitivity and convergence problems in the softening state. Some benchmark and practical examples are presented to verify the accuracy and performance of the proposed model for predicting different modes of failure and non-linear behaviour with respect to orientation of the fibres and loading.

An Amended 8-Chain Model for Rubber-Like Materials

A new constitutive equation for rubber-like materials model was proposed. The model is based on the Arruda-Boyce (AB) 8-chain non-Gaussian molecular network to which two new terms were added. Taking into account the effects of molecular chain entanglement and the topological constraint on the transverse motions of a single chain, the new added components combine the phenomenological theory of elasticity for rubber vulcanizates at large deformation and the tube theory for topological constraint. The new model contains five parameters which are obtained by fitting the uniaxial Treloar extension data, the predictions from the proposed model for equi-biaxial extension and pure shear are in good agreement with test data. Moreover, compared with some available models, the proposed model is more suitable for characterization of the uniaxial tension mechanical behavior of carbon black particles filled rubber material.

Influence of crack micro-roughness on the plasticity-induced fatigue propagation in high strength steel

This article deals with the locally multiaxial fatigue behaviour of high strength steel. To this end, the influence of the cracking path deflections (at the micro level) on the plasticity-induced fatigue crack growth is analyzed. With regard to this, a modelling by means of the finite element method was performed for a given stress intensity factor in the Paris regime, considering the existence of micro-roughness in the crack path (local micro-deflections with distinct micro-angles as a function of the microstructure of the material). The numerical results allow one to obtain the fatigue crack propagation rate and compare it with that for the same material in the absence of micro-roughness (with no micro-crack deflections, i.e., uniaxial fatigue behaviour).

Impact of repeated uniaxial mechanical strain on flexible a-IGZO thin film transistors with symmetric and asymmetric structures

This letter investigates repeated uniaxial mechanical stress-induced degradation behavior in flexible amorphous In-Ga-Zn-O thin-film transistors (TFTs) of different geometric structures. Two types of via-contact structure TFTs are investigated: symmetrical and UI structure (TFTs with I- and U-shaped asymmetric electrodes). After repeated mechanical stress, I-V curves for the symmetrical structure show a significant negative threshold voltage (VT) shift, due to mechanical stress-induced oxygen vacancy generation. However, degradation in the UI structure TFTs after stress is a negative VT shift along with the parasitic transistor characteristic in the forward-operation mode, with this hump not evident in the reverse-operation mode. This asymmetrical degradation is clarified by the mechanical strain simulation of the UI TFTs.