Axial Strain Control Research Articles

Rock fractures developed in the Class II post-peak unloading

In this study, post-peak tests were conducted on sandstone specimens subjected to uniaxial compression under two different loading conditions: axial strain control and lateral strain control. The fractures developed in both loading regimes were investigated. Under axial strain control, Class I post-peak stress–strain curves (with negative slope) were obtained until the complete failure of the specimens. The observed macroscopic fractures have mixed orientations: sub-parallel and inclined to the axial loading direction. Under lateral strain control, the specimens were tested until different residual post-peak stresses; the tests yielded consistent Class II post-peak stress–strain curves (with positive slope). Interestingly, the tested specimens were found to be relatively intact, with only non-penetrating fractures observed on the lateral surfaces. X-ray CT scanning revealed that the subvertical macroscopic fractures were developed close to the lateral surfaces of the specimens in the post-peak stages, while the internal central parts remained visually undamaged. Analysis of longitudinal and shear wave velocities measured in the axial direction of sandstone specimens before and after post-peak tests under lateral strain control provided evidence of internal fracture (crack) development parallel to the loading axis in the internal central parts of the tested specimens. Concentration of these small fractures (cracks) is high, but it only marginally increases as the loading proceeds in post-peak region from 90% to 70% of the peak stress. This suggests that the Class II post-peak regime obtained under lateral strain control cannot be explained by pure unloading; also, the Class II post-peak unloading is mainly produced by the development of the large subvertical tensile fractures identified in the CT scans, rather than by the growth of these small fractures (cracks).

Mechanical properties of rock under uniaxial compression tests of different control modes and loading rates

To study the influence of control mode and loading rate on mechanical property of rock, uniaxial compression tests of four types of rocks (gray sandstone, red sandstone, mudstone and granite) are carried out under axial strain control mode and lateral strain control mode respectively. The characteristics of complete stress and strain curves, strength, brittleness and failure modes are analyzed. The results show that control mode has little influence on the pre-peak deformation, stress thresholds, while it has a greater impact on post-peak stress and strain curve, which makes the post-peak deformation stable and controllable, and shows the feature of Class II behavior. With lateral loading rates decrease, post-peak stress and strain curves appear more and more obvious fluctuations in the post-peak stage, and the time required for rock failure increases sharply, but the lateral control rate has little effect on the brittleness of rock. The failure mode of rock samples under axial strain control mode is mainly splitting failure, while that under lateral strain control is gradually changed to shear failure. The smaller the lateral loading control rate is, the more obvious the characteristics of shear failure is. Compared with uniaxial compression tests, under high confining pressure, the lateral dilation deformation is restricted, so peak strength is larger and stress redistribution can be better adjusted and stress fluctuation reduced accordingly in post-peak stage. The research results are an effective supplement to rate-dependent property of rocks and can provide some reference for deformation and strength characteristics research of brittle rock under lateral control mode.

Cycle life prediction under low cycle fatigue using nonlinear Marco - Starkey model

The process of deformation and fracture of structural alloys under low-cycle fatigue in conditions of uniaxial loading with axial strain control under complex cycle shape and block loading has been studied. The obtained results of experimental studies of structural alloys were used to assess the possibility of using the nonlinear Marco - Starkey damage accumulation model. The processing of the nickel alloy cyclic tests results with a simple and complex form of the cycle has been carried out. A combination of exponents included in the non-linear Marco - Starkey model was selected which for the case of low-cycle fatigue with a complex M-shaped cycle made it possible to predict durability which is in good agreement with experimental test data. The main problem of using the nonlinear Marco - Starkey model for predicting cyclic durability with an M-shaped cycle is the presence of many combinations of exponents that allow predicting cyclic durability with the same accuracy. To determine the uniqueness of the solution it is proposed to carry out a set of tests under simple and block loading. Experimental results have been obtained on the processes of deformation, fracture of D16T aluminum alloy under low-cycle fatigue conditions under simple cycle forms with constant parameters, and block loading with variable cycle parameters in tests for uniaxial loading with axial strain control. Based on the obtained experimental results according to the new method combinations of the m degrees coefficients were selected, which was carried out in the range of exponents from 0.2 to 10 with a step of 0.2. Comparison of the forecasting results for blocks consisting of three groups made it possible to find several combinations of general exponents m for two groups that are present in all blocks. The final choice of a pair of coefficients was carried out on the condition that the predicted damage is close to unity in the first (test) block. The selected values of the coefficients made it possible to predict the durability under block low-cycle loading using the nonlinear Marco - Starkey model.

