Low Cycle Fatigue Tests Research Articles

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.

Fatigue Reliability Prediction Method of Large Aviation Planetary System Based on Hierarchical Finite Element

The reliability of planetary equipment determines the economic affordability and service safety, to a large extent, for a helicopter transmission system. However, with the continuous improvement of the progressiveness and large-scale degree of new aviation planetary equipment, the contradiction between reliability design indexes and R&D economy is also gradually highlighted. This paper takes the large aviation planetary system as a research object, aims to accurately evaluate the system reliability level formed in design processes, and deeply excavates the inherent characteristics of the planetary system in functional realization and builds a system fatigue reliability evaluation model accordingly. An advanced hierarchical finite element technology is used to calculate dangerous tooth load histories under the influence of system global elastic behavior, and the tooth probability fatigue strength is obtained through the gear low-cycle fatigue test and life distribution transformation method, so as to provide economic load and strength input variables, respectively, for the reliability model. This prediction method can provide targeted structural optimization guidance in the development and design of the large aviation planetary system and significantly reduce the cost of reliability index realization for this kind of large-scale, high-end equipment in design iteration processes.

Experimental and Numerical Study on LCF Crack Propagation of Coiled Tubing Steel

Coiled tubing (CT) is a joint-less long oil pipe that is wound around a reel and can be run and pulled continuously. Due to the particularity of the operating process, low-cycle fatigue (LCF) failure of the CT constitutes the main production cost. Aiming at the characteristics of small diameter and thin wall of CT, a single-edge-notched (SEN) arc specimen was designed and machined. LCF tests were conducted with force-controlled mode. Cyclic softening of the CT steel was presented and crack growing rates were measured. Meanwhile, finite element simulation was carried out to obtain the relationships among J-integral, crack size and load. Based on the experimental and numerical results, the speed of the LCF crack growth of the CT steel is expressed as an explicit function of the J-integral. It provides a basis for predicting the LCF life of the CT under working conditions from the perspective of crack propagation.

Experimental study on flexural fatigue behavior of composite T-beams in ultra-high performance concrete reinforced and normal-strength concrete

The purpose of this study is to investigate the flexural fatigue properties of reinforced ultra-high performance concrete (RUHPC) T-beams, and composite T-beams of normal-strength concrete-RUHPC (NC-RUHPC) and damaged reinforced concrete (RC) strengthened with RUHPC (RC-RUHPC) under constant-amplitude low-cycle fatigue tests. Firstly, the loading value in the fatigue test was obtained by static load tests. Then, the relationship between fatigue deflection, bending stiffness, fatigue crack width, fatigue crack width growth rate, and fatigue life of RUHPC, NC-RUHPC, and RC-RUHPC T-beams were studied with the fatigue tests. The test results show that one main crack is only formed near the midspan in the later stage of fatigue loading, other bending cracks are micro-cracks, and their propagation is slow and short. The fatigue fracture of steel bars embedded in tested beams occurs after more than 3.7 million cycles. When the steel bars are undergoing fatigue fracture, their stress is significantly lower than the yield strength, and two kinds of fracture surfaces are produced: intergranular fracture and transgranular fracture; the mechanical properties of steel bars are changed in the strength index and plasticity index after fatigue fracture. The cost of UHPC and other materials can be reduced by using NC-RUHPC or RC-RUHPC T-beams in practice.

Effects of Vanadium Addition on Strain Distribution, Crack Initiation and Propagation during Low-cycle Fatigue Test in Ferrite and Martensite Dual-phase Steel

Effects of Vanadium Addition on Strain Distribution, Crack Initiation and Propagation during Low-cycle Fatigue Test in Ferrite and Martensite Dual-phase Steel

Effect of hydrogen on low-cycle fatigue behavior of HRB400 steel under asymmetric cyclic loading

This paper focuses on the study of ductility changes, ratcheting, and low-cycle fatigue behavior of materials after hydrogen charging. Hot-rolled ribbed bar 400 (HRB400) steel was electrochemically charged with H2SO4 solution, and the samples subjected to different hydrogen charging times were subjected to quasi-static tensile and low-cycle fatigue tests under asymmetric cyclic loading. The tests indicated that hydrogen charging changed the initial yield stress of the material, but the ultimate strength remained almost unchanged; hydrogen charging significantly changed the cyclic hysteresis behavior of the material, which increased the elastic range of the stable hysteresis curve; hydrogen charging changed the pattern of ratcheting deformation. The pattern of ratcheting deformation in the later stage was very different from that of the sample without hydrogen charging; hydrogen charging reduced the ductility of the material and resulted in a significant decrease in fatigue life. The fracturing of the hydrogen-charged samples exhibited apparent quasi-cleavage and intergranular fracture characteristics. The non-hydrogen-charged samples had two failure modes (fatigue failure and increased ratcheting deformation resulting in necking instability), whereas the hydrogen-charged samples had only one failure mode of fatigue failure. Based on the experimental analysis, a fatigue life equation that simultaneously considers the effect of hydrogen embrittlement index, mean stress, and stress amplitude is proposed. The equation was used to predict the low-cycle fatigue life of hydrogen-charged specimens under asymmetric cyclic loading, and the error between the results and the measured values was in the two-factor band.

