Low Cycle Fatigue Process Research Articles

Low-Cycle Fatigue Behavior and Fracture Characteristics of Low-Cost Ti-2Fe-0.1B Alloy

In recent decades, the effect of Fe element addition on titanium alloy has been investigated extensively due to the development of low-cost titanium alloys, as well as B microalloying, which could decrease the grain size of titanium alloys during the casting process. As a key structural material, the study of the fatigue behavior of titanium alloys is crucial and always attractive for scientists. Hence, in this paper, the low cycle fatigue (LCF) behavior and fracture characteristics of a low-cost Ti-2Fe-0.1B alloy with a lamellar structure were investigated systematically, five different strain amplitudes (Δεt/2) in the range from 0.6% to 1.4% were selected to control the LCF process. It was found that the Ti-2Fe-0.1B alloy exhibits continuous cyclic softening behavior in the cycle as a whole at Δεt/2 ≤ 1.2%, while at Δεt/2 = 1.4%, it exhibits slight cyclic hardening at the initial stage of the cycle, then shows cyclic softening. Compared with pure titanium and other typical titanium alloys, the Ti-2Fe-0.1B alloy indicated maximum fatigue life under the same strain amplitude, it can be attributed to the fine grain size result from the effect of Fe element and trace B, which could hinder the dislocation movement and crack propagation.

Loosening and fracture behavior of hybrid titanium-to-steel threaded connection under cyclic loading condition

Hybrid threaded connection involving titanium alloy and conventional steel components is introduced and studied in the face of lightweight metals application trend in modern machinery design. Pre-tightening procedure and loosening test were conducted based on specially designed specimen regarding cyclic pulling load condition. The loosening and fracture behavior were intentionally investigated through both mechanical analysis and related microstructure observation. The loosening evolution tendency along with the corresponding low cycle fatigue process was obtained, which was then further related with the examined structural mechanical responses and thread wear mechanism. Results have shown that in the early stage of fatigue process the pretension would experience a rapid and mass reduction accompanied with a high rate cyclic hardening response of structure. The cyclic load induced thread wear occurred mainly in the titanium nut component, which involved abrasive wear, fatigue wear and oxidative wear mechanism. Besides, the thread root turn closest to the bearing surface was demonstrated as major position of stress concentration and strain accumulation, making it critical source for loosening initiation and fatigue fracture, which should be properly optimized in both designing and fabricating process.

Enhanced synergy of strength-ductility and low-cycle fatigue resistance of high-entropy alloy through harmonic structure design

Compared with homogeneous structured materials, heterogeneous structured materials exhibit unprecedented performances. In the present work, a series of harmonic structured CoCrFeMnNi high-entropy alloys (HEAs) with different shell fractions were prepared by mechanical milling (MM) and spark plasma sintering (SPS) process. The effects of shell fraction on mechanical properties and low-cycle fatigue (LCF) performances were investigated at room temperature. Results show that the harmonic structured HEAs with a shell fraction between 20% and 40% demonstrate a combination of superior strength-ductility synergy and good LCF resistance. Development and rearrangement of dislocations dominantly occur in the coarse-grained areas (cores) during LCF process, while the ultrafine-grained areas (shells) show good cyclic stability. The enhanced strain hardening leads to a good balance of high strength and high ductility in the harmonic structured HEAs. Moreover, the enhanced ductility also restrains LCF failure.

Effects of Machined Surface Integrity on High-Temperature Low-Cycle Fatigue Life and Process Parameters Optimization of Turning Superalloy Inconel 718

