Strain-controlled Tests Research Articles

High-temperature low-cycle fatigue behavior of a 9Cr-ODS steel: Part 1 - pure fatigue, microstructure evolution and damage characteristics

Low cycle fatigue (LCF) behavior of a tempered martensitic Fe-9%Cr-based oxide dispersion strengthened (ODS) steel at elevated temperatures is correlated to the microstructural evolution after cyclic straining. The fully reversed strain-controlled tests were conducted in air at 550°C and 650°C for different strain amplitude values ranging from ± 0.4% to ± 0.9%. Apart from the higher cyclic stress levels, the steel manifests complex cyclic softening which is significantly lower in comparison to that observed for similar non-ODS steels. This is due to the fact that highly stable nano-oxide particles act as persistent barriers for dislocation motion which as a consequence slows down the typical microstructural evolution realized in the conventional non-ODS steels. Upon cycling at 550°C, microstructure exhibits only minor changes. The main modifications are in respect to the dislocations rearrangement and/or annihilation which finally result in their reduced density. At 650°C, microstructural evolution hastens and becomes prominent mainly in nano-oxides/carbides deficient regions. Here, in addition to the reduced dislocation density, partially eliminated original sub-grain structures, grain growth, M23C6 carbides coarsening and Cr-W enriched Laves phase precipitation were evident. The microstructural modifications, concerning dislocation density and sub-grain structures, intensify even further with increase in applied strain amplitude. Nevertheless, annealing at 650°C for similar duration has no major influence on microstructure. Damage studies revealed expeditious as well as pronounced damage with increase in applied strain amplitude. Cracks initiation, eventuates at the early stage of the test, and their propagation were further assisted by oxidation. The stable crack growth region manifests secondary cracks, and at higher magnification the classical fine-scale transgranular ductile fatigue fracture features called striations. In addition, at 650°C, crack path also acquires an intergranular tendency under higher strain amplitude.

Strain energy based low cycle fatigue damage analysis in a plain C-Mn rail steel

Strain-controlled low cycle fatigue tests were performed on 60kg/m 90-UTS plain C-Mn rail steel with fully pearlitic microstructure. Analysis of hysteresis loop shape and properties revealed that the steel deviated from the ideal Masing type material behaviour to a little extent. To evaluate the fatigue damage in rail steel, energy-based analytical approach proposed by Morrow based on Masing hypothesis was employed. The Morrow energy model resulted in reasonably good approximation of both average plastic strain energy and fatigue life in comparison to experimentally observed values. In addition to that the plastic strain energy density (ΔWp) was correlated to fatigue life (2Nf) through a simple power law and very well represented by Coffin-Mansion type relationship similar to conventional strain-life (Δεp – 2Nf) relationship. Finally, the concept of fatigue toughness, an energy parameter, was introduced to life prediction in rail steel which was proved to be a suitable and reliable alternative to strain-life approach in fatigue damage evaluation with high degree of accuracy.

Effect of nonplastic silt content on undrained shear strength of sand\u2013silt mixtures

To assess the behavior of sand–silt mixtures, strain-controlled monotonic triaxial tests were conducted on sand–silt mixtures of specimen size 71 mm in diameter and 142 mm in height at various relative densities but same isotropic effective confining pressure of 100 kPa. Concept of limiting fines content (LFC) was verified by these undrained monotonic triaxial tests. LFC was found to be the very important parameter to understand the behavior of sand–silt mixtures. The behavior of sand–silt mixture changes approximately at LFC. At constant relative density, increase in silt content decreases the undrained peak shear strength till LFC. After LFC the strength becomes near about same till pure silt sample. The reason of behavior could not be explained. At constant global void ratio, the peak shear strength decreases with increase in silt content till LFC and for further increment of silt content the peak shear strength increases. Sand–silt mixtures containing certain amount of silt which is near to the LFC showed flow type as well as brittle behavior. Failure of structure on this type of soil will be catastrophic during earthquake. In the case of permeability decreased with increasing silt content up to LFC. After the LFC, dry density is decreasing with increasing silt content but permeability remains constant till pure silt.

