Strain-controlled Fatigue Tests Research Articles

Fatigue behavior and microstructural evolution of additively manufactured Inconel 718 under cyclic loading at elevated temperature

This study investigates the fatigue behavior of Inconel 718 (IN718) fabricated through laser beam directed energy deposition (LB-DED) process. Fully-reversed (Rε = −1) strain-controlled fatigue tests are conducted on LB-DED IN718 specimens at 650 °C and the results are compared with those of LB-DED IN718 specimens tested at room temperature as well as the ones obtained from wrought counterparts at both room temperature and 650 °C. The microstructure is characterized after fabrication, heat treatment, and fatigue tests at elevated temperature. Microstructure characterizations reveal that as-built LB-DED IN718 has a dendritic structure. The heat treatment applied could not eliminate the dendritic structure, and needle-like δ phases are formed on the grain boundaries. The comparison of LB-DED and wrought fatigue results show that the LB-DED IN718 possesses slightly lower fatigue resistance at room temperature, while exhibits a comparable fatigue strength at elevated temperature. In the low cycle regime, the fatigue resistance of LB-DED IN718 tested at elevated temperature is somewhat inferior to the ones tested at room temperature, which can be explained by the formation and growth of brittle needle-like δ and Laves phases on the grain boundaries during fatigue testing at elevated temperature and their effects on increasing the fatigue crack growth rate. In the high cycle regime, however, LB-DED IN718 specimens tested at elevated temperature had somewhat similar fatigue lives to the ones tested at room temperature. This may be partially explained by the surface oxidation during fatigue testing at elevated temperature, which can retard the microstructurally short cracks to grow.

Assessment of potential service-life performance for MarBN steel power plant header under flexible thermomechanical operations

This paper is concerned with the service-life assessment of 9Cr steels superheater outlet steam header subjected to realistic subcritical and future ultra-super critical flexible operating conditions. The proposed methodology is achieved via a combined program of high temperature strain-controlled fatigue tests, temperature- and time-dependent unified viscoplastic model for thermomechanical fatigue analysis, and Smith-Watson-Topper critical plane criterion for multi-axial life prediction. Samples of idealised operational transients with particular attention on the starting-up cycle is fully coupled with the computational modelling of header component for the high temperature performance assessment. The predicted results indicate that: (i) the header shell inner-bore saddles at weld regions are the critical locations that lead to earlier potential fatigue crack initiation, and (ii) the predicted lifetime under subcritical conditions correlates reasonably with the industrial experience. A steam header manufactured from MarBN operating under ultra-super critical condition is shown to have comparable life performance with the P91 header operating under subcritical condition.

Effect of ultrasonic shot peening on strain controlled fatigue behaviour of low nickel austenitic stainless steel

In the present investigation effect of Ultrasonic shot peening (USP) treatment on strain controlled fatigue behavior of low nickel austenitic stainless steel (Low Ni ASS) was studied at room temperature. The gauge section of fatigue samples were USPed with 2 mm diameter hard steel shots for 3 min. Microstructural characterization of the USP treated samples were carried out using XRD, SEM and TEM techniques. Refinement of grains in the surface region was observed and grain size was found to be ≈17 nm from quantitative analysis of XRD patterns. Strain controlled fatigue tests were carried out at different total strain amplitudes (Δεt/2) ranging from ±0.4% to ±0.8%. Fatigue life was increased significantly by 200% at lowest Δεt/2 of ±0.4% whereas it was reduced at highest Δεt/2 of ±0.8% by 50%. Fracture surfaces were characterized using SEM.

Fatigue of Tungsten Fiber/Copper Matrix Composite—An Interlaboratory Study

Abstract Seven laboratories participated in an interlaboratory study to assess the applicability of ASTM E606, Test Method for Strain-Controlled Fatigue Testing, to strain-controlled fatigue of a tungsten fiber/copper matrix composite (Battelle Columbus, University of Iowa, IITRI, MCL, MarTest, Rockwell International Science Center and University of Twente). Because of the material and specimen fabrication methods, the number of fibers in the 9, 25, and 36 % fiber volume fraction specimens is approximately the same and the strain-life data are nearly identical for each of the volume fractions. Although the specimen aspect ratio is within the nominal range, the specimen diameter is less than the nominal value specified in ASTM E606. Laboratories reporting data noted that a small specimen diameter and a large gage section aspect ratio made specimen alignment difficult. Thus, specimen buckling was problematic at high strain ranges. The study data are reported and analyzed with nonparametric and semiparametric statistical methods to assess the effect of study covariates on fatigue life. The median fatigue life, median absolute deviation from the median, and interquartile range are reported as measures of central tendency and variability. ASTM E606 may be used as a guide to evaluate the fatigue response of this type of composite material if the length-to-diameter aspect ratio, specimen alignment in the fatigue machine, and specimen surface finish are closely monitored to ensure valid data.

