Ultra-low Cycle Fatigue Research Articles

Study on hysteresis behavior of two kinds of reinforced CHS X-joints

Ultra-low cycle fatigue loading tests were conducted on two reinforced CHS X-joints (external stiffening rings and plates) and an unreinforced joint to analyze their post-yield failure modes, fracture mechanisms, and hysteretic properties. The findings indicate that these three joints exhibit varying failure modes post-yield: the unreinforced joints fail primarily due to chord fracture at the weld; the ring-reinforced joints initially see ring fracture, followed by chord crack at the weld; while the plate-reinforced joints initially fail from chord fracture and subsequently develop cracks at the plate-chord interface. The reinforced joints display plumper hysteretic loops and higher ultimate bearing capacities than the unreinforced joint. Compared to X-1, the tensile and compressive ultimate bearing capacities of RX-1 have increased significantly by 56.9 % and 74.1 %, respectively. In comparison, those of TX-1 have increased moderately by 2.2 % and 67.6 %, respectively, albeit with a slight compromise in ductility. The cumulative energy dissipation of RX-1 and TX-1 has risen by 22.62 % and 131.99 %, respectively, compared to the unreinforced joint. Notably, the dissipated energy before cracking accounts for 16.50 %, 18.16 %, and 22.29 % of the cumulative energy dissipationfor the three joints, respectively, highlighting the substantial contribution of crack propagation to energy dissipation under large deformations. The VUSDFLD subroutine based on Cyclic Void Growth Model (CVGM) is embedded into ABAQUS, this subroutine considers the initiation and propagation of joint cracks. Three finite element models with the same dimensions of the specimens were built to simulate the damage process of the joint. The results show that this method can accurately simulate such joints’ cracking and crack growth behavior under cyclic axial load. The simulated hysteresis curve is in good agreement with the test curve.

Experiment and prediction of the strain-range dependent ultra-low-cycle fatigue based on micromechanical cyclic void growth model

Under severe seismic action, the local stress concentration region in steel structures is prone to the ultra-low-cycle fatigue (ULCF) under high triaxiality and irregular cyclic deformation with intensively varying strain amplitudes. It emphasizes the necessity of the jointed effects of the stress triaxiality and strain range incorporated in the ULCF life prediction. Given the limited relevant studies in terms of the underlying micro-mechanisms, this paper focuses on experimental and modeling studies of the combined effects of stress triaxiality and strain range on the ULCF failure, based on Gurson-Tvergaard-Needleman (GTN) microvoid evolution theories. Monotonic tensile tests on three types of notched round bars suggest that fracture strain decreases with increasing triaxiality. Cyclic tests on two types of notched bars under various strain ranges confirm the combined effects of stress triaxiality and strain range. Given a specific triaxiality, the ULCF damage accumulation rate increases linearly as the tensile plastic strain range ramps up. For the same tensile plastic strain range, higher triaxiality results in a greater damage accumulation rate. In the modeling studies, the consistency of the GTN and uncoupled damage models has been proven in terms of ULCF failure of structural steel. Based on GTN theories, a new micromechanical cyclic void growth model (GCVGM) is proposed, considering the detailed processes of void nucleation, growth, and coalescence under monotonic and cyclic loadings. Moreover, the joint effects of stress triaxiality and strain range can be explicitly described within a unified framework, without additional empirical assumptions. Based on experimental and simulation results, the proposed GCVGM shows significant advantages over conventional models in accurately predicting ULCF failure under various loading protocols, with low average errors of 2.73 % and −0.91 % for two types of notched bar specimens, respectively.

Ultra-low cycle fatigue life prediction of stainless steel based on transfer learning guided artificial neural network

Determining ultra-low cycle fatigue (ULCF) life of stainless steel typically involves laborious and time-consuming tests. While machine learning offers an efficient solution for fatigue life prediction, the inherent demand for sufficient training data remains unaddressed. To solve this challenge, a novel method to predict ULCF life using small datasets was proposed by combining artificial neural network (ANN) and transfer learning (TL). A TL-guided ANN (TLNN) framework was developed, wherein the knowledge gained from a pre-trained ANN model for structural steels was transferred to ULCF life prediction of stainless steel. The TLNN model exhibits enhanced predictive capabilities for stainless steel under small amounts of training datasets, with mean R2 (coefficient of determination) value of 0.88 and average MAPE (mean absolute percentage error) value of 25.52%. In comparison, the directly trained ANN model shows lower performance, possessing that the average R2 and MAPE are 0.69 and 47.58%, respectively. We pioneeringly explored and found that adding synthetic data enhancement had no positive effect on the predictive ability of the TLNN model. Furthermore, the dependence of predictive capacity by the TLNN model on the number of training data availability was studied. The prediction process of the TLNN model was interpreted by SHAP (SHapley Additive exPlanations) method. In general, the developed TLNN model can efficiently predict the ULCF life of stainless steel with high accuracy and reduce the cost of consecutive fatigue property evaluation.

Shaking table test-based numerical study on post-buckling ULCF fracture behavior of partially concrete filled steel tubular piers

Steel structures may undergo local buckling deformation or ultra-low cycle fatigue (ULCF) fracture due to stress concentration under earthquake excitations, and the stress concentration may further develop after buckling deformation, thus leading to ULCF fracture. To investigate the post-buckling fracture behavior of partially concrete-filled steel tube (PCFST) piers under earthquake exactions, a new ULCF analysis method that integrates three fracture indexes of cyclic void growth model (CVGM) was proposed in this study based on a shaking table test of PCFST piers, and then the index for quantifying the buckling deformation of PCFST piers was developed to study the post-buckling ULCF mechanism. Finally, the effect of each design parