Low Cycle Fatigue Tests Research Articles

Microstructure, Tensile, and Fatigue Properties of Large-Scale Austenitic Lightweight Steel.

High-Mn lightweight steel, Fe-0.9C-29Mn-8Al, was manufactured using steelmaking, ingot-making, forging, and rolling processes. After the final rolling process, a typical austenite single phase was observed on all sides of the thick plate. The microstructural changes after annealing and aging heat-treatments were observed, using optical and transmission electron microscopy. The annealed coupon exhibited a typical austenite single phase, including annealing twins in several grains; the average grain size was 153 μm. After aging heat treatment, κ-carbide was observed within the grains and on the grain boundaries. Additionally, the effect of aging heat treatment on the mechanical properties was analyzed, using a tensile test. The fine κ-carbide that precipitated within the grains in the aged coupon improved the 0.2% offset yield and the tensile stresses, as compared to the as-annealed coupon. To estimate the applicability of high-Mn lightweight steel for low-pressure (LP) steam turbine blades, a low-cycle fatigue (LCF) test was carried out at room temperature. At a total strain amplitude of 0.5 to 1.2%, the LCF life of high-Mn lightweight steel was approximately three times that of 12% Cr steel, which is used in commercial LP steam turbine blades. The LCF behavior of high-Mn lightweight steel followed the Coffin-Manson equation. The LCF life enhancement in the high-Mn lightweight steel results from the planar dislocation gliding behavior.

Combining 2D and 3D Characterization Techniques for Determining Effects of HIP Rejuvenation After Fatigue Testing of SX Microstructures

A hot isostatic pressing rejuvenation heat treatment is applied to a CMSX-4 type SX superalloy after it has been subjected to a low-cycle fatigue test to rupture. The evolution of microstructural defects, such as pores and cracks which are present after fatigue, has been tracked in 3D by X-ray tomography before and after rejuvenation. From the rejuvenated specimen, series of metallographic cross sections were prepared and investigated by scanning electron microscopy for getting complementary 2D information at high resolution. The micrographs were stitched to a panorama which was then matched into the 3D representation of the specimen volume. By combining 3D and 2D data, statistical volume-related quantities were achieved, while detailed characteristics have been assigned to individual defects present in the 2D panorama micrograph. This technique is in general appropriate for length-scale bridging microstructural investigations. Results of the performed investigations concerning the rejuvenation effect on the microstructure are presented and discussed.

Using Direct Current Potential Drop Technique to Estimate Fatigue Crack Growth Rates in Solid Bar Specimens under Environmental Assisted Fatigue in Simulated Pressurized Water Reactor Conditions

The direct current potential drop (DCPD) technique may be used in crack propagation tests to measure the crack growth rate (CGR). Potential probes attached to the specimen allow the variation of the crack length to be estimated. In this research, the DCPD technique using one single potential probe was applied to solid bar specimens (i.e., without any initial notch or crack) subjected to low-cycle fatigue testing in a simulated pressurized water reactor (PWR) environment. This particular analysis had two associated difficulties, the first one being the fact that crack initiation sites are not known beforehand, and the second one consisting in the experimental difficulties and conditioning factors associated with the simulation of the PWR environment. Nine solid bar specimens were tested to fatigue failure under different strain amplitudes and frequencies, while also measuring the corresponding DCPD signal during the fatigue process. It was observed that the initiation of multiple cracks was detected by the DCPD measurements. Moreover, as fatigue continued, one of the cracks became dominant and progressed to cause the specimen failure. The DCPD technique allowed the average CGR of the dominant crack to be estimated. Finally, the obtained average CGRs were validated by comparing them with average CGRs derived from striation spacing measurements, obtained from scanning electron microscopy (SEM) and from literature values gathered in the NUREG/CR-6909 document.

Effect of Strain Range and Hold Time on High Temperature Fatigue Life of G17CrMoV5-10 Cast Alloy Steel

Abstract In this work, cast steel G17CrMoV5-10 was investigated. The material subject to investigation as part of this study is commonly used to manufacture steam turbine casings. Modern steam turbines operate under elevated temperature and complex oscillated loads. Thus, the focus of this study was to investigate material under behavior during low cycle fatigue (LCF) test performance at 500°C with and without hold time in tension. During all types of test, cyclic softening of cast steel was noticed. Increasing of total strain rate and applying hold time significantly reduce fatigue life. During hold time, due to temperature and tension the material creep what is confirmed by increasing inelastic stain accommodation.