Fatigue characteristics of the extruded AZ80 automotive wheel

In order to provide a reliable theoretical foundation for fatigue resistant design of the magnesium alloy automobile wheel, specimens machined from rim and disc of the extruded AZ80 wheel were subjected to strain-controlled fatigue tests. Various characterizations (such as microstructure and texture analysis, stress-strain response monitoring, twinning/detwinning behavior evolution, and fatigue fracture morphology analysis) have been carried out to investigate the static and fatigue performance of the AZ80 wheel. The obtained microstructural analysis revealed that the rim sample exhibited finer and more homogeneous dynamic recrystallized grains with the average grain size of ~17.2 μm, while ~30.5 μm for the disc sample. The rim specimen achieved the ultimate tensile strength of ~339 MPa with a noticeable elongation to fracture of ~14.6%. In addition, the rim sample also showed superior fatigue properties as compared with the disc sample, which was related to the smaller grains and the weaker texture intensity. The twinning/detwinning behavior participated in the cyclic deformation above the strain amplitude of 0.4% for both samples, resulting in asymmetric hysteresis loops and residual twins. The fatigue crack initiation site was observed with second phase particles and secondary cracks near the surface for all specimens, and in particular, cleavage-like faceted area with twins was identified at strain amplitude of 1%.

Investigation on fatigue crack resistance of Q370qE bridge steel at a low ambient temperature

The Q370qE steel has a wide application in the medium and long span railway steel bridges in China, due to its excellent material characteristics including high strength, high toughness and good weldability at room temperature. Nevertheless, the limited research work on the fatigue properties of the Q370qE steel challenges the safety of steel bridges in cold and severe cold regions under the cyclic actions. This study first conducts a series of strain-controlled fatigue tests of coupon specimens at room temperature and −60 °C to characterize the cyclic stress-strain curve, hysteresis loops, Masing behavior and fatigue crack initiation resistance by Coffin-Manson relationship. Thereafter, the fatigue crack propagation tests of the compact tension specimens under different stress ratios and different temperatures reveal the effect due to stress ratios and temperatures on the fatigue crack propagation resistance of the Q370qE steel through the Paris formula. Results demonstrate that the low temperatures enhance the resistance to the fatigue crack initiation and propagation of the Q370qE steel. Finally, to better quantify the performance of the Q370qE steel, this paper collects different bridge steels and compares the fatigue crack initiation resistance and fatigue crack propagation between the Q370qE steel and other bridge steels. The results quantify the excellent higher fatigue resistance of Q370qE steel in comparison with other bridge steels.

Mechanical characterization of laser-welded double-lap joints in ultra-high and low strength steels for sandwich panel applications

This study is to investigate the microstructure and the mechanical properties (hardness, strength, fatigue resistance) of laser welded double-lap joints from two different steels, ultra-high strength abrasion resistant steel (AR400) and low strength structural (S355 MC) steels. The laser welding was conducted at two different energy density 170 and 958 J/mm2 and constant power 3 kW. The morphology and the microstructure of the cross-section lap joints have examined by optical microscopy. The shear strength and the fatigue resistance of the double-lap joints were evaluated by uniaxial tensile tests and fully reversed tension–compression strain-controlled fatigue tests, respectively. Whereas the hardness of the welded lap joints of UHS steel has been decreased due to the tempered martensite formation. The microstructure of the laser welded lap joints of LS steel shows martensite formation in the fusion zone. Hence, the hardness has been significantly increased to 300 Hv. The shear strengths of the welded lap joints have significantly influenced by the laser power density. Significant improvement in the strength could be observed in the strength of the lap joints of UHS steel at energy density 958 J/mm2. Fatigue cracking along steels sheets boundaries that intersect the weld zone in the welded lap joints and the crack propagation towards the weld zone observed to be the fatigue-induced microstructural features in the lap joints welded by laser.

