Strain-controlled Fatigue Tests Research Articles

Study on low cycle fatigue properties of Cr-Mo steel in 50 MPa gaseous hydrogen

At present, a lack of fatigue design curves for Cr-Mo steel in high-pressure gaseous hydrogen worldwide impedes the use of the design fatigue curve (S-N curve) evaluation for vessel fatigue design, necessitating reliance on fracture mechanics methods related to the initial crack size. In this study, strain-controlled fatigue tests with a strain ratio of −1 were carried out in 50 MPa gaseous hydrogen to study the low cycle fatigue properties of Cr-Mo steel. Results indicate that hydrogen reduces the fatigue life of 4130X under high strain amplitudes. Under 0.5 % strain amplitude, different from characteristics of ductile fracture in air, the fracture in 50 MPa gaseous hydrogen showed quasi-cleavage and intergranular fracture characteristics. Based on the test results, the S-N curve in 50 MPa gaseous hydrogen was obtained with fatigue life safety factor 20 and stress amplitude safety factor 2. The S-N curve in 50 MPa gaseous hydrogen is under the curve in KD-320.2 M of the 2023 ASME Boiler & Pressure Vessel Code VIII-3, which does not consider the effect of hydrogen, when the number of cycles to failure is less than 1×106. This indicates that the fatigue life calculated by the S-N curve in KD-320.2 M cannot ensure that 50 MPa high-pressure hydrogen storage vessels avoid fatigue failure under high stress amplitudes.

Influence of friction and back stresses evolution on cyclic softening of laser powder bed fusion Ti-6Al-4V ELI alloy

Laser Powder Bed Fusion (LPBF) of Ti-6Al-4V ELI enables the fabrication of complex and individualized load-bearing patient-specific implants (PSI). These PSI are subjected to cyclic loading, which necessitates their fatigue performance evaluation. In this work, the fatigue properties of LPBF Ti-6Al-4V ELI in heat-treated conditions were characterized by studying the low cycle fatigue (LCF) behavior and underlying deformation mechanism. The fully reversed strain-controlled fatigue tests were performed at various strain amplitudes at room temperature. The ratio of fatigue life of conventional wrought alloy and LPBF Ti-6Al-4V ELI at the lowest (0.8 %) and the highest (1.8 %) strain amplitudes were found to be approximately 2.7 and 4.5, respectively. Further, the softening response during fatigue loading was correlated to the variation in the back and friction stresses. The TEM observations revealed that the dislocation pileup along α/β interfaces caused an increase in dislocation density, which drives the increase in back stress. However, the ease of slip transmission at high strain amplitude (1.8 %) reduced the heterogeneity between α and β phases, causing a decrease in the back stress. The TEM observations further suggested that the dislocation annihilation and subsequent rise in dislocation-free zones in α-phase reduced the friction stress at all strain amplitudes, which resulted in cyclic softening of LPBF Ti-6Al-4V ELI.

Analysis of viscoelastic behaviour in asphalt pavement through four-point beam bending tests

This study, conducted in accordance with ASTM T321-14 standards, offers crucial insights into the behaviour of asphalt materials subjected to cyclic loading. For proper maintenance and pavement design, it is essential to understand the material response under different loading conditions. This study focuses on the four-point beam bending test to investigate the viscoelastic behaviour of asphalt pavement. The four-point beam bending test is a useful method for determining the material's ability to withstand cyclic loading and deformation, which the material experiences during field traffic conditions. The experimental setup involves subjecting asphalt samples to cyclic loading using a four-point bending apparatus. The imposed load causes the specimen to experience bending strains, representing the actual loading conditions that pavements endure. The data gathered during testing include stress, strain, and deformation properties under various loading conditions. The stress-strain response demonstrates the material's resilience to fatigue, with a gradual decrease in stiffness beyond 10,300 cycles. Fatigue failure criteria include a 50% reduction in initial stiffness for strain-controlled fatigue tests and cracking in stress-controlled tests. The dynamic modulus in a compressive-type, repeated load test follows a three-phase pattern, highlighting the impact of temperature and binder characterization methods on sample performance. The results provide information about the material's resilience to rutting and fatigue cracking, the most significant distresses indicated in asphalt pavements. The findings from this study contribute to an in-depth understanding of the viscoelastic behaviour of asphalt pavement and can aid in the development of improved design guidelines and maintenance strategies characterizing the material response to cyclic loading. Engineers and researchers can make better decisions on the durability and performance of asphalt pavements, resulting in more cost-effective and sustainable road infrastructure.

