Strain-controlled Tests Research Articles

A combined critical distance and highly-stressed-volume model to evaluate the statistical size effect of the stress concentrator on low cycle fatigue of TA19 plate

Titanium alloy has been widely used for compressor discs in aero-engines due to its high specific strength. As a common geometry feature in compressor discs, bolt hole with stress concentration can cause compressor discs to fail under low cycle fatigue (LCF). In this study, the statistical size effect of such stress concentrators was investigated by LCF experimentations and theoretical predictions for titanium alloy plate specimens with a central circular hole (CHP). Four different scales of test sections without the influence of geometrical size were specially designed and experimental tests were conducted under LCF loading at 180°C. The strain-controlled LCF tests of smooth specimen were also carried out at the same temperature. It is found that the predicted LCF lifetime from the theory of critical distance (TCD) correlates well with experimental results for the 100%-scale CHP specimens. However, it fails to accurately predict the dependence of LCF lifetime on the statistical size effect for CHP specimens at the other scales i.e., 40%-scale, 60%-scale, and 80%-scale. Furthermore, a novel methodology combining the TCD and highly-stressed-volume models is proposed to evaluate the statistical size effect, which exhibits much improved accuracy than the TCD method alone. The new model not only inherits the versatility of the TCD method but providing an essential assessment of the statistical size effect.

Finite element-based micromechanical modeling of the influence of phase properties on the elastic response of cementitious mortars

This study reports the influence of inclusion stiffness and its distribution on the stress distributions in the microstructural phases of different cementitious mortars using microstructure-guided finite element simulations. Randomly generated periodic microstructures with single/multiple inclusion sizes and random spatial distribution, subjected to periodic boundary conditions and a strain-controlled virtual testing regime are chosen for final analysis. Numerical simulations reveal: (i) the differences in locations/magnitudes of stress concentrations as a function of inclusion stiffness and size distribution, and (ii) the sometimes detrimental influence of matrix and interface stiffening/strengthening on the overall composite response, leading to material design strategies when non-conventional inclusions are used in cementitious systems for special properties. The constitutive behavior in the linear elastic regime is extracted based on the predicted dominant principal stresses and strains in the representative area element. Thus, in addition to the microstructural phase stresses, this methodology also provides predictions of the composite elastic modulus, which are observed to be more reliable than those obtained from analytical prediction models.

Load history and sequence effects on cyclic deformation and fatigue behavior of a thermoplastic polymer

The effect of load history and sequence on the cyclic deformation and fatigue behavior of polyether ether ketone (PEEK) thermoplastic was experimentally investigated. Various uniaxial strain-controlled multi-block fatigue tests were conducted, including (1) fully-reversed (Rε = −1) two-, three- and four-block loadings with adjusted frequency to maintain a nominal temperature rise on the specimen gage surface, (2) fully-reversed two-block loading to study the frequency effect, and (3) pulsating tension (Rε = 0) two-block loading to investigate the combine effect of pre-loading and mean stress/strain on cyclic deformation and fatigue behavior. For all fatigue tests under fully-reversed (Rε = −1) multi-block loading, cyclic stability of PEEK specimens was achieved at approximately the same reversal and with similar stress amplitudes as those without pre-loading under identical test conditions. Additionally, for tests with Rε = −1, pre-loading was found to have a significant beneficial effect on PEEK fatigue resistance, irrespective of the load sequence (i.e. high-low or low-high). However, no obvious load history and sequence effect on fatigue behavior of PEEK was observed for pulsating tension (Rε = 0) block loading tests. Experimental results also revealed that increasing test frequency generally leads to higher fatigue lives.

Effect of relative humidity on uniaxial cyclic softening/hardening and intrinsic heat generation of polyamide-6 polymer

Strain-controlled uniaxial cyclic tests on a polyamide-6 (PA6) polymer with various relative humidity (RH) levels were performed in a room environment. The effects of RH on the cyclic softening/hardening and intrinsic heat generation of the PA6 are discussed by using the experimental data obtained in the cyclic tests with various applied strain amplitudes, mean strains and strain rates. It is observed that the cyclic softening/hardening of PA6 depends greatly on the applied strain amplitude and strain rate, and the cyclic softening observed in the cyclic tests with a larger strain amplitude and at a higher strain rate is mainly caused by the remarkable temperature rise due to the intrinsic heat generation. The variation of relative humidity does not change the cyclic softening/hardening feature of PA6, but does influence the extent of cyclic softening/hardening; an apparent mean stress relaxation occurs in the asymmetrical strain-controlled cyclic tests, and its extent increases with increasing mean strain but decreases with increasing relative humidity.