Modelling the test methods used to determine material compatibility for hydrogen pressure vessel service

This work aims to create finite element models to simulate the three ISO 11114-4 test methods applicable to hydrogen gas cylinders, coupled with calibrated constitutive models, to predict the deformation response of each. Experimental measurements are used to calibrate a monotonic constitutive model and a constitutive model of cyclic deformation. Six finite element solid models are discussed: monotonic tensile test of dog bone-shaped specimens, strain-controlled fatigue test of dog bone-shaped specimens, ISO test Method A, ISO test Method B, and ISO Method C (from ISO 11114-4), and a gas cylinder. Each finite element solid model is paired with the appropriate constitutive model based upon loading conditions. The modeling results are then combined with a new damage parameter in an attempt to compare each of the test methods to the others, as well as to in-service conditions. It is shown that the proposed damage parameter may be used to correlate all test methods considered (except for ISO Method A, a burst-disc test) as well as in-service conditions. The calibrated damage parameter may be coupled with any geometry, loading condition, and boundary condition modeled within a finite element package to predict the onset of critical damage in the material for which the coupled constitutive model is calibrated to. Parametric modelling study results provide estimated cycles to the onset of crack extension for DOT 3AA cylinders having varying sizes of internal thumbnail-shaped cracks. This work provides the baseline for measurements and models in air, with similar work in hydrogen to follow.

Non-destructive testing derived parameters for microstructure-based residual service life assessment of aging metallic materials in nuclear engineering

Abstract Metallic components in nuclear engineering are exposed to extensive loads such as pressurization and temperature changes which can affect the properties of the material significantly depending on the load spectrum applied. In view of developing a procedure to evaluate the residual service life of metallic components in nuclear power plants aged during service, metastable austenitic steel AISI 347 (German designation: X6CrNiNb18−10) has been considered as an example. To this purpose, total strain-controlled fatigue tests were carried out under different environmental conditions and monitored by continuously measuring thermometric, resistometric, electromagnetic and electrochemical parameters. These parameters provide an information gain in terms of material characterization when compared to conventional strain measurements. Based on these parameters, the short time evaluation procedure StrainLife has been developed, which allows the determination of local S-N curves with a significantly reduced effort as compared with traditional procedures. This method has been implemented into the structural simulation program PROST for the integrity assessment of the components while considering local fatigue properties. This very effective method allows for the determination of local fatigue properties including the strain-specific local scatter of the metallic microstructure properties of the material which has not been possible by traditional means.

Low cycle fatigue performance of DP600 steel under various pre-straining paths

The present work concerns the assessment of low cycle fatigue performance of dual phase steel subjected to change in uniaxial pre-strain path. An uniaxial pre-strain of 12.5% in the rolling or transverse direction were imposed on the dual phase steel blanks, thereafter fully reversed strain controlled fatigue tests were conducted parallel or orthogonal to the initial pre-straining direction. As received specimens depict mild cyclic hardening in the initial cycles followed by gradual cyclic softening until failure. However, all pre-strained materials exhibit continuous cyclic softening with higher magnitude and rate throughout the life as compared to the as received specimen. Relaxation of mean stresses were observed for the fatigue tests with pre-straining. Rotation of principal axis due to orthogonal reloading (i.e. low cycle fatigue) after initial pre-straining introduces non-proportionality, thereby leading to significant reduction of fatigue lives. In-grain misorientation and lattice rotation analyses are noticed during electron backscattered diffraction investigation, and which is due to the non-proportional loading effect.

Effect of cold-working process on cyclic deformation of electrolytic copper

Abstract Cyclic softening and hardening processes are expressed by the change in the stress amplitude necessary to cause a given strain amplitude. Understanding the cyclic stress-strain behavior of materials is an important step in the complex study of their fatigue behavior. The potential differences between the defect structures of cold-formed materials may also be related to internal stress changes. Considering that wiredrawing and rotary swaging apply distinct forces on the material in order to obtain the product, differences may arise in the defect structures that can, consequently, affect its mechanical behavior. This study aims at evaluating and comparing, by means of strain-controlled fatigue tests, the cyclic behavior of polycrystalline electrolytic copper cold-formed by wiredrawing (WD) and rotary swaging (RS) with 87% area reduction. The fatigue test results evidenced the highest resistance to cyclic deformation presented by WD material in the low cycle regime. It was observed that the strain hardening for both cold forming conditions is related to a great increase of long-range stresses in the defect structure and the cyclic softening is related, mainly, to the subsequent drop of that stresses. The WD internal stresses resulted slightly bigger than those of the RS condition.

Strain-controlled fatigue and fracture of AISI 410 stainless steel

Microstructural characteristics, monotonic and strain-controlled cyclic axial behaviors of AISI 410 stainless steel were investigated. Grain size and distribution is uniform throughout the alloy's microstructure. Tensile behavior is characterized with high ductility yet mild strain hardening. Strain-controlled fatigue testing was conducted at strain amplitudes ranging from 0.3% to 0.9%. Cyclic stress softening was observed at all applied strain amplitudes. Hysteresis loops show that stress-strain stabilization was not reached by the alloy. Cyclic softening rate decreases with fatigue life and its ratio was independent of strain amplitude above 0.5%. An accumulating plastic damage behavior was exhibited by the alloy. Fatigue life data were well correlated with Coffin-Manson and plastic energy density damage equations. Multiple cracks were observed in specimens under monotonic and cyclic loadings.

