Strain-controlled Tests Research Articles

Comparisons of low cycle fatigue behavior of CP-Ti under stress and strain-controlled modes in transverse direction

Low cycle fatigue (LCF) behavior of CP-Ti along transverse direction under symmetrical strain and stress-controlled modes was compared systematically. For the strain-controlled tests, tension-compression asymmetry (TCA) has little effect on LCF behavior due to the negligible compressive mean stress. For the stress-controlled tests, TCA is detrimental to LCF resistance. The microstructure evolution during cycling deformation was characterized by EBSD and TEM to identify different deformation mechanisms. It was found that strain cycling deformation was dominated by dislocation slip and exhibited the transition from planar slip to wavy slip with the increase of strain amplitude. For the stress-controlled tests, tensile twinning together with dislocation slip accommodated the increased ratcheting deformation. The strain-life curve under stress-controlled tests is lower than that at strain-controlled tests due to the heterogeneous plastic deformation caused by ratcheting deformation. In order to obtain a unified fatigue life prediction model for different control modes, a novel effective strain range related model was proposed by introducing the TCA factor into the original Manson–Coffin relation. Furthermore, the application of this effective strain range related model was extended to the condition of asymmetrical stress cycling by correcting fatigue ductility coefficient and exponent.

Fatigue analysis of cast iron components considering the influence of casting skin

The assessment of the fatigue strength of castings with remaining casting skin still constitutes a challenge. Until now, only the roughness of the casting skin has been considered through a reduction factor during the design process. This reduction factor, however, neglects the influence of discontinuities such as inhomogeneous microstructures, imperfections, pores etc. In this work, the casting skin represents the rim zone of the casting, consisting of surface roughness and a deviating microstructure.To assess the influence of the casting skin on the fatigue strength, stress- and strain-controlled fatigue tests were conducted on EN-GJS-400-15 specimens with deviating casting skins. The cyclic behavior of the rim zone microstructure was determined via cyclic axial strain-controlled tests under alternating loading Rε = -1 on small flat specimens removed from the rim zone. For the integration of the results from the axial fatigue tests with the influence of the surface roughness, as well as stress gradients present in cast components, on fatigue strength, cyclic bending tests under alternating Rσ = -1 and pulsating loading Rσ = 0 were performed. The test results are used to develop a fatigue approach to consider the influence of the rim zone on the fatigue life of cast iron components.

Investigation of the liquefaction potential of fiber-reinforced sand

In the present, the liquefaction potential of fiber-reinforced sandy soils was investigated through the energy-based approach by conducting a series of strain-controlled cyclic simple shear tests. In the tests, the effects of the fiber properties, such as the fiber content, fiber length, relative density and effective stress, and the test parameters on sandy soil improvement were investigated. The results indicated that the fiber inclusion yields to higher cumulative liquefaction energy values compared to the unreinforced (plain) ground by increasing the number of cycles and shear strength needed for the liquefaction of the soil. This result reveals that the fiber inclusion increases the resistance of the soil to liquefaction. However, the increase in the fiber content was determined to be more effective on the test results compared to the fiber length. Furthermore, the increase in the relative density of the soil increases the efficiency of the fibers on soil strengthening.

Effects of different heat treatments on the cyclic material behavior of additively manufactured Inconel®718