An LCF lifetime model for PM superalloy considering equivalent ellipsoidal inclusion

The non-metallic inclusions are one of the main defects in powder metallurgy (PM) superalloy, which may promote crack initiation under the cyclic loading and seriously affect the safety of aero engines. To quantify the effects of inclusions to the low cycle fatigue (LCF) lifetime, the LCF tests on PM FGH96 superalloy are firstly carried out to observe the inclusions on fracture. By equating the inclusion to an ellipsoid, the relationship between the inclusion's feature and the stress field is established based on Eshelby's theory and finite element method (FEM). Then, the ellipse-equated fracture is determined according to the maximum shear stress's location. Subsequently, key inclusion-related parameters influencing the LCF lifetime are introduced to construct a modified SWT model. Finally, the model's accuracy is validated by comparing with the LCF experimental data. It is demonstrated that the modified SWT model has a good accuracy, with an improvement of scattering band from ±15 to ±4 compared with the original SWT model.

Low Cycle Fatigue of G20Mn5 Cast Steel Relation between Microstructure and Fatigue Life.

Cast steel is commonly used to produce structural and safety parts. Foundry processes allow producing parts from scrap steel directly to the required dimensions without any forming operation. Cast components may, however, exhibit macro- and micro-shrinkage porosities. The combined effect of macro- and micro-shrinkages on the fatigue behavior of cast steel has been characterized in the literature. Macro-shrinkages may nowadays be eliminated by adequate positioning of risers. However, micro-shrinkages will always be present in cast steel components. Present work addresses the influence of micro-shrinkage porosity on a G20Mn5 cast steel. G20Mn5 (normalized) ingots have been cast under industrial conditions, but ensuring the absence of macro-porosities. Solidification leads to two very different microstructures prior to the normalization treatment: columnar dendrites beneath the surface (Skin) and equiaxed microstructures close to the center (Core). First, metallographic observations of the whole ingot revealed the same grain size in both areas. Fatigue samples were extracted, by differentiating two sampling volumes corresponding to columnar (S) and equiaxed solidification (C), respectively. The distribution of micro-porosities was determined on all samples by Micro-CT-scans. Core samples exhibit micro-porosities with volumes 1.7 larger than Skin samples. Low cycle fatigue tests (3 levels of fixed plastic strain) were run on both sample series (C, S). Results follow a Manson–Coffin law. Core specimens exhibit lower fatigue life than Skin specimens. The differences in fatigue life have been related successfully to the differences in micro-porosities sizes.

Challenge of Establishing Materials Testing Standards with Miniature Specimen for Solders

Abstract Solder is an essential material for connecting parts inside electrical devices. Although there is an international standard for the chemical composition of solder, there is no testing standard for evaluating the mechanical properties and characteristics of solder as a structural material. The Committee on High Temperature Strength of Materials, the Society of Materials Science, Japan, proposed some testing standards for solders in the early 2000s based on the results from research activities of the Solder Strength Evaluation Method Working Group. The primary objective of the standardization at that time was to acquire stable testing data and to eliminate institutional differences. The recommended specimen diameter is 10 mm. To verify the size difference between the 10-mm-diameter specimen and an actual solder joint inside an electrical device, the working group has been investigating materials testing techniques using a specimen with a reduced diameter of 3 mm. The research results were published in 2020 as Standard for Miniature Testing of Solders. We have also conducted a series of systematic experiments based on the testing standards for Sn-3.0Ag-0.5Cu. The results obtained so far are as follows: In the tensile tests, the tensile strength of the 10-mm- and 3-mm-diameter specimens are almost identical, independent of the specimen size, although the casting method and heat treatment conditions are different. The effect of specimen size on the creep rupture life is insignificant. In the low-cycle fatigue tests, stable fatigue data can be obtained even at high temperatures using the 3-mm-diameter specimen.

Softening response of cast Al‐Si‐Cu samples tested under low‐cycle‐fatigue and fatigue‐creep regime and elevated temperate: Experimental and analysis

AbstractIn this paper, the strain‐controlled low‐cycle‐fatigue (LCF) test and low‐cycle‐fatigue‐creep (LCFC) test of a cast Al‐Si‐Cu alloy at 350°C were investigated. A significant softening behavior can be observed in the tests, the strain amplitude has an effect on softening speed, and the dwell time has an effect on both softening ratio and softening speed. Microscopic observation indicated that the compressive load of creep could result in a significant amount of plastic deformation and lead to the eutectic silicon particles rupture and the propagation of dimples, causing the softening behavior and the decrease of failure life of Al‐Si‐Cu alloy. Based on the experiment data, a modified visco‐plasticity model combining Chaboche model and θ‐projection model was proposed to capture the cyclic stress response. The modified model could simulate the softening behavior and hysteresis loop for LCF test and LCFC test of Al‐Si‐Cu alloy accurately.

Low cycle fatigue behavior of Cu‐Cr‐Zr alloy with different cold deformation

AbstractThe present study addressed the cyclic deformation behavior and fatigue properties of Cu‐0.69Cr‐0.07Zr alloy with different cold deformation (ε = 64%, 75%, and 84%) using low cycle fatigue test. Low cycle fatigue tests were conducted under fully‐reversed conditions at different total strain amplitudes. The microstructure changes and fatigue fracture characteristics were analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM). The main findings suggest that the Bauschinger effect was significantly stronger with larger deformation at low total strain amplitude. And it was proved that the relationship between the total strain amplitude and the low cycle fatigue life of Cu‐Cr‐Zr alloy with different deformation can be expressed by the Manson–Coffin–Basquin formula. Further, the reason for the fatigue life was shorter and the cyclic softening rate decreased faster at high applied total strain amplitude was that the dislocation density decreased due to the rearrangement of the dislocations.

Investigation of extremely low cycle fatigue behavior of low yield strength steel LY225 under different stress states

• Monotonic tensile and ELCF tests were conducted for LY225 low yield point steel. • ELCF fracture behavior between LY225 steel and Q235 steel was compared. • The LCVGM parameters of LY225 steel were calibrated based on the experimental results. • Buckling and ELCF behavior of LY225 steel dog bone specimen were experimental studied. • The LCVGM can well simulate the post buckling ELCF fracture of dog bone specimen. Low yield point steel is frequently utilized in the energy dissipation components of structures because of its good seismic performance. A low yield point steel LY225 subjected to different stress states is experimental investigated in the study by the monotonic tensile tests and extremely low cycle fatigue tests. The test results demonstrate that LY225 steel shows cyclic hardening characteristic as well as dimple fracture failure mode. In addition, LY225 steel exhibits higher deformability and energy dissipation capacity than Q235 low carbon steel, especially in pure shear stress state, the deformation capacity and extremely low cycle fatigue life of LY225 steel are more than twice that of Q235 steel. The Lode parameter enhanced cyclic void growth model (LCVGM) comprehensively considering the influence of stress triaxiality and Lode parameter can be employed to predict the extremely low cycle fatigue fracture. The LCVGM parameters of LY225 steel were determined based on the experimental results. It indicates that the fracture failure of LY225 steel is free from the influence of Lode parameter due to its good shear performance. The monotonic tensile test and extremely low cycle fatigue test were carried out on the LY225 dog bone specimens, and the fracture failure phenomenon after buckling occurred in the test. The post buckling crack initiation and growth process were numerically simulated by the LCVGM of LY225 steel. The prediction accuracy of the LCVGM was verified by comparing the test results and simulations. This study may provide valuable information about the fracture behavior of LY225 steel under various stress conditions, as well as an effective approach to analyze the post buckling fracture process on the member level.

The optimisation of hot isostatic pressing treatments for enhanced mechanical and corrosion performance of stainless steel 316L produced by laser powder bed fusion

This work compares the mechanical and corrosion properties of 316L steel manufactured by Laser Powder Bed Fusion (LPBF) and post treated by Hot Isostatic Pressing (HIP) to wrought 316L. HIP is often used by default on LPBF components to reduce porosity and obtain the best mechanical properties, however, if the HIP temperatures are too high, there is a risk of reducing mechanical strength and corrosion resistance. The purpose of this work was to investigate the HIP parameters and understand the trade-off in properties. By choosing various HIP temperatures (700 °C, 1125 °C, 1200 °C), pressures (100 MPa, 137 MPa and 200 MPa) and hold times, optimal cycles were investigated based on the most favourable mechanical properties (density, hardness, tensile and low-cycle fatigue), and pitting corrosion resistance. Microstructural features associated with LPBF such as melt pools, melt pool boundaries and sub granular cells were observed. These features were found to disappear with longer and higher temperature treatments, accompanied by increased grain sizes. Low and mid temperature point HIP treatments resulted in higher ultimate tensile strength but lower fracture elongation. The decreasing hardness and tensile strength trends were consistent with decreased grain boundary strengthening and decreased dislocation strengthening (with disappearing sub grain boundary and granular cells). Only one HIP condition, consisting of a low temperature and medium pressure, produced samples that achieved runout under low cycle fatigue testing for both the lower and higher stresses. Despite this, most higher temperature HIP cycles reduced the fatigue resistance. This was again attributed to the coarsening of the microstructure at the higher temperature treatments. The spread of the pitting potentials of HIP treated samples was reduced by 52.46 % compared to the as-built material, although none were better overall compared to the wrought material. Of all the properties, porosity appears to play the most influential role on pitting corrosion, and to this extent, despite having a larger variation in results, some of the treated parts demonstrated improved pitting resistance and some demonstrated improved repassivation potentials compared to wrought 316L.

Experiments on static and fatigue behaviour of corroded Q235B and 42CrMo steels

This paper studies the mechanical properties and low-cycle fatigue behaviour of corroded Q235B and 42CrMo steels. Tensile test and low-cycle fatigue test were conducted on the corroded specimens after the accelerated corrosion process in the 25 °C salt spray, 25 °C dry/wet cycle or 50 °C dry/wet cycle conditions. Based on the accelerated corrosion test, the corrosion rate and ratio of weight loss were calculated and their relationships with the exposure time were described by a time-dependent model. The tensile test results indicate that the 50 °C dry/wet cycle condition caused the most degradation on the strength and ductility of both Q235B and 42CrMo steels. The modified three-stage constitutive model is used to describe the stress-strain curve of specimens under different conditions and the key parameters in the model were obtained by regression analysis on the experimental data. The low-cycle fatigue test results demonstrate that the coupling effect of corrosion environment, such as chloride ion and higher temperature, and applied cyclic load can lead to decrease in the fatigue life of these two metallic materials. Comparisons between the test fatigue life and the design values demonstrate that the current design codes cannot provide accurate fatigue life predictions for the Q235B and 42CrMo steels in the salt spray and dry/wet cycle conditions.

Investigation on low-cycle fatigue performance of high-strength steel bars including the effect of inelastic buckling

Bar buckling and subsequently low-cycle fatigue fracture is one of the main failure modes of RC columns/piers under strong earthquakes. Due to the adoption of new metallurgical technology and possibly enduring higher stress, high-strength steel bars (HSSB) may be more prone to occur buckling and low-cycle fatigue failure, which will detrimentally affect the seismic performance of RC columns/piers. In view of this, this paper studies the buckling and low-cycle fatigue performance of high-strength steel bars (HSSB) HTRB600 and compares with normal steel bars (NSB) HRB400E. At first, the monotonic tensile and compressive tests of HTRB600 and HRB400E steel bars with different slenderness ratios (L/D = 6, 8, 10, 12, 15) were conducted to study their monotonic tensile and compressive properties. Then, the low-cycle fatigue tests of HTRB600 and HRB400E reinforcing steel with different strain amplitudes and slenderness ratios were conducted to evaluate their low-cycle fatigue performance. The fatigue life prediction formulas for HTRB600 and HRB400E steel bars taking into account the effects of inelastic buckling were proposed through the regression analysis of test results. Moreover, the fractured surfaces obtained by the scanning electron microscope (SEM) were used to compare the low-cycle fatigue failure features of HTRB600 and HRB400E steel bars. Finally, a finite element model (FEM) was developed to simulate the nonlinear mechanical behaviors of HTRB600 and HRB400E steel bars under monotonic and cyclic loading. Test results show that HTRB600 steel bars present lower tensile ductility, more severe post-buckling softening and pinching effect than HRB400E steel bars. The increase in yield strength, buckling length and strain amplitude will reduce the low-cycle fatigue life of steel bars. The range of striation marks observed from low-cycle fatigue fractured surfaces by SEM for HTRB600 steel bar is smaller than HRB400E steel. The calibrated buckling and low-cycle fatigue parameters by the validated finite element model (FEM) can be adopted to accurately simulate the inelastic seismic response of RC structures.

Influence of additional static stresses on biaxial low-cycle fatigue of 2024 aluminum alloy

In this paper, a previously developed modification of the Sainz model of multiaxial fatigue is reduced to an invariant form. Model constants were determined for different sets of setup experiments. It was supposed to introduce an additional summand to account for the phase shift between loading modes. The model is used to describe the fatigue behavior of the D16T aluminum alloy. Low-cycle fatigue tests under biaxial loading conditions are presented, with one mode changing cyclically and the other mode remaining constant in magnitude throughout the test. The results of cyclic durability prediction by the modified model provide good convergence.