Cumulative nonlinearity as a parameter to quantify mechanical fatigue

AbstractThe cumulative nonlinearity (Qf) is proposed as a new parameter to quantify mechanical fatigue. The parameter Qf represents the (cycle number) integral of the ratio of the third harmonic to the fundamental one (I3/1), obtained via Fourier transform of the stress (torque), normalized by the square of the strain amplitude (γ0). The validation is performed on different polymers: polystyrene (PS) with different molecular weights, styrene‐acrylonitrile (SAN), polymethylmethacrylate (PMMA), and polytertbuthylmethacrylate (PtBMA) under strain controlled fatigue tests in a torsion rectangular set‐up. A power‐law correlation between the number of cycles to failure (Nf) and the cumulative nonlinearity was found for Nf > 1000 cycles, and the results were compared with well‐established failure criteria such as the cumulative stress and the dissipated energy density. It was found that the cumulative nonlinearity has superior prediction ability since it can quantify material changes during a test.

Analysis of the Fatigue Damage Behavior of AW2099-T83 Al-Li Alloy under Strain-Controlled Fatigue

Microstructural characteristics, monotonic and strain-controlled cyclic axial behaviors of AW2099-T83 Aluminum-Lithium alloy were investigated. Grain sizes and structures are not uniform in the different orientations studied. High strength and low ductility characterize the tensile behavior of the alloy under static loading. Strain-controlled fatigue testing was conducted at strain amplitudes ranging from 0.3% to 0.7%. Over this range, macro plastic deformation was only observed at 0.7%. Cyclic stress evolution was found to be dependent on both the applied strain amplitude and the number of cycles. Limited strain hardening was observed at low number of cycles, followed by softening, due probably to damage initiation. With low plastic strain, analytical approach was adopted to profile the damaging mechanism for the different applied strain amplitude. Because of the absence of fatigue ductility parameters due to low plasticity, a three-parameter equation was used to correlate fatigue life. Fractured specimens were studied under SEM to characterize the fracture surface and determine the controlling fracture mechanisms. The fractography analysis revealed that fracture at low strain amplitudes was shear controlled while multiple secondary cracks were observed at high strain amplitude. Intergranular failure was found to be the dominant crack propagation mode.

Effect of LCF on behavior and microstructure of microalloyed HSLA steel and its simulated CGHAZ

Numerous structural components made of high strength low-alloyed steels are often exposed to low-cycle fatigue loading during their exploitation life. In order to study the fatigue behavior of one grade of structural steels produced in Yugoslavia during the eighties of the last century, a push-pull constant amplitude strain-controlled fatigue tests were conducted at room temperatures. This paper analyses behavior of two different materials, parent steel (PM) and its coarse-grained (CG) simulated heat-affected zone (SHAZ) exposed to the action of low-cycle fatigue (LCF) loading performed in the range from 1 to 104 cycles. LCF tests were conducted at strain ratio R = -1 at strain amplitude range from 0.3% to 0.8%. The materials' Coffin-Manson parameters appearing in the strain-life and cyclic stress-strain curves were calculated. Fracture surface analysis of both materials was done. The results showed that plasticity under dynamic load conditions is satisfactory for both materials which give them good resistance to crack initiation and propagation.

Optimization of Post-processing Annealing Conditions of the Laser Powder Bed-Fused Ti–18Zr–14Nb Shape Memory Alloy: Structure and Functional Properties

Ti–18Zr–14Nb (at%) shape memory alloy was processed by laser powder bed fusion (LPBF) and subjected to post-processing annealing treatments in the 500–800 °C temperature range. The microstructure, crystallographic texture, static mechanical properties, and low-cycle fatigue behavior of this alloy in the as-built state and after different post-fusion annealings have been studied. It was found that a strongly columnar microstructure developed during LPBF processing morphed into a predominantly equiaxed grain structure after 800 °C recrystallization annealing. However, the highest number of cycles to failure during high-intensity strain-controlled fatigue testing (2% of strain in a cycle) was obtained after annealing at 500 °C, whereas the lowest number of cycles was found after annealing at 700 °C. A beneficial combination of static and fatigue mechanical properties with a relatively low Young’s modulus makes 500 °C-annealed LPBF Ti–18Zr–14Nb components suitable for biomedical applications, especially where the capacity of LPBF to manufacture geometrically complex and patient-specific load-bearing components makes a difference.