The flexibility in design offered by advanced additive manufacturing (AM) technologies makes these processes more and more attractive for automotive and aircraft applications and also for the manufacture of safety relevant metal components. The high-strength, thermally resistant Nickel-based alloy Inconel®718 is widely used by the aircraft industry. Its reduced machinability makes it an optimal candidate for AM technologies. The challenge, together with improving the process, is now to build the path that will bring AM technologies from rapid prototyping to series production. Therefore, it is essential to investigate additively manufactured materials and the effect of different subsequent heat treatments on their properties and microstructure. While the static properties of additively manufactured Inconel®718 have already been investigated, this work aims to describe its cyclic stress-strain behavior, to be used for a fatigue assessment. Small scale flat specimens are produced by Laser Powder Bed Fusion (LPBF) of Inconel®718 powder. To state the initial condition, the specimens are printed with different orientations with respect to the build platform using standard process parameters and individually adapted support structures, when needed, are subsequently removed. The material, in the as-built state, shows columnar grains and a highly heterogeneous microstructure. Therefore, it undergoes three different heat treatments corresponding to maximum temperatures of 650°C, 720°C or 965°C. The specimens are then subjected to incremental, variable amplitude strain-controlled testing (Incremental Step Tests) in order to evaluate the cyclic stress-strain behavior of the material. The combined effect of the different heat treatments and build orientations on the cyclic stress-strain behavior and fatigue life are evaluated and discussed through metallographic investigations and analysis of the stabilized hystereses.

Evaluation of ratcheting behaviour in cyclically stable steels through use of a combined kinematic-isotropic hardening rule and a genetic algorithm optimization technique

Modeling of material behaviour under strain-controlled and stress-controlled loading for cyclically stable materials (CSMs) is generally done using kinematic hardening, neglecting the isotropic hardening component. Although this approach is able to simulate the hysteresis loops, the quantitative ratcheting characteristics are often not of the desired accuracy; thus earlier investigators have introduced different modifications in the kinematic hardening component for case to case basis to obtain better agreement to the experimentally observed cyclic-plastic behaviour. Combined hardening models, though used for cyclic softening/hardening materials, have not been used for cyclically stable materials to the best knowledge of the authors. The primary aim of this study is to improve the ratcheting predictions for CSMs considering the well-known Chaboche's combined hardening model using a new generalized methodology. The initial estimates of the hardening components are obtained from symmetric strain-controlled tests subjected to their refinement by genetic algorithm based optimization technique. The Chaboche's parameters have been evaluated using the proposed methodology for available reported experimental data on cyclically stable steels to demonstrate the response of materials under stress-controlled and strain-controlled loading conditions.

Shear modulus of sand–tyre chip mixtures

This paper presents experimental results on the shear modulus of sand–tyre chip (STCh) mixtures. A series of bender element tests was carried out on specimens of sand mixed with varying proportions of tyre chips (TCh). Tests were carried out on STCh mixtures at a constant initial relative density of 50% for different initial effective confining pressures. The bender element test results indicate that the maximum shear modulus of the STCh mixtures increases with effective confining pressure and decreases with the gravimetric proportion of TCh, which ranged from 23 to 138 kPa and 0 to 40%, respectively. However, the strain-controlled cyclic triaxial test results show that the shear modulus at large shear strains decreases with increasing single-amplitude shear strain as a function of the proportion of TCh in the mixture. The lower the proportion of TCh, the larger the degradation observed. The modified Hardin and Drnevich mathematical formulation adequately captures the variation in shear modulus of the STCh mixtures for a wide range of shear strain amplitudes.

Understanding strain controlled low cycle fatigue response of P91 steel through experiment and cyclic plasticity modeling

In this paper, constitutive models for cyclic plasticity, namely, Chaboche and the Ohno-Wang models, are used to simulate the low cycle fatigue response of tempered ferritic-martensitic P91steel. A series of uniaxial strain-controlled low cycle fatigue tests are conducted at room temperature to evaluate various fatigue parameters. P91 steel shows cyclic softening behavior at all strain amplitudes. The analysis of the shape of hysteresis loops reveals that P91 steel deviates from Masing behavior. Plastic strain energy is used as a parameter to characterize the fatigue damage. The Morrow energy model estimates experimental average plastic strain energy density accurately. Remnant tensile properties are evaluated to quantify the damage during low cycle fatigue loading. The evaluation of back stress components and material Jacobian are implemented through a user defined subroutine UMAT into the commercial finite element package ABAQUS. Ohno-Wang material model predicts the hysteresis loop shape, area, and cyclic softening nature better than the Chaboche model. The predicted fatigue lives by simulation correlates well with the experimental fatigue lives within ± 200 cycles. These results suggest that energy based parameters can provide a reliable alternative to strain based approach in evaluating the fatigue damage of P91 steel.

Microstructure analysis and low-cycle fatigue behavior of spray-formed Al–Li alloy 2195 extruded plate

Microstructure and low-cycle fatigue behavior of spray-formed Al–Li alloy 2195 extruded plate were investigated in this work. The spray-formed alloy after hot extrusion experiment was treated with solid solution treatment and artificial aging. Microstructure analysis indicated the aged plate was dominated by elongated unrecrystallized grains, and had a rolling-type texture along extrusion direction with the highest intensity at Brass component. The existence of T1 phase strengthened the alloy crucially, but δ′ phase was basically absent. Then, the fully-reversed strain-controlled low-cycle fatigue tests were conducted at total strain amplitudes ranging from 0.4% to 1.0% for samples along two orthogonal directions. The stress-strain hysteresis loops were acquired, and the cyclic stress response curves were derived. At low strain amplitudes (0.4–0.5%), the initial cyclic hardening was slight and followed by a cyclic stability, while at higher strain amplitudes (0.6–1.0%), the alloy merely presented a continuously increasing cycle hardening behavior. Moreover, the fatigue life model based on the total strain energy was built and found to be suitable to predict life. Finally, the fatigue fractography observation showed that the fatigue source is relatively concentrated and the fracture surface had typical fatigue striations at 0.5% strain amplitude, while multiple cracks originated on the sample surface and the final fracture zone showed a ductile characteristic at 1.0%. The deformed microstructure near fracture surfaces were observed, and it was found that the cyclic hardening and stability were closely associated with the interaction between moving dislocations and obstacles including (sub)grain boundaries and secondary phase particles against them.

Elastoplastic response of as-built SLM and wrought Ti-6Al-4V under symmetric and asymmetric strain-controlled cyclic loading

PurposeThe purpose of this paper is to examine the mechanical behaviour of additively manufactured Ti-6Al-4V under cyclic loading. Using as-built selective laser melting (SLM) Ti-6Al-4V in engineering applications requires a detailed understanding of its elastoplastic behaviour. This preliminary study intends to create a better understanding on the cyclic plasticity phenomena exhibited by this material under symmetric and asymmetric strain-controlled cyclic loading.Design/methodology/approachThis paper investigates experimentally the cyclic elastoplastic behaviour of as-built SLM Ti-6Al-4V under symmetric and asymmetric strain-controlled loading histories and compares it to that of wrought Ti-6Al-4V. Moreover, a plasticity model has been customised to simulate effectively the mechanical behaviour of the as-built SLM Ti-6Al-4V. This model is formulated to account for the SLM Ti-6Al-4V-specific characteristics, under the strain-controlled experiments.FindingsThe elastoplastic behaviour of the as-built SLM Ti-6Al-4V has been compared to that of the wrought material, enabling characterisation of the cyclic transient phenomena under symmetric and asymmetric strain-controlled loadings. The test results have identified a difference in the strain-controlled cyclic phenomena in the as-build SLM Ti-6Al-4V when compared to its wrought counterpart, because of a difference in their microstructure. The plasticity model offers accurate simulation of the observed experimental behaviour in the SLM material.Research limitations/implicationsFurther investigation through a more extensive test campaign involving a wider set of strain-controlled loading cases, including multiaxial (biaxial) histories, is required for a more complete characterisation of the material performance.Originality/valueThe present investigation offers an advancement in the knowledge of cyclic transient effects exhibited by a typical α’ martensite SLM Ti-6Al-4V under symmetric and asymmetric strain-controlled tests. The research data and findings reported are among the very few reported so far in the literature.

A stress-based model for fatigue life prediction of high density polyethylene under complicated loading conditions

Fatigue life prediction is crucial to the long-term durability evaluation of high density polyethylene (HDPE) which may experience stress-controlled fatigue or strain-controlled fatigue depending on the external loading conditions. Although abundant studies have been carried out on stress-controlled fatigue, investigations on strain-controlled fatigue are insufficient, and the relationship between these two types of fatigues is unclear. In addition, approaches for strain-controlled fatigue life prediction of HDPE are scarce. The present paper aims at revealing the relationship between strain-controlled fatigue and stress-controlled fatigue and proposing a method for fatigue life prediction. Firstly, the stress-controlled fatigue tests were performed to study the influence of stress ratio on fatigue life, and the strain-controlled fatigue tests were successfully conducted thanks to the application of anti-buckling devices. Furthermore, our results showed that the stress level (i.e. stress amplitude and stress ratio) determined the fatigue life for both stress-controlled test and strain-controlled test. More importantly, a stress-based numerical model was established to predict the fatigue life under complicated loading conditions. The numerical model was built based on the constitutive relation for the computation of the stress response, the Walker method for the evaluation of the stress ratio influence, and the damage accumulation for the calculation of the fatigue life. The proposed numerical method was validated by comparing the predicted fatigue lives with the experimental data of the strain-controlled tests.

A comparative study on the cyclic plasticity and fatigue failure behavior of different subzones in CrNiMoV steel welded joint

The cyclic plasticity and the low cycle fatigue failure behavior of the weld metal (WM) and base metal (BM) of the CrNiMoV steel welded joint under the strain and stress-control modes were investigated respectively. Significant cyclic softening was observed for both the WM and BM under the low cycle fatigue tests with the two control modes. Besides, obvious ratcheting happened in the WM and BM under the stress-controlled cyclic loading conditions. It is shown that both the WM and BM exhibited lower fatigue strength at the stress control mode than that at the strain control mode due to the influence of tension-compression asymmetry. Meanwhile, the WM showed larger cyclic softening rate, lower ratchetting deformation and fatigue strength than the BM under the same loading levels. The failure location of the WM specimens shifted from BM region (nearby the heat affected zone) to the center of WM with the increasing of strain amplitude under the strain-controlled tests, which can be explained with the similar maximum equivalent plastic strain amplitude location shifting behavior observed from the corresponding finite element simulations.

The Mechanical Properties of Naturally Deposited Soft Soil under True Three-Dimensional Stress States

Abstract Foundation instability and collapse accidents often occur during construction in soft soil regions. The mechanical properties of naturally deposited soft soil in Shanghai were discussed in light of experimental observations from a series of major principal strain-controlled true triaxial undrained shearing tests performed on cubical undisturbed soft soil specimens. The results show that the three-dimensional stress state has a strong influence on the stress and pore pressure properties of soft soil. The initial gradients of deviator stress–strain curves and strength of soft soil obviously increase with the increasing intermediate principal stress and confining pressure. The measured mechanical responses of soft soil are subject to the influence of stress path effects, changes in the initial mean effective stress, and relative magnitudes of intermediate principal stress b. The effective stress paths are manifested by concave or convex shape caused by excess pore pressure, which increases with the increasing b value. The failure envelopes in the p′- q space are a series of straight lines with various slopes at various b values that intersect at the same point in the p′-axis. Considering the effects of the intermediate principal stress and structural characteristics, the failure function with a shape function g(b) for Shanghai soft soil is presented. The results calculated by the equation g(b) presented are in good agreement with experimental data.

Cyclic stress responses of a newly developed nickel-base superalloy at elevated temperatures

Total strain-controlled low cycle fatigue tests were conducted on the newly designed nickel-base superalloy M951G under different testing conditions; the relationship among cyclic stress responses, microstructural degradations, deformation mechanisms and testing conditions has been established. Results show that both the cyclic hardening and softening behaviors are dependent on the testing temperature and strain amplitude. As the strain amplitude increases both at 900 and 1000 °C, M951G alloy exhibits cyclic hardening under low strain amplitudes and cyclic softening under higher strain amplitudes. At 900 °C, the initial cyclic hardening is related to the coherent γ/γ′ interface, parallel dislocation arrays and dislocation bypassing the tiny γ′ particles. At higher strain amplitudes, the initial cyclic softening is due to the higher density of shearing dislocations in γ′ precipitates. At 1000 °C, plenty of parallel dislocation arrays present in γ channels, which reduces the possibility of dislocation interactions from different slip systems and results in initial cyclic hardening. Under higher strain amplitudes, apart from microstructural degradations, dislocations shearing into γ′ precipitates and formation of dislocation networks, the dislocation annihilation and partial loss of coherency of γ′ precipitates are also responsible for the initial cyclic softening at 1000 °C.

Comparative study between small punch test and hydraulic bulge test

The systematically comparison between small punch test and hydraulic bulge test is carried out, which include specimen geometry, the test device, testing procedure, testing results interpretation. The specimen geometry, test device and testing procedure are three major aspects of the experiments. Both the testing fixtures of SPT and HBT are redesigned to meet the specimen geometry. The testing procedure of SPT is similar to strain-controlled tension test, which control the displacement of the puncher and measure the corresponding reaction load. The testing procedure of HBT is similar to stress-controlled tension test, which control the pressure of the hydraulic oil and measure the displacement of the deformed specimen. The finite element method is used to study the deformation pattern of the specimen. The initial plastic zone distribution is different. The plastic zone of SPT is concentrated in a small area under the puncher and the initial plastic zone is a circle around the corner of the fixture for HBT. The deformation pattern play an important role in the yield stress interpretation, it is suggest that the interpretation method should include the parameters from the testing fixtures for HBT, and radius of the puncher for SPT.

Energy Dissipation and Pore Pressure Generation in Stress- and Strain-Controlled Cyclic Triaxial Tests

Abstract The pore pressures generated in soils subjected to cyclic loadings have been shown to be proportional to the energy dissipated in the soil during that loading. Both stress-controlled and strain-controlled cyclic triaxial tests can be used to develop parameters for energy-based pore pressure models; however, the patterns of their energy dissipation into the soil are quite different. In this study, the two patterns of energy-dissipation associated with stress-controlled and strain-controlled cyclic triaxial tests were developed and then confirmed using sixteen stress-controlled and sixteen strain-controlled cyclic triaxial tests. Additionally, it was shown that the patterns of pore-pressure generation for the two tests, which differ greatly when evaluated relative to the cycles of loading, were very similar when evaluated in terms of their energy dissipation.

NONLINEAR VISCOELASTIC DISSIPATION IN VULCANIZATES CONTAINING CARBON BLACK AND SILANIZED SILICA HYBRID FILLERS

ABSTRACT Different mechanisms of large-strain viscoelastic dissipation in vulcanizates containing carbon black or silanized silica and their synergistic effects in hybrid filler tread vulcanizates were investigated. In addition, the pure effects of fillers were examined while the degree of chemical cross-linking in the rubber matrix was similar in the vulcanizates. The results revealed that nonlinear viscoelastic loss modulus under strain-controlled dynamic tests, as a measure of dissipation mechanism, is high in both carbon black and silanized silica tread vulcanizates. However, a synergistic effect in reducing loss modulus was observed in the hybrid filler vulcanizates. Conversely, storage modulus in the vulcanizates containing more silanized silica is distinguishably higher and results in a lower loss factor representing load-controlled cyclic deformation of vulcanizates, such as in the heat buildup test. Both loss factor and heat build-up reduced nonlinearly as the amount of silanized silica increased in the vulcanizates, verifying the synergistic effect of hybrid carbon black–silanized silica filler in reducing dissipative mechanisms in large-strain dynamic loadings. This feature is highly favorable for the tire industry, where the lowering of viscoelastic dissipation in tread vulcanizates is of great importance.