Fatigue assessment for seismic loads considering material degradation due to stress corrosion cracking

This study was aimed at showing a fatigue damage assessment procedure for seismic loads considering material degradation, specifically stress corrosion cracking (SCC), caused during long term operation of nuclear power plants. To superpose the damage due to cyclic loading and SCC, crack growth analysis was applied to the fatigue damage assessment, although this assessment is conventionally performed using the usage factor. To show applicability of the crack growth prediction to seismic loads, fully-reversed stress- or strain-controlled crack growth tests were conducted using plate specimens made of Type 316 stainless steel. A relatively large load amplitude was applied to simulate a fast growth rate, which was 0.32mm/cycle for the maximum rate case. It was shown that obtained crack growth rates correlated well with the strain intensity factor range. Furthermore, the effect of crack closure was corrected by using the effective strain intensity factor range. Fatigue life estimated by the crack growth prediction with an initial depth of 50μm correlated well with the fatigue life obtained by low-cycle and extremely low-cycle fatigue tests, for which the maximum strain range was 12%. Finally, a fatigue damage assessment procedure considering the effect of SCC was presented, in which the initial crack depth was determined by SCC initiation and growth predictions.

Cyclic plasticity and microstructure of as-built SLM Ti-6Al-4V: The effect of build orientation

This paper investigates the cyclic elastoplastic anisotropy of Ti-6Al-4V manufactured via selective laser melting, caused by different build orientations. Tensile monotonic and cyclic stepped symmetric strain-controlled tests on coupons manufactured at 0°, 45°, and 90° build orientations were performed for both the SLM material and for a mill annealed material. The microstructure characteristics of the SLM material were examined through optical and electron microscopy revealing a unique α' martensite microstructure. The examination of the evolving tensile and compressive maximum stresses identified an interesting phenomenon, that of asymmetric cyclic softening. This phenomenon was observed only in the SLM Ti-6Al-4V, while its wrought counterpart confirmed the findings of past research reported in the literature. The residual stresses presented in the SLM coupons had a significant influence on the cyclic behaviour of the material. Mechanical anisotropy in both monotonic and cyclic tests was noticed with the diagonal (45°) coupon having the largest yield stress in both loading conditions. The findings of this research study can be very useful in engineering applications utilising as-built SLM materials.

Anisotropic in-plane fatigue behavior of rolled magnesium alloy with {10−12} twins

The effects of initial {10−12} twins on the in-plane fatigue characteristics of a rolled Mg alloy were investigated by introducing these twins through precompression along the rolling direction of the material and then performing strain-controlled low-cycle fatigue tests along the rolling direction, 45° to the rolling direction, and transverse direction. The as-rolled sample with a strong basal texture was found to exhibit the same fatigue behavior in all the loading directions, whereas the precompressed samples exhibited anisotropic in-plane fatigue behaviors because the specific crystallographic orientation of the twinned region resulted in a variation in the active deformation modes during cyclic loading in the applied loading directions. An increase in the amount of precompression was also found to enhance the anisotropy of the in-plane fatigue behavior via an increase in the area fraction of the twinned region. The variation in the dominant deformation mechanism with a change in the loading direction was investigated by analyzing the crystallographic angle relationship and activation stresses, and the resultant evolutions of the cyclic flow curve and fatigue life were discussed in detail.

Influence of heat treatment on cyclic deformation and low-cycle fatigue behavior of sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy

Abstract

Evaluation of dynamic properties of sandy soil at high cyclic strains

Dynamic loading conditions, such as earthquakes, may result in the generation of high shear strain (>5%) in the soil. Conventionally, dynamic properties of soils are estimated from the tests conducted up to a shear strain of 1% by considering Symmetrical Hysteresis Loop (SHL). However, it is commonly observed that the hysteresis loops become progressively asymmetric with increasing shear strain, which leads to the over- or under-estimation of the conventionally evaluated dynamic properties. Hence, it is necessary to adopt a modified methodology of evaluating the dynamic properties of saturated sands based on the actual Asymmetrical Hysteresis Loop (ASHL). Strain-controlled cyclic triaxial tests have been conducted, for a peak shear strain range of 0.015–4.5% at 1Hz loading frequency, on test specimens prepared at different relative density (30–90%) and confining stress (50–150kPa). Although, the shear modulus evaluated considering SHL and ASHL are on close agreement, the damping ratio evaluated considering SHL is approximately 40–70% lesser than that obtained by considering ASHL. Moreover, in contrast to the classical curves as largely applied in geotechnical engineering, a noticeable decrement of the damping ratio is observed beyond 0.75% shear strain.

Effect of organic content and frequency on degradation and pore pressure in marine organic soils

ABSTRACTThe effects of organic content and the frequency of cyclic shear straining on the cyclic degradation and the normalized pore water pressure in 27 fibrous marine organic soil specimens were investigated with the two-way cyclic strain-controlled dynamic triaxial testing system. The testing program included three cyclic shear strain amplitudes, γc = 0.25, 0.5, and 1%, three ranges of the organic contents, OC = 13.3–18.4%, 23.3–27.9%, and 36.9–45.3%, and three loading frequencies, f = 0.05, 0.25, and 1 Hz. The results clearly exhibit that the degradation parameter, t, which means the rate of the cyclic degradation with the number of loading cycles, N, decreases with the OC and increases with the f. At the γc = 0.25–1%, if the OC decreases from 40.8 to 13.3%, the degradation parameter t may increase by 82.9–221.1%. If f is increased 20 times, from 0.05 to 1 Hz, then t may increases by 34.7–117.9%. The cyclic pore water pressure normalized by the effective consolidation pressure, , increases with the OC and decreases with the f was also observed.

Ratcheting Behavior of SA508-3 Steel at Elevated Temperature: Experimental Observation and Simulation

A series of monotonic uniaxial tensile tests, strain-controlled and stress-controlled cyclic tests of SA508-3 steel were conducted from 25 to 350 °C. Results showed that the steel exhibited temperature-dependent cyclic softening characteristic and obvious ratcheting behavior, and dynamic strain aging was observed in the range of 250–350 °C. Based on experimental observations, a temperature-dependent cyclic plastic constitutive model was proposed by introducing the nonlinear cyclic softening and kinematic hardening rules, and the dynamic strain aging was also considered into the constitutive model. Comparisons between experiments and simulations were carried out to validate the proposed model at elevated temperature.

Multiaxial low cycle fatigue life of Ti-6Al-4V under non-proportional loading with mean strain

The mean strain effect on the multiaxial low cycle fatigue behavior of Ti-6Al-4V was investigated. Strain-controlled multiaxial low cycle fatigue tests at room temperature were carried out using a tubular specimen in tension and torsion loadings. Strain paths employed were four types of proportional and non-proportional loadings. Failure lives are affected by not only the non-proportionality of stress and strain histories but also the mean stress and strain. This study discusses the failure life properties based on the test results obtained and also shows strain parameters for the life evaluation under non-proportional loading with mean strain. The proposed strain parameters, where Findley and Walker models were applied within a non-proportional strain range, were suitable parameters to evaluate the life for Ti-6Al-4V under non-proportional straining with mean strain.

Viscoelastic behaviour of hydrogel-based composites for tissue engineering under mechanical load