Utilization of motion capture systems for low cycle fatigue tests on induction-hardened steel

Motion capture systems were used to control the low cycle fatigue test of heated and unheated specimens obtained from induction-hardened curved steel braces. The two motion capture systems used in this study demonstrated that the axial-strain control was achieved, and the strain distributions were visualized using digital image correlation. Additionally, the test results demonstrated that the fatigue lives of the unheated and heated specimens can be predicted using Manson's universal slopes method. However, the obtained fatigue lives were slightly shorter. Induction hardening proved to be an effective method to increase the fatigue life of specimens subjected to small strain.

Effect of surface nanostructuring in solution treated and thermally aged condition on LCF life of AA7075

Low cycle fatigue (LCF) behavior of the AA7075 was studied in the solution treated-ultrasonic shot peened and the peak aged (ST-USSP-PA) condition and was compared with that of the peak aged and ultrasonic shot peened (PA-USSP) as well the peak aged-ultrasonic shot peened and stress relieved (PA-USSP-SR) condition, studied earlier. LCF tests were carried out under fully reversed axial strain control at different total strain amplitudes (Δεt/2) from ±0.38% to ±0.60%. LCF life was highest in the ST-USSP-PA followed by the PA-USSP and PA-USSP-SR conditions. It was found that nanostructure of comparable size (~22 nm) was developed up to a depth of ~50 μm in all the three conditions, however, there was much variation in the type, size, distribution and number density of the metastable of η precipitates in the three conditions. While the size of the strengthening precipitates η′ was ~20 nm in the ST-USSP-PA, it was much smaller (~10 nm) in the other two conditions. High resolution TEM examination revealed high density of dislocations in the matrix and also within the equilibrium η precipitates in the PA-USSP condition whereas dislocations were not observed in η precipitates of the PA-USSP-SR and ST-USSP-PA conditions. Highest fatigue life in the ST-USSP-PA condition is attributed to more homogeneous distribution, larger size and higher volume fraction of the strengthening precipitates η′ and consequent increase in the resistance against fatigue crack initiation.

Low-cycle fatigue crack initiation and propagation from controlled surface imperfections in nuclear steels

The impact of a controlled surface imperfection on the low-cycle fatigue life in two different steels is investigated. After introduction of a single imperfection with a depth varying from 50 to 350 µm in cylindrical samples, fatigue tests were conducted under fully-reversed total axial strain control in air at ambient temperature. It is shown that the fatigue life is significantly reduced, even in presence of small imperfections. In addition, the use of the potential drop methods provided an assesment of the initiation life and the determination of crack growth rates. Two characteristic crack growth domains were thus identified.

Low cycle fatigue properties of 7050-T7451 aluminum alloy under different strain ratios

The low cycle fatigue properties of 7050-T7451 aluminum alloy was studied. The low cycle fatigue properties of 7050-T7451 aluminum alloy at three strain ratios R = -1, 0.02, and 0.5 were measured by axial strain control method on MTS electro-hydraulic servo testing machine. In this research, the relationship between cyclic stress and strain at strain ratio R = -1 and the average stress relaxation effect of R = -1, 0.05 were given. The equivalent strain model was used to draw the strain-life curve for low cycle fatigue life data. The results show that under different strain ratios of 7050-T7451 aluminum alloy, the same strain amplitude in the high strain region leads to approximate fatigue life, and the effect of the strain ratio is only reflected in the low strain region.

