Cyclic Material Behavior Research Articles

Nonlocal damage propagation in the dynamics of masonry elements

In this work, a nonlocal damage-plasticity model for dynamic finite element analyses of cohesive structural elements is presented. The proposed cohesive model is able to reproduce the main relevant behaviors of quasi-brittle materials despite being quite simple, i.e. governed by only a few parameters which can be determined by standard laboratory tests. In particular, the model is able to reproduce the mechanisms of cohesive materials under static or dynamic loads: degradation of the mechanical properties (damage) and accumulation of irreversible strains (plasticity). Moreover, the model also simulates the cyclic macroscopic behavior of quasi-brittle materials, taking into account the loss and recovery of stiffness due to crack closure and reopening. The latter effect represents a particularly important characteristic in the case of dynamic loads. The proposed formulation is implemented as a constitutive model for two-dimensional plane stress four-node quadrilateral elements. The second order equations of motion are solved adopting the implicit Newmark time integration scheme. The proposed model is validated and its dynamic performance is numerically demonstrated through the analysis of a large-scale structural element.

Fatigue life and cyclic material behavior of butt-welded high-strength steels in the LCF regime*

Abstract Fatigue life approach methods based on general local concepts for welded joints assign universal material properties to geometrically well-defined weld toes or weld roots, in order to evaluate local stresses and strains, ignoring any metallurgical discrepancies. In this study, analysis of the main influences on these concepts, linear damage accumulation and cyclic material behavior, of the high-strength and ultra-high-strength steels S960QL, S960M and S1100QL was performed for the base material and for butt-welded specimens. Results of stress-controlled tests under constant amplitude loading (CAL) and variable amplitude loading (VAL) show that a linear damage accumulation with a damage sum of Dal = 0.5 is conservative. Effects of the geometrical and metallurgical notch on the cyclic material behavior are shown by strain controlled testing of both base material and welded specimens. Cycles to crack initiation, presented in strain-life curves, are consequently reduced in the as-welded state. Cyclic stress-strain curves show a cyclic softening of the base material as well as of the welded joints. Furthermore, the advantages of the ultra-high-strength steel S1100QL disappear in welded connections.

Estimation of local cyclic plasticity based on a coupled EBSD and TEM analysis*

Abstract In the present work, a novel approach is adopted to evaluate the low cycle fatigue (LCF) damage at a mesoscopic scale. The fatigue behavior of a ferritic steel has been studied and damage criteria were assessed by a metallurgical approach that combines two analysis techniques: transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). After characterizing the cyclic behavior of the material for different loading parameters at a total strain range varying from 0.3 % to 0.7 %, the microstructure evolution induced by the cyclic plasticity was studied in a multi-scale investigation. Here, TEM and SEM-EBSD give complementary information about the link between macroscopic fatigue behavior and microstructure changes. TEM shows a clear indication of the dislocations structures related to fatigue damage. SEM-EBSD completes the investigation by giving quantitative data associated to local deformation. This relation enables to determine plastic strain assessment, for example, of industrial components.

Fatigue Life Design of Components under Variable Amplitude Loading with Respect to Cyclic Material Behaviour

Load carrying components are subjected to variable amplitude loading for an extensive proportion of their life time. The available methods for fatigue life design can be divided into global and local approaches with reference to the system in which they evaluate the stress vs. endurance relation. The so called material based approach considers the elastic-plastic material behaviour through a combination of the local and the global approach, improving the transferability of material fatigue properties to arbitrary geometries while reducing the numerical effort for fatigue life estimation.

New Bainitic Steel for Cyclic Loaded Safety Parts with Improved Cyclic Material Behaviour

A bainitic forging steel, which enables the TRIP-effect, has been developed and can be used for lightweight-construction for forged components under cyclic loading. The high potential under cyclic loading conditions of the new generation of TRIP forging steels is shown and also the improved results under service loads, especially under overloads and misuse. A comparison of linear damage accumulations by Palmgren-Miner will depict the differences in characteristic damage sums between quenched and tempered and the new bainitic steel.For TRIP steels properties the process parameters are very important. Hence, the influence of the cooling conditions of TRIP will be discussed.

Fatigue of 316L stainless steel notched [formula omitted]m-size components

The aim of the present paper is to provide an in-depth analysis of the fatigue-life assessment for μm-size 316L stainless steel components. Such components find typical applications in the biomedical field, e.g., in cardiovascular stents. To this purpose, the present work analyzes experimental data on 316L stainless steel from literature for smooth and notched μm-size components using a global computational approach. Several aspects are discussed: (i) the choice of an appropriate constitutive law for cyclic material behavior, (ii) fatigue criteria based on shakedown concepts for finite and infinite lifetime, in particular distinguishing between low, high and very high-cycle fatigue regimes (denoted as LCF, HCF and VHCF, respectively), and (iii) gradient effects in relation with hot-spot as well as average or mean volume approaches for the lifetime estimation. The results give a new insight into the lifetime design of μm-size components and could be directly applied for the fatigue-life assessment of small size structures as, for instance, cardiovascular 316L stainless steel stents.

Fundamental influences on quasistatic and cyclic material behavior of short glass fiber reinforced polyamide illustrated on microscopic scale

ABSTRACTIn this work, the influences of fiber orientation and weld lines on the morphological structures and the mechanical behavior of polyamide 6.6 (PA6.6‐GF35) are investigated. In quasistatic and fatigue tests tensile and 3‐point‐bending loads are applied. Test temperatures vary between RT and 150°C. Two different specimen types are produced by using injection moulding process to create different fiber orientations as well as weld lines. Fiber orientations are determined using computer tomography. Scanning electron microscopy is used to investigate fracture surfaces of tested specimens. Results show that mechanical properties and morphological structures depend highly on fiber orientation and temperature. Transversely oriented fibers in weld lines result in brittle failure mechanisms and decreased mechanical properties. Different stress distributions in the specimens under tensile and flexural loads have influence on the material behavior as well. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40842.

Cyclic Viscoplasticity Testing and Modeling of a Service-Aged P91 Steel

A service-aged P91 steel was used to perform an experimental program of cyclic mechanical testing in the temperature range of 400 °C–600 °C, under isothermal conditions, using both saw-tooth and dwell (inclusion of a constant strain dwell period at the maximum (tensile) strain within the cycle) waveforms. The results of this testing were used to identify the material constants for a modified Chaboche, unified viscoplasticity model, which can deal with rate-dependant cyclic effects, such as combined isotropic and kinematic hardening, and time-dependent effects, such as creep, associated with viscoplasticity. The model has been modified in order that the two-stage (nonlinear primary and linear secondary) softening which occurs within the cyclic response of the service-aged P91 material is accounted for and accurately predicted. The characterization of the cyclic viscoplasticity behavior of the service-aged P91 material at 500 °C is presented and compared to experimental stress–strain loops, cyclic softening and creep relaxation, obtained from the cyclic isothermal tests.

AN INVESTIGATION ON THE EFFECT OF CYCLIC DISPLACEMENT ON THE INTEGRAL BRIDGE ABUTMENT

The integral bridge abutment, as a special type of retaining wall, is subject to cyclic displacement, which is due to the daily and seasonal temperature variations. The frame of this type of bridges is rigid and jointless. This requires that the slab of the bridge to be longitudinally continuous without expansion joints. This causes cyclic displacement to be imposed to the backfill material of integral bridge abutment. It should be pointed out that the omission of expansion joints helps to provide a fluent traffic and a reduction in maintenance and repair of the bridges. To investigate the impact of cyclic displacement on the loose backfill soil behaviour, an innovative laboratory retaining wall model has been designed and constructed to imitate the cyclic behaviour of backfill granular material. In addition, a numerical model, based on finite element method, has been developed to interpret the experimental results. This model was calibrated using the laboratory test data. The results indicate that the passive pressure, except for low amplitude displacement, escalates with progressive number of cycles and its distribution is not linear, which is due to the forming arch.

A new test to study the cyclic hardening behaviour of a range of high strength rail materials

Plastic ratcheting plays a key role in wear and rolling contact fatigue crack formation at the wheel–rail interface. Tests to examine the wear and rolling contact fatigue behaviour of rail materials over a wide range of service conditions are expensive and can be impractical. A physical simulation of the deformation behaviour associated with ratcheting is an attractive replacement for such tests. In this work, the Plane Stress Local Torsion (PSLT) test is proposed as a novel mechanical testing method to physically simulate near-surface deformation in rails and to characterize the cyclic deformation behaviour of rail materials. Contrary to the orthodox mechanical tests, the proposed method is capable of producing a nonlinear strain gradient in test samples which resembles the real gradient in the rail–wheel system. The PSLT testing was performed on specimens of the nominated rail steels in a strain-controlled fashion to simulate the unidirectional as well as the fully reversing strain cycles. The test was used to examine the cyclic hardening behaviour and ratcheting characteristics of a range of high strength rail materials under cyclic loading at room temperature. The effect of the cyclic strain amplitude under symmetrical strain cycling on the cyclic hardening behaviour was investigated. Experimental torque–twist data were used to compare the plastic flow behaviour of commonly used rail materials in heavy haul applications under cyclic loading. The cyclic and ratcheting strain accumulation behaviour in the test samples was characterized based on the torque–twist data to allow a comparative study of the mechanical properties. Optical microscopy of the tested samples was also performed to compare the microstructures at the flow localization zone for different materials subjected to cyclic strain accumulation.

Effect of deformation frequency on temperature and stress oscillations in cyclic phase transition of NiTi shape memory alloy

Distinctive temperature and stress oscillations can be observed in superelastic shape memory alloys (SMAs) when they subject to displacement-controlled cyclic phase transition. In this paper, we examine the effect of the deformation frequency on the thermal and mechanical responses of the polycrystalline superelastic NiTi rods under stress-induced cyclic phase transition. By synchronized measurement of the evolutions in overall temperature and stress–strain curve over the frequency range of 0.0004–1Hz (corresponding average strain rate range of 4.8×10−5/s–1.2×10−1/s) in stagnant air, it was found that both the temperature evolution and the stress–strain curve vary significantly with the frequency and the number of cycles. For each frequency, steady-state cyclic thermal and mechanical responses of the specimen were reached after a transient stage, exhibiting stabilization. In the steady-state, the average temperature oscillated around a mean temperature plateau which increased monotonically with the frequency and rose rapidly in the high frequency range due to the rapid accumulation of hysteresis heat. The oscillation was mainly caused by the release and absorption of latent heat and increased with the frequency, eventually reaching a saturation value. The variations in the stress responses followed similar frequency dependence as the temperature. The steady-state stress–strain hysteresis loop area, as a measure of the material׳s damping capacity, first increased then decreased with the frequency in a non-monotonic manner. The experimental data were analyzed and discussed based on the simplified lumped heat transfer analysis and the Clausius–Clapeyron relationship, incorporating the inherent thermomechanical coupling in the material׳s response. We found that, for given material's properties and specimen geometries, all such frequency-dependent variations in temperature, stress and damping capacity were essentially determined by the competition between the time scale of the heat release (i.e. the phase transition frequency) and the time scale of the heat transfer to the ambient. The results emphasize that, the two time scales of loading and heat transfer must be clearly specified when characterizing and modeling the cyclic behavior of SMA materials.

High Temperature, Low Cycle Fatigue Characterization of P91 Weld and Heat Affected Zone Material

The high temperature low cycle fatigue behavior of P91 weld metal (WM) and weld joints (cross-weld) is presented. Strain-controlled tests have been carried out at 400 °C and 500 °C. The cyclic behavior of the weld material (WM) and cross-weld (CW) specimens are compared with previously published base material (BM) tests. The weld material is shown to give a significantly harder and stiffer stress–strain response than both the base material and the cross-weld material. The cross-weld tests exhibited a cyclic stress–strain response, which was similar to that of the base material. All specimen types exhibited cyclic softening but the degree of softening exhibited by the cross-weld specimens was lower than that of the base material and all-weld tests. Finite element models of the base metal, weld metal and cross-weld test specimens are developed and employed for identification of the cyclic viscoplasticity material parameters. Heat affected zone (HAZ) cracking was observed for the cross-weld tests.

