Bilinear Behavior Research Articles

Irradiation creep of the US Heat 832665 of V–4Cr–4Ti

The paper presents irradiation creep data for V –4Cr –4Ti irradiated to 3.7 dpa at 425 and 600 °C in the HFIR-17J experiment. Creep deformation was characterized by measuring diametral changes of pressurized creep tubes before and after irradiation. It was found that the creep strain rate of the US Heat 832665 of V –4Cr –4Ti exhibited a linear relationship with stress up to ∼180 MPa at 425 °C with a creep coefficient of 2.50 × 10 −6 MPa −1 dpa −1. A linear relationship between creep rate and applied stress was observed below ∼110 MPa at 600 °C with a creep coefficient of 5.41 × 10 −6 MPa −1 dpa −1; non-linear creep behavior was observed above ∼110 MPa, and it may not be fully accounted by invoking thermal creep. The bilinear creep behavior observed in the same alloy irradiated in BR-10 was not observed in this study.

Free Volume Dependence of the Internal Rotation of a Molecular Rotor Probe in Room Temperature Ionic Liquids

The fluorescence efficiency of a well-known microviscosity probe, 9-(dicyanovinyl)julolidine (DCVJ), which is highly sensitive to the viscosity of the medium, has been studied in seven imidazolium ionic liquids (ILs) of varying viscosities over a temperature range of 10-60 degrees C. The microviscosities around the probe in different ILs have been estimated from the linear dependence of the logarithm of fluorescence quantum yield (log phi(f)) on the logarithm of the bulk viscosity (log eta) in various conventional solvents of different viscosities at room temperature. These microviscosities, which represent the local environments around the probe, are found to be significantly different from the directly measured bulk viscosities of these ILs. The log phi(f) vs log (eta/T) plots, which are also expected to be linear, interestingly show a bilinear behavior in more viscous ILs with a break around 28-30 degrees C. The observation of a similar break in the Arrhenius plots of the rate constant of the internal rotation in DCVJ and absence of any such break in the temperature dependence of the mobility of the ILs allow us to determine the important role of the free volume around the probe in dictating the nonradiative deactivation rate or the fluorescence efficiency of DCVJ. The break in the plots, which implies a change in the available free volume around the probe at approximately 28-30 degrees C, presumably arises from the repositioning of the probe from one environment to a different one of these microheterogeneous ILs with change of temperature.

The effect of foundation embedment on inelastic response of structures

AbstractIn this research, a parametric study is carried out on the effect of soil–structure interaction on the ductility and strength demand of buildings with embedded foundation. Both kinematic interaction (KI) and inertial interaction effects are considered. The sub‐structure method is used in which the structure is modeled by a simplified single degree of freedom system with idealized bilinear behavior. Besides, the soil sub‐structure is considered as a homogeneous half‐space and is modeled by a discrete model based on the concept of cone models. The foundation is modeled as a rigid cylinder embedded in the soil with different embedment ratios. The soil–structure system is then analyzed subjected to a suit of 24 selected accelerograms recorded on alluvium deposits. An extensive parametric study is performed for a wide range of the introduced non‐dimensional key parameters, which control the problem. It is concluded that foundation embedment may increase the structural demands for slender buildings especially for the case of relatively soft soils. However, the increase in ductility demands may not be significant for shallow foundations with embedment depth to radius of foundation ratios up to one. Comparing the results with and without inclusion of KI reveals that the rocking input motion due to KI plays the main role in this phenomenon. Copyright © 2008 John Wiley & Sons, Ltd.

Direct use of PGV for estimating peak nonlinear oscillator displacements

AbstractA predictive model is presented for estimating the peak inelastic oscillator displacements (Sd,ie) from peak ground velocity (PGV). The proposed model accounts for the variation of Sd,ie for bilinear hysteretic behavior under constant ductility (µ) and normalized lateral strength ratio (R) associated with postyield stiffness ratios of α=0 and 5%. The regression coefficients are based on a ground‐motion database that contains dense‐to‐stiff soil site recordings at distances of up to 30 km from the causative fault. The moment magnitude (M) range of the database is 5.2⩽M⩽7.6 and the ground motions do not exhibit pulse‐dominant signals. Confined to the limitations imposed by the ground‐motion database, the model can estimate Sd,ie by employing the PGV predictions obtained from the attenuation relationships (ground‐motion prediction equations). In this way, the influence of important seismological parameters can be incorporated to the variation of Sd,ie in a fairly rationale manner. This feature of the predictive model advocates its implementation in the probabilistic seismic hazard analysis that employs scalar ground‐motion intensity indices. Various case studies are presented to show the consistent estimations of Sd,ie by the proposed model. The error propagation in the Sd,ie estimations is also discussed when the proposed model is associated with attenuation relationships. Copyright © 2008 John Wiley & Sons, Ltd.