Molecular dynamics simulation of fatigue damage formation in single-crystal/polycrystalline aluminum

The fatigue plasticity mechanisms and local defect characteristics of engineering materials are critical for addressing fatigue damage. This study aims to identify the formation mechanisms of fatigue damage and delineate the microstructural characteristics that resisted or facilitated fatigue-induced plastic accumulation. The fatigue mechanical properties and microstructural evolution processes of single-crystal/polycrystalline aluminum were investigated through strain-controlled fatigue testing, considering three distinct strain amplitudes. The initiation and cumulative occurrence of local plasticity in single-crystal aluminum occur at the intersections of slip planes, which serve as sources of dislocations. The cumulative formation of irreversible surface steps results in local stress concentration, ultimately leading to fatigue damage at the free surface. Polycrystalline aluminum undergoes grain rotation and merging during cyclic loading, leading to a gradual increase in grain size during plastic deformation. The initiation and cumulative occurrence of local plasticity occur at grain boundaries, ultimately leading to fatigue damage at the grain boundaries.

Creep-Fatigue Life Evaluation for Grade 91 Steels with Various Origins and Service Histories

Grade 91 steel is widely used in the boilers and piping of thermal power plants. There has been significant research interest in understanding the variations in creep characteristics among different heats of this steel for effective plant management. In recent years, thermal power plants have been subjected to frequent load changes and startup/shutdown to adjust power supply and demand and stabilize frequencies. These operational shifts have raised concerns regarding the potential for creep-fatigue damage in high-temperature components. Therefore, this research focuses on creep-fatigue properties of Grade 91 steel and their predictability. Tensile, creep, strain-controlled fatigue, and strain-controlled creep-fatigue tests were performed on six Grade 91 steels with different heats and/or histories, and the characteristics in each test were compared. As a result, the variations in creep-fatigue life among the materials were found to be correlated with the difference in creep characteristics and stress level during stress relaxation. Furthermore, the study involved a comparative assessment of the predictive performance of creep-fatigue life using five different approaches: time fraction, classical ductility exhaustion, modified ductility exhaustion, energy-based, and hybrid approaches. Among these approaches, the hybrid approach, based on inelastic strain energy density at fracture formulated as a function of inelastic strain rate, exhibited the most accurate predictive performance.

Effect of Post-Deposition Heat Treatment on the Mechanical Behavior and Deformation Mechanisms of a Solid-State Additively Manufactured Al–Mg–Si Alloy

Abstract The effects of post-deposition heat treatment on the fatigue behavior of AA6061 processed by additive friction stir deposition (AFSD) were investigated for the first time in this work. A heat treatment to recover the T6 temper was performed on AFSD AA6061 is then subjected to strain-controlled fatigue and monotonic tension testing. Microstructural analysis revealed abnormal grain growth resulting in bimodal grain size distribution. Mechanical testing indicated a full recovery of the strength of the AA6061-T6 temper with comparable fatigue performance to the as-deposited AFSD AA6061. Fractography revealed deformation mechanisms in the post-deposition heat treatment not observed in the as-deposited samples, however, the fatigue resistance remained unchanged. A microstructure-sensitive fatigue model was implemented to capture the effects of the heat treatment process on the fatigue performance of the post-deposition heat-treated AFSD AA6061.

Qualification of thermal barrier coating for semi-cryogenic engine thrust chamber through strain-controlled fatigue tests

