Cyclic Stress-strain Behavior Research Articles

Influence of the microstructure on the cyclic stress-strain behaviour and fatigue life in hypo-eutectic Al-Si-Mg cast alloys

Aluminium alloys are promising candidates for energy-and cost-efficient components in automotive and aerospace industries, due to their excellent strength-to-weight ratio and relatively low cost compared to titanium alloys. As modern cast processing and post-processing, e.g. hot isostatic pressing, result in decreased frequency and size of defects, the weakest link depends on microstructural characteristics, e.g. secondary dendrite arm spacing (SDAS), Si eutectic morphology and α-Al solid solution hardness. Hereby, fatigue investigations of the effect of the microstructure characteristics on the cyclic stress-strain behaviour as well as fatigue mechanisms in the low cycle and high cycle fatigue regime are performed. For this purpose, samples of the aluminium cast alloy EN AC-AlSi7Mg0.3 with different Si eutectic morphology and α-Al solid solution hardness were investigated. To compare the monotonic and cyclic stress-strain curves, quasistatic tensile tests and incremental step tests were performed on two microstructure conditions. The results show that the cyclic loading leads to a hardening of the material compared to monotonic loading. Based on damage parameter Woehler curves, it is possible to predict the damage progression and fatigue life for monotonic and cyclic loading in hypo-eutectic Al-Si-Mg cast alloys by one power law.

Thermomechanical fatigue – Mechanism-based considerations on the challenge of life assessment

The combination of cyclic mechanical and cyclic thermal loading leads to thermomechanical fatigue (TMF) which is considered to be the primary life-limiting factor for engineering components in many high-temperature applications. Extensive low-cycle fatigue (LCF) data, which is traditionally used for design purposes, has been generated isothermally on various high-temperature materials, and thus, it is tempting to try to predict TMF life based mainly on isothermal LCF data. In this contribution, studies on different metallic structural high-temperature materials, which have mainly been carried out in the author's laboratory, are reviewed addressing the question, in which way and to which extent a reliable, unerring and robust TMF life assessment is possible on the basis of isothermally obtained fatigue life data. It is shown by means of examples that a sound TMF life prediction first of all requires a detailed mechanistic understanding of the isothermal cyclic stress-strain response and the relevant damage mechanisms. Furthermore, the TMF-specific peculiarities in both the non-isothermal cyclic stress-strain behaviour and the non-isothermal damage evolution process must be known. If all these requirements are fulfilled and reflected in the TMF life assessment methodology applied, a reasonable predictive accuracy can be attained.

Microstructure stability and micro-mechanical behavior of as-cast gamma-TiAl alloy during high-temperature low cycle fatigue

This study systematically investigated the low cycle fatigue deformation of a high Nb-containing TiAl alloy with a nominal chemical composition of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y at 850 °C by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and synchrotron-based high-energy X-ray diffraction (HE-XRD) techniques. Cyclic stress-strain (CSS) behavior, lattice strain, and peak broadening of {100}α2, {201}γ, and {202}ωo planes, phase transformations, and crack propagation behavior were obtained for samples with three total strain amplitudes: Δεt/2 = ±0.25%, Δεt/2 = ±0.28%, and Δεt/2 = ±0.30%. At early deformation stages, α2lamellae transformed into ωo phase with a distinct orientation relationship, and a certain orientation relationship (OR) between them was observed after the following cyclic deformation. Furthermore, γ particles precipitated within the single ωo area. In addition, according to the peak intensity and peak broadening results, the ωo → B2 phase transformation occurred, leading to the appearance of single B2-phase areas. The lattice strains in the ωo phase were always in tension during the cyclic deformation and large differences of the lattice strains were found in the γ phase and α2 phase, not only the values but also the directions, which resulted in crack nucleated at and propagated along the α2/γ lamellar interface. This study provides a better understanding of the low cycle fatigue deformation of TiAl alloys.

Fatigue crack propagation prediction of a pressure vessel mild steel based on a strain energy density model

Fatigue crack growth (FCG) rates have traditionally been formulated from fracture mechanics, whereas fatigue crack initiation has been empirically described using stress-life or strain-life methods. More recently, there has been efforts towards the use of the local stress-strain and similitude concepts to formulate fatigue crack growth rates. A new model has been developed which derives stress-life, strain-life and fatigue crack growth rates from strain energy density concepts. This new model has the advantage to predict an intrinsic stress ratio effect of the form ?ar=(?amp)?·(?max )(1-?), which is dependent on the cyclic stress-strain behaviour of the material. This new fatigue crack propagation model was proposed by Huffman based on Walkerlike strain-life relation. This model is applied to FCG data available for the P355NL1 pressure vessel steel. A comparison of the experimental results and the Huffman crack propagation model is made.

Fatigue crack propagation prediction of a pressure vessel mild steel based on a strain energy density model

Fatigue crack growth (FCG) rates have traditionally been formulated from fracture mechanics, whereas fatigue crack initiation has been empirically described using stress-life or strain-life methods. More recently, there has been efforts towards the use of the local stress-strain and similitude concepts to formulate fatigue crack growth rates. A new model has been developed which derives stress-life, strain-life and fatigue crack growth rates from strain energy density concepts. This new model has the advantage to predict an intrinsic stress ratio effect of the form ?ar=(?amp)?·(?max )(1-?), which is dependent on the cyclic stress-strain behaviour of the material. This new fatigue crack propagation model was proposed by Huffman based on Walkerlike strain-life relation. This model is applied to FCG data available for the P355NL1 pressure vessel steel. A comparison of the experimental results and the Huffman crack propagation model is made.

Cyclic stress-strain behavior of polymeric nonwoven structures for the use as artificial leaflet material for transcatheter heart valve prostheses

Abstract Xenogenic leaflet material, bovine and porcine pericardium, is widely used for the fabrication of surgically implanted and transcatheter heart valve prostheses. As a biological material, long term durability of pericardium is limited due to calcification, degeneration and homogeneity. Therefore, polymeric materials represent a promising approach for a next generation of artificial heart valve leaflets with improved durability. Within the current study we analyzed the mechanical performance of polymeric structures based on elastomeric materials. Polymeric cast films were prepared and nonwovens were manufactured in an electrospinning process. Analysis of cyclic stress-strain behavior was performed, using a universal testing machine. The uniaxial cyclic tensile experiments of the elastomeric samples yielded a non-linear elastic response due to viscoelastic behavior with hysteresis. Equilibrium of stress-strain curves was found after a specific number of cycles, for cast films and nonwovens, respectively. In conclusion, preconditioning was found obligatory for the evaluation of the mechanical performance of polymeric materials for the use as artificial leaflet material for heart valve prostheses.

Dependence of hardening and saturation stress in persistent slip bands on strain amplitude during cyclic fatigue loading

Cyclic loading of FCC single crystals leads to the formation of persistent slip bands (PSBs) over a wide range of applied strain amplitudes. The hardening and saturation stress associated with cyclic loading of Cu single crystals are reproduced with 3-D discrete dislocation dynamics simulations. Evolution of the dislocation microstructure in PSB channels and walls during cyclic loading is shown to explain key features of plastic deformation. Screw dislocation segments are found to deposit edge components near PSB channel walls resulting in the nucleation of half-loops that expand into neighbouring channels. The saturation stress is found to be 34 MPa at an applied strain amplitude, , within the range of experimental observations. It is shown that once PSBs are formed, subsequent cycling at lower strain amplitudes does not lead to their elimination, but the cyclic stress–strain behaviour is modified. At lower values of of previously formed PSBs, the saturation stress is found to decrease to 24 MPa at . Plastic energy dissipation in hysteresis loops is also significantly reduced but not removed. However, some reverse plasticity is shown to take place upon unloading at low values of , and is a direct result of strain recovery of loops that expand into neighbouring channels during the loading phase.

Stress softening and hardening during compression and tensile consecutive cyclic loading of Mn18Cr18N austenitic stainless steel

The compression and tensile consecutive deformation behavior and microstructure evolution of Mn18Cr18N steel were investigated by cyclic loading tests at room temperature and strain amplitude from 0.005 to 0.15. The results indicated that the cyclic loading stress-strain curves of the steel show plastic deformation characterized by almost the same working hardening rates without yield plateaus at various strain amplitudes. At the lower strain amplitudes from 0.005 to 0.01, the stress amplitudes and subsequent yield surface radius decreased with cycles, which suggest that stress softening occurs during cyclic loading. At the strain amplitudes from 0.02 to 0.15, the stress amplitudes and subsequent yield surface radius increased with cycles, which imply that stress hardening occurs during cyclic loading. With the increasing of strain amplitudes, the cyclic hardening coefficients monotonically increased, while the cyclic softening coefficients rapidly decreased. The maximum flow stress increased up to 1549.6MPa after three cycles at the strain amplitude of 0.15, which increased by 395.4MPa compared to the maximum uniaxial tensile flow stress. It was also demonstrated that the cyclic loading stress-strain behaviors (cyclic softening and hardening) depend on internal stress component rather than effective stress component. TEM observation shows that planar slip with parallel substructures along one direction at high number cycles evolved from various dislocation configurations at low number cycles occurs at the lower strain amplitude of 0.005; double planar slip with grid substructures along two directions at high number cycles evolved from parallel substructures along one direction at low number cycles occurs at the higher strain amplitude of 0.04. Internal stress could be released by dislocation rearrangement and activation of cross slips during cyclic loading process, which induce stress softening at lower strain amplitudes; while slipping and twining alone two intersectional directions caused stress hardening at higher strain amplitudes.

PARC_CL 2.0 crack model for NLFEA of reinforced concrete structures under cyclic loadings

This paper presents the development of an updated version of PARC_CL 2.0 crack model implemented in Abaqus Code using subroutine UMAT.for. The PARC_CL 2.0 model is a fixed cracked model for the response prediction of reinforced concrete members. More refined material constitutive relationships were incorporated in the formulation of the model to take into account plastic deformations. Finally, to validate the model, reinforced concrete (RC) panels subjected to cyclic loads, available in literature, are modeled and compared with experimental observations. The PARC_CL 2.0 model was able to capture, with reasonable accuracy, the overall behaviour including cyclic shear stress vs. shear strain behaviour, shear stiffness, cyclic stiffness degradation, energy dissipation during the cycles and pinching effect.

The Simulation of a Unified Viscoplastic Model at Elevated Temperature

In this paper, a Chaboche unified constitutive model has been used to describe the cyclic plasticity and viscoplasticity of Nimonic alloy 75. A non-linear optimization algorithm has been applied in numerically simulation of this alloy at elevated temperature. Optimization algorithm has facilitated a step-by-step method to obtain the initial material parameters, while a non-linear least-square approach were used to obtain the optimized material parameters. Uniaxial experiments were carried to obtain the full cyclic stress-strain and stress relaxation data at 600ï‚°C. Satisfactory results have been obtained for the simulation of the transient and steady state cyclic stress-strain and stress relaxation behavior of dwell cyclic test.

Cyclic deformation and microstructure evolution of high Nb containing TiAl alloy during high temperature low cycle fatigue

The cyclic stress-strain (CSS) behavior of high Nb containing TiAl alloy with a duplex microstructure was investigated at 850°C. Transmission electron microscope (TEM) observations and electron backscattered diffraction (EBSD) techniques were carried out to obtain insight into the microstructure evolution governing this behavior. At low strain amplitude, the material exhibits a rapid saturation of stress amplitude. At intermediate and high strain amplitude, the CSS behavior is characterized by generally cyclic softening. The changes of microstructure are strain-induced phase transformations and dynamic recrystallization, which lead to a degradation of lamellar microstructure. Twin boundaries can promote discontinuous dynamic recrystallization of γ phase, for processing relatively high energy. α2+γ→B2, α2→B2, and α2lamellae→γ phase transformations are detrimental to fatigue life of the material, because fracture propagation are usually along B2 phase boundaries and γ grain boundaries, thus, the fracture mode is a combination of ductile fracture and intergranular cleavage fracture.

Micromechanical modeling of fatigue behavior of DP steels

The effect of martensitic phase fraction on the cyclic stress-strain behavior of two DP steels is investigated via a micromechanical model based on a representative volume element (RVE). A dislocation density based model and a Chaboche hardening model are used to identify the isotropic and kinematic hardening behavior of constituent phases, respectively. The Chaboche parameters obtained by fitting flow curves computed from a dislocation density based model for both ferrite and martensite phases of each steel are incorporated into a Finite Element code ABAQUS to simulate the low cycle fatigue with a combined hardening behavior. Based on experimental observations reported in the literature, fatigue crack initiates in ferrite phase. A ductile damage model, therefore, is used to simulate damage initiation in ferrite. The results show that the martensite fraction has a significant influence on cyclic plastic strain accumulation during the cyclic deformation. It is also concluded that with an increase in the martensite volume fraction in DP steel, the elastic component of the total strain amplitude increases and higher fatigue strength is, subsequently, observed.

Cyclic Stress–Strain Behavior of Concrete Confined with NiTiNb-Shape Memory Alloy Spirals

AbstractRecent studies showed that concrete confinement using shape memory alloy (SMA) spirals is a promising technique for seismic retrofitting of reinforced concrete columns that lack flexural du...

Cyclic response of FRP-confined concrete with post-peak strain softening behavior

The majority of experimental studies investigating the cyclic stress-strain behavior of fiber-reinforced polymer (FRP) confined concrete feature a post-peak strain hardening. Research featuring post-peak strain softening is rare, and the development of this type of stress-strain model is currently impossible due to the lack of test results. This work addresses the gap through experimental testing and analyses of test results. Sixty FRP confined concrete cylinders were tested. Concrete strength and FRP thickness were selected as the main test variables. The test results indicated that confinement rigidity and concrete grade have a more pronounced effect on stress-strain behavior for specimens with strain softening, than those with strain hardening, in terms of the overall stress-strain curve, the unloading and reloading paths, and the plastic strain. The test results provided a database for the development of general stress-strain model that features both strain-hardening and strain-softening curves.

PREDICTIONS OF STRESS-STRAIN CURVE AND FATIGUE LIFE FOR AISI 316 STAINLESS STEEL IN CYCLIC STRAINING

In this study, experimental observations have been performed to understand the cyclic stress-amplitude response, the cyclic hardening/softening characteristics and the cyclic fatigue properties of AISI 316 stainless steel. Based on the experimental observations, these results are found so that the tested AISI 316 stainless steel possesses cyclic hardening characteristics. The shape of the stable hysteresis loop is symmetrical in tension and compression. The Massing cyclic stress-strain behavior is obviously absent. For the case of the absence of the Massing behavior, the power-law type equation is successfully applied to simulate the shape of the stable hysteresis loop at all applied levels of fully-reversed cyclic straining. For the �������� -based and ���������������� �������� -based fatigue life curves, the typical expressions are developed via cyclic fatigue properties. Besides, a power-law type relationship is also used to develop the fatigue life curve. In contrast to the typical expression, it is found that the power-law relationship is a viable and valid expression in fatigue life predictions. Similarly, the power-law relationship is also used to describe the correlation between the cumulative plastic strain energy W_f with the corresponding the fatigue life cycle N_f. Based on the comparison between the predicted and experiment cycles, it is confirmed that the predicted value of W_f calculated on the basis of the simulated hysteresis loop is capable of yielding reasonable life predictions via the W_f- N_f curve.

A constitutive model of nanocomposite hydrogels with nanoparticle crosslinkers

Nanocomposite hydrogels with only nanoparticle crosslinkers exhibit extraordinarily higher stretchability and toughness than the conventional organically crosslinked hydrogels, thus showing great potential in the applications of artificial muscles and cartilages. Despite their potential, the microscopic mechanics details underlying their mechanical performance have remained largely elusive. Here, we develop a constitutive model of the nanoparticle hydrogels to elucidate the microscopic mechanics behaviors, including the microarchitecture and evolution of the nanoparticle crosslinked polymer chains during the mechanical deformation. The constitutive model enables us to understand the Mullins effect of the nanocomposite hydrogels, and the effects of nanoparticle concentrations and sizes on their cyclic stress–strain behaviors. The theory is quantitatively validated by the tensile tests on a nanocomposite hydrogel with nanosilica crosslinkers. The theory can also be extended to explain the mechanical behaviors of existing hydrogels with nanoclay crosslinkers, and the necking instability of the composite hydrogels with both nanoparticle crosslinkers and organic crosslinkers. We expect that this constitutive model can be further exploited to reveal mechanics behaviors of novel particle-polymer chain interactions, and to design unprecedented hydrogels with both high stretchability and toughness.