Low Cycle Fatigue Properties of Extruded 6061-T6 Aluminum Alloy

This paper presents the low cycle fatigue properties of an extruded 6061 aluminum alloy. The cyclic strain-controlled fatigue tests were performed under fully reversed total strain amplitudes ranging from 0.5% to 1.3% at a fixed strain rate of 0.004 s−1 using smooth specimens cyclically loaded along with the extrusion directions. The fatigue test results represented by the hysteresis curves of stress-strain show slight Bauschinger effect at the total strain amplitudes (Δεat) ≤ 0.7 %. The corresponding cyclic strain response revealed that the alloy underwent cyclic softening particularly at the lower strain amplitude (Δεat ≤0.7%) and at the higher strain amplitudes, the alloy tended to undergo cyclic hardening. The strain-fatigue life models were empirically determined by the Coffin-Manson-Basquin relationships.

Fatigue resistance of dual-phase NiTi wires at different maximum strain amplitudes

Known by their functional shape memory and superelastic properties, aligned with an excellent biocompatibility, near equiatomic NiTi alloys are vastly used as biomaterials. In various applications, such as endodontic files and cardiovascular stents, NiTi components are subjected to aggressive fatigue conditions, making fatigue resistance and mechanisms relevant to the materiaĺs development. In this context, heat treatments can be a powerful tool to improve the fatigue life and efforts have been made to understand and predict the fatigue behavior of these alloys in different conditions. In this work, initially superelastic near equiatomic Ni-rich wires were heat-treated at 400 °C and 450 °C and the ε × Nf curve was obtained. Rotating-bending strain-controlled fatigue tests were performed at maximum strains varying from 1% to 5% at 37 °C. The results showed that both samples exhibited large fatigue resistance and similar fatigue behavior through the different maximum strains, and thus, treatments within the range of temperature from 400 °C to 450 °C are recommended to obtain NiTi components with high fatigue life for large maximum strain amplitudes.

A strategy to fast determine Chaboche elasto-plastic model parameters by considering ratcheting

Ratcheting simulation for steel material is of importance especially when it is subjected to asymmetrical cyclic loading. A strategy to fast determine the Chaboche elasto-plastic model parameters is proposed by considering the ratcheting effect. First, one cyclic ascend experimental test with an equal strain increment of ±0.1% after each twenty-cycle loading phase up to ±1.2% is performed to observe the Masing effect. Then, a strain-controlled fatigue test is conducted at a strain range of ±0.8% to identify the parameters of Chaboche combined hardening model. Besides, a ratcheting test with stress variation from −361.6 MPa to 441.6 Mpa is executed. Based on these experiments, two computational programs are carried out to simulate the cyclic stress-strain response and ratcheting, respectively. In the parameters identification process, a pseudo-percent allocation method is developed to reduce the stress overestimation. Results indicate that: (i) 304SS presents Masing effect when the strain range is bound within ±0.9%, and it follows the previous experimental results about the serrated flow and ratcheting; (ii) The Armstrong-Frederick rules reverse in a small strain when the isotropic hardening considered, so the predicted ratcheting strains for the first few cycles are inevitably underestimated; (iii) Not only the third Armstrong-Frederick hardening rule, but the second Armstrong-Frederick hardening rule has also an effect on the ratcheting simulation.

Experiments and Modeling of Fatigue Behavior of Friction Stir Welded Aluminum Lithium Alloy

An extensive experimental and computational investigation of the fatigue behavior of friction stir welding (FSW) of aluminum–lithium alloy (AA2099) is presented. In this study, friction stir butt welds were created by joining AA2099 using two different welding parameter sets. After FSW, microstructure characterization was carried out using microhardness testing, scanning electron microscopy, and transmission electron microscopy techniques. In particular, the metastable strengthening precipitates T1 (Al2CuLi) and δ’(Al3Li) seen in the base metal were observed to coarsen and dissolve due to the FSW process. In order to evaluate the static and fatigue behavior of the FSW of the AA2099, monotonic tensile and fully-reversed strain-controlled fatigue testing were performed. Mechanical testing of the FSW specimens found a decrease in the ultimate tensile strength and fatigue life compared to the base metal. While the process parameters had an effect on the monotonic properties, no significant difference was observed in the number of cycles to failure between the FSW parameters explored in this study. Furthermore, post-mortem fractography analysis of the FSW specimens displayed crack deflection, transgranular fracture, and delamination failure features commonly observed in other parent Al–Li alloys. Lastly, a microstructurally-sensitive fatigue model was used to elucidate the influence of the FSW process on fatigue life based on variations in grain size, microhardness, and particle size in the AA2099 FSW.