The combustion chamber of semi-cryogenic engines experiences elevated temperatures which are beyond the capability of most structural materials. To solve this issue, an effective cooling mechanism to be implemented. In one of the developmental launch vehicles of Indian Space Research Organisation, the wall temperature in the combustion chamber is reduced by means of employing regenerative cooling along with thermal barrier coating applied on the inner wall made of special grade copper alloy. This special coating made of layers of Nickel and Nickel-Chromium also protect the thrust chamber from oxidation. The coating forms an integral part of the inner wall and are subject to a complex pattern of loads involving both thermal and pressure loads. This paper presents details about the qualification process used to evaluate integrity of the coating. Firstly, the stresses and strains on the inner wall during the engine operating conditions are arrived at based on detailed finite element analysis. Test specimens are fabricated from copper alloy and the thermal barrier coating as in the actual thrust chamber is applied on it. The specimens are tested under tensile and compressive loading that will induce the expected strains obtained from finite element analysis. The tests are done by means of strain-controlled fatigue tests at elevated temperature in tensile and compressive strain conditions. The integrity of the coating is assessed by means of Non-Destructive Test after each cycle of test and based on these, modifications were made for better performance. The modified coating was again tested for the same loading conditions and evaluated the coating integrity.

The Effect of Buckling Restraint on Axial Strain Life Fatigue Data

Abstract Focused on the common problem of easy specimen buckling in the axial strain–controlled fatigue testing of thin automobile steel sheets, a strain fatigue test method for thicknesses less than 2.5 mm is proposed. Because of the unrestrained grip end of specimens between the anti-buckling device and the testing machine fixture, specimens were prone to buckling in this area. In addition, if the designed specimen sizes were unreasonable, buckling may have occurred in the width and thickness directions of the gauge length of the specimen. Both situations can lead to invalid tests. This paper adopted a testing machine fixture and an anti-buckling plate as a mortise-and-tenon structure for the anti-buckling method; the upper and lower fixtures of the testing machine were cut in straight grooves, and the left and right anti-buckling plates were designed to be connected with a mortise and tenon. This is equivalent to narrowing the width of the unrestrained part of the specimen’s grip end to avoid buckling failure due to plane strain. The factors affecting the stiffness of fatigue specimens are also discussed herein. In order to avoid common instability in the thickness and width directions of the gauge length and unconstrained part of the specimen’s grip end at a strain ratio of Re = −1 during testing, six recommended specimen sizes are given on the premise of ensuring maximum specimen stiffness. When using the anti-buckling device, the recommended specimen sizes and test points of this study (i.e., the lower limits for yield strength and thickness of strain control testing) are 110 MPa and 0.5 mm, respectively, with which continuous and smooth stress–strain curves that result in accurate and reliable experimental data can be obtained.

Fatigue behavior of a nodular cast iron subjected to a variable amplitude strain-based load spectrum applied in bench tests of an Off-highway axle

Nodular cast irons are commonly adopted in off-highway applications, owing to the possibility to shape complex components and to guarantee good mechanical properties. In the fatigue strength assessment of axles made of nodular cast irons, engineers have to deal with external loads that turn into local strain spectra at the fatigue crack initiation zones, which are usually located in areas weakened by notched geometrical details. Moreover, off-highway axles are subjected to variable amplitude load histories during their service life; during the design phase, the field load histories are usually applied to the axles in the form of simplified bench load spectra. In this paper, the mechanical properties of EN-GJS-500-7 have been investigated through static tensile tests and strain-controlled fatigue tests performed on specimens with machined surface extracted from an axle. Furthermore, a local strain-based spectrum has been derived by means of strain gauges which have been applied at the failure location of the axle during the bench tests. Finally, the damage index to failure relevant to the simplified bench load spectrum has been calculated by adopting Miner's linear damage rule and the Smith, Watson and Topper mean stress corrected life equation.

Research on fatigue life prediction model for 2A70-T6 aluminum alloy at different strain ratios

PurposeIn practical engineering, oil filters often work under asymmetric cyclic loading. In order to improve the prediction accuracy of fatigue life of the oil filters under asymmetric cyclic loading, the effect of strain ratio and low cycle fatigue plastic deformation on fatigue life need to be considered. This paper aims to discuss the aforementioned objective.Design/methodology/approachFirst, strain-controlled fatigue tests with strain ratios of 0, 0.5 and −1 were carried out on the oil filter material 2A70-T6 aluminum alloy, and the test data were used to obtain strain fatigue life curves at three strain ratios. Then, based on the idea of the constant life curve method, the average value of the ratio of the strain amplitude corresponding to different strain ratios under the same partial life was defined as the strain ratio factor. Finally, the elastic-plastic factor was modified by the strain ratio factor, and a new fatigue life prediction model considering the effect of strain ratio was proposed.FindingsThe proposed model was validated, respectively, by fatigue test data of 2A70-T6 aluminum alloy, 2124-T851 aluminum alloy and oil filter and the results of the proposed model were compared with the Coffin–Manson equation, Morrow model and Smith–Watson–Topper (SWT) model, showing that the proposed model had higher applicability and accuracy.Originality/valueIn this work, a strain ratio factor is established based on the idea of the constant life curve method, and the strain ratio factor is used to modify the introduced elastic-plastic factor, and then a new fatigue life prediction model considering the influence of strain ratio and low cycle fatigue plastic deformation on material fatigue damage accumulation is proposed. The results show that the prediction results of the proposed model are in good agreement with the experimental data, and the proposed model has good fatigue life prediction ability considering the influence of strain ratio and lays a foundation for the fatigue life prediction of the oil filter.

