Large Cyclic Strains Research Articles

A generalized model of lead rubber bearing considering large strain stiffening and degradation

Lead rubber bearings (LRBs) have been widely used in seismic isolation systems to mitigate earthquake damage in regions characterized by high seismic intensity. However, effective isolation is achieved at the expense of largely concentrated displacement in the isolation layer. Past experimental studies have confirmed that rubber stiffening and degradation properties are essential for LRBs at large cyclic strains. That is, accurately simulating the complex nonlinear behavior of LRBs is significant to evaluate the performance of isolated structures under strong earthquakes. Nevertheless, most LRB models in engineering practice cannot capture the strong nonlinear properties and the existing models for elaborate modeling are not widely used due to the complicated shape parameters. For this reason, the present study proposes a generalized Bouc–Wen model that can reasonably reflect the large strain stiffening and strength degradation properties of rubber. The traditional Bouc–Wen model of LRBs was introduced first. Subsequently, a generalized Bouc–Wen model of LRBs that is capable of simulating the observed large cyclic behavior with reasonable accuracy was developed. The model was further verified by the experimental results of LRBs with different design parameters under various loading conditions. Moreover, time history analyses were conducted to evaluate the earthquake responses of the single-degree-of-freedom system with the traditional Bouc–Wen and generalized Bouc–Wen model. Comparison proves that the two bearing models can closely estimate the peak deformation and shear force in minor and moderate earthquakes. By contrast, the generalized model can accurately capture the LRB properties under large cyclic strain that can restrain the bearing deformation but possibly increase the shear force significantly under strong earthquakes.

Distributed Fiber-Optic Strain Sensing of an Innovative Reinforced Concrete Beam-Column Connection.

Distributed fiber-optic sensing (DFOS) technologies have been used for decades to detect damage in infrastructure. One recent DFOS technology, Optical Frequency Domain Reflectometry (OFDR), has attracted attention from the structural engineering community because its high spatial resolution and refined accuracy could enable new monitoring possibilities and new insight regarding the behavior of reinforced concrete (RC) structures. The current research project explores the ability and potential of OFDR to measure distributed strain in RC structures through laboratory tests on an innovative beam–column connection, in which a partial slot joint was introduced between the beam and the column to control damage. In the test specimen, fiber-optic cables were embedded in both the steel reinforcement and concrete. The specimen was tested under quasi-static cyclic loading with increasing displacement demand at the structural laboratory of the Pacific Earthquake Engineering Research (PEER) Center of UC Berkeley. Different types of fiber-optic cables were embedded both in the concrete and the rebar. The influence of the cable coating and cable position are discussed. The DFOS results are compared with traditional measurements (DIC and LVDT). The high resolution of DFOS at small deformations provides new insights regarding the mechanical behavior of the slotted RC beam–column connection, including direct measurement of beam curvature, rebar deformation, and slot opening and closing. A major contribution of this work is the quantification of the performance and limitations of the DFOS system under large cyclic strains. Performance is quantified in terms of non-valid points (which occur in large strains when the DFOS analyzer does not return a strain value), maximum strain that can be reliably measured, crack width that causes cable rupture, and the effect of the cable coating on the measurements. Structural damage indices are also proposed based on the DFOS results. These damage indices correlate reasonably well with the maximum sustained drift, indicating the potential of using DFOS for RC structural damage assessment. The experimental data set is made publicly available.

Modeling of Lead Rubber Bearings under Large Cyclic Material Strains

AbstractLead rubber bearings (LRBs) have been widely used in seismic isolation systems for buildings and other structures to effectively mitigate the damaging effects of horizontal earthquake groun...

Numerical modeling of soil liquefaction and lateral spreading using the SANISAND-Sf model in the LEAP experiments

Laboratory element and centrifuge tests from LEAP-UCD-2017 and LEAP-Asia-2019 were used for model calibration and evaluation in a dynamic coupled analysis of a saturated and gently sloped deposit of sand subjected to base excitation. The paper focuses on using a recently proposed novel constitutive ingredient for modeling the post-liquefaction large cyclic shear strains in sands. An existing critical state compatible, bounding surface plasticity reference model is used, with and without this new constitutive ingredient, to explore its improved modeling capabilities. The constitutive model was first calibrated based on available laboratory element tests on Ottawa-F65 sand, and compared to the reference model showed significantly improved performance in capturing the strain-based liquefaction strength curve of a series of undrained hollow cylinder cyclic torsional shear tests. The calibrated models were used in Class-C prediction of the slope surface deformation in five centrifuge tests on a mildly sloping liquefiable ground of the same soil subjected to dynamic loading, in the three-dimensional finite difference program FLAC3D. The simulation results were compared with experiments in terms of acceleration history, spectral response, excess pore water pressure development, and horizontal displacement evolution at specified control points. The vectors and contours of displacements at the end of shaking also revealed a pattern of slope deformation consistent with that of a gently sloped liquefiable ground. Following the insights from the performance of the models in simulation of the slope response, the calibration was adjusted to more realistically account for the impact of initial static shear stress on the development of post-liquefaction shear strains. The models were again used for Class-C1 prediction of the slope deformation of the same centrifuge tests. The overall assessment revealed the capabilities and limitations of the models in simulating the soil liquefaction strength and its post-liquefaction response.