The Role of Load Control Modes in Determination of Mechanical Properties of Granite

Circumferential strain-control tests were suggested to obtain the complete stress–strain curves (SSCs) of intact rock. However, compression test performed by the axial strain-control loading is mostly used to obtain the strength parameters and the SSCs. The influences of load control modes on the mechanical behavior of rock have not been fully investigated and understood. In this study, triaxial compression tests are conducted on granite specimens loaded by using both axial strain-control and circumferential strain-control modes to examine influences of load control modes on the determined mechanical properties. The occurrence mechanism and influencing factors of class II behavior are studied, and the potential application of the class II SSC in the evaluation of brittle failure of hard rock is also discussed. Results show that the peak strength, elastic modulus and fracture angle are generally higher in the axial strain-control test than in the circumferential strain-control test, and the failure in the former is also more violent than the latter under the same confining pressure. The much lower and constant circumferential deformation rate applied in the circumferential strain-control test is the dominant factor that favors the recovery of the complete post-peak SSC. The lower measured values of strength and elastic modulus are also associated with the lower deformation rate in the circumferential strain-control test, with a higher degree of damage and cohesion weakening occurring inside the rock. The development of the class II SSC is attributed to the axial strain recovery during the release of the stored elastic energy, which is influenced by the ratio of elastic energy to dissipated energy in the pre-peak stage, and is also associated with the consumed energy with respect to plastic deformation or initiation and propagation of new cracks in the post-peak stage. The class II SSC can aid the differentiation of the energy accumulation and consumption of different brittle rocks, which are usually characterized by a more abrupt and larger stress drop in the axial strain-control test. It can then be used to evaluate the strain rockburst potential of a particular rock type.

Initiation and propagation of fatigue cracks from surface imperfections on 304L austenitic stainless steel

Given the stringent requirements of high levels of safety in nuclear components, stakeholders of French nuclear industry must anticipate the presence of residual surface imperfections in these components. Such imperfections could be introduced during manufacturing or maintenance operations. The incidence of surface irregularities on the fatigue strength of metallic components must be considered. Meanwhile, nuclear components are generally loaded under low-cycle fatigue and large-scale plasticity conditions. The present work aims at assessing the impact of a controlled surface irregularity on the fatigue life of typical nuclear materials. The influence of characteristic parameters under low-cycle fatigue conditions is also investigated. A 304L austenitic stainless steel used in components of French nuclear power plants was studied. Fatigue tests were conducted under fully-reversed total axial strain control in air at ambient temperature with total strain amplitudes ranging from Δεt/2=0.2% to Δεt/2=0.6%. Surface irregularities, whose depth varies between 100 and 350 micrometers, have been introduced on polished cylindrical samples. The direct current potential drop method has been used to monitor the crack propagation, and thereafter, to derive crack growth rate data. Experimental markings of the crack front for different cycle numbers have been carried out to calibrate the variation of the potential as a function of crack depth. It appears that the fatigue life is strongly reduced in presence of a surface irregularity.

High-Temperature Creep-Fatigue Behavior of Alloy 617

This paper presents the high-temperature creep-fatigue testing of a Ni-based superalloy of Alloy 617 base metal and weldments at 900 °C. Creep-fatigue tests were conducted with fully reversed axial strain control at a total strain range of 0.6%, 1.2%, and 1.5%, and peak tensile hold time of 60, 180, and 300 s. The effects of different constituents on the combined creep-fatigue endurance such as hold time, strain range, and stress relaxation behavior are discussed. Under all creep-fatigue tests, weldments’ creep-fatigue life was less than base metal. In comparison with the low-cycle fatigue condition, the introduction of hold time decreased the cycle number of both base metal and weldments. Creep-fatigue lifetime in the base metal was continually decreased by increasing the tension hold time, except for weldments under longer hold time (>180 s). In all creep-fatigue tests, intergranular brittle cracks near the crack tip and thick oxide scales at the surface were formed, which were linked to the mixed-mode creep and fatigue cracks. Creep-fatigue interaction in the damage-diagram (D-Diagram) (i.e., linear damage summation) was evaluated from the experimental results. The linear damage summation was found to be suitable for the current limited test conditions, and one can enclose all the data points within the proposed scatter band.