Influence of grain size on fatigue strength of austenitic stainless steel (Investigation of ultimate strength dependency of fatigue strength)

Grain size dependency of the fatigue strength was investigated using Type 316 stainless steel. The main interest of this study was to clarify the root cause of the ultimate strength dependency of fatigue strength, particularly in the high-cycle fatigue regime. Four kinds of heat treatment were applied to obtain different grain sizes. The increase in the grain size caused smaller ultimate strength. The strain-controlled fatigue test and the crack growth test were conducted at room temperature. It was shown that the materials of larger grain size tended to have a shorter fatigue life, although the grain size had little influence on the crack growth rate. The reduced fatigue life due to large grain size was deduced to have been brought about by an increase in initial crack size after an incubation period before crack initiation. It was concluded that the grain size was not the dominant factor which caused the ultimate strength dependency of the fatigue strength, although the grain size surely affected both the fatigue strength and the ultimate strength.

Multiaxial non-proportional fatigue behaviour and microstructural deformation mechanisms of 9%Cr steel at elevated temperature

High-temperature components utilised in practical service are often subjected to multiaxial non-proportional loading due to their complex geometry. In the present study, a set of strain-controlled fatigue tests were conducted on P92 steel at 600 °C, including uniaxial, multiaxial proportional, and multiaxial non-proportional loading conditions. Results reveal that the axial and shear stresses exhibit varied responses to the different non-proportionalities, with proportional loading resulting in the shortest fatigue life when the strain ratio is small. The non-proportional loading induces accelerated cyclic softening at axial direction while the shear stress is enhanced. Electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) analysis indicate that the increased role of shear stress promotes the dislocation motion from the planar slip to cross-slip under non-proportional loading, which in turn facilitates the microstructure evolution. It is also important to note that the evident development of microstructure and the combined effect of more crack initiations and more secondary cracks contribute to the lowest fatigue life under proportional loading.

On the low-cycle fatigue behavior of a novel high-strength mold steel

Mold steels, usually being used for dies in low-temperature die casting or in molds for plastic injection, can additionally be considered as material of choice in precision optomechatronics. In any case, these steels will suffer from cyclic loading during their service life. Thus, despite of the envisaged application, the fatigue behavior must be studied comprehensively. In the present study, microstructure and mechanical properties, especially the low-cycle fatigue behavior, of a novel high-strength mold steel were investigated focusing on two different heat treatment conditions, i.e., low- and high-temperature annealing (LTA and HTA). The mechanical behavior being elaborated by tensile tests and strain controlled fatigue tests is discussed based on microstructural insights revealed by electron backscatter diffraction and fracture surface analysis. The results obtained indicate, that a more energy- and cost-intensive HTA treatment can be avoided without pronounced loss of mechanical performance.

Cyclic deformation behavior of an equiatomic CrFeNi multi-principal element alloy