Along with biocompatibility, bioinductivity and appropriate biodegradation, mechanical properties are also of crucial importance for tissue engineering scaffolds. Hydrogels, such as gellan gum (GG), are usually soft materials, which may benefit from the incorporation of inorganic particles, e.g. bioactive glass, not only due to the acquired bioactivity, but also due to improved mechanical properties. They exhibit complex viscoelastic properties, which can be evaluated in various ways. In this work, to reliably evaluate the effect of the bioactive glass (BAG) addition on viscoelastic properties of the composite hydrogel, we employed and compared the three most commonly used techniques, analyzing their advantages and limitations: monotonic uniaxial unconfined compression, small amplitude oscillatory shear (SAOS) rheology and dynamic mechanical analysis (DMA). Creep and small amplitude dynamic strain-controlled tests in DMA are suggested as the best ways for the characterization of mechanical properties of hydrogel composites, whereas the SAOS rheology is more useful for studying the hydrogel’s processing kinetics, as it does not induce volumetric changes even at very high strains. Overall, the results confirmed a beneficial effect of BAG (nano)particles on the elastic modulus of the GG–BAG composite hydrogel. The Young’s modulus of 6.6 ± 0.8 kPa for the GG hydrogel increased by two orders of magnitude after the addition of 2 wt.% BAG particles (500–800 kPa).

Low-cycle fatigue behavior of fiber-laser welded, corrosion-resistant, high-strength low alloy sheet steel

Advanced laser welding techniques such as fiber laser welding are desirable for joining high-strength steels in ground vehicle applications since they can greatly increase productivity. Weld fatigue is a primary design consideration for implementation of laser-welded sheet into ground vehicle structures. However, much of the previous fatigue work on fiber laser welded sheet steel has utilized stress-controlled fatigue testing. Therefore, this paper presents an optimized fiber laser welding procedure for butt welds of high-strength low-alloy steel sheet, and subsequent fully-reversed, strain-controlled low-cycle fatigue testing on weld material. Existing low cycle fatigue models adequately describe fatigue behavior of the laser welds. The results show that weld low cycle fatigue strength is significantly increased relative to base metal due to formation of martensite in the weld caused by rapid cooling inherent to laser welding. Weld fatigue life was shorter than base metal when plastic strains were high, but was close to base metal at lower plastic strains. Fatigue specimens containing weld failed at both base metal and in the weld indicating that there is some competition between weld cyclic fatigue strength and stress concentration effect at the weld fusion zone.

Fatigue crack growth in two TWIP steels with different stacking fault energies

Fatigue crack growth tests with R=0 and 0.4 were carried out on Fe-22Mn-0.6C and Fe-22Mn-0.6C-3Al (wt.%) Twinning-Induced Plasticity (TWIP) steels with stacking fault energies around 21.5 and 37mJ/m2, respectively. The former exhibits more crack closure effects, partly due to stronger asperity-induced closure. Strain-controlled push-pull tests followed by scanning electron microscope observations show that both steels are prone to mechanical twinning under low-cycle fatigue, associated with an increasing kinematic hardening. Twinning is however more profuse in Fe-22Mn-0.6C steel. Elastic-plastic finite elements simulations of crack growth, using specific constitutive equations able to capture the increasing kinematic hardening suggest that plasticity-induced crack closure is lower in Fe-22Mn-0.6C steel. Even after closure corrections, the Al-free steel, exhibits a lower resistance to fatigue crack growth, which is attributed to a pronounced strain localisation at the crack tip, and maybe also to environment effects.

Application of a new distortional yield surface model in cyclic uniaxial and multiaxial loading

This paper is concerned with the ratcheting prediction for steels. The yield surface evolution is integrated in the elastoplasticity analysis by a new proposed model. First, the new proposed model of Distortional Yield Surface (DYS) is developed and then ratcheting under different uniaxial and multiaxial cyclic loadings is investigated. The capabilities of the DYS model is assessed by using the results of various stress-controlled and strain-controlled tests under uniaxial loading condition and the cyclic uniaxial experimental results of Hassan and Kyriakides (1994a). In addition, the model prediction under multi-steps uniaxial loadings is also studied. Effects of DYS consideration in different uniaxial loadings are shown. Strain or stress controlled tests and combined stress-strain control experimental tests (Hassan and Kyriakides, 1994b) are used to be modeled by the present distortional yield surface approach. The biaxial experiments on CS 1018 thin-walled tubes (Hassan and Kyriakides, 1994b) are chosen to be compared with the present model results. With the incorporation of DYS, The new proposed model shows better prediction of uniaxial and multiaxial ratchetting in all ranges of experimentally tested parameters.