Bauschinger Effect of Mn18Cr18N Austenitic Stainless Steel

The experimental study of Bauschinger effect in Mn18Cr18N austenitic stainless steel was presented by compression-tensile cyclic loading tests with the prestrains ranging from 0.005 to 0.1, which was illustrated utilizing stress-strain curves and analysed by TEM images from aspects of microstructural mechanisms. Moreover, the Bauschinger effect and its associated roundness phenomenon in reverse flow curve with respect to different cycles and cyclic strain amplitudes were evaluated in a quantitative manner. The experimental results indicate that Bauschinger effect is apparent during the test. At smaller cyclic strain amplitude, intergranular backstress is the main source of Bauschinger effect. With further increasing of cycles, dislocation density increases and dislocation movement is hindered in the reverse deformation. Therefore, Bauschinger effect is weakened to some extent. At large cyclic strain amplitude, backstress originating from the dislocation pile-up at grain boundaries and the continuous formation of deformation twins dominate the Bauschinger effect. In addition, the backstress results in the roundness of reverse curve during cyclic loading. The larger value of Δep*, the more obvious the roundness of the reverse curve, and the more significant the Bauschinger effect.

Unveiling the Stress–Strain Behavior of Conjugated Polymer Thin Films for Stretchable Device Applications

The success of stretchable electronics based on conjugated polymers relies on having a thorough understanding of the polymer’s mechanical behavior over conditions likely encountered during operation. To meet this need, a novel approach to capture the stress–strain response of thin conjugated polymer films is introduced. This is achieved by laminating the polymer film of interest on a thin elastomer substrate and testing the composite specimen in a dynamic mechanical analyzer in a tensile test configuration. We term this approach as film laminated on thin elastomer (FLOTE) method. The benefits of this method include the ability to (1) determine the viscoelastic behavior of the conjugated polymer by testing over a broad range of temperatures and strain rates, (2) measure the film behavior over large cyclic strains, including under in-plane compression, and (3) capture the impact of the neighboring elastomer on the behavior of the polymer film. The focus is on the widely studied poly­(3-hexylthiophene) (P3HT) as a model system. We find that the viscoelastic characteristics of P3HT, varied by changing the specimen temperature, significantly impact film stability under cyclic strain. This includes showing that the hysteresis behavior of the films under cyclic strain changes significantly with sample temperature. In addition, it is found that, under cyclic loading, the composite has features consistent with Mullins’ effect. Based on these results, insights into polymer viscoelastic characteristics necessary to achieve high-performance stretchable electronics are gained.

Low-cycle fatigue of stainless steel plates under large plastic strain demands

The low-cycle characteristics of structural subassemblies under large cyclic strains play an important role in contemporary seismic design. Obtaining these characteristics can lead to an understanding of a structure's degradation and nonlinear response behaviour, and can serve as a basis for developing efficient numerical models to predict seismic collapse mechanisms. The austenitic stainless tubular grade of steel has shown promise in terms of strain hardening character, structural overstrength and ductility, but existing test data is limited or constrained to small plastic strains, which is hardly useful in earthquake engineering applications. This article presents an experimental study designed to characterize the hysteresis of stainless steel plates under large inelastic cyclic strains and to assess their potential use in buckling-restrained brace components for seismic applications. Axial coupons machined from austenitic Grade 304L stainless steel and from regular carbon steel Grade 350WT were tested under uniaxial tensile loading, as well as constant and variable strain amplitude cyclic loadings. Results of the uniaxial tests confirmed higher ductility and strain hardening capacity for the stainless steel plates compared to those of carbon steels. These results were subsequently used to validate a novel technique based on image analysis to derive the true stress-strain characteristics. The stainless steel Grade 304L plates showed higher cyclic hardening but shorter low-cycle fatigue life compared to the carbon steel. Parameters for representing the low-cycle fatigue behavior of these materials, useful for developing a cyclic plasticity hardening numerical model, were derived from the test data.