For multi-principal element alloys (MPEAs) potential engineering applications, understanding their low-cycle fatigue (LCF) behavior is of decisive importance. Recently, an equiatomic face-centered cubic (FCC) CrFeNi has been shown to offer an excellent combination of monotonic properties. In the present study, we report on its LCF behavior at room temperature. Fully reversed strain-controlled fatigue tests were conducted in air under three different strain amplitudes (±0.3 %, ± 0.5 % and ± 0.7 %). The measured cyclic stress response reveals a rapid increase (i.e., cyclic hardening) followed by a relatively gradual decrease of peak stresses (i.e., cyclic softening) until failure. Electron microscopy investigations on post-fatigue samples revealed strain amplitude dependent dislocations slip-mode and resulting substructures evolution. These observations are linked to the observed cyclic stress response and lifetime. Furthermore, the origin of CrFeNi’s cyclic stress response is analyzed by partitioning hysteresis loops. Lastly, a comparison with similar grain-sized (60–67 µm) equiatomic CoCrFeMnNi, and 316L alloys pinpoints the peculiarities of CrFeNi LCF response, which is discussed in terms of the difference in their solid solution strengthening, grain boundary strengthening and stacking fault energy.

Generalized strain energy density-based fatigue indicator parameter

In the history of fatigue study, the potentialities of strain energy density (SED) based approaches were extensively discovered; however, there was limited usage in engineering practice, largely due to the lacking of a widely accepted fatigue model and operating instruction. Accordingly, referring to previous studies, a normative form of total strain energy density (TSED) based fatigue life equation with two power function terms is given, together with a standard flow to determine the corresponding SED-related material fatigue parameters, intending to popularize its application in engineering practice. In particular, the positive elastic and plastic SEDs play the dominant role in high and low cycle fatigue domains, respectively. Strain-controlled fatigue tests of virgin Al 7075-T6511 alloy and an in-service puddle iron are conducted to check the effectiveness of the TSED as the fatigue indicator parameter to correlate fatigue life. Results show that the TSED formula works better than the strain-based Coffin–Manson–Basquin equation.

Large-strain functional fatigue properties of superelastic metastable β titanium and NiTi alloys: A comparative study

This paper investigates the fatigue behavior of superelastic NiTi and two metastable β titanium alloys - commercial Beta III (Ti-11.5Mo-6Zr-4.5Sn wt%) and Ti2448 (Ti–24Nb–4Zr–8Sn wt%) alloys. In situ cyclic tensile tests performed under synchrotron X-ray radiation were used to precisely characterize the stress induced martensitic (SIM) transformation occurring in these alloys. For the NiTi alloy, an intermediate B2-R SIM transformation was detected before the B2-B19′ SIM transformation and no plastic deformation occurred until failure. All metastable β titanium alloys that were solution-treated before testing underwent a reversible β-α" SIM transformation and plastic deformation prior to failure. Low-cycle strain-controlled fatigue tests were performed in tension-tension strain-controlled mode at 37 °C to evaluate large-strain functional fatigue properties. The fatigue life of metastable β titanium alloys was found to be much better than that of NiTi alloy at large applied strains. After a rapid evolution during the first cycles, the mechanical response was found to be constant for NiTi alloy while it evolved continuously for metastable β titanium alloys. In addition, failure occurred suddenly in NiTi, whereas cyclic softening was observed before failure in metastable β titanium alloys. Fatigue properties at higher applied strains are mainly hindered by SIM transformation and defects generated at austenite/martensite interfaces during cycling. This explains why the studied NiTi alloy showed lower fatigue properties than metastable β titanium alloys. In fact, while SIM transformation is homogeneously nucleated in metastable β titanium alloys, SIM transformation is highly localized in NiTi, resulting in higher concentration of defects that promote crack nucleation and, in turn, degrade the functional fatigue properties.

An investigation on the cyclic deformation and service life of a reusable liquid rocket engine thrust chamber wall

