Multiaxial Low Cycle Fatigue Lives Research Articles

Multiaxial low-cycle fatigue life model for notched specimens considering small sample characteristics

PurposeSufficient sample data are the necessary condition to ensure high reliability; however, there are relatively poor fatigue test data in the engineering, which affects fatigue life's prediction accuracy. Based on this, this research intends to analyze the fatigue data with small sample characteristics, and then realize the life assessment under different stress levels.Design/methodology/approachFirstly, the Bootstrap method and the principle of fatigue life percentile consistency are used to realize sample aggregation and information fusion. Secondly, the classical outlier detection algorithm (DBSCAN) is used to check the sample data. Then, based on the stress field intensity method, the influence of the non-uniform stress field near the notch root on the fatigue life is analyzed, and the calculation methods of the fatigue damage zone radius and the weighting function are revised. Finally, combined with Weibull distribution, a framework for assessing multiaxial low-cycle fatigue life has been developed.FindingsThe experimental data of Q355(D) material verified the model and compared it with the Yao’s stress field intensity method. The results show that the predictions of the model put forward in this research are all located within the double dispersion zone, with better prediction accuracies than the Yao’s stress field intensity method.Originality/valueAiming at the fatigue test data with small sample characteristics, this research has presented a new method of notch fatigue analysis based on the stress field intensity method, which is combined with the Weibull distribution to construct a low-cycle fatigue life analysis framework, to promote the development of multiaxial fatigue from experimental studies to practical engineering applications.

An enhanced multiaxial low-cycle fatigue life model

To improve the accuracy of predicting multiaxial fatigue life, an enhanced multiaxial low-cycle fatigue life model is proposed based on the FS model. This model introduces a correction parameter for the stress-related term to consider the influence of normal stress and shear stress on the critical plane. The feasibility of the model is investigated, and it is validated using fatigue test data from eight different materials. The results indicate that the proposed model is applicable for both symmetric and asymmetric loading conditions under constant amplitude loading, with highly accurate fatigue life prediction results.

The strain ratio-dependent multiaxial low cycle fatigue behaviour and life prediction of 316L stainless steel based on critical plane at elevated temperature

The study mainly investigates the effect of strain ratio on the multiaxial low cycle fatigue behaviour of 316L at 550 °C. The evolution of both axial and torsional stress amplitude is observed to go through three stages: cyclic hardening, softening and fracture. What's different is that the torsional loading experiences fewer cyclic hardening cycles compared to the axial loading. Regarding the interaction between axial and torsional stresses, it is observed that as the torsional strain increases, the axial deformation response presents a lower peak stress and a higher softening rate, while the torsional peak stress and softening rate hardly changes with the increase of axial strain amplitude. The fracture pattern is obtained through three-dimensional (3D) depth-of-field observation. It is discovered that as the strain ratio increases, the fracture pattern transits from tensile failure mode to shear failure mode. Furthermore, the orientation angle of the fracture surface increases and lies between the critical plane angle of maximum shear strain and maximum normal strain, indicating a mixed failure mode. Additionally, a comparison of nine representative critical plane models reveals that the energy-based models can better predict the multiaxial fatigue life influenced by the strain ratio. Finally, based on the analysis of deformation and failure mechanisms, a normal damage parameter weakening factor (NDWF) is introduced into the KBM-R model, and an improved normal stress component (INSC) is introduced into the Wu-R model. The two modified models well capture the impact of strain ratio and accurately predict the multiaxial fatigue life of both 316L and P92 steel.

Multiaxial low cycle fatigue behavior and life prediction of wire arc additive manufactured 308L stainless steel

Wire arc additive manufacturing (WAAM) is a technique that has gained popularity in various industrial sectors due to its high productivity and flexibility. However, the comprehension of cyclic deformation and fatigue behaviors in WAAM austenitic stainless steel (SS), particularly under complex multiaxial loading conditions, is much less advanced compared to conventional manufacturing methods. This study aims to fill some of these gaps by performing fatigue tests of WAAM 308L SS under axial, torsional and axial–torsional multiaxial loadings. Similar tests were carried out on the hot-rolled 308L SS as a basis for comparison. WAAM 308L SS exhibits notable distinctions in cyclic deformation, fatigue life, cracking mode and fatigue failure mechanisms compared to its hot-rolled counterparts as a result of its distinct manufacturing technologies and resulting microstructures. To assess all fatigue life data, three critical plane criteria, namely FS, SWT and CXH criteria, were employed. The CXH model, which considers the effects of both tensile and shear damage on fatigue properties, has the ability to predict the fatigue life of both hot-rolled and WAAM 308L SS.