Modeling cyclic shearing of sands in the semifluidized state

SummaryLiquefaction is associated with the loss of mean effective stress and increase of the pore water pressure in saturated granular materials due to their contractive tendency under cyclic shear loading. The loss of mean effective stress is linked to loss of grain contacts, bringing the granular material to a “semifluidized state” and leading to development and accumulation of large cyclic shear strains. Constitutive modeling of the cyclic stress‐strain response in earthquake‐induced liquefaction and post‐liquefaction is complex and yet very important for stress‐deformation and performance‐based analysis of sand deposits. A new state internal variable named strain liquefaction factor is introduced that evolves at low mean effective stresses, and its constitutive role is to reduce the plastic shear stiffness and dilatancy while maintaining the same plastic volumetric strain rate in the semifluidized state. This new constitutive ingredient is added to an existing critical state compatible, bounding surface plasticity reference model, that is well established for constitutive modeling of cyclic response of sands in the pre‐liquefaction state. The roles of the key components of the proposed formulation are examined in a series of sensitivity analyses. Their combined effects in improving the performance of the reference model are examined by simulating undrained cyclic simple shear tests on Ottawa sand, with focus on reproducing the increasing shear strain amplitude as well as its saturation in the post‐liquefaction response.

Flexible Lead‐Free Piezoelectric Composite Materials for Energy Harvesting Applications

AbstractVibrational piezoelectric energy harvesters are being investigated to replace batteries in embedded sensor systems. The energy density that can be harvested depends on the figure of merit, d33g33, where d33 and g33 are the piezoelectric charge and voltage coefficient. Commonly used piezoelectric materials are based on inorganic ceramics, such as lead zirconium titanate (PZT), as they exhibit high piezoelectric coefficients. However, ceramics are brittle, leading to mechanical failure under large cyclic strains and, furthermore, PZT is classified as a Substance of Very High Concern (SVHC). To circumvent these drawbacks, we fabricated quasi 1–3 potassium sodium lithium niobate (KNLN) ceramic fibers in a flexible polydimethylsiloxane (PDMS) matrix. The fibers were aligned by dielectrophoresis. We demonstrate for the structured composites values of d33g33 approaching 18 pm3 J−1, comparable to that of state‐of‐the‐art ceramic PZT. This relatively high value is due to the reduced inter‐particle distance in the direction of the electric field. As a confirmation, the stored electrical energy for both material systems was measured under identical mechanical loading conditions. The similar values for KNLN/PDMS and PZT demonstrate that environmentally friendly, lead‐free, mechanically compliant materials can replace state‐of‐the‐art environmentally‐less‐desirable ceramic materials in piezoelectric vibrational energy harvesters.

Pronounced grain boundary network evolution in nanocrystalline Cu subjected to large cyclic strains

The grain boundary network of nanocrystalline Cu foils was modified by the application of cyclic loadings and elevated temperatures. Broadly, the changes to the boundary network were directly correlated with the applied temperature and accumulated strain, including a 300% increase in the twin length fraction. By independently varying each treatment variable, a matrix of grain boundary statistics was built to check the plausibility of hypothesized mechanisms against their expected temperature and stress/strain dependences. These comparisons allow the field of candidate mechanisms to be significantly narrowed. Most importantly, the effects of temperature and strain on twin length fraction were found to be strongly synergistic, with the combined effect being ∼150% that of the summed individual contributions. Looking beyond scalar metrics, an analysis of the grain boundary network showed that twin related domain formation favored larger sizes and repeated twin variant selection over the creation of many small domains with diverse variants.

Crystal plasticity simulation of the effect of grain size on the fatigue behavior of polycrystalline Inconel 718

A microstructure-based model that accounts for the effect of grain size has been developed to study the effect of grain size on the fatigue life of Inconel 718 alloys. The fatigue behavior of two alloys with different grain size was determined by means of uniaxial cyclic deformation tests under fully-reversed deformation (Rε=-1) at 400 °C in the low cycle fatigue regime. The model was based in the determination of the fatigue indicator parameter (based on the local crystallographic strain energy dissipated per cycle) by means of computational homogenization of a representative volume element of the microstructure. The mechanical response of the single crystal within the polycrystal was modelled through a phenomenological crystal plasticity model which was modified to account for the effect of grain size on the monotonic and cyclic hardening/softening mechanisms. The microstructure-based crack initiation model parameters were calibrated from the experimental tests of the material with fine grain size. The results of the fatigue simulations were in good agreement with the experimental results in terms of the cyclic stress-strain curves and of the number of cycles for fatigue crack initiation. The model did not show any grain size effect on the fatigue life for the largest cyclic strain ranges while the predicted fatigue life predicted was considerably longer in the case of the microstructure with fine grain size for the lowest strain ranges, in quantitative agreement with experimental data. These differences were attributed to changes in the deformation modes between homogeneous plastic deformation at large cyclic strain ranges and localized plasticity in a few grains at low cyclic strain ranges.

