High Strain Amplitudes Research Articles

Low cycle fatigue response and fracture mechanism in a novel Al-3.5Si-0.5Mg-0.4Cu casting alloy

The LCF (low-cycle fatigue) behavior of a T6-treated low silicon cast aluminum alloy Al-3.5Si-0.5Mg-0.4Cu was investigated under axial symmetric tension-compression cyclic loading conditions. LCF behavior and results of experiment (under 6 different total strain amplitudes from 0.25 % to 0.5 %) are included. OM (Optical Microscopy), SEM (Scanning Electron Microscope), XRD (X-Ray Diffraction) and TEM (Transmission Electron Microscope) are employed to observe and analyze the fracture morphology and microstructure evolution. The results demonstrate that cracks were initiated from surface or subsurface defects. A higher total strain amplitude led to a smaller sum area of fatigue crack initiation region as well as a steady crack propagation regime, and a larger fatigue striation bandwidth. Furthermore, crack propagates along the interface between eutectic silicon and α-Al matrix under lower strain amplitudes. Otherwise, it extends through eutectic silicon particles if higher strain amplitudes are applied.

Dynamic Compression Simulation of Magnetorheological Elastomer Using Bouc-Wen Hysteretic Model

This paper explores the versatility of Bouc-Wen hysteresis model in simulating the dynamic behaviour of magnetorheological elastomer (MRE) material. Bouc-Wen model have been used in many field of science including modelling the hysteresis phenomenon happen in magnetic material, elastomer, base isolation of structures and many more. Introduced by the Robert Bouc, this nonlinear hysteretic model has been modified by many researchers to suit different applications. Compression testing of MRE material under high strain amplitude produces nonlinear hysteresis curve based on stress-strain data. Bouc-Wen hysteretic model has been found to be able to simulate the hysteresis curve of MRE material using parameter identification method within MATLAB Simulink.

Low cycle fatigue behavior of 316LN stainless steel hollow specimen in air and liquid lead–bismuth eutectic

A hollow specimen, which can be filled with lead–bismuth eutectic (LBE), was designed to study the low cycle fatigue behavior of 316LN stainless steel (SS) in air and LBE at 400 °C. The fatigue life of 316LN SS in liquid LBE only reduced at high strain amplitude (≥0.8%). The fatigue cracks initiated at outer surface in liquid LBE at low strain amplitude and the opposite is true at high strain amplitude. The oxygen concentration has no impact on the fatigue life of 316LN SS in liquid LBE. The corrosion fatigue damage mechanism for 316LN SS in liquid LBE is discussed.

Low cycle fatigue behavior of 316LN stainless steel: Effects of temperature, strain rate and strain amplitude

Continuous and interrupted LCF tests were conducted to reveal the effects of temperature, strain rate and strain amplitude on the LCF behavior of a nitrogen-alloyed 316LN stainless steel. The cyclic hardening and softening behavior at high and low strain amplitudes were elaborately elucidated based on the method of stress decomposition and characterization of dislocation configuration evolution. In addition, the dominant damage mechanism responsible for the different cracking modes under various loading conditions was determined based on the observations of fracture surface and secondary cracks on the longitudinal section.

Micromechanical modeling of the low-cycle fatigue behavior of additively manufactured AlSi10Mg

Factors such as high degrees of design freedom and flexibility in the production process contribute to a continued interest in the additive manufacturing (AM) process in industrial and academic research. The AM-processed components are widely used in industries; however, their use for production of parts under cyclic loading is still limited due to a significant variance in the cyclic behavior and the effect of the AM-associated defects, like porosity or unmelted powder, on the fatigue resistance. Micromechanical modeling approaches can be used to understand the relationship between the microstructure and the cyclic properties of the AM-processed material and thus expedite its employment in safety-critical applications. By using experiments and microstructure-sensitive models, this study aims to examine and to predict the low-cycle fatigue (LCF) behavior of AlSi10Mg parts produced by laser-based powder bed fusion in both the as-built and the direct-aged condition. Fatigue testing reveals higher stress amplitudes and cyclic hardening capabilities for the as-built condition. Direct-aged specimens demonstrate a higher number of cycles to failure at the highest strain amplitude, albeit at lower stress amplitudes when compared to the as-built condition. Both conditions show significant mean stress relaxation during LCF testing at a load ratio of R = 0. The applied modeling framework consists of a J2-plasticity model for the Si-rich phase and a crystal plasticity model for the Al phase. This hybrid model accurately predicts the LCF behavior under various strain amplitudes and ratios for both the as-built and direct-aged conditions of the AlSi10Mg.