High Temperature Low-Cycle Fatigue, Deformation Microstructure and Final Fracture Behavior of a Nickel-Base Superalloy

The low-cycle fatigue (LCF) properties of a nickel-base precipitation-strengthened superalloy (GH4145/SQ), obtained at a temperature of 538 o C, were reported and discussed in this paper. The properties investigated include cyclic stress response, fatigue life, deformation microstructure and final fracture features as a function of applied strain amplitude. It was shown that the alloy exhibited a pronounced initial hardening followed by continuous softening to failure at high plastic strain amplitudes ( > 0.2% ap ε ), while at low plastic strain amplitudes ( < 0.2% ap ε ) the initial hardening was followed by a well-defined saturation stage. Bilinear behavior with a change of slope at a plastic strain amplitude of about 0.2% was observed in the cyclic stress-strain (CSS) and Coffin-Manson (C-M) plots. TEM observations revealed that slip band density increased with increasing total strain amplitude and precipitate degradation resulting from dislocation-precipitate interactions took place with continuous cyclic straining. The change in the microstructure during cycling is thus responsible for the fatigue hardening / softening behavior of the alloy. SEM examinations indicated that at low plastic strain amplitudes ( < 0.2% ap ε ) crack propagation was basically transgranular, while at high plastic strain amplitudes ( > 0.2% ap ε ) crack propagation exhibited intergranular features, as a whole. The variation in both the number of operating slip systems and the fracture modes with the strain amplitude employed was used to explain the observed two-stage LCF behavior of the present investigated superalloy.

Flexural Strengthening of Steel Bridges with High Modulus CFRP Strips

Acceptance of carbon fiber-reinforced polymer (CFRP) materials for strengthening concrete structures, together with the recent availability of higher modulus CFRP strips, has resulted in the possibility to also strengthen steel structures. Steel bridge girders and building frames may require strengthening due to corrosion induced cross-section losses or changes in use. An experimental study investigating the feasibility of different strengthening approaches was conducted. Large-scale steel-concrete composite beams, typical of bridge structures, were used to consider the effect of CFRP modulus, prestressing of the CFRP strips, and splicing finite lengths of CFRP strips. All of the techniques examined were effective in utilizing the full capacity of the CFRP material, and increasing the elastic stiffness and ultimate strength of the beams. Results of the experimental program were compared to an analytical model that requires only the beam geometry and the constitutive properties of the CFRP, steel, and concrete. This model was used to investigate the importance of several key parameters. Finally, an approach for design is proposed that considers the bilinear behavior of a typical strengthened beam to the elastic-plastic behavior of the same beam before strengthening.

A comparison of dynamic nonlinear behavior of ordinary and buckling restrained braced frames subjected to strong ground motion

AbstractIn this paper nonlinear dynamic analyses of 4‐ to 12‐story frames with two types of bracing behavior are performed. In the first series of analyses, the bracing post‐buckling behavior is modeled using a Jain‐phenomenological model. In this model bracing buckled against compressive loading and yielded when subjected to tensile loading. In the second series, buckling restrained brace frames subjected to strong ground motion have been analyzed using elements with a bilinear behavior. The results in terms of story drifts, story shears, story shear versus drift hysteresis behavior and plastic hinge locations were compared. By comparing the response of the frame with the buckling restrained brace and the ordinary brace, better performance was observed for the buckling restrained braced frames. Copyright © 2007 John Wiley & Sons, Ltd.

Elimination of Dual Slope from the Coffin–Manson Relationship of Low-Cycle Fatigue in the Titanium Alloy Timetal 834, by Cold Rolling

Cold rolling of the titanium alloy Timetal 834 was found to cause marked enhancement in low-cycle fatigue (LCF) life at low strain amplitude and to eliminate bilinear behavior from the Coffin–Manson (C-M) relationship. It was due to work hardening of surface grains of soft orientation and consequent increase in resistance of the material against crack initiation. The observed effect was not associated with texture.

Force-Deformation Behavior of Isolation Bearings

The isolation bearings are widely used in earthquake prone areas to protect the structure from seismic forces. The isolation bearing consists of an isolator to increase the natural period of the structure away from the high-energy periods of the earthquake, and a damper to absorb energy in order to reduce the seismic force. The most common isolation bearings used are lead–rubber bearings. They combine the function of isolation and energy dissipation in a single compact unit, giving structural support, horizontal flexibility, damping, and a centering force in a single unit. The relation between the horizontal force and horizontal displacement of the isolation bearings is nonlinear; to calculate the stiffness and the damping constant, which correspond to effective design displacement, the nonlinear behavior is expressed by bilinear behavior. This technical note presents new relations to calculate yield force, horizontal displacement, and damping.