Fatigue Deformation of Polycrystalline Cu Using Molecular Dynamics Simulations

Molecular dynamics (MD) simulations have been performed to investigate the fatigue deformation behaviour of polycrystalline Cu with grain size of 5.4 nm. The samples were prepared using Voronoi algorithm with random grain orientations. Fatigue simulations were carried out by employing fully reversed, total strain controlled cyclic loading at strain amplitude of $\pm4$\% for 10 cycles. The MD simulation results indicated that the deformation behaviour under cyclic loading is dominated by the slip of partial dislocations enclosing the stacking faults. At higher number of cycles, the grain boundary migration leading to coarsening of larger grains at the expense of the smaller grains has been observed. The cyclic stress-strain behaviour, the deformation mechanisms and the variation of dislocation density as a function of cyclic deformation have been discussed.

Microstructure, Cyclic Deformation and Corrosion Behavior of Laser Welded NiTi Shape Memory Wires

The present paper reports the effects of Nd:YAG laser welding on the microstructure, phase transformation, cyclic deformation behavior, and corrosion resistance of Ti-55 wt.% Ni wire. The results showed that the laser welding altered the microstructure of the weld metal which mainly composed of columnar dendrites grown epitaxially from the fusion line. DSC results indicated that the onset of the transformation temperatures of the weld metal differed from that of the base metal. Cyclic stress-strain behavior of laser-welded NiTi wire was comparable to the as-received material; while a little reduction in the pseudo-elastic property was noted. The weld metal exhibited higher corrosion potential, lower corrosion current density, higher breakdown potential and wider passive region than the base metal. The weld metal was therefore more resistant to corrosion than the base metal.

An in situ Study of NiTi Powder Sintering Using Neutron Diffraction

This study investigates phase transformation and mechanical properties of porous NiTi alloys using two different powder compacts (i.e., Ni/Ti and Ni/TiH2) by a conventional press-and-sinter means. The compacted powder mixtures were sintered in vacuum at a final temperature of 1373 K. The phase evolution was performed by in situ neutron diffraction upon sintering and cooling. The predominant phase identified in all the produced porous NiTi alloys after being sintered at 1373 K is B2 NiTi phase with the presence of other minor phases. It is found that dehydrogenation of TiH2 significantly affects the sintering behavior and resultant microstructure. In comparison to the Ni/Ti compact, dehydrogenation occurring in the Ni/TiH2 compact leads to less densification, yet higher chemical homogenization, after high temperature sintering but not in the case of low temperature sintering. Moreover, there is a direct evidence of the eutectoid decomposition of NiTi at ca. 847 and 823 K for Ni/Ti and Ni/TiH2, respectively, during furnace cooling. The static and cyclic stress-strain behaviors of the porous NiTi alloys made from the Ni/Ti and Ni/TiH2 compacts were also investigated. As compared with the Ni/Ti sintered samples, the samplessintered from the Ni/TiH2 compact exhibited a much higher porosity, a higher close-to-total porosity, a larger pore size and lower tensile and compressive fracture strength.

A pressure-sensitive kinematic hardening model incorporating Masing’s law

A model is proposed to simulate the stress–strain behavior of sands subjected to cyclic loads in general three-dimensional stress space. A novel formulation is developed using Prager’s kinematic hardening rule and extending the one-dimensional Masing’s rule to general three-dimensional stress space. The hysteretic stress–strain curves are constructed based on skeleton curves. In order to do this, the Masing’s rule is generalized to the proportional rule, which consists of two sets of rules: the internal and external rule. Subsequently, a drag rule is introduced to simulate cyclic stress–strain behavior in which the stress amplitude increases at a decreasing rate during cyclic loading with a constant strain amplitude. The dimensionless kinematic hardening rate is assumed to depend on the current stress value along the stress path. When the direction of loading is reversed, the initial rate of hardening is restored. The rate of variation of hardening is scaled according to an extended Masing’s law. As a result, a closed hysteretic stress–strain loop is obtained during cyclic loading.