Machined surface integrity characteristics, including surface stresses, physical-mechanical properties and metallographic structures, play important roles in the fatigue performance of machined components. This work aimed at investigating the effects of machined surface integrity on high-temperature low-cycle fatigue life. The process parameters were optimized to obtain required surface integrity and fatigue life of the turning superalloy Inconel 718. The relationships between low-cycle fatigue life and machined surface integrity characterization parameters were established based on the low-cycle fatigue tests at a high temperature (650 °C). The sensitivities of turning process parameters to high-temperature low-cycle fatigue life were analyzed, and the optimization parameters were proposed with the goal of antifatigue manufacturing. Experimental results indicated that the impact order of the characterization parameters of machined surface integrity on the high-temperature low-cycle fatigue life were the degree of work hardening RHV, the residual stress in the cutting speed direction S22, the fatigue stress concentration factor Kf, the degree of grain refinement RD and the residual stress in the feed direction S33. In the range of turning parameters of the experiments in this research, the cutting speeds could be 80~110 m/min, and the feed rate could be 0.10~0.12 mm/rev to achieve a longer high-temperature low-cycle fatigue life. The results can be used for guiding the fatigue-resistant manufacturing research of aeroengine superalloy turbine disks.

The investigation of low-cycle fatigue crack propagation of 16 Mn eccentric bar based on the acoustic emission technique

Compared to the traditional cropping technology, low-cycle fatigue cropping can reduce load and improve cross-section quality. Geometric factors of V-shaped notch and processing parameters (such as load amplitude and load frequency) influence the cropping efficiency and section quality. By changing the factors, the sufficient efficiency of cropping and quality of cross-section get together. However, the influences of material defects, geometric parameters, loading frequency and other factors are different in each stage of the whole cropping process. If the effects of the parameters on the each stage of cropping process are clear, the higher productivity and the better cross-section quality will be obtained by applying suitable parameter on each stage of cropping process. To investigate the effects of eccentric ratio on each stage of low –cycle fatigue cropping process, a suitable monitoring method is needed. This study proposes acoustic emission (AE) technique to detect the low-cycle fatigue cropping process of 16 Mn eccentric bar (The bar prefabricated eccentric notches. The prefabricated notch improve the efficiency of cropping). The parameters of signals such as counts and kurtosis during the low–cycle fatigue process are obtained. According to the counts changes over time, the process of the 16 Mn metal bar cropping can be divided into three stages: the crack initiation stage, the crack propagation stage, and the fracture stage. Based on the cumulative counts, the eccentric ratio’s influence on the time of each cropping process stage is obtained. The time of the crack initiation stage and the final fracture stage doesn’t increase with the eccentric ratio. The time of the propagation stage influenced by the eccentric ratio greatly. Besides, the eccentric ratio’s influence on the cross-section quality is studied by using an advanced optical microscope system. The cross-section quality was influenced by the eccentric ratio significantly. The results of the paper indicate the acoustic emission (AE) monitoring technique is a useful method to detect the process of low-cycle fatigue cropping. Especially, it provide effective information to investigate the effects of the notch eccentric ratio during the low-cycle fatigue cropping process. The eccentric ratio’s effects on each stage of low-cycle fatigue cropping process offer guidance to improve low-stress fatigue cropping efficiency and cross-section quality.

Failure Analysis and Service Life Prediction of 80SH Casing Steel under Thermal Cycle Service Environment

The main exploitation process of heavy oil is cyclic steam huff and puff. Cyclic operation causes cumulative damage to thermal recovery casing and causes a large number of failures. Failure analysis shows that the main failure factor of thermal recovery casing is the low cycle fatigue process caused by thermal cycle, which results in the decrease of strength of casing material, thus causing casing fracture and failure. Based on the failure analysis results, the low cycle fatigue tests of 80SH thermal recovery casing steel under different temperature, strain, pre-strain and other conditions were carried out. The influencing factors, life prediction model and criterion of service safety of casing materials under thermal recovery service conditions were first proposed, which provides a strong theoretical basis for the service safety design theory of thermal recovery casing materials. The research shows that the strain limit and low cycle fatigue life are the two core issues of service life. Life predictions need to satisfy two criteria. The first is the strain criterion. The strain limit should be lower than the total strain during the long service life. And the second is the low cycle fatigue criterion, which satisfies the expectation of low cycle fatigue life under double conditions of strain amplitude and mean strain.