Deformation and damage mechanisms of a bimodal 12Cr-ODS steel under high-temperature cyclic loading

The present study reports on the high-temperature low-cycle fatigue (LCF) behavior of a ferritic bimodal 12Cr oxide dispersion strengthened steel. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The measured cyclic stress response showed four distinct stages which include instant initial cyclic softening followed by gradual cyclic hardening thereafter continuous linear cyclic softening and finally crack initiation and growth stage. The rate and amount of these stages depend on both strain amplitude and testing temperature. The cyclic stress-strain and strain-life relationships were obtained through the test results, and related LCF parameters were calculated. Microstructural investigations, using electron microscopy, were carried out to shed light on the deformation mechanisms. At 550°C, no appreciable microstructural evolution occurred, which is consistent with the manifestation of Masing behavior. A propensity towards cross-slip and hence the formation of three-dimensional dislocation structures increased with increase in loading amplitude and testing temperature, which assisted in accumulating higher inelastic strain and led to slightly higher cyclic hardening at these conditions. Furthermore, W-enriched Laves phase particles precipitated at grain boundaries. The evolution of cell structures with strain amplitude at 650°C resulted in a small deviation from Masing behavior. The damage studies revealed multiple surface initiated transgranular cracks under higher strain amplitudes and a single surface initiated transgranular crack under lower strain amplitudes. The different stages of the crack path, whose durations depend on the testing conditions, revealed distinct fatigue fracture features.

Experimental fatigue characterization and elasto-plastic finite element analysis of notched specimens made of direct-quenched ultra-high-strength steel

The low-cycle fatigue behavior of a direct-quenched ultra-high-strength steel was experimentally characterized and numerically modeled. Fatigue and cyclic parameters were obtained by conducting strain-controlled low-cycle fatigue tests on uniform-gage specimens. Surface residual stresses were minimized and axial deflection eliminated by optimization of machining parameters and post-machining electro-polishing. The steel material showed cyclic softening and decrease in yield strength. Cyclic softening, stabilized response, and the cyclic stress–strain curve were numerically simulated using finite element analysis with a model capable of describing nonlinear kinematic-isotropic hardening. The results showed good agreement with experimental values and validated the model’s ability to simulate the softening and cyclic stabilization of the material under investigation. The same numerical method was then used in elasto-plastic stress–strain analysis of notched specimens made of the same material to make fatigue life predictions. Estimated lives were compared with predictions made by analytical approximations such as the linear rule, Neuber’s rule, and the strain energy density method and verified by experimental data. Finite element analysis using stabilized cyclic response yields the most accurate predictions and, thus, provides an effective tool for the fatigue analysis of this material.

Behavior of 321 stainless steel under engineering stress and strain controlled fatigue

Cyclic deformation and fracture behavior of the 321 stainless steel are investigated at 633K under engineering-stress and strain controlled low cycle fatigue (LCF) cycling, and the fatigue life data are compared with ASME section-III Division-1 S-N curve. In stress-controlled LCF tests, ratcheting-strain accumulation is observed even under zero mean-stress, and the direction of ratcheting is found to strongly depend on initial-ramp direction of first-cycle. In addition, plastic-strain range and striation width are observed to be higher than those in strain-controlled tests. The stress-life curve obtained from stress-controlled tests is 2–3 orders of magnitude lower in comparison to stress-life curve derived from strain-controlled LCF tests as per the ASME procedure. The discrepancy is attributed to the differences in deformation and damage development in stress and strain-controlled cycling.