Cyclic strain-induced crack growth in graphite during electrochemical testing in propylene carbonate-based Li-ion battery electrolytes

Lithiation/de-lithiation cycles induced cracks in isostatically pressed graphite samples subjected to constant-load bending tests while simultaneously conducting cyclic voltammetry (CV) experiments in propylene carbonate-based Li-ion battery electrolyte solutions. A large cyclic strain of Δe = 0.95% was generated by lithiation/de-lithiation cycles as determined by micro-Raman measurements carried out concurrently with CV experiments. In-situ optical microscopy measurements of crack lengths, a, showed that the crack-growth rate, da/dt, depended on the stress intensity factor at the crack tip and could be expressed as da/dt = AΔK I . A two-stage crack-growth behaviour was determined with n = 51.3 in the first stage. However, lower crack propagation rates observed in the second stage (n = 9.9) were affected by crack closure due to (1) a rough fracture surface morphology of the graphite cracks, and (2) the deposition of solid electrolyte reduction products on facetted crack surfaces, where both the factors likely contributed to reducing ΔK to ΔK eff.

Fatigue crack growth prediction of welded joints in low-cycle fatigue region

Low-cycle fatigue is one of the failure modes in steel structures during earthquakes. With a focus on crack growth in the low-cycle fatigue region, this study developed a fatigue crack growth curve and verified its applicability to crack growth prediction in welded joints. Fatigue crack growth tests under highly plastic conditions were performed using compact tension specimens with side grooves. The results indicate that the crack growth rate in the low-cycle fatigue region correlates with the cyclic J-integral range, which can be calculated using finite element analysis. Additionally, the results for both plain steel and weld metal were distributed in the same region within a narrow band. Based on the results, a formula for the fatigue crack growth rate under large cyclic strains was proposed. Low-cycle fatigue tests were then performed on welded joints to clarify their crack growth behavior, and the crack growth observed in these tests was compared with the crack growth calculated using the proposed formula. The calculation results were found to be in relatively good agreement with the experimental results, verifying the applicability of the formula.

Experimental and theoretical methods on determination of stress-dependent natural frequency of marine sedimentary clay

ABSTRACTExcited by the vibration sources in dynamic engineering, the natural frequency and damping factor of the saturated marine sedimentary clay are key dynamic parameters that influence the responses under cyclic loads. Experimental and theoretical methods are proposed in this paper to analyze the natural frequency and the stress-dependent nonlinearity. The experimental method shows that the natural frequency of soils with specific stress state subjected to large cyclic shear strain can be estimated from the data of dynamic triaxial tests based on the amplitude–frequency response curve. Trial and error by the criterion from the half-power bandwidth method is used to determine the optimal fitting. The results of a theoretical study on the free vibration of soil layers are then presented to derive the analytic solution of natural frequency. In addition to the two frequency-independent elements (a lumped mass matrix and a stiffness matrix), the system’s equivalent damping coefficient matrix is iteratively determined based upon the forced vibration experimentally. Finally, the impacts of the resonance phenomenon on the dynamic shear modulus and hysteretic loop are discussed.

Connections for Integral Jointless Bridges in Seismic Regions Suitable for Accelerated Bridge Construction

Prefabricated jointless bridges consisting of pretensioned girders, posttensioned spliced girders, trapezoidal open box girders, and other types of superstructure members are often used for accelerated bridge construction. Connections in precast concrete substructures are typically made at the beam–column and column–foundation interfaces to facilitate fabrication and transportation. However, for structures in seismic regions, those interfaces represent locations of high moments and shears and large inelastic cyclic strain reversals. Jointless bridge superstructures are constructed to work integrally with the abutments. Movements due to creep, shrinkage, and temperature changes are accommodated by using flexible bearings or foundation and through incorporating relief joints at the ends of the approach slabs. In addition to reduced maintenance costs, other advantages of jointless bridges include improved structural integrity, reliability and redundancy, improved long-term serviceability, improved riding surface, reduced initial cost, and improved aesthetics. In recent times, jointless bridges have been built in seismically sensitive areas. Developing connections that can accommodate inelastic cyclic deformations and are readily constructible is the primary challenge for accelerated bridge construction in seismic regions. The AASHTO load and resistance factor design specifications do not explicitly address jointless precast, pretensioned, or posttensioned elements. The seismic design and detailing, accomplished research, and construction practices of jointless bridges, and implementation of a precast concrete bridge bent system that is intended to meet all those challenges, are presented. This paper attempts to capture the state of practice of jointless continuous bridges in seismic regions.