A study of low cycle fatigue life prediction method for 1Cr11Ni2W2MoV at 360°C

The high temperature low cycle fatigue tests of 1Cr11Ni2W2MoV are conducted under total axial straincontrol. The specimens are tested under fully reversed strain. The relationships of cyclic strain-life, stress-strain and stress-life are analyzed. Manson-Coffin method and a new method based on power transformation theory are evaluated by the low cycle fatigue data. The results showed that two methods can effective predict low cycle fatigue life of 1Cr11Ni2W2MoV at 360 °C. Prediction accuracy within ± 1.65 times scatter band. The life prediction capability of new methods proves more effective and accurate than Manson-Coffin method. The new method has smaller standard deviation than Manson-Coffin method. Manson-Coffin method and new method have standard deviations of 0.32 and 0.18 respectively.

Low cycle fatigue behaviour of Grade 92 steel weld joints

In the present study, low cycle fatigue (LCF) behaviour of tungsten (W) alloyed 9Cr steel (Grade 92 steel) base metal and its weld joints has been investigated. LCF tests on both base metal and weld joint were conducted under fully reversed, total axial strain control mode employing a triangular waveform at 823K, under a constant strain rate of 3×10−3s−1 with strain amplitudes varying from ±0.25% to ±1%. Both base metal and weld joint exhibited continuous softening with the weld joint exhibiting a lower stress response than base metal. At lower strain amplitudes, weld joint exhibited lower fatigue life than that of base metal whereas at higher strain amplitude, both the materials exhibited comparable fatigue life. Weld joints failed in the base metal for all the strain amplitudes at 823K except at the strain amplitude ±0.25% where weld joint failed in inter-critical region. Effect of temperature on LCF behaviour of weld joints and base metal at various temperatures ranging from 300K to 873K under constant strain amplitude of ±0.6% and strain rate 3×10−3 has been investigated. At all the temperatures, weld joints failed in the base metal region. The material exhibited dynamic strain ageing phenomenon in the temperature range 473–673K. Oxidation was found to show more adverse effects on fatigue life beyond 673K.

Improvement of low-cycle fatigue resistance of Co-Ni alloys by silicon addition

The effect of silicon alloying on the low-cycle fatigue (LCF) resistance and fatigued microstructure of the Co–31Ni, Co–31Ni–3Si, Co–33Ni, and Co–33Ni–3Si (in wt%) alloys was studied under an axial strain control mode at the total strain amplitude, Δεt/2, of 0.01. The addition of 3wt% of Si increases the fatigue life, Nf, of the examined Co alloys by more than 2.5 times. For instance, Nf of ~ 4000 and 10,000 cycles were achieved in the Co–31Ni and Co–31Ni–3Si alloys, respectively. It was observed that improved LCF resistance of the Si added Co alloys results from the moderate kinetics of the deformation-induced ε-martensitic transformation (ε-MT) and inhibited cross-slip of perfect dislocations provided by solid solution hardening of the austenite. The similarity in the fatigue behavior and fatigued microstructure of the Si added Co alloys and Fe-based high-Mn alloys is discussed.

Effect of strain amplitude on the low-cycle fatigue behavior of a new Fe–15Mn–10Cr–8Ni–4Si seismic damping alloy