PurposeRegeneratively cooled thrust chamber is a key component of reusable liquid rocket engines. Subjected to cyclic thermal-mechanical loadings, its failure can seriously affect the service life of engines. QCr0.8 copper alloy is widely used in thrust chamber walls due to its excellent thermal conductivity, and its mechanical and fatigue properties are essential for the evaluation of thrust chamber life. This paper contributes to the understanding of the damage mechanism and material selection of regeneratively cooled thrust chambers for reusable liquid rocket engines.Design/methodology/approachIn this paper, tensile and low-cycle fatigue (LCF) tests were conducted for QCr0.8 alloy, and a Chaboche combined hardening model was established to describe the elastic-plastic behavior of QCr0.8 at different temperatures and strain levels. In addition, an LCF life prediction model was established based on the Manson–Coffin formula. The reliability and accuracy of models were then verified by simulations in ABAQUS. Finally, the service life was evaluated for a regenerative cooling thrust chamber, under the condition of cyclic startup and shutdown.FindingsIn this paper, a Chaboche combined hardening model was established to describe the elastoplastic behavior of QCr0.8 alloy at different temperatures and strain levels through LCF experiments. The parameters of the fitted Chaboche model were simulated in ABAQUS, and the simulation results were compared with the experimental results. The results show that the model has high reliability and accuracy in characterizing the viscoplastic behavior of QCr0.8 alloy.Originality/value(1)The parameters of a Chaboche combined hardening constitutive model and LCF life equation were optimized by tensile and strain-controlled fatigue tests of QCr0.8 copper alloy. (2) Based on the Manson–Coffin formula, the reliability and accuracy of constitutive model were then verified by simulations in ABAQUS. (3)Thermal-mechanical analysis was carried out for regeneratively cooled thrust chamber wall of a reusable liquid rocket engine, and the service life considering LCF, creep and ratcheting damage was analyzed.

Two-fold improvement of low cycle fatigue resistance of steel by bidirectional transformation-induced plasticity

A new deformation mechanism via bidirectional transformation between γ-austenite and ε-martensite [B-transformation-induced plasticity (TRIP)] provides a great possibility to develop fatigue-resistant steel due to its high reversibility in dislocation motion. The Fe-18Mn-11Cr-7.5Ni-4Si B-TRIP steel was designed by modifying the previously developed Fe-15Mn-11Cr-7.5Ni-4Si steel. Strain-controlled fatigue test was conducted at the total strain range of 1%, and newly developed steel demonstrated two-fold fatigue life (21,994 cycles in average) compared with the previous one. Microstructural analysis after fatigue fracture showed that the enhanced irreversible α’-martensitic transformation at the crack tip in the previous steel negatively affected fatigue durability by locally invaliding B-TRIP.

Creep-fatigue interaction behavior of simulated microstructures and the actual weldment of P91 steel

The paper presents first of its kind study on the creep-fatigue interaction (CFI) behavior of simulated coarse grain (CG), fine grain (FG) and inter-critical (IC) heat-affected zones (HAZs) of P91 steel weldment. Total axial strain-controlled fatigue tests were conducted at 550 °C using trapezoidal waveform and introducing 30-min hold at peak tensile strain of ±0.6%. Irrespective of the initial microstructure, the alloy exhibited continuous cyclic softening throughout the deformation under CFI condition. Strain localization together with interlinkage of subsurface creep cavities caused enhanced crack propagation that resulted in the lowest fatigue life in the case of ICHAZ. The actual weldment failed in the interface between FGHAZ and ICHAZ. The mode of failure was found to be largely governed by Type IV cracking.

Unveiling the damage evolution of SAC305 during fatigue by entropy generation

Low-cycle thermal-mechanical fatigue loadings induce progressive and permanent degradation of mechanical properties of lead-free solder materials, and thus reduce the fatigue life of electronic devices. In this study, damage evolution and accumulation of Sn-3.0Ag-0.5Cu (SAC305), the most successfully commercialized lead-free solder material, was investigated by performing strain-controlled fatigue tests at different temperatures (288–373 K) and strain rates (0.001–0.004 s−1). Unlike existing empirical models, a fatigue damage model was proposed based on entropy generation related to the thermodynamic nature of fatigue damage. To be intrinsic to entropy generation, the proposed model was calibrated with the peak stress degradation at different temperatures and strain rates. Our findings showed that the damage parameter is closely related to temperature and strain rate and monotonically increases from 0 to 1 during the low-cycle fatigue loading, which unveiled the fact regarding the irreversibility of the internal entropy generation. For the first time, the damage evolution is found to be more associated with the applied strain rate than the temperature. By observations using an optical microscopy, the physical damage mechanism is elucidated for SAC305 solder by correlating microstructures and damage evolutions. The evolving dendritic β-Sn phase and the surrounding Sn-Ag-Cu ternary eutectic network also explained the effects of temperature and strain rate based on the energy dissipation. Our proposed damage model reconciled the damage accumulation of SAC305 solder subjected to the low-cycle fatigue loading, which is readily adopted to predict the fatigue life of the electronic packaging structures.