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.

Cyclic Behaviors of SHS Columns in Low Cycle Fatigue Tests

ABSTRACT The damage caused by small inelastic loadings cannot be neglected when the number of cycles is large. Previous research has mainly focused on the low cycle fatigue (LCF) behaviors of beam-to-column connections, ignoring those of columns. Here, cyclic loading tests were conducted on 22 cold-roll-formed square hollow section columns under multiple small-amplitude loadings. All the specimens failed owing to flange fracture. The effects of the axial force ratio, shear span ratio, and loading amplitude on the LCF behaviors were investigated. Fatigue life prediction methods were developed, and two methods for predicting the lateral resistance deterioration of the specimens were proposed.

Planar-Biaxial Thermo-mechanical Fatigue Behavior of Nickel-Base Superalloy Inconel 718 Under Proportional Loading

The planar-biaxial thermo-mechanical fatigue behavior of nickel-base superalloy Inconel 718 was studied for selected proportional loading conditions, in particular biaxial strain ratios of 1.0, 0.6, and − 1.0. The cyclic temperature loading with minimum and maximum temperatures of 400 °C and 630 °C and a duration of 250 seconds was either In-Phase or Out-of-Phase to the mechanical axes. Besides the multiaxial tests, uniaxial thermo-mechanical fatigue tests were conducted In-Phase and Out-of-Phase with the same temperature cycle and cycle duration. The performed thermo-mechanical fatigue tests were analyzed regarding the deformation and lifetime behavior and compared with high-temperature isothermal low-cycle fatigue tests from a previous work of the authors. On the side of the lifetime description, a strain- and a stress-based approach were presented. For the planar-biaxial tests, the crack initiation mechanism and crack paths were shown.

Effect of Infill Density in FDM 3D Printing on Low-Cycle Stress of Bamboo-Filled PLA-Based Material.

In this paper, the fatigue behavior of polylactic acid (PLA) material with bamboo filler printed by 3D additive printing using fused deposition modelling (FDM) technology at different infill densities and print nozzle diameters is investigated. The mechanical test results are supported by the findings from SEM image analysis. The fatigue behavior was tested at four consecutive 250 cycles at loads ranging from 5 to 20, 30, 40, and 50% based on the limits found in the static tensile test. The results of the static tensile and low-cycle fatigue tests confirmed significant effects of infill density of 60%, 80%, and 100% on the tensile strength of the tested specimens. In particular, the research results show a significant effect of infill density on the fatigue properties of the tested materials. The influence of cyclic tests resulted in the strengthening of the tested material, and at the same time, its viscoelastic behavior was manifested. SEM analysis of the fracture surface confirmed a good interaction between the PLA matrix and the bamboo-based filler using nozzle diameters of 0.4 and 0.6 mm and infill densities of 60%, 80%, and 100%. Low-cycle testing showed no reductions in the mechanical properties and fatigue lives of the 3D printed samples.

Low cycle fatigue behavior of micro-grain casting K4169 superalloy at room temperature

The strain-controlled low cycle fatigue tests were carried out at 298 ​K for the newly developed micro-grain casting K4169 superalloy, and the cyclic stress response, deformation and fracture behaviors of the alloy were studied. The results show that the fatigue life of the alloy was increased by more than two times compared with other casting K4169 alloys. This can be partly attributed to grain refinement impeding crack initiation and propagation. When the strain amplitude was in plastic range (Δεt/2 ​≥ ​0.5%), the alloy displayed a short period of initial cyclic stability followed by cyclic softening to fracture. By observing dislocation configurations, only a few dislocations moved around grain boundaries at initial cycles, leading to short cyclic stability. Afterwards, dislocations in the slip bands cut γ" phase continuously leading to cyclic softening. When the strain amplitude was in elastic range (Δεt/2 ​≤ ​0.4%), the alloy exhibited elastic deformation and the peak stress kept stable. In addition, the crack initiation site changed from the MC carbides to the persistent slip bands on surface with increasing strain amplitudes.