Effect of Grain Size on Low Cycle Fatigue Life in Compressor Disc Superalloy GH4169 at 600 °C

Nickel-based superalloy GH4169 in Chinese series (similar to Inconel 718 in the U.S. and NC19FeNb in France) has been widely used for aero-engine discs due to its excellent fatigue, oxidation and corrosion resistance at high temperature. The effect of grain size on the low cycle fatigue (LCF) lifetime was investigated in this study. Smooth specimens cut from an actual compressor disc in a civil aero-engine were conducted to strain-controlled LCF tests at 600 °C, and results showed a significant scattering in the LCF lifetime. Then grain sizes of specimens were estimated using optical microscope (OM) and Image-Pro Plus (IPP) software. A negative correlation between the grain size and LCF lifetime was found by inducing the grain refinement strengthening theory. Afterwards, a modified Smith-Watson-Topper (SWT) model was developed to describe the dependence of LCF lifetime on grain size. Finally, the possible microscopic mechanism of the grain size-dependent LCF behavior of GH4169 superalloy was discussed based on fracture analysis.

Constitutive modelling of a T74 multi-step creep ageing behaviour of AA7050 and its application to stress relaxation ageing in age formed aluminium components

Abstract This work focuses on validating a model, which is defined using load controlled creep ageing data, from strain-controlled stress relaxation ageing tests. A set of phenomenological constitutive equations were proposed. The material constants within the equations were determined from multi-step (120 °C× 6 h +177 °C ×7 h) creep-ageing test data. Temperature effects on the internal state variables (i.e. precipitate size, volume fraction and dislocation density) were also considered. This model was further used to predict the stress relaxation phenomena under the same ageing condition. Although similar stress reduction trends and features were observed, the stress reduction was significantly over-predicted. The over-prediction of the stress reduction magnitude suggests that improvements should be applied to the current modelling method for simulating spring-back after creep age forming.

Adaptation of Broadband Simple Shear Device for Constant Volume and Stress-Controlled Testing

Abstract We adapted an existing digitally controlled simple shear device, originally designed for drained testing with strain-control, to perform constant height testing under strain- or stress-controlled conditions. The re-designed system provided PID control of horizontal displacements or loads coupled with either vertical force or displacement control (for drained and constant-volume testing, respectively). The new system had substantially similar horizontal displacement control capabilities as the previous system. The tracking error under stress-controlled loading depends on amplitude and frequency of the load; however, its value was almost the same as errors in strain-controlled tests for common shear strain/stress amplitudes. High-precision vertical control was achieved during constant height testing in two respects: (1) height change was small (less than 0.05 % of the specimen height for large shear-strain tests), even at high frequencies; and (2) top cap rocking was small, with the vertical displacements due to rocking at the specimen edge being half or less than average changes in specimen height.

On the application of polar representation for investigating high and low cycle fatigue of metals

Stress and strain analyses on eight materials tested under multiaxial stress- and strain-controlled conditions are performed. Stress and/or strain fields are generated using plane stress-strain transformation. Maximum stress and strain loci are plotted in polar diagrams. For low cycle fatigue tests, normal stress and shear strain are normalized with their yield counterparts to allow plotting them in one polar diagram. The roles of shear strain and normal stress in fatigue can be examined using such a representation. For high cycle fatigue, normal and shear stresses are plotted in a single polar diagram. Observed crack planes are superimposed on all polar diagrams. In general, observed cracks are found to be aligned with the plane of both maximum shear strain and normal stress. Polar representations of Fatemi-Socie fatigue damage for strain-controlled tests show that significant numbers of planes experience high damage value especially for 90° out-of-phase loading.

Identification of Fatigue Constants by Means of 3D Method

Abstract The conventional method for evaluation of the fatigue constants uses one set of experimental data from strain-controlled uniaxial fatigue tests. However, these constants do not ensure the compatibility conditions. The new 3D method retains the mathematical and physical relationships between curves considered. This paper presents a way of implementation of the identification procedure and shows results obtained for three types of materials.