Multiaxial Viscoplasticity Modelling of Power Plant Steel

The thermo-mechanical fatigue (TMF) of power plant components is caused by the cyclic operation of power plant. A time-dependent plasticity model can be used to simulate the component response under cyclic thermo-mechanical loading. This paper is concerned with the modelling of thermo-mechanical behaviour of power plant steel under various loading conditions. Fully-reversed, strain-controlled tests were conducted on a parent material of P91 steel at high temperatures in order to determine material constants. A unified, Chaboche viscoplasticity model, was used to model the TMF behaviour of the steel. The multiaxial form of the Chaboche constitutive equations have been implemented in the finite element software and validated by comparing to experimental data. Simulation results have been compared with the results of anisothermal cyclic testing in order to validate the performance of the model in cyclic temperature conditions. The model’s performance under multiaxial stress conditions was investigated by testing and simulating the notched bar specimen under load-controlled conditions.

Experimental Study of Shear Behavior of Interfaces Between Crushed Sandstone-Mudstone Particle Mixture and Smooth Steel Plate

Shear behavior of interfaces between steel and soil composed of crushed sandstone and mudstone greatly influences the side resistance of RSCFST pile, a new kind of pile widely used in deep water wharf in western China. This paper presents the results of experimental studies on the shearing behavior of the soil-steel interfaces. Strain-controlled shearing tests are carried out with a direct shear apparatus under constant normal load condition. The influence of water content on the response of the interfaces is investigated. The test results reveal nonlinear relations between the shear strain and the shear stress. It indicates that the shear strength of the interfaces obey the Mohr-Coulomb criterion. The strength parameters, including cohesive force and friction angle are influenced greatly by the soil properties as well.

Assessment of the Poisson Ratio Effect on Low Cycle Fatigue (LCF) Behavior of Shear-Cracking Mode Materials

Abstract This paper evaluates the effect of different Poisson's ratios and an equivalent Poisson's ratio formula on the crack initiation angle and life estimation by using the three shear-strain based fatigue damage models (Brown–Miller, Fatemi–Socie, and Liu's Virtual Energy) on strain-controlled biaxial cyclic loading tests of specific tubular material specimens. The four shear failure mode materials studied were Inconel 718, 304L stainless steel, normalized 1050, and quenched/tempered 1050 steels. This study was motivated by the fact that for all the strain-controlled fatigue testing, the induced transverse strain was not measured, but assumed to be a negative product of the axial strain and the Poisson ratio, where the axial strain was known and given, and the Poisson ratio was either assumed to be a constant or a variable represented by a function of elastic and plastic equivalent strains. Therefore, the assumption of a Poisson's ratio in the transverse strain calculation was assessed in this study. It is concluded that the use of different Poisson's ratios in the three fatigue damage models would have some effects on the crack angle prediction, depending on the material type, and little effect on the fatigue life estimation for all materials investigated in this study.

Cyclic Behavior and Liquefaction Potential of Renusagar Pond Ash Reinforced with Geotextiles

AbstractIn this study, cyclic behavior and dynamic properties of Renusagar pond ash samples reinforced with geotextiles have been studied. Pond ash samples were collected from the upstream side of the pond ash embankment. Laboratory tests have been carried out to obtain the material properties of the pond ash. Strain-controlled cyclic triaxial tests have been carried out on reconstituted pond ash samples with and without geotextiles, at varying confining pressures (70, 100, and 150 kPa), loading frequencies (1 and 0.5 Hz), and cyclic shear-strain amplitudes (0.5, 0.8, and 1.05%). The woven geotextiles were used as reinforcing materials, which were placed in the pond ash samples in one and three layers. Results show that the pond ash samples reinforced with geotextiles exhibit better friction angle, drainage properties, and liquefaction resistance. It may be concluded that pond ash reinforced with geotextiles can be used effectively as embankment construction material.

Role of loading direction on cyclic behaviour characteristics of AM30 extrusion and its fatigue damage modelling

Anisotropic fatigue and cyclic behaviour of AM30 Mg alloy extrusion is investigated by performing fully-reversed strain-controlled tension-compression cyclic tests at strain amplitudes between 0.3% and 2.3%, along extrusion (ED) and transverse (TD) directions. The shapes of half-life hysteresis loops suggest the predominance of slip and twinning/de-twinning mechanisms below and above the strain amplitude of 0.5%, respectively. The twinning/de-twinning occurrence is found to be more extensive during straining along ED, which results in higher asymmetry of hysteresis loops, and thereby, higher induced mean stress. This adversely affects the fatigue resistance and yields to less number of cycles before failure in ED. Optical microscopy and texture analysis are employed to validate the findings. In addition, fracture surfaces are studied by scanning electron microscopy to identify the sources of fatigue crack initiation. Persistent slip bands (PSBs) and twin lamellae interfaces are evidenced as crack initiation sites at low and high strain amplitudes, respectively. Cracks emanated from debonded inclusion interface are also observed. Lastly, estimated fatigue life by Smith-Watson-Topper (SWT) and Jahed-Varvani (JV) fatigue models are compared with experimental life obtained through this study as well as the ones reported in the literature. The JV energy model is proven to yield better life predictions.