Structure and fatigue behavior of electroless plated Ni-Zn-P films using pretreatments, annealing and an applied magnetic field

Electroless ternary Ni-Zn-P films are plated onto a low carbon steel substrate. The surface morphology, microstructure, hardness, fatigue life and corrosion properties of the Ni-Zn-P coatings are respectively determined using scanning electron microscopy, X-ray diffraction, nano-indentation, high-strain low-cycle fatigue and polarization tests. The substrates pretreated using the proposed roughening (15 % H2SO4, 20 min) and activation (10 % H2SO4, 1 min) produce good Ni-Zn-P film deposition. When the annealing temperature reaches 400 °C, the Ni–P-Zn films contain crystalline Ni, FeNi and a precipitation Ni3P structures phase. Applied magnetic field improves the film growth rate, mechanical properties and corrosion resistance.

Microstructure, Fatigue Properties and Stress Concentration Analysis of 6005 Aluminum Alloy MIG Welded Lap Joint.

This paper studies the microstructure and mechanical properties of MIG (Melt Inert Gas) lap welded 6005 aluminum alloy plates. Microstructure analysis (OM) of the joint showed that 15~30 μm small grains were observed at the fusion line. Mechanical analysis shows that the small grains are broken by shielding gas and molten pool flow force. Hardness test shows that there is a softening zone (41~43 HV) in HAZ much lower than BM and WZ. The low cycle fatigue test showed that the performance of lap joint decreased sharply, and the fatigue strength of weld decreased significantly, which was only 27.34% of the base metal. The fatigue fracture (SEM) of the weld observed slip band cracking and a large number of brittle fracture characteristics. Using the stress concentration factor Kt for analysis, it was found that the cause of brittle fracture was mostly stress concentration. Lap joint stress concentration model appears in two ways: firstly, at the weld toe, the weld is subjected to eccentric force, secondly, there is a small gap between the two plates at the weld root, which cracks along the direction of 45° of the maximum shear stress.

In-Situ Characterization of Microstructural Changes in Alloy 718 during High-Temperature Low-Cycle Fatigue

Components made of nickel-based alloys are typically used for high-temperature applications because of their high corrosion resistance and very good creep and fatigue strength, even at temperatures around 1000 °C. Corrosive damage can significantly reduce the mechanical properties and the expected remaining service life of components. In the present study, a new method was introduced to continuously determine the change in microstructure occurring as a result of exposure to high temperature and cyclic mechanical loading. For this purpose, the conventional low-cycle fatigue test procedure was modified and a non-destructive, electromagnetic testing technique was integrated into a servohydraulic test rig to monitor the microstructural changes. The measured values correlate with the magnetic material properties of the specimen, allowing the microstructural changes in the specimen’s subsurface zone to be analyzed upon high-temperature fatigue. Specifically, it was possible to show how different loading parameters affect the maximum chromium depletion as well as the depth of chromium depletion, which influences the magnetic properties of the nickel-based material. It was also observed that specimen failure is preceded by a certain degree of microstructural change in the subsurface zone. Thus, the integration of the testing technology into a test rig opens up new possibilities for improved prediction of fatigue failure via the continuous recording of the microstructural changes.

Development of a novel buckling-restrained damper with additional friction energy dissipation: Component tests and structural verification

Aiming at solving the issue of the conventional buckling-restrained brace, i.e., the fixed yield strength and residual deformation, a novel buckling-restrained damper with additional friction energy dissipation (BFD) was proposed and tested in the present study. By fabricating an inner core with a small yield segment (SYS) and a large yield segment (LYS) connected in series and setting a friction device in parallel with the SYS, the dual stiffness and stable energy dissipation capacity of the BFD were achieved. Low-cycle fatigue tests were conducted on nine BFD specimens to investigate the influences of the length of the LYS, the length of the slotted holes, the bolt torque and the loading protocol on the hysteretic behavior of the BFD. The experimental results demonstrated that the strength of the BFD increased with the maximum strain of the LYS, whereas the low-cycle fatigue life of the BFD was negatively affected by the maximum strain of the LYS. The friction energy dissipation device could enhance the energy dissipation capability of the BFD throughout the working process. Moreover, the results from the numerical case study determined that the BFD could effectively control the displacement response of the SDOF system under different earthquake magnitudes.