The effect of β′ precipitates on mechanical properties and damping capacity in the Mg–Gd alloy

In this work, the effect of β′ precipitates on mechanical properties and damping capacity in Mg–10Gd alloys were investigated systematically. The results show that compared with the as-extruded alloy, the aged alloy has a higher strength and damping capacity due to large number of nano-β′ precipitates. After aging treatment, the ultimate yield strength and ultimate tensile strength increase obviously, and the damping capacity is improved, especially, in high strain amplitude region. After comparing the applied stress corresponding to the damping capacity test with the increment of yield strength contributing from β′ phases, the nano-β′ precipitates act as strong pining points in dislocations damping mechanism, which is not beneficial to improving damping capacity. However, the β′ phases/Mg interface will cause local strain. When theexternal stress reaches certain level, these local dislocations start to move, lead to the relationship between the strain amplitude and damping value fit the Granato-Lücke (G-L) dislocation theory for the aged alloys not well and enhance the damping capacity. Aging phases not only have great contribution on strength, but also have positive effect on damping capacity, so precipitate strength may become an effective strategy to moderate the contradiction between the strength and damping capacity.

Creep–Fatigue Interaction of Inconel 718 Manufactured by Electron Beam Melting

Electron beam melting of Ni‐base superalloy Inconel 718 allows producing a columnar‐grained microstructure with a pronounced texture, which offers exceptional resistance against high‐temperature loading with severe creep–fatigue interaction arising in components of aircraft jet engines. This study considers the deformation, damage, and lifetime behavior of electron‐beam‐melted Inconel 718 under in‐phase thermomechanical fatigue loading with varying amounts of creep–fatigue interaction. Strain‐controlled thermomechanical fatigue tests with equal‐ramp cycles, slow–fast cycles, and dwell time cycles are conducted in the temperature range from 300 to 650 °C. Results show that both dwell time and slow–fast cycles promote intergranular cracking, gradual tensile stress relaxation, as well as precipitate dissolution and coarsening giving rise to cyclic softening. The interplay of these mechanisms leads to increased lifetimes in both dwell time and slow–fast tests compared to equal ramp tests at higher strain amplitudes. Conversely, at lower mechanical strain amplitudes, the opposite is observed. A comparison with results of conventional Inconel 718 indicates that the electron‐beam‐melted material exhibits superior resistance against strain‐controlled loading at elevated temperatures such as thermomechanical fatigue.

Cyclic deformation mechanism and fracture behaviour of 316L stainless steel under thermomechanical fatigue loading

Thermomechanical fatigue (TMF) behaviour of 316L is investigated based on the internal stress and microstructural characterization at various cyclic stages. Results reveal that the cyclic stress response shows initial hardening followed by cyclic softening at a high strain amplitude, while presenting continuous cyclic hardening as the strain amplitude decreases. Among these, Out-of-phase TMF always shows cyclic softening irrespective of strain amplitude. It is important to note that the hardening of back stress and friction stress contribute to the initial and subsequent cyclic hardening. In contrast, the subsequent cyclic softening is mainly dominated by the reduced friction stress. It is observed that the proliferation of dislocations at grain boundaries and the enhanced dislocation interactions within the grains are primarily responsible for the hardening of back stress and friction stress, respectively. Specifically, the cyclic softening of friction stress is attributed to the annihilated dislocations as a result of cross-slip. It is also found from the hysteresis curves that the intensity of dynamic strain ageing is related to strain amplitude, loading history and phase angle, which correlates to the occurrence of cross-slip, the diffusion rate of the solute atom, the density of point defect and the mobility of vacancies. Finally, the effect of loading conditions on fracture behaviour is demonstrated from the point view of crack sources, secondary cracks, creep cavities and oxidation damage.

On the low-cycle fatigue behavior of a multi-phase high entropy alloy with enhanced plasticity

A multi-phase non-equiatomic FeCrNiMnCo high entropy alloy (HEA) was fabricated using vacuum induction melting. Thermo-mechanical treatments consisting of cold rolling and annealing at 750 °C and 850 °C were employed to improve the mechanical properties of the HEA in focus. Tensile experiments revealed that yield strength and ultimate tensile strength levels can be enhanced significantly after thermo-mechanical processing (TMP). At the same time, ductility remains at an adequate level. Strain-controlled low-cycle fatigue (LCF) experiments were carried out in order to assess the mechanical properties of this HEA under cyclic loading conditions. At the same strain amplitude, the stress levels of the processed samples were considerably higher than that of the as-received counterpart. Similarly, fatigue lives of the former could surpass the base condition at the strain amplitudes of 0.2% and 0.4%; however, at the higher strain amplitudes, cyclic softening was observed. Electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) results revealed that phase transformation from face-centered cubic (FCC) to body-centered cubic (BCC/B2) took place at a higher occurrence with increasing strain amplitude (0.2% to 0.6%). Furthermore, transmission electron microscopy (TEM) studies confirm that upon tensile deformation additional plasticity mechanisms, i.e., deformation twinning and phase transformation, contribute to the overall mechanical behavior of the multi-phase HEA.