The low-cycle fatigue (LCF) properties and post-fatigue microstructure of a Fe–15Mn–10Cr–8Ni–4Si austenitic alloy were investigated under an axial strain control mode with total strain amplitudes, Δεt/2, ranging from 2.5×10−3 to 2×10−2. The fatigue resistance of the alloy was described by Coffin–Manson’s and Basquin’s relationships, and the corresponding fatigue parameters were evaluated. In addition, the Masing behavior, which is associated with a constant deformation mode during fatigue, was revealed at the examined strain amplitudes. Microstructural observations of the fatigue fractured samples showed that the strain induced ε-martensitic transformation accompanied by a planar slip of the Shockley partial dislocations in the austenite is the main deformation mode controlling the fatigue behavior of the studied alloy at Δεt/2<2×10−2. However, at Δεt/2=2×10−2, the formation of a cell structure was found in the austenite in addition to ε-martensitic transformation. The LCF resistance of the alloy was compared with conventional Cr–Ni austenitic stainless steels, ferrous base TRIP and TWIP steels and low yield point damping steels. It was found that at the studied strain amplitudes the alloy possessed a higher LCF resistance compared to conventional Fe-base alloys and steels. Remarkably, the fatigue ductility coefficient, εf′, of the studied alloy is 1.3–6 times higher than that of the stainless steels because of a cyclic deformation-induced ε-martensitic transformation. The results showed that the ε-martensitic transformation that occurred in the studied alloy during LCF is the main reason for the improved LCF resistance.

Fatigue Life of the Strain Hardened Austenitic Stainless Steel in Simulated Pressurized Water Reactor Primary Water

Over the last 20 years or so, many studies have revealed the deleterious effect of the environment on fatigue life of austenitic stainless steels in primary water reactor (PWR) primary water. The fatigue life correlation factor, so-called Fen, which corresponds to the ratio of fatigue life in air at room temperature to that in water under reactor operating conditions, has been standardized to consider the effect on fatigue life evaluation, and the formulations are function of strain rate and temperature due to their noticeable negative effect compared with other factors (Chopra and Shack, 2007, “Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials,” Final Report, Report No. NUREG/CR-6909, ANL-06/08; Codes for Nuclear Power Generation Facilities, 2009, "Environmental Fatigue Evaluation Method for Nuclear Power Plants," JSME S NF1-2009, The Japan Society of Mechanical Engineers, Tokyo, Japan). However, mechanism causing fatigue life reduction remains to be cleared. As one of the possible approaches to examine the underlying mechanism of environmental effect, the authors focused on the effect of plastic strain, because it could lead microstructural evolution on the material. In addition, in the case of stress corrosion cracking (SCC), it is well known that the strain-hardening prior to exposure to the primary water can lead to remarkable increase of the susceptibility to cracking (Vaillant et al., 2009, “Stress Corrosion Cracking Propagation of Cold-Worked Austenitic Stainless Steels in PWR Environment,” 14th International Conference on Environmental Degradation of Materials in Nuclear Power Systems; Couvant et al., 2009, "Development of Understanding of the Interaction Between Localized Deformation and SCC of Austenitic Stainless Steels Exposed to Primary PWR Environment," 14th International Conference on Environmental Degradation of Materials in Nuclear Power Systems). However, its effect on fatigue life has not necessarily been cleared yet. The main effort in this study addressed the effect of the prior strain-hardening on low cycle fatigue life in the primary water. A plate of 304LSS was strain hardened by cold rolling or tension prior to fatigue testing. The tests were performed under axial strain control at 300 °C in primary water including B/Li and hydrogen, and in air. The effect on environmental fatigue life was investigated through a comparison of Fen in experiments and in regulations, and also the effect on the fatigue limit defined at 106 cycles was discussed.

Experimental investigation of the effect of change in control modes on the post-failure behavior of coal and coal measures rock

This paper presents post-failure tests on rock and coal specimens obtained from various coal mines in West Virginia and Hiawatha. Berea sandstone was used for comparing post-failure behavior of coal measures rock. Two feedback-control modes: axial strain control and lateral strain control modes were used to study the post-failure behaviors of the rock and coal specimens. The results obtained from the experiments show that coal measure rocks show significant change in post-failure behavior when the control modes were changed from axial strain to lateral strain. It was found that when the lateral strain control mode was used, in the post-failure region, coal measures rock exhibited class II behavior. Also coal specimen under lateral strain control showed class II behavior. However, it was observed that when the same coal specimens are tested under confining pressure, the class II behavior is replaced by Class I behavior.