EBSD analysis of strain distribution and evolution in ferritic-Pearlitic steel under cyclic deformation at intermediate temperature

Strain accumulation as a result of cyclic loading affects the microstructure and mechanical performance of low-alloy steels under cyclic operation conditition. This manuscript presents a comprehensive analysis of a ferritic-pearlitic SA204 Grade C steel under strain-controlled low-cycle fatigue tests to understand the microstructural deformation and strain localization in the ferritic-pearlitic steel. Low-cycle fatigue tests were performed in a Gleeble® thermo-mechanical simulator at 0.008, 0.01, 0.015, and 0.02 strain amplitudes under an isothermal condition at 250 °C. The microstructure evolution during cyclic loading was evaluated using the samples from interrupted 0.01 strain amplitude tests. The LCF analysis shows strain localization at the ferrite-cementite interfaces and ferrite-pearlite interfaces. The characterization results using OM, SEM, EBSD, and nanoindentation analysis reveals the grain misorientation in the ferritic-pearlitic microstructure due to strain accumulation increases with the strain amplitudes and the loading cycles.

On the microstructural evolution and failure mechanism in laser powder bed fusioned Ti–6Al–4V during low cycle fatigue at room and elevated temperatures

Microstructural features and their evolution during cyclic deformation directly impact the low cycle fatigue (LCF) life of additively manufactured Laser Powder Bed Fusion (LPBF) Ti–6Al–4V. Tensile and strain controlled LCF tests were performed at room (RT) and elevated temperature (ET, @ 400 °C) to study the cyclic softening behaviour and failure mechanism of LPBF Ti–6Al–4V. The evolution of α′ grains and free dislocation density were studied using Electron Backscatter Diffraction (EBSD). LPBF Ti–6Al–4V has greater tensile strength than conventionally manufactured wrought Ti–6Al–4V due to its microstructure, with fine α′ needles which provide small slip lengths. For cyclic loading at ET, the interaction between the dislocations increases which in-turn increases the ability of material to overcome the obstacles to dislocation motion, resulting in higher cyclic softening compared to the RT test. During cyclic deformation, evolution of dislocation substructures takes place to subsequently produce Low Angle Boundaries (LABs) inside the prior α’ grains. The LABs progressively lead to nucleation and coalescence of voids with fatigue cycles, eventually leading to fracture. An increase in strain range (i.e. plasticity level) causes more significant dislocation pile-up, contributing to a greater amount of cyclic softening. The lack of fusion voids or pores, present at or near the surface, and microcracks, present at the rough surface, act as the crack initiation locations which propagate to cause fracture of the LPBF material under LCF loading, where the primary mode of fatigue fracture observed is intergranular.

The Influence of Process Parameters on the Low-Cycle Fatigue Properties of 316L Steel Parts Produced by Powder Bed Fusion

In this paper, the influence of the additive manufacturing (AM), powder bed fusion (PBF) process parameters on the low-cycle fatigue (LCF) properties of 316L steel samples is shown. Based on the previous research, five parameter groups were selected. To make this analysis broader, research results of AM parts have been compared to the conventionally made counterparts. Such an approach allowed analyzing the manner different parameters affect the tensile and LCF behavior. The preliminary tests indicated that AM specimens are characterized by 65 pct of the total LCF strength in comparison to the conventionally made material. Further LCF tests indicated differences in the dissipated energy of some samples, which was visible in the hysteresis loops generated during testing in the total strain amplitude range from 0.30 to 0.45 pct. Based on the Morrow approach, it was possible to register an increased share of the plastic component during the fracture process in the Additive Manufacturing (AM) parts in the LCF tests with the total strain amplitude above 0.45 pct. The final microscopical investigation of parts’ fractures surfaces indicated the influence of the layered structure, and internal imperfections (such as unmelted powder particles and lack of fusion) of the as-built AM parts on the cracking process, which caused an increased number of multiplanar cracks and generation of the complex fracture morphology characterized by the layered structure of AM parts and share of imperfections—mostly porosity caused by unmelted powder particles which potentially was a base of secondary stage cracks.

Amount of hydrogen absorbed into 316L austenitic stainless steel and its correlation with properties of oxide film, dissolved oxygen concentration and low-cycle fatigue behaviors of this material in borated and lithiated high temperature water

It was found that the low-cycle fatigue (LCF) life of 316L austenitic stainless steel (ASS) in high temperature water had a negative correlation with the amount of hydrogen absorbed into the material per cycle of the LCF test. Effects of dissolved oxygen (DO) on reducing the amount of hydrogen absorbed into the materials started to appear even though only 7 ppb DO was added to the water. Instead of oxide film properties and the corrosion rate, the DO concentrations of the bulk water and the water inside the cracks were considered the key factors affecting the amount of hydrogen absorbed.