Cyclic stress response and microcrack initiation mechanism of modified 9Cr‐1Mo steel under low cycle fatigue at room temperature and 350°C

AbstractTo investigate the effect of temperature on microcracking mechanisms of modified 9Cr‐1Mo steel, low cycle fatigue tests were performed at 350°C and room temperature (RT). At RT, the cyclic stress response at low strain amplitude (<0.5%) is controlled by friction stress while dominated by back stress at high strain amplitude. By comparison, the disappearance of sub‐grain boundaries induced by dislocation annihilation occurs at 350°C, which contributes to the decrease in back stress, resulting in more significant effects of back stress compared to RT. As a result of the accumulated fatigue damage, the distance between two parallel extrusions at 350°C is larger than that at RT, which is ascribed to the disappearance of low angle sub‐grain boundaries due to dislocation annihilation at 350°C. Due to the disappearance of small‐angle grain boundaries at 350°C, microcracks initiate along high‐angle grain boundaries at 350°C instead of along low‐angle grain boundaries at RT.

Cyclic deformation and dynamically induced short-range ordering in small particles reinforced Al composite

Cyclic deformation of an Al-Cu-Mg composite strengthened by small non-shareable particles was firstly investigated through strain controlled low-cycle fatigue (LCF) tests. In comparison with the reported Al-Cu-Mg composites, the in-situ TiB2/Al-Cu-Mg composite presented higher fatigue life at all the tested strain amplitudes. The composite exhibited cyclic hardening response, which was contributed by non-shareable particles, dislocations and dynamically deformation-induced short-range orderings in the Al matrix. Considering plastic strain evolution, the cyclic hardening model was established and well fitted with experimental results. When cyclic loading was controlled at higher strain amplitudes, the serrated flow occurred on stress-strain hysteresis loops and then it disappeared after the first few cycles related to the occurrence of dynamic strain aging. Onset of the stress serration on stress-strain hysteresis loop was greatly dependent on the strain amplitude rather than cumulative plastic strain. Finally, the correlative cyclic hardening model of small particles reinforced Al composite was analytically discussed based on microstructure evolution and cyclic stress-strain responses.

Thermo-Mechanical Fatigue Behavior and Resultant Microstructure Evolution in Al-Si 319 and 356 Cast Alloys.

The out-of-phase thermo-mechanical fatigue (TMF) behavior of the two Al-Si cast alloys most widely used for engine applications (319 and 356) were investigated under temperature cycling (60-300 °C) and various strain amplitudes (0.1-0.6%). The relationship between the microstructural evolution and TMF behavior was closely studied. Both alloys exhibited asymmetric hysteresis loops with a higher portion in the tensile mode during TMF cycling. The two alloys showed cyclic softening behavior with regard to the maximum stress, but an earlier inflection of cyclic stress was found in the 356 alloy. The TMF lifetime of the 319 alloy was generally longer than that of the 356 alloy, especially at higher strain amplitudes. All the precipitates (β'-MgSi in 356 and θ'-Al2Cu in 319) coarsened during the TMF tests; however, the coarsening rate per cycle in the 356 alloy was significantly higher than that in the 319 alloy. An energy-based model was applied to predict the fatigue lifetime, which corresponded well with the experimental data. However, the parameters in the model varied with the alloys, and the 356 alloy exhibited a lower fatigue damage capacity and a higher fatigue damage exponent.