A generalization ability-enhanced image recognition based multiaxial fatigue life prediction method for complex loading conditions

A generalization ability-enhanced image recognition based multiaxial low-cycle fatigue life prediction is proposed for complex conditions, including multiaxial variable amplitude fatigue, fatigue-creep interaction, and anisotropic materials. Notably, a novel modified dynamic hysteresis image plotting method is introduced for multiaxial variable amplitude loading. Building on a pre-trained convolution neural network (CNN) model, fine-tuning is used across three cases. It makes the error decrease to 25%. Besides, fine-tuning improves training efficiency and generalization. Based on fine-tuning results, each layer of the model is attributed with physics significance. Furthermore, regression activation map (RAM) is used for interpretation, illustrating that the model focuses on the inner and edges of hysteresis images.

Asymmetrical multiaxial low-cycle fatigue behavior and life prediction of CP-Ti under non-proportional stress-controlled mode

Asymmetrical multiaxial fatigue tests were performed. A new quantification parameter is proposed to represent the difference in velocity during ratcheting accumulation. Although the stress–strain response under asymmetric loading is similar to that under symmetric loading, substantial deteriorations of life are observed. The threshold value for ratcheting strain accumulation of axial stress amplitude is closer to 287 MPa. The brand-new stress parameters-based KBM-PH model with mean stress correction is the best overall among a series of models based on strain and stress parameters with or without mean strain and stress corrections with the best accuracy and ease of use.

A multiaxial low-cycle fatigue prediction method under irregular loading by ANN model with knowledge-based features

An ANN model with knowledge-based features is proposed to predict multiaxial low-cycle fatigue life under irregular loading and verified on 304L stainless steel. Feature selection for knowledge-based features solves overfitting problem, improves model performance, and selects the genetic knowledge-based features. With help of genetic knowledge-based features, the proposed model combines physics knowledge and machine learning, being able to predict fatigue life of irregular cases through training only with regular cases. Most predicted results are located within 2-factor band. Besides, SHAP method interprets the ANN model, shows the contribution of features, and illustrates effectiveness of the model through the curves that fit facts.

An Improved Multiaxial Low-Cycle Fatigue Life Prediction Model Based on Equivalent Strain Approach

The fatigue life of the materials is significantly reduced under non-proportional loading. In this study, the factors affecting additional hardening are explored, and a hardening function is proposed. Firstly, the stress and strain states of the specimen under multiaxial loading are analyzed, and the deficiencies of the equivalent strain models are discussed. Secondly, the factors affecting the additional hardening are analyzed from both stress and strain perspectives, and the effect of phase differences on fatigue life is investigated. The stress on the critical plane is considered to reflect its effect on crack initiation and growth. An improved multiaxial low-cycle fatigue life prediction model is developed based on the equivalent strain approach. Finally, experimental data from five metals are used to verify the established model and are compared with existing classical models. The results show that the proposed model has good accuracy.

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.

An image recognition based multiaxial low-cycle fatigue life prediction method with CNN model

A CNN model for hysteresis loop image recognition is proposed to predict multiaxial low-cycle fatigue life. The samples are two-channel dynamic hysteresis images made by half-life hysteresis loop. The CNN model with three convolution layers has the best performance, with the training of 316L experimental data. By freezing the first two convolution layers, the well-trained model is transferred to the application of 316LN, 304L, and 40# steel. All the predicted results are almost located within the 1.5-factor band. Furthermore, the regression activation map and integrate gradient methods are used to interpret and unfold the black box of the CNN model.

Multiaxial low cycle fatigue behavior and life prediction method of 316LN stainless steel at 550 °C

This work studied the uniaxial, torsional and multiaxial low cycle fatigue behavior of 316LN stainless steel at 550 °C. Results show that the normalized kernel average misorientation value can be used to describe the loading path non-proportionality quantitatively. The formation of equiaxed and small-sized dislocation cells leads to the occurrence of continuously cyclic hardening under circular loading path. The crack initiation and early crack growth is dominated by intergranular mode in uniaxial test, whereas the transgranular cracking is observed in multiaxial test. Moreover, a weighting factor, evaluating the contribution of shear fatigue damage to the total multiaxial damage is proposed to modify the damage parameter of Chen-Xu-Huang (CXH) life prediction model, which enables all life data falling into the scatter band of 1.5.

Multiaxial low cycle fatigue assessment of notch specimens for 10CrNi3MoV steel and its welds

Due to the complex multiaxial loading service conditions for welded structures, it is a fundamental step to accurately compute the fatigue damage under complex stress states. Multiaxial low cycle fatigue behaviors of notch specimens considering geometry variations and strength heterogeneity for 10CrNi3MoV high strength steel and weldments under proportional and non-proportional loading modes were investigated in our study. The experimental results show that the crack initiates from the notch tip. The characteristic states of normal stress, shear stress and energy are analyzed to express the notch gradient behaviors. Furthermore, a new simplified fatigue damage parameter is proposed to predict the multiaxial low cycle fatigue life of notch components. This damage parameter is combined with Smith-Watson-Topper (SWT) damage parameter and notch stress concentration factor in the proximity of the notch tip. Finally, the proposed model gives more satisfactory estimations than other notch energy damage models for low cycle multiaxial fatigue life.

Evaluation of multiaxial low cycle fatigue life under non-proportional loading considering effect of mean strain (Loading simulated thermal fatigue due to temperature fluctuation in junction piping)

Evaluation of multiaxial low cycle fatigue life under non-proportional loading considering effect of mean strain (Loading simulated thermal fatigue due to temperature fluctuation in junction piping)

Effect of Pre-deformation on the Multiaxial Low-Cycle Fatigue Resistance of 2024-T4 Aluminum Alloy

Abstract The effect of pre-deformation on the fatigue properties of 2024-T4 aluminum alloy is evaluated, including pre-tension of 1, 2, 3, and 4 % as well as pre-torsion of 10°, 20°, 30°, and 40°. Multiaxial low-cycle fatigue (LCF) tests are carried out under a constant amplitude sinusoidal wave loading with a tension-torsion ratio √3 in air at room temperature. The results indicate that the multiaxial LCF life of the specimen reduces with the pre-deformation level. The observed mechanical behavior and associated phenomena are directly linked to microstructure characteristics, such as dislocation density, surface defects, and microcracks. The analysis of mechanism reveals a competition between beneficial effects that are due to hardening and detrimental influences that are due to defects that are caused by the pre-deformation. However, the multiaxial loading makes the detrimental influence more prominent. The typical fracture morphology of multiaxial fatigue can be observed. With the consideration of the effect of pre-deformation on fatigue limit, a modified criterion is developed to predict the multiaxial fatigue life of 2024-T4 aluminum alloy.

Effects of work hardening models on low-cycle fatigue evaluations of coiled tubing with CT-100 steel

In the present study, low-cycle fatigue life of a coiled tubing (CT) with a CT-100 steel was evaluated by using various work hardening models. Tensile and low-cycle fatigue tests were performed, and experimental results were used to calibrate material model constants. A nonlinear finite element model was constructed in the ABAQUS program by using a CT fatigue test machine. During the test cycles, bending and straightening conditions were repeated and histories of strains were collected. The multiaxial low-cycle fatigue life was calculated by using Manson–Coffin relation and Tresca criterion. The kinematic and combined hardening models can be used to evaluate the fatigue life of CT, and their results are conservative compared with the fatigue test results. Results of the present study can be used as the basic data in establishing CT fatigue analysis.

Effect of alternate corrosion factors on multiaxial low-cycle fatigue life of 2024-T4 aluminum alloy