Material parameter optimisation of Ohno-Wang kinematic hardening model using multi objective genetic algorithm

Ohno-Wang hardening model is an advanced constitutive model to evaluate the cyclic plasticity behaviour of material. This model has capability to simulate uniaxial and biaxial ratcheting response of the material. But, it is required to determine large number of material parameters from several experimental responses in order to simulate this phenomenon. Material parameters for constitutive models are generally determined manually through trial and error approach which is tedious and less accurate. Due to arbitrariness and complexity of cyclic loading, advanced constitutive material models become non-linear and multimodal in functional and parameter space. To overcome this problem, an automated parameter optimisation approach using genetic algorithm has been proposed in the present work to identify Ohno-Wang material parameters of 304LN, stainless steel for uniaxial simulation. Optimisation by this approach has improved the model prediction in uniaxial low cycle and ratcheting fatigue simulations after comparison with the experimental response.

Characterization of viscoplasticity behaviour of P91 and P92 power plant steels

This paper deals with the determination of material constitutive model for P91 and P92 steels at high temperatures. An isothermal, strain-controlled test programme was conducted for both steels for a temperature range between 400 and 675 °C. The experimental data from these tests were used to obtain the material constants in a viscoplasticity model. The model includes the effects of isotropic and kinematic hardening, as well as time-dependent effects, and has been used to model the cyclic material behaviour of each material. Material constants were initially determined from initial cycle stress–strain data, maximum stress evolution data and stress relaxation data. The material constants were improved by use of a least-squares optimisation algorithm. The constitutive models have been implemented into the ABAQUS finite element (FE) code by using the Z-mat software. The performances of the material models for both steels have been assessed by comparing predictions with experimental data obtained from the tests.

Modeling Cyclic Behavior of Rockfill Materials in a Framework of Generalized Plasticity

Typical triaxial compression experiments were revisited to investigate the essential mechanical behavior of rockfill materials to be reflected in constitutive modeling, such as the nonlinear dependence of the strength and the dilation on the confining pressure and the accumulation of permanent strains during cyclic loading. The mathematical descriptions of the axial stress-strain behavior during initial loading, unloading, and reloading were formulated, respectively, which enables us to represent the hysteresis loops directly without recourse to complex concepts and parameters. The axial stress-strain model was then incorporated into the constitutive framework of generalized plasticity for the modeling of cyclic behavior of rockfill materials. This task was fulfilled by defining the elastic modulus, the plastic flow direction tensor, the loading direction tensor, and the plastic modulus for different loading conditions. In particular, the plastic flow direction tensor was derived based on a stress-dilatancy equation considering the influence of loading direction, and the representation of the plastic modulus was established in terms of the tangential modulus and the elastic modulus by using the special constitutive equations under axisymmetric stress states. The cyclic model proposed in this study has three distinct features. First, the hysteresis behavior and the accumulation of permanent strains were unified and described under the framework of generalized plasticity. Second, all the loading phases were treated as elastoplastic processes so that no purely elastic regions exist in the principal stress space. Third, the introduction of two aging functions for the consideration of the hardening effect facilitates the controlling of the magnitudes of permanent strains. There are in total 13 parameters in the model, all of which can be determined easily from (cyclic) triaxial compression experiments. To check the capabilities of the model in reproducing the monotonic and cyclic behavior, typical triaxial compression experiments were simulated with the constitutive equation. Satisfactory agreement between the experimental results and the corresponding model predictions lent sufficient creditability to the effectiveness of the proposed model, which further motivates us to extend the model for more complex stress paths and apply the model in practical engineering in the future.

807 高温における一方向CFRP積層板の非主軸サイクル変形挙動(OS8 オーガナイズドセッション《複合材料の変形と破壊およびマルチスケール計算技術》)

Strain-controlled off-axis cyclic behavior of unidirectional CFRP laminates at high temperature has been studied.Accuracy of strain control using a mechanical extensometer was evaluated by comparing the results measured with strain gauges, indicating that strain-controlled tests can successfully be achieved. Then, strain-controlled cyclic tests for different strain amplitudes and strain rates were carried out on coupon specimens with different fiber orientations, respectively. Hysteretic stress-strain response was clearly observed for cyclic straining. Hysteretic loops tend to move to compressive stress side, which is similar to cyclic stress relaxation behavior of metallic materials. Such an off-axis cyclic stress response tends to saturate with cyclic straining. The cyclic stress range for each hysteresis that depends on the strain rate of cyclic straining as well as the strain amplitude tends to increase with cyclic straining, suggesting cyclic hardening of the composite.