Mechanical Behaviour of Human and Porcine Urethra: Experimental Results, Numerical Simulation and Qualitative Analysis

Low urinary tract dysfunctions and symptoms (LUTS) affect both men and woman, with the incidence increasing with age. Among the LUTS, urinary incontinence (UI) is a common dysfunction, characterised by the involuntary loss of urine. These medical conditions become debilitating, with a severe impact on patients’ routines and overall well-being. To mitigate LUTS-associated symptoms, the mechanical behaviour of both normal and LUTS-affected urethrae can be an important tool. The current work approaches the porcine urethra as a mechanical replacement candidate for the human urethra. It aims to provide a framework based on in silico (numerical) simulations and experimental data, to compare the candidate’s mechanical behaviour against the human urethra. Porcine urethral samples were mechanically evaluated through low-cycle fatigue tests in both circumferential and longitudinal orientations. The specimens were collected from porcine urethrae from crossbred pigs raised for human consumption. The experimental results were compared with human references found in the literature, with similar experimental conditions. The experimental data were used as the input for the mechanical properties estimation (nonlinear fitting to hyperelastic constitutive models) and for the simulation of the urethral tensile behaviour, using those models. In the longitudinal orientation, the results for the porcine and human urethra were in good agreement, while in the circumferential direction, the differences increased with deformation. Previous data on the mechanical behaviour of the equine urethra is in line with these findings. The nonlinear mechanical behaviour of a porcine urethra was modelled using the finite element method (FEM) and hyperelastic constitutive models. For the longitudinal urethra, the simulation results approximate experimental data for stretches up to λ≈1.5 (50% deformations), whereas for the circumferential urethra, the same was true for stretches up to λ≈1.35 (35% deformations). The hyperelastic models with a higher number of parameters performed better with the third-order Ogden model (six parameters), displaying the best performance among the studied models. The pig urethra is a suitable candidate for an implant targeted at human urethra replacement or as a model to study the human urinary system. Nevertheless, the data available on the circumferential mechanical behaviour need to be consolidated with additional mechanical tests. The tensile behaviour of the porcine urethra over large deformations can be modelled using the third-order Ogden model; however, to extend the modelling capabilities to larger deformations requires the use of hyperelastic models more adequate to soft tissue behaviour.

Creep-fatigue properties of austenitic cast iron D5S with tension and compression dwell: A dislocation density-based crystal plasticity study

To predict and better understand the creep-fatigue behaviour of austenitic cast iron D5S under tension and compression dwell at 800∘C, a physics-based crystal plasticity model that describes the complex rate- and temperature-dependent deformation of the material as a function of the dislocation density is implemented. In addition to the tension and compression dwell direction, the effect of three different dwell times (30, 180 and 600 s) on the creep-fatigue properties is investigated. The dislocation density-based crystal plasticity simulations are compared to experimental tests from a prior work. While relaxation tests and low-cycle fatigue (LCF) tests without dwell assist in systematically identifying the material parameters, creep-fatigue (CF) data is used to validate the predictions. The virtual testing is performed on a large-scale representation of the actual test specimen with a polycrystalline structure. To analyse the fatigue damage mechanism, small-scale predictions are also conducted using a micromechanical unit cell approach. Here, a single graphite nodule frequently found in the material is embedded into the austenitic matrix. In the present work, a close agreement is achieved between the predicted CF behaviour and the experimental results. Consistent with the experimental findings, the simulation results show that the addition of compression dwell leads to an uplift of the overall tensile stress level, which significantly reduces the fatigue life of the material. The unit cell studies demonstrate that during this uplift, a strong localisation of stresses and strains arises at the graphite/matrix interface, triggering the nucleation and growth of cavities and/or debonding.

Multiaxial low cycle fatigue behavior and life prediction of CP‐Ti under non‐proportional stress‐controlled mode

AbstractMultiaxial low cycle fatigue tests under non‐proportional stress (NPSS) controlled mode were performed on commercial pure titanium (CP‐Ti). Strain responses of axial and torsional channels under highly applied stress amplitudes show an initial hardening phenomenon. Non‐proportional hardening coefficient of CP‐Ti is independent of the controlled mode. The critical plane of CP‐Ti under NPSS controlled mode is aligned with the maximum principal stress plane proved by optical microscopy observation. Optimized FSM model and KBM‐PM model with mean axial and torsional strain are established. These models are further integrated into equations related to multiaxial stress ratio with high accuracy of life prediction for CP‐Ti under NPSS controlled mode.