Mechanisms of intergranular fatigue crack initiation in polycrystalline materials

Fatigue failure in polycrystalline materials occurs by initiation and propagation of fatigue cracks. Fatigue cracks can initiate transgranularly or intergranularly. While considerable progress has been achieved in the explanation of the mechanism of transgranular fatigue crack initiation intergranular initiation is still the subject of controversy. A recent study of the crack initiation in superaustenitic Sanicro 25 steel [1] revealed the important role of persistent slip markings (PSMs) in the grains adjoining the grain boundary. A novel mechanism of intergranular fatigue crack initiation has been proposed. Provided the cyclic slip cannot proceed from one grain to the neighbour one, extrusions and intrusions arise not only on the surface but also on the grain boundary. Growing intrusions represent void-like defects and growing intrusions push neighbour grain apart. A systematic study of intergranular cracking has been performed on polycrystalline copper cycled with high and low strain amplitudes. SEM observations of the early stages of fatigue crack initiation on the grain boundaries, EBSD study of grains orientation, FIB cutting of the grain boundaries and simultaneous documentation of the effect of extrusions and intrusions on the grain boundary cohesion are reported simultaneously with the conditions favourable for the initiation of the fatigue cracks.

Exploring low cycle fatigue anisotropy and the failure mechanism of the DD412 single crystal alloy for aeroengines

In this paper, the low cycle fatigue properties of the DD412 single crystal alloy with [001], [011] and [111] orientation at 760℃ were studied. A correlation between low cycle fatigue properties and orientation was found, and the microstructure evolution and fatigue failure mechanism were studied, which lays a solid theoretical foundation for the engineering application of the DD412 single crystal alloy. The fatigue testing was uniaxial. The experimental results show that the low cycle fatigue properties of the DD412 alloy with different orientations have significant anisotropy at 760℃. The single crystal alloy with a [001] orientation parallel to the loading axis exhibited the longest fatigue life at high strains, and the alloy with [111] orientation exhibited the shortest fatigue life at high strains, while the alloy with [011] orientation was located between these two durations, which is mainly related to the difference of elastic modulus. For the alloy with [001] orientation, elastic deformation is the main fatigue damage mechanism, and cleavage-like fracture occurs along a specific crystallographic plane. For the alloy with [011] orientation, under high strain amplitude, the fatigue fracture is caused by the joint action of plastic deformation and elastic deformation, and cleavage tear mixed fracture occurs. For the alloy with [111] orientation, under high strain amplitude, plastic deformation is the main fatigue damage mechanism. The dislocation configurations of the DD412 alloy with different orientations after low cycle fatigue at 760℃ are different. The main observation in the alloy with [001] orientation is the dislocations cross-slip in the γ channel to form serrated dislocations; while in the alloy with [011] orientation, there are a large number of dislocations entangled in the γ channel; and in the alloy with [111] orientation, there are two high density parallel dislocation bands in the γ channel.

Angiography-derived radial wall strain predicts coronary lesion progression in non-culprit intermediate stenosis.

Intermediate coronary lesions (ICLs) are highly prevalent but ported mixed prognosis. Radial strain has been associated with plaque vulnerability, yet its role in predicting lesion progression is largely unknown. The purpose of this study was to determine the predictive value of angiography-derived radial wall strain (RWS) for progression of untreated non-culprit ICLs. Post-hoc analysis was conducted in a study cohort including 603 consecutive patients with 808 ICLs identified at index procedure with angiographic follow-up of up to two years. RWS analysis was performed on selected angiographic frames with minimal foreshortening and vessel overlap. Lesion progression was defined as ≥ 20% increase in percent diameter stenosis. Lesion progression occurred in 49 ICLs (6.1%) with a median follow-up period of 16.8 months. Maximal RWS (RWSmax), frequently located at the proximal and throat plaque regions, distinguished progressive ICLs from silent ones. The largest area under the curve value of 0.75 (95% CI: 0.67-0.82, P < 0.001) was reached at the optimal RWSmax cutoff value of > 12.6%. According to this threshold, 178 ICLs were classified as having a high strain pattern. Exposure to a high strain amplitude with RWSmax > 12.6% was independently associated with an increased risk of lesion progression (adjusted HR = 6.82, 95% CI: 3.67-12.66, P < 0.001). Assessment of RWS from coronary angiography is feasible and provides independent prognostic value in patients with untreated ICLs.

Effect of texture on damping behaviour in ZK60 magnesium alloy

ABSTRACT Effect of texture on the damping capacity of ZK60 magnesium alloy is investigated through compression experiments with a deformation of 10% at room temperature. It is found that both the dislocation networks, twins and the basal texture intensity affect the damping capacity. At the low strain amplitudes, the dislocation networks and twins are dominant and the damping capacity decreases with the increase in the dislocation networks and twins. At the high strain amplitudes, the basal texture intensity is dominant and the damping capacity decreases with the increase in the basal texture intensity. Based on the anisotropy of the damping capacity of the wrought magnesium alloys, this research provides provide a direction for the study of high-strength and high-damping magnesium alloys.