Anisotropic Hardening of Sheet Metals at Elevated Temperature: Tension-Compressions Test Development and Validation

New test equipment has been developed to measure the in-plane cyclic behavior of sheet metals at elevated temperatures. The tester has clamping dies with adjustable side force to prevent the sheet specimens from buckling during compressive loading. In addition to the room temperature experiment, cartridge type heaters are inserted in the clamping dies so that the specimen can be heated up to 400 °C during the cyclic tests. For the strain measurement, a non-contact type laser extensometer is used. In order to validate the newly developed test device, the tension-compression (and compression-tension) tests under pre-strains and various temperatures have been performed. As model materials, the aluminum alloy sheet which exhibits a large Bauschinger effect and the magnesium alloy sheet which exhibits different amounts of asymmetry under cyclic loading are used. The developed device can be well-suited to measure the cyclic material behavior, especially the anisotropic and asymmetric hardening of light-weight materials.

Multiaxial fatigue models for short glass fibre reinforced polyamide. Part II: Fatigue life estimation

Components made of short fibre reinforced thermoplastics are increasingly used in the automotive industry, and more frequently subjected to fatigue loadings during their service life. The determination of a predictive fatigue criterion is therefore a serious issue for the designers, and requires the knowledge of the local mechanical response. As the cyclic behaviour of polymeric material is reckoned to be highly nonlinear, even at room temperature, an accurate constitutive model is a preliminary step for confident fatigue design.Constitutive equations for the cyclic behaviour, developed and validated by the authors in Part I of this paper [Launay et al., Int J Fatigue, in press], are applied to the mechanical analysis of fatigue campaigns carried out by Klimkeit et al. and De Monte et al. on specimens made out of polyamide 66 reinforced with 35wt.% of short glass fibres. Both studies are performed at room temperature, with material conditioned at the equilibrium with air containing 50% of relative humidity (RH50) or dry-as-moulded (DAM).Post-processing the cyclic response in steady-state allows the comparison of several fatigue criteria. The fatigue databases involve various loadings, including the study of multiaxiality and mean–stress effects on different microstructures. Among all physical quantities, the dissipated energy density per cycle ΔWdiss displays the best correlation with the fatigue life.

Experimental evaluation of the mechanical behavior of elastomeric materials for seismic applications at different air temperatures

This paper presents the results of an experimental study aimed at investigating the sensitivity of the cyclic behavior of elastomeric materials to air temperature variations. The experimental tests have been performed on several elastomeric specimens, purchased from two different Italian manufacturers of structural bearings and seismic isolation devices. Six different types of elastomers, with shear modulus (G) ranging from 0.5MPa to 1.2MPa at 100% shear strain, have been examined. During the tests, the specimens have been cycled under shear strains of increasing amplitude (from 25% to 125%), at three different frequencies of loading (0.01Hz, 0.1Hz and 0.5Hz). The tests have been carried out at six different air temperatures, ranging between −20°C and 40°C. The effects of prolonged exposure to low temperatures have been also investigated.The cyclic behavior of the specimens is described in terms of (i) shear stress at zero cyclic strain, (ii) maximum shear stress, (iii) secant shear modulus at the maximum strain amplitude, (iv) energy loss in one fully reversed cycle and (v) corresponding equivalent viscous damping ratio. In the paper, the most important results of the experimental investigation are described.

Hysteretic behavior of rectangular tube (box) sections based on Preisach model

In the present paper, the Preisach model of hysteresis is applied to model cyclic behavior of elasto-plastic material. Rate of loading and viscous effects will not be considered. The problem of axial loading of rectangular cross-section and cyclic bending of rectangular tube (box) will be studied in details. Hysteretic stress–strain loop for prescribed history of stress change is plotted for material modeled by series connection of three unite element. Also moment–curvature hysteretic loop is obtained for a prescribed curvature change of rectangular tube (box). All obtained results clearly show advantages of the Preisach model for describing cyclic behavior of elasto-plastic material.