Experimental and modeling results of creep–fatigue life of Inconel 617 and Haynes 230 at 850 °C

Creep–fatigue testing of Ni-based superalloy Inconel 617 and Haynes 230 were conducted in the air at 850°C. Tests were performed with fully reversed axial strain control at a total strain range of 0.5%, 1.0% or 1.5% and hold time at maximum tensile strain for 3, 10 or 30min. In addition, two creep–fatigue life prediction methods, i.e. linear damage summation and frequency-modified tensile hysteresis energy modeling, were evaluated and compared with experimental results. Under all creep–fatigue tests, Haynes 230 performed better than Inconel 617. Compared to the low cycle fatigue life, the cycles to failure for both materials decreased under creep–fatigue test conditions. Longer hold time at maximum tensile strain would cause a further reduction in both material creep–fatigue life. The linear damage summation could predict the creep–fatigue life of Inconel 617 for limited test conditions, but considerably underestimated the creep–fatigue life of Haynes 230. In contrast, frequency-modified tensile hysteresis energy modeling showed promising creep–fatigue life prediction results for both materials.

Low cycle fatigue behavior of a type 304L austenitic stainless steel in air or in vacuum, at 20 °C or at 300 °C: Relative effect of strain rate and environment

The austenitic stainless steel type 304L is widely used for the manufacturing of nuclear power plants components. Some of the reactor components are subjected to cyclic thermal stresses due to temperature gradients during start-ups, shutdowns or operating transients. In these applications, the low cycle fatigue (LCF) properties of the material have to be taken into account for the design and life analysis of such components. In this context, a study of the fatigue behavior of 304L cylindrical specimens is carried out and the aim of the work is to analyze the respective roles of environment and strain rate, with the occurrence of dynamic strain aging. Thus, tests are performed in both ambient air and vacuum, at 20 and 300 °C under total axial strain control using a triangular waveform at strain amplitudes of ±0.3 or ±0.6% and at strain rates of 4×10−3, 1×10−4 or 1×10−5 s−1. The cyclic stress strain response of this steel is analyzed and SEM examinations of specimens’ surface are conducted. The negative influence of air environment on fatigue life is found to be much pronounced at 300 °C and at high strain rates. A detrimental effect of lowering strain rate is observed on the LCF resistance at 300 °C in vacuum as in air due to an enhancement of surface crack damage.

Low cycle fatigue behavior and life evaluation of a P/M nickel base superalloy under different dwell conditions

Low cycle fatigue (LCF) and fatigue-creep tests were conducted under different dwell conditions at 650 °C on a P/M nickel base superalloy, which was used widely as aero-engine turbine disc. The specimens, taken from a hot isostatic pressing (HIP) disc (Φ180X50mm), were tested under total fully reversed axial strain control. Compared with continuous cycling tests without dwell, the ones with tensile, compressive or tensile-compressive strain dwells showed some shorter fatigue lives. The difference of fatigue lives under different strain dwells reflects the fatigue-creep interactions at elevated temperature. The cyclic deformation and life distribution features were analysed and a lifing model was proposed to evaluate the fatigue life of the alloy. The prediction agrees well with the experimental results.

Creep-fatigue–environment interactions in INCONEL 617

Creep-fatigue testing of alloy 617 was performed in air, vacuum, and purified Ar environments at 1000 °C. Tests were performed in axial strain control at total strain ranges of 0.3% and 1.0% (fully reversed) with hold times at maximum tensile strain ranging from 0 to 1800 s. Introduction of a tensile hold period led to reduced creep-fatigue life at both strain ranges in all environments; the effect was greater at 0.3% than 1.0%. The hold time effect clearly saturated for tests at 1.0% strain range; the behavior at 0.3% was not clear. Decarburization occurred in specimens tested in vacuum and purified Ar, but not in air. Although fatigue lives were longer in the inert environments than in air for most test conditions, quantitative assessment of the differences was not possible because cracking frequently did not occur before test termination due to load drop for tests in inert environment. Cavitation damage was observed for tests with tensile hold periods in all environments.