Low cycle fatigue properties and microstructure of P92 ferritic-martensitic steel at room temperature and 873 K

Low cycle fatigue tests for P92 heat-resistant steels (P92 HRS) were conducted at room temperature (RT) and elevated temperature (873 K). The fatigue test results showed that P92 steel showed softening characteristics during fatigue at RT and 873 K, and the increase of temperature and strain amplitude was the main reason for the decrease of uniaxial fatigue life. The specimens before and after fatigue test at two temperatures were observed and analyzed by transmission electron microscope (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED). Microstructure observations revealed that the cyclic softening was caused by the annihilation of dislocations, as well as the fragmentation and polygonization of lath structure. High temperature promoted the formation of subgrains by accelerating the motion of dislocations and migration of low-angle grain boundaries (LABs). With the continuously decreasing of dislocation density, the growth of equiaxed subgrains and the weakening of the precipitation strengthening, the softening rate of the material was further increased until the final fracture occurs. Therefore, the evolution of microstructure of 9Cr martensitic steel during the low cycle fatigue process was deduced and described.

Low-cycle fatigue of single crystal nickel-based superalloy – mechanical testing and TEM characterisation

Low-cycle fatigue (LCF) is studied for a nickel-based single-crystal superalloy in this paper, with a focus on the effect of crystal orientation and temperature. Specifically, cyclic deformation of the alloy was compared for [001]- and [111]-oriented samples tested under strain-controlled conditions at room temperature and 825 °C. Either cyclic hardening or softening was observed during the LCF process, depending on the strain amplitude, crystallographic orientation and temperature. LCF life was also reduced significantly by changing loading orientation from [001] to [111] or increasing temperature to 825 °C. Employing a comprehensive study with transmission electron microscopy (TEM), a connection between microstructure and mechanical behaviour of the alloy is discussed. It was found that the processes of γ′-precipitate dissolution and dislocation recovery were responsible for cyclic softening. Alignments and pile-ups of dislocations in the γ matrix, which prohibited their movement and reduced the interaction of dislocations on different slip systems, contributed to cyclic hardening.

Low-cycle fatigue of ship hull damaged in collision

Abstract This work aims to extend the conventional post-accidental structural safety assessment by investigating the possibility of damage propagation in a damaged ship during the salvage period as a low-cycle high-stress fatigue process caused by fluctuating wave loads. Low-cycle loading can cause high-amplitude stresses at the edge of the damage opening, which consequently lead to high-stress concentrations. In the present study, a finite element model is created for an intact ship and 50 random damage scenarios for which the stress concentration factors are calculated. Random damage scenarios are generated using damage parameters obtained from historical data on ship accidents. The number of constant-amplitude wave load cycles to initiate a fatigue crack is calculated using the strain-life method defined according to Det Norske Veritas classification notes. Accumulated low-cycle fatigue damage is then estimated by Monte Carlo simulation, where individual vertical wave bending moment amplitudes are drawn as random numbers according to Weibull distribution. Parametric analysis is also performed in order to investigate the influence of cyclic material properties, sea environment and the number of towing days on the overall low-cycle fatigue damage accumulation. Realistic sea state scenarios are selected in order to investigate the influence of low-cycle fatigue damage accumulation during some specific constant sea states, enabling to determine marginal sea conditions for a safe rescue operation.

Tuning Low Cycle Fatigue Properties of Cu-Be-Co-Ni Alloy by Precipitation Design

As material for key parts applied in the aerospace field, the Cu-Be-Co-Ni alloy sustains cyclic plastic deformation in service, resulting in the low cycle fatigue (LCF) failure. The LCF behaviors are closely related to the precipitation states of the alloy, but the specific relevance is still unknown. To provide reasonable regulation of the LCF properties for various service conditions, the effect of precipitation states on the LCF behaviors of the alloy was investigated. It is found that the alloy composed fully of non-shearable γ′ precipitates has higher fatigue crack initiation resistance, resulting in a longer fatigue life under LCF process with low total strain amplitude. The alloy with fine shearable γ′I precipitates presents higher fatigue crack propagation resistance, leading to a longer fatigue life under LCF process with high total strain amplitude. The cyclic stress response behavior of the alloy depends on the competition between the kinematic hardening and isotropic softening. The fine shearable γ′I precipitates retard the decrease of effective stress during cyclic loading, causing cyclic hardening of the alloy. The present work would help to design reasonable precipitation states of the alloy for various cyclic loading conditions to guarantee its safety in service.

Numerical investigation of low cycle fatigue mechanism in nodular cast iron

The failure mechanism in ductile cast iron under static loading is nucleation, growth, and coalescence of voids. It is known from the literature that elastic–plastic porous materials under strain controlled cyclic loading show an increase of void volume fraction called void ratchetting effect. From this observation, we suggest that void ratchetting is the relevant mechanism for low cycle fatigue in nodular cast iron as well. The low cycle fatigue process in the microstructure of ductile cast iron has been simulated cycle by cycle using unit cell models. The strain-life approach for fatigue is followed in this work. The matrix material behavior is modeled as an elastic–plastic one. From the simulations, void ratchetting is identified as a potential mechanism of low cycle fatigue in nodular cast iron which finally leads to failure. The lifetime of different load ratio and different matrix material behavior is extracted from the simulations. The simulated strain-life curves are compared with experimental data from literature.

Stability of Residual Stresses in Ultrasonic Surface Deep Rolling Treated Ti-6Al-4V Alloy under Cyclic Loading

Compressive residual stresses have been found to affect fatigue crack growth behavior by delaying the crack initiation and by decelerating the crack propagation rate. Therefore, various mechanical surface treatment techniques have been developed to produce the compressive residual stresses on the surface of components. However, the residual stresses will relax due to cyclic loading. Hence, the stability of residual stress during fatigue process is a great importance aspect for design of components. In this paper, the ultrasonic surface deep rolling was used to generate the compressive residual stress near the surface of Ti-6Al-4V. The stress relaxation behavior was identified during the low cycle fatigue process. The X-ray diffraction method was used to determine the magnitude and sign of residual stress. Results showed that under cyclic loading, the residual stress relaxation occurred fast in the first few cycles then became stable. Furthermore, it was found that relaxation rates of residual stress were depended on the applied stress.

Parameters influencing thermomechanical fatigue of a directionally-solidified Ni-base superalloy

Thermomechanical fatigue (TMF) is a low cycle fatigue process in which material life is correlated to the mechanical strain amplitude. However, it is well known that several other factors influence this life. This paper examines several of these parameters and their influence on life using experiments conducted on a second generation directionally-solidified (DS) Ni-base superalloy. The parameters considered include the influence of the temperature extremums (Tmax of either 750 or 950°C and Tmin of either 100 or 500°C), strain ratio (R∊), the strain-temperature phasing (in-phase (IP) and out-of-phase (OP)), the influence of dwells at the high temperature end of the cycle resulting in a creep–fatigue (CF) interaction, and material anisotropy associated with the grain growth direction (longitudinal versus transverse). Results suggest that the phasing has a primary role in controlling the mechanism of degradation. IP TMF is dominated by crack formation in volumes surrounding debonded carbides for both continuously cycling (CC) and CF at 950 and 750°C, while OP TMF is dominated by surface oxidation and repetitive cracking of the oxide that reforms at the crack tip at 950°C. Decreasing the Tmax to 750°C the environmental and creep effects are reduced resulting in virtually pure fatigue exposure under OP conditions. With decreasing Tmin from 500°C to 100°C was observed an increase in inelastic strain amplitude and corresponding decrease in life. Variations in R∊ were found to have no significant influence on life or stabilized stress behavior. TMF loading in the transverse orientation resulted in life reductions over the longitudinal orientation due to cracks propagating in a transgranular manner. Lastly, only in material exposed to CF with a Tmax of 950°C rafting of the γ′ precipitates was observed.

The relation between the strain energy release in fatigue and quasi-static crack growth

This work proposes to use an average Strain Energy Release Rate (SERR) to characterise similarly fatigue and quasi-static delamination growth. Mode I quasi-static and fatigue tests were performed. The quasi-static crack extension was considered as a low-cycle fatigue process, discretized to different levels and correlated to the fatigue data. Fracture surfaces were analysed and damage mechanisms were related to average SERRs for each case. The strain energy released in crack extension showed to be dependent on the decohesion mechanisms, and it is demonstrated how the values of the SERR for fatigue and quasi-static loading can be linked through physical principles.

High Temperature Low Cycle Fatigue Behavior of GH4742 Alloy

High temperature low cycle fatigue tests on GH4742 superalloy were studied under the total strain-controlled conditions at 650 °C. Combined with fatigue test data, fatigue properties of the alloy were analyzed. Fracture morphology and dislocation structure were observed by scanning electron microscopy and transmission electron microscopy. The results showed that fatigue life and fatigue resistance of GH4742 alloy decreased significantly with increasing total strain amplitude. The cyclic hardening, cyclic softening and cyclic stability phenomena of the alloy occurred during the low cycle fatigue process. The increasing total strain amplitude is conducive to the formation of γ′ phase. Fatigue crack propagation is controlled jointly by ductile and brittle fracture. Inhomogeneous deformation and deformation restricted in slip bands are the main reasons for the reduction of fatigue life of GH4742 alloy.

Cyclic deformation behavior of a nickel-base superalloy under fatigue loading

Abstract Total strain-controlled low cycle fatigue (LCF) tests of a nickel-base superalloy were performed at 650 °C. Combined with fatigue test data, cyclic deformation behaviors of the alloy were analyzed. Fatigue cycle life decreases significantly with the increasing total strain amplitude. The cyclic hardening and cyclic softening phenomena occur during the LCF process, which are associated with the total strain amplitude. Fracture morphologies and dislocation characteristics were observed through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The combined effects of brittle fracture and ductile fracture are the main LCF fracture mechanism of the alloy.

Low Cycle Fatigue Behavior and Deformation Mechanism of TWIP Steel

Low cycle fatigue behavior of TWIP (twinning induced plasticity) steel was investigated in axial symmetric tension-compression cyclic loading pattern. Fracture surfaces and microstructures were examined by optical, scanning electron and transmission electron microscopes. It was found that the fatigue life at the strain amplitude of 0.4% is up to 15000 cycles, which is much longer than TRIP780 and HSLA800 steels. The strain hardening and softening features are significant until the strain amplitude comes to 1.25%. Persistent slip bands and tiny mechanical twinning layers were observed after fatigue deformation. Deformation mechanism of TWIP steel at low cycle fatigue process is not only twinning, but a complex of both twinning and persistent slip bands.

142 低サイクル疲労過程におけるSUS316NGの表面性状評価(材料力学・機械材料)

142 低サイクル疲労過程におけるSUS316NGの表面性状評価(材料力学・機械材料)

Cyclic Hardening Behavior of Extruded AZ31B Magnesium Alloy during Low Cycle Fatigue Process

The strain-controlled fatigue tests on extruded AZ31B magnesium alloy were conducted under the uniaxial loading with strain ratio Rε=-∞, frequency of 0.1 Hz and strain amplitude of 2% at room temperature. The cyclic hardening behavior was investigated. It was found that, during the low cycle fatigue (LCF) process, as the number of cycles increases, the stress amplitude increases corresponding to the decrease of the plastic strain amplitude. The development of dislocation density can be described as the function of the number of fatigue cycles, and the behavior can be explained well based on the dislocation density development model.

Low-cycle fatigue behavior of extruded Mg–10Gd–2Y–0.5Zr alloys

The low-cycle fatigue properties of extruded Mg–10Gd–2Y–0.5Zr (GW102k) magnesium alloy have been studied and compared to those of conventional extruded AZ31 magnesium alloy. Typical post-fatigue microstructure and surface morphology features are presented for both the GW102k and AZ31 alloy. It is found that GW102k alloy contains a large amount of precipitated particles and possesses a relatively weak basal texture, which gives rise to near symmetric yield asymmetry. Different from AZ31 alloy, the GW102k alloy shows near-symmetric stress–strain hysteresis loops and marginal cyclic hardening. This symmetry significantly reduces tensile mean stress during low-cycle fatigue process. Due to the small reversible plastic zone size, the GW102k alloy shows rough faceted fracture surfaces in the fatigue crack propagation zone. Accordingly, the low-cycle fatigue life of GW102k alloy is found to be longer than that of AZ31 alloy.