Actual Stress Fatigue of Welded Joint of a Steel Structure Subjected to Large Cyclic Strain

Actual Stress Fatigue of Welded Joint of a Steel Structure Subjected to Large Cyclic Strain

Molecular dynamics investigation of the grain boundary migration hysteresis of nanocrystalline Ni under cyclic shear loading

The deformation behavior and grain boundary (GB) response of nanocrystalline Ni under cyclic shear loading are investigated by molecular dynamics simulations. The GB migration hysteresis phenomenon, in which the GB migration displacement lags behind the change in nominal shear strain, is observed in the symmetric tilt GBs for the first time. The elementary structure transformation occurring at the two end segments of the observed GB during GB migration produces a disordered and irreversible state, while the transformation in the middle segment is reversible. Both dislocation retraction and nucleation occur during unloading. Relatively large cyclic strain amplitudes lead to disordered GB segments of greater length, such that the residual GB migration displacement increases with increasing cyclic amplitude. GB migration hysteresis vanishes after the GB becomes immobile owing to a cyclic shear induced transition to a disordered state along its entire length.

Thermomechanical fatigue life prediction of 316L compact heat exchanger

Compact welded heat exchangers are designed to be used in severe operating conditions (temperature, pressure, aggressive fluid, etc.). Thus fatigue failure can be observed after large cyclic strain due to thermomechanical cyclic loads. In this paper, a 316L stainless steel structure solicited in the (extremely) low cycle fatigue regime is analyzed through a multi-scale approach. A finite element analysis method has been developed and correlated to experiments. The thereto-elastic response of the heat exchanger to thermal cycling of various amplitudes has been firstly investigated. The stress concentration locations are identified and the local thermo-elastic stored energy on these points is calculated. By the use of a combined isotropic/ldnematic hardening previously determined by alternated bending tests, these data are then used in a micromechanic approach. It consists in the consideration of the material elastic-plastic behavior under uniaxial mechanical solicitation. Based on energy equivalence, a local nonlinear description is so adopted to an expected equivalent strain amplitude. This estimation is used to predict the lifetime of the heat exchangers through an adapted rule relying this deformation to cycle number to failure through the Coffin Manson law. The methodology, simple to handle, is thought to improve the design of heat exchangers depending on their operating conditions. (C) 2016 Elsevier Ltd. All rights reserved.

Graphene Reinforced Carbon Nanotube Networks for Wearable Strain Sensors

Transparent, stretchable films of carbon nanotubes (CNTs) have attracted significant attention for applications in flexible electronics, while the lack of structural strength in CNT networks leads to deformation and failure under high mechanical load. In this work, enhancement of the strength and load transfer capabilities of CNT networks by chemical vapor deposition of graphene in the nanotube voids is proposed. The graphene hybridization significantly strengthens the CNT networks, especially at nanotube joints, and enhances their resistance to buckling and bundling under large cyclic strain up to 20%. The hybridized films show linear and reproducible responses to tensile strains, which have been applied in strain sensors to detect human motions with fast response, high sensitivity, and durability.

Contact Deformation Analysis of Elastic–Plastic Asperity on Rough Roll Surface in a Strip Steel Mill

In this paper, the deformation behaviors of elastic–plastic asperity on rough roll surface in a strip steel mill were simulated by a three-dimensional finite element (FE) model. The asperity characterized a sinusoidal profile and all objects in the FE model were elastic–plastic with linear strain hardening. The deformation behaviors including contact stresses, strains (including elastic and plastic strains), and contact radii of different rolling force densities and different geometrical shapes and sizes were calculated and analyzed. It revealed that the tall-thin type asperity caused larger plastic strain in the asperity itself and larger Von Mises stress in the mating roll subsurface. Long-term repeated effects of large cyclic strain and stress were liable to spark subsurface cracks and increased the risk of fatigue failure of rolls in strip rolling mills.