Dynamic Behavior of a Geotextile-Reinforced Pond Ash Embankment

ABSTRACT The present study investigates the applicability of woven geotextile in improving the seismic performance of a pond ash embankment located in Renusagar, in seismic zone III of India. Reinforcing materials used are woven geotextiles, placed at 1 and 3 layers of pond ash embankment. To obtain the material models, strain-controlled cyclic triaxial tests had been carried out. A general expression was developed for shear modulus and damping ratio of pond ashes reinforced with and without woven geotextiles considering the data taken from seismic zones of III and IV. The proposed equations have been compared with the reported results of other pond ashes located in India. Finite element model of the pond ash embankment reinforced with geotextiles has been developed. A fluid solid porous material model has been employed to incorporate pore pressure build up during cyclic loading. A parametric study has also been performed to study the effect of reinforcements on the seismic behavior of Renusagar pond ash embankment. Results showed that using the three layers of geotextile, the lateral displacement reduces up to 17%, and the excess pore pressure reduces as much as 30–45%. From the study, it can be concluded that use of woven geotextile in pond ash dykes can be beneficial for its application as reinforcing material in highway embankments, building fills, and earth dams.

Comparative Study of the Uniaxial Cyclic Behaviour of Carbide-Bearing and Carbide-Free Bainitic Steels

Bainitic steels play an important role in the modern automotive and rail industries because of their balanced properties. Understanding the relationship between the bainitic microstructure features and the fatigue performance is a fundamental ingredient in developing safer and durable products. However, so far this relationship is not sufficiently clear. Therefore, there is the need to strengthen the knowledge within this field. The present paper aims at comparing the uniaxial cyclic behaviour of carbide-bearing and carbide-free bainitic steels. To meet this goal, fully-reversed strain-controlled tests at various strain amplitudes were performed. After the final failure, fracture surfaces were observed by transmission electron microscopy to relate the bainitic morphology to the fatigue performance. The main findings of this work show that the carbide-free lower bainite has superior fatigue performance compared to the carbide-bearing lower bainite. This is explained by the presence of stable carbides and thick bainite ferrite plates.

High temperature fatigue and creep-fatigue behaviors in a Ni-based superalloy: Damage mechanisms and life assessment

The low cycle fatigue (LCF) and creep-fatigue behaviors of Ni-based GH4169 superalloy are investigated by uniaxial strain-controlled fully-reversed testing at 650 °C. Compared with LCF tests, the effects of tensile and compressive strain hold times on creep-fatigue lifetimes are experimentally explored with varying total strain ranges in the present work. In order to elucidate the damage mechanisms under complex loading waveforms, an additional series of tests with both tensile and compressive hold times are carried out at a given total strain range of 2.0%. Posterior to the cyclic tests, main-crack-failure modes and secondary cracking modes are studied via optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) techniques. Main-crack failure mechanisms are examined by the fracture appearance observations. Cracking modes are explored through quantitative characterization on the distributions of secondary cracks in the longitudinal cross sections under different loading waveforms. Moreover, a generalized life model based on linear damage summation (LDS) framework and energy dissipation criterion (EDC) is elaborated to estimate the damage mechanisms of fatigue, creep and oxidation. The prediction results can well establish the correlations between the reductions of numbers of cycles to failure and the presences of different damage mechanisms under respective loading waveforms.

Evaluation of Shear Strength Parameters of Sandy Soils upon Microbial Treatment

ABSTRACTA new method for soil stabilization known as microbial-induced calcite precipitation (MICP) has been the focus of research in this area during the last decade. In this method, the reaction of microorganisms in the presence of urea and calcium chloride is used to produce calcite. Despite the large numbers of bacteria in soil, Sporobacillus pasteurii (previously known as Bacillus pasteurii) has the most capability to create cementation between soil particles in the MICP method due to its high urease activity. In this paper, the effect of MICP treatment on the shear strength characteristics of a sandy soil was studied. The change in the shear strength of sandy soil upon MICP treatment was measured using a strain-controlled direct shear test before and after treatment of the soil samples. The results showed an increase of 44–86% in the shear strength of the sandy soil after 15 days of MICP treatment compared to the untreated soil. The enhanced shear strength was the result of an increase in both the cohesion intercept and angle of internal friction. The increase in the cohesion intercept was more significant than the increase in the angle of internal friction.