Piezoresistance Behaviour of CFRP Cross-Ply Laminates with Transverse Cracking

This paper describes the piezoresistance behavior in CFRP cross-ply laminates with transverse cracking loaded in tension. The resistance change due to transverse cracking and the gage factor (the rate of resistance change per mechanical strain) for each transverse crack density were experimentally measured during loading/unloading cycles. The resistance change-strain curves in the unloading/ reloading processes show the bilinear behavior where two gage factors are defined as the slopes of the bilinear curve. The residual resistance change after full unloading increases almost linearly with the mechanical strain while the gage factors do not always increase with the strain. The residual resistance change and gage factors are associated with transverse crack density on the basis of an equivalent resistance circuit.

Indentation Size Effects in the Nano and Microhardness of FCC Single Crystal Metals

This paper presents the results of the experimental study of nano and micro indentation size effect on selected (001) oriented face centered cubic (FCC) crystals (Ni, Au, and Ag). Following a detailed description of experimental techniques and correction factors, indentation size effects in (001) oriented Au, Ag, and Ni single crystals are elucidated. Material pile-up phenomena are discussed before analyzing the results within the context of mechanism-based strain gradient plasticity (SGP) theories. Microstructure length scales are calculated based on the analysis. These are shown to vary with indentation size. A bi-linear behavior is shown to describe the indentation size effects between the micro- and nano-scales. This is rationalized by considering the limiting cases of plastic deformation by source-limited and established dislocation substructures. A multi-scale framework is proposed for modeling small contacts.

A Model for Piezoresistance Behavior in a CFRP Cross-Ply Laminate with Transverse Cracking

An electromechanical model is proposed for predicting the resistance change in CFRP cross-ply laminates with transverse cracking loaded in tension. A one-dimensional equivalent circuit model was established for describing the relation between the resistance change and mechanical strain at a given transverse crack density. A shear lag model was employed for obtaining the mechanical strain distribution. The relative resistance change-strain curve was measured for various transverse crack density and the slope and secant of the curve were defined as the gage factor and residual resistance change, respectively. Unknown functions in the model were determined using the measured residual resistance changes. The model can predict the bilinear behavior in the relative resistance change-strain curves during loading/unloading. It is found that the second residual resistance change can be the parameter for predicting transverse crack density because it increases monotonically with transverse crack density compared with the gage factors.

Self-centring capacity of seismic isolation systems

SUMMARY The ability of seismic isolators to develop restoring forces, also referred as self-centring capacity, is commonly associated with the system ability to prevent excessively large unit deformations and possible isolator failure. This concept is emphasized in some of the current standards or guide specifications for the design of seismic isolated structures that usually impose minimum values of the isolator restoring force capacity. In this paper a numerical study is performed to investigate the influence of the parameters that characterize the model of a structure–isolator system on its self-centring capacity. The system is idealized as a rigid mass mounted on a single isolator in which a bilinear force–displacement relation, which approximates the nearly bilinear behaviour exhibited by most of the currently available devices, characterizes the isolator response. Results from this study provide insights on the merit of current guide specifications and on the overall isolation behaviour of the models. Copyright # 2006 John Wiley & Sons, Ltd.

Temperature and stress analysis and simulation in fractal scanning-based laser sintering

A model for calculating the evolution of temperature and thermal stresses within a single metallic layer formed on the powder bed using different scanning patterns in selective laser sintering (SLS) was proposed. The model allows for the non-linear behavior of thermal conductivity, specific heat, and elastic modulus due to temperature changes and plastic deformation capabilities with a bilinear isotropic hardening behavior. The effect of laser scanning patterns on temperature, residual thermal stresses and distortion were investigated. It was shown that the distortion and transient stresses of a layer processed by a moving laser beam is decreased with fractal scanning pattern.

Indentation size effects in the nano- and micro-hardness of fcc single crystal metals

This paper presents the results of the experimental study of nano- and micro-indentation size effect on selected (0 0 1) oriented face-centered cubic (fcc) crystals (Ni, Au and Ag). Following a detailed description of experimental techniques and correction factors, indentation size effects in (0 0 1) oriented Au, Ag and Ni single crystals are elucidated. Material pile-up phenomena are discussed before analyzing the results within the context of mechanism-based strain gradient plasticity (SGP) theories. Microstructure length scales are calculated based on the analysis. These are shown to vary with indentation size. A bi-linear behavior is shown to describe the indentation size effects between the micro- and nano-scales. This is rationalized by considering the limiting cases of plastic deformation by source-limited and established dislocation substructures. A multi-scale framework is proposed for modeling small contacts.