High-Temperature Low-Cycle Fatigue Behavior of MarBN at 600 °C

This paper presents the high-temperature low-cycle fatigue (HTLCF) behavior of a precipitate strengthened 9Cr martensitic steel, MarBN, designed to provide enhanced creep strength and precipitate stability at high temperature. The strain-controlled test program addresses the cyclic effects of strain-rate and strain-range at 600 °C, as well as tensile stress-relaxation response. A recently developed unified cyclic viscoplastic material model is implemented to characterize the complex cyclic and relaxation plasticity response, including cyclic softening and kinematic hardening effects. The measured response is compared to that of P91 steel, a current power plant material, and shows enhanced cyclic strength relative to P91.

Dynamic Behaviour of MSW Materials Under Cyclic Triaxial Testing: A Case of Kahrizak Landfill, Tehran, Iran

Kahrizak landfill could be called the largest dumpsite not only in the Tehran province but also in the country. In some parts of this dumpsite, there are piles of municipal solid waste (MSW), with considerable height and very steep slopes, the stability of which is a constant matter of concern not only in static loading condition but also under dynamic loading. As this region is subjected to a high seismic activity, the dynamic properties of the dumped MSW would be necessary to perform stress–strain analysis and estimate the deformation in the body of a landfill cell. The estimation of deformations in a cell mainly at the crown of landfills is vital, as different types of facilities like gas drainage and leachate re-circulation systems are installed in landfills. This paper presents the results of some strain-controlled cyclic triaxial test on fresh MSW materials with a diameter of 100 mm to estimate the shear stiffness and damping of these materials. To normalize the estimated shear stiffness from laboratory tests, the measured G max from field continuous surface wave system was employed. The results showed that the damping ratio for MSW materials is considerably higher when soils. The results of this study showed that in the range of applied shear strains, G-reduction curve has a good agreement with the graph proposed in the literature; however, variation of damping ratio by shear strain seems to be lower than values proposed by these researchers.

Fatigue Assessment of Cement-Treated Base for Roads: An Examination of Beam-Fatigue Tests

AbstractThis paper presents an examination of the fatigue characteristics of cement-treated base (CTB), a cement-stabilized material for use in road pavement construction based on CTB beam test specimens prepared and tested under laboratory conditions. The beam fatigue protocol for asphalt concrete, following the Austroads standard test method of AG:PT/T233, were adapted to examine the fatigue characteristics of the CTB test specimens. Standard road base (Granite/Diorite) from the local quarry in Western Australia was employed as the parent material of CTB. The beam-fatigue tests were conducted under the strain-controlled (constant strain) and stress-controlled (constant stress) testing conditions with varying cement contents (3–10%) of CTB test specimens. Two different loading waveforms of sinusoidal and haversine load curves were also investigated. The results show that for the strain-controlled testing condition, the fatigue characteristics of the CTB test specimens were not affected by the difference ...

Characterisation of head-hardened rail steel in terms of cyclic plasticity response and microstructure for improved material modelling

Stress- and strain-controlled tests of heat treated high-strength rail steel (Australian Standard AS1085.1) have been performed in order to improve the characterisation of the said material׳s ratcheting and fatigue wear behaviour. The hardness of the rail head material has also been studied and it has been found that hardness reduces considerably below four-millimetres from the rail top surface. Historically, researchers have used test coupons with circular cross-sections to conduct cyclic load tests. Such test coupons, typically five-millimetres in gauge diameter and ten‐millimetres in grip diameter, are usually taken from the rail head sample. When there is considerable variation of material properties over the cross-section it becomes likely that localised properties of the rail material will be missed. In another case from the literature, disks 47mm in diameter for a twin-disk rolling contact test machine were obtained directly from the rail sample and used to validate ratcheting and rolling contact fatigue wear models. The question arises: How accurate are such tests, especially when large material property gradients exist? In this research paper, the effects of rail sampling location on the ratcheting behaviour of AS1085.1 rail steel were investigated using rectangular-shaped specimens obtained at four different depths to observe their respective cyclic plasticity behaviour. The microstructural features of the test coupons were also analysed, especially the pearlite inter-lamellar spacing which showed strong correlation with both hardness and cyclic plasticity behaviour of the material. This work ultimately provides new data and testing methodology to aid the selection of valid parameters for material constitutive models to better understand rail surface ratcheting and wear.

Comparison of 15Mo3 Strain Curves Obtained for Strain-Controlled Cyclic Bending and Tension-Compression Tests

The paper presents a comparison of the fatigue life curves based on test of 15Mo3 steel under cyclic, pendulum bending and tension-compression. These studies were analyzed in terms of a large and small number of cycles where strain amplitude is dependent on the fatigue life. It has been shown that commonly used Manson-Coffin-Basquin model cannot be used for tests under cyclic bending due to the impossibility of separating elastic and plastic strains. For this purpose, some well-known models of Langer and Kandil and one new model of authors, where strain amplitude is dependent on the number of cycles, were proposed. Comparing the results of bending with tension-compression it was shown that for smaller strain amplitudes the fatigue life for both test methods were similar, for higher strain amplitudes fatigue life for bending tests was greater than for tension-compression.

Seismic ground response at Lotung: Hysteretic elasto-plastic-based 3D analyses

This paper presents a non-linear finite element study to back-interpret the free field seismic response recorded at the Lotung Large-Scale Seismic Test site. The study is carried out in the time domain by the Finite Element (FE) code PLAXIS 3D, considering the vertical wave propagation of both the horizontal components of motion. The non-linear soil behaviour is simulated through a constitutive model, the Hardening Soil model with Small-Strain Stiffness (HSsmall), capable of describing the cyclic response of the material at different strain levels. In the paper, the constitutive response of the HSsmall model is firstly investigated through numerical simulations of strain-controlled cyclic shear tests under single and multi-directional conditions at low strain levels. Then, it is adopted to back-analyse the recorded free field seismic response, comparing the FE numerical results to the in-situ down-hole and surface signals recorded during two earthquakes occurred on May 20th and July 17th 1986, characterized by different peak ground accelerations.

Approach for Estimating Seismic Compression Using Site Response Analyses

AbstractSeismic compression is the accrual of volumetric strains in unsaturated soils caused by cyclic loading and has caused significant damages to buildings and other structures during earthquakes. To date, the available methods for predicting the severity of seismic compression have mainly been simplified procedures, in which a number of equivalent cycles are used to represent the duration of earthquake loading. Often, however, the number of equivalent cycles is computed inconsistently with the underlying mechanics of seismic compression. This paper proposes a non-simplified procedure for predicting the severity of seismic compression. The procedure is based on a modified version of the Richart-Newmark cumulative damage hypothesis, wherein volumetric strain is used as the damage metric. The proposed model was calibrated using data from 425 constant-amplitude sinusoidal strain-controlled cyclic simple shear tests performed on clean sand and validated using test data from samples subjected to variable-am...

Experimental observations on rate-dependent cyclic deformation of super-elastic NiTi shape memory alloy

Based on the strain-controlled cyclic tension-unloading tests at various strain rates (3.3×10−4–3.3×10−2/s), the effect of strain rate on the uniaxial cyclic deformation of a super-elastic NiTi shape memory alloy (SMA) was investigated. It is concluded that apparent degeneration of super-elasticity occurs during the cyclic loading, i.e., the residual strain and transformation hardening increase, but the start stress of forward transformation and the maximum responding stress decrease with the increasing number of cycles and strain rate, and finally reach to their saturated states after certain cycles. Although the dissipation energy per cycle decreases progressively during the cyclic loading and is saturated after certain cycles, it does not change monotonically with the increasing strain rate. Moreover, the temperature oscillation is observed due to the internal heat production from the inelastic dissipation and transformation latent heat, and the extent of temperature variation increases monotonically with the increasing strain rate, which is the physical nature of rate-dependent cyclic deformation of the NiTi SMA.