Due to the seawater splash, rain water scouring and prolonged exposure, the aircrafts serviced in sea-coast or marine environment always suffered liquid corrosion effects under certain temperature. 2024-T4 aluminum alloy had been widely used in aircrafts fuselage. During service, the actual process was close to the alternate mode of “Ground Corrosion + Air Fatigue”, in which alternate corrosion factors had greatly effects on multiaxial fatigue life of 2024-T4 aluminum alloy. For the purpose of evaluating the influence of alternate corrosion time, corrosion temperature, corrosion solution flow rate and pH value on the fatigue performance, a multi-parameter regulated liquid corrosion device was designed. Multiaxial fatigue tests were performed under constant amplitude sinusoidal wave loading with constant tension-torsion ratio 3 in air. The fracture was observed by scanning electron microscope. The results indicated a reduction tendency of multiaxial fatigue life with the increase of alternate corrosion time, corrosion temperature and corrosion solution flow rate as well as the decrease of pH value. The influence order of relevant factors was: alternate corrosion time > corrosion temperature > corrosion solution flow rate > solution pH value. The effects of alternate time and temperature immersed in solution on corrosion resistance of 2024-T4 aluminum alloy are investigated by potentiodynamic polarization experiments and electrochemical impedance spectroscopy (EIS) tests. The observed mechanical behavior and associated phenomena were directly linked to microstructure characteristics such as corrosion pits and micro-cracks. The cyclic hardening appeared in both axial and tangential direction at the beginning stage. With the consideration of corrosion effect on the fatigue property, the equivalent corrosion life was proposed to characterize the influence of corrosion and to predict the alternate corrosion - fatigue life. Fine results were obtained with all data in 2× scatter band.

Cracking directions in multiaxial low cycle fatigue at high and room temperatures

Cracking direction in multiaxial low cycle fatigue is an important research subject because crack initiation and propagation behavior is a physical background for developing an estimation method of multiaxial low cycle fatigue lives. However, there are a few open questions on cracking direction in multiaxial low cycle fatigue because cracking direction in multiaxial low cycle fatigue is complex and changes depending on stress multiaxiality, strain range, notch and material. This paper overviews cracking directions in tension-torsion low cycle fatigue of low alloy steels and nickel base superalloys. Two types of cracking directions in these materials, maximum shear direction and maximum principal direction, are discussed in relation with strain multiaxiality and an existence of notch and precrack. The two cracking directions in torsion low cycle fatigue of SUS 304 stainless steel are also discussed in relation with strain range. Detailed micro crack observations are finally presented to discuss the two cracking directions in torsion low cycle fatigue of a SUS 304 unnotched specimen.

Multiaxial low cycle fatigue life of Ti-6Al-4V under non-proportional loading with mean strain

The mean strain effect on the multiaxial low cycle fatigue behavior of Ti-6Al-4V was investigated. Strain-controlled multiaxial low cycle fatigue tests at room temperature were carried out using a tubular specimen in tension and torsion loadings. Strain paths employed were four types of proportional and non-proportional loadings. Failure lives are affected by not only the non-proportionality of stress and strain histories but also the mean stress and strain. This study discusses the failure life properties based on the test results obtained and also shows strain parameters for the life evaluation under non-proportional loading with mean strain. The proposed strain parameters, where Findley and Walker models were applied within a non-proportional strain range, were suitable parameters to evaluate the life for Ti-6Al-4V under non-proportional straining with mean strain.

非比例負荷におけるSn-8Zn-3Biはんだの多軸低サイクル疲労寿命

非比例負荷におけるSn-8Zn-3Biはんだの多軸低サイクル疲労寿命

Evaluation of multiaxial low cycle fatigue life for type 316L stainless steel notched specimen under non-proportional loading

This study discusses a multiaxial low cycle fatigue (LCF) life of notched specimen under proportional and non-proportional loading conditions at room temperature. Multiaxial LCF tests controlled by strain were carried out using a smooth and a circumferentially notched round-bar specimens of type 316L stainless steel (316LSS). Four kinds of notched specimens of which elastic stress concentration factors Kt are 1.5, 2.5, 4.2 and 6.0 were employed. The strain paths employed were proportional loading and non-proportional loading. The former is push–pull loading and reversed torsion loading and the latter is circular loading achieved by the strain path of which axial and shear strains are loaded by 90° sinusoidal out-of-phase. Total axial strain and total shear strain ranges were the same ranges based on von Mises. An evaluation of the fatigue life for the notched specimen is discussed based on the experimental results and also by employing an inelastic finite element analysis (FEA). The fatigue life is decreased with increase in Kt at each strain path and is underestimated by a maximum local strain range evaluated by FEA result. A proposed strain range defined by a local mean strain value near the notch calculated from FEA by taking into account the non-proportional effect is an appropriate parameter for life evaluation of 316LSS notched specimen under non-proportional loading.