Detection and Evaluation of Localized Damage under Creep-Fatigue Loading at 1000°F Using Acoustic Emission Technique

Acoustic emission (AE) technique was applied as a non-destructive test method to detect and evaluate the localized damage at high temperature environment. The creep-fatigue crack growth tests were carried out with the acquisition of AE signal at 1000°F. Under trapezoidal waveform loadings, AE results showed different features according to each damage mode. During the creep period, low and steady emissions were shown, while emissions were burst and high counts rate was recorded during the fatigue loading. Based on these characteristics, damage contribution was expressed in terms of acoustic emission parameter as a part-time monitoring method. Comparisons of damage contribution with respect to lifetime showed the transient behavior from cycle-dependent to time-dependent process. In case of full-time monitoring, bilinear behavior between AE counts and life was represented. From both monitoring results, it was confirmed that creep and fatigue damage can be characterized by means of emission features and AE is possible way to evaluate the localized damage at elevated temperature.

Soft Quaternary Anticholinergics: Comprehensive Quantitative Structure−Activity Relationship (QSAR) with a Linearized Biexponential (LinBiExp) Model

A comprehensive quantitative structure-activity relationship (QSAR) study is presented for quaternary soft anticholinergics including two distinctly different classes designed on the basis of the soft analogue and the inactive metabolite approaches. Because of the clear biphasic (bilinear) nature of the activity data when all structures (n = 76) were considered as a function of molecular size (volume), a nonlinear model had to be used, and a linearized biexponential (LinBiExp) model proved very adequate. LinBiExp can fit activity data that show a maximum (or a minimum) around a given parameter value but tend to show linearity away from this turning point. Contrary to Hansch-type parabolic models, LinBiExp represents a natural extension of linear models, and a direct correspondence between its parameters and those obtained earlier by linear regression on compound subsets covering more limited parameter ranges could be easily established. Stereospecificity was confirmed as important, and the presence of an acid moiety was found to essentially eliminate activity. The consideration of bilinear behavior, which most likely results from size limitations at the binding site, can also explain the embarrassingly low activity found for a relatively large compound predicted as highly active by Lien, Ariëns, and co-workers based on their QSAR study.

Investigation of the Bi-linear Behavior of the Indentation Size Effects in Single and Polycrystalline Ni Thin Films/MEMS Thin Films

AbstractThis paper presents the results of an experimental study of the indentation size effects at the nano, sub-micron, and micron-scales. The size dependence of the indentation hardness in these regimes is shown to exhibit a bi-linear behavior when the data are fitted to Taylor Dislocation Hardening (TDH) model. The deformation in indent sizes corresponding to the onset of the transition from micro and deep nano indents to shallow nano indents represent dislocation substructures at the sub-micron and micron scales whereas the deformation at the nano-scale represents dislocation source-limited behavior.

A hybrid numerical method to calculate the sag of composite conductors

A modification to the numerical sag method (NSM) for predicting conductor sag is presented that accurately predicts the observed bilinear sag behavior of composite conductors. The modified method, called the hybrid sag method (HSM), uses the stress–strain curves of conductors to calculate the initial load distribution between the components of the composite conductors. In this study, the HSM is used to predict sag of conductors with conventional designs. The HSM predictions are compared with those obtained using a conventional graphical sag method as well as NSM. The HSM shows superior accuracy over NSM and virtually the same accuracy as the graphical method for predicting sag for composite conductors operated under specific conditions. The HSM predictions of sag are validated by comparisons with experimental measurements.

Effect of interface roughness on exchange coupling in synthetic antiferromagnetic multilayers

FeCo/Ru/FeCo synthetic antiferromagnetic (SAF) multilayers with different FeCo/Ru interface roughnesses were prepared by varying the sputtering conditions. The films were characterized by hysteresis loops and torque magnetometry. A large remanence observed in the hard direction for the SAF with interface root-mean-square (RMS) roughness of 0.83 nm suggested a large biquadratic exchange coupling, while the SAF with RMS interface roughness of 0.49 showed primarily bilinear behavior. From field dependent torque curves, assuming coherent rotation, the values of J/sub 1/ and J/sub 2/ were found to be 0.35 and 0.05 erg/cm/sup 2/, respectively, in the smooth SAF compared to 0.13 and 0.16 erg/cm/sup 2/ in the rough SAF. The results are in reasonable agreement with the values obtained from the hysteresis loops and are consistent with the model for apparent biquadratic exchange arising from ferromagnetic coupling through pinholes in the Ru spacer layer.