Investigation of Low Cycle Fatigue Behaviors of Inertia-Friction-Welded Joints of the TC21 Titanium Alloy

As a new highly damage-tolerant structural material, the TC21 titanium alloy has been widely used in aerospace applications. Inertial friction welding (IFW) is a form of pressure welding technology with less welding parameters and high welding joint performance, which is especially suitable for the connection of rotors of aero-compressors and engines. In this paper, inertia friction welding of TC21 titanium alloys was successfully carried out, and the microhardness, tensile properties and low cycle fatigue (LCF) behaviors of IFW joints were studied. Based on the mechanical parametric results of the tensile test, the true stress–strain curves of the IFW joint of TC21 titanium alloys are obtained by further calculation. Based on the LCF test results under different strain amplitudes, life prediction of IFW joints was investigated. The results of the LCF test show that there is no obvious cyclic hardening and cyclic softening of the IFW joints. Moreover, the fracture morphology of LCF samples under high strain amplitude (0.9%) and low strain amplitude (0.6%) was observed. The results show that the fatigue cracks initiate and propagate at multiple points in the LCF samples, and the transient fracture zone is larger under high strain amplitude. However, under low strain amplitude, a fatigue crack nucleates and propagates at a single point, and the crack propagation zone is larger.

Low cycle fatigue behaviour study of a nano precipitate strengthened ferrite-bainite 780 steel

Low-cycle fatigue behaviour of a newly developed ferrite-bainite 780 steel with the ferrite phase, strengthened by nanosized precipitates was investigated. The LCF tests were conducted in fully reversed strain-controlled mode to determine the cyclic stress response and strain life. The steel showed predominantly cyclic softening behaviour at lower strain amplitudes (up to 0.25%). However, at higher strain amplitudes (≥ 0.35%), cyclic hardening was observed with increase in the number of loading cycles. EBSD study done on the fatigue tested samples revealed grain fragmentation and texture weakening during cyclic deformation. TEM study elucidated the evolution of dislocation structure responsible for cyclic softening/hardening behaviour during cyclic deformation. At lower strain amplitude (0.2%), concurrent presence of random arrangement of dislocations and dislocation tangles were observed with some grains having weakly developed persistent slip bands (PSBs) and veins. At high strain amplitude (0.55%), subgrain structure was found to be the dominant feature along with occurrence of PSBs and veins at few locations. Further to this the role of nano-precipitates in influencing the fatigue behaviour was also investigated.

Addressing the healing compensation on fatigue damage of asphalt binder using TSH and LASH tests

This paper investigated the damage healing, local life compensation and intrinsic healing characteristics of asphalt binder. The damage healing (ΔS) is independent of damage level and the percent local life compensation (%ΔN) increases with a higher strain amplitude in TSH test. The integrated ΔS-ΔN analysis is recommended for the healing compensation characterization. The totally stored pseudo strain energy (PSE) to counteract healing recovery is linearly linked to the ΔS regardless of the loading modes, strain amplitudes and rest periods. A new parameter of average stored PSE (QH) is proposed to reveal the intrinsic healing potential and characteristics.

Numerical and Analytical Studies of Low Cycle Fatigue Behavior of 316 LN Austenitic Stainless Steel

Abstract Mechanical components are frequently subjected to severe cyclic pressure and/or temperature loadings. Therefore, numerical and analytical low cycle fatigue methods become widely used in the field of engineering to estimate the design fatigue lives. The primary aim of this work is to evaluate the accuracy of the most commonly used numerical and analytical low cycle fatigue life methods for specimens made of 316 LN austenitic stainless steel and subjected to fully reversed uniaxial tension–compression loading, in the room temperature condition. It was found that both maximum shear strain and Brown–Miller criterions result in a very conservative estimation for uniaxially loaded specimens. However, maximum shear strain criteria provide better results compared to the Brown–Miller criteria. The total strain energy density approach was also used, and both the Masing and non-Masing analysis were adopted in this study. It is found that the Masing model provides conservative fatigue lives, and non-Masing model results in a more realistic fatigue life prediction for 316 LN stainless steel for both low and high strain amplitudes. The fatigue design curves obtained from the commonly used analytical low cycle fatigue equations were reexamined for 316 LN SS. The obtained design curves from Langer model and its modified versions are nonconservative for this type of material. Consequently, the authors suggest new optimized parameters to fit the given test data. The obtained curve using the currently suggested parameters is in better agreement with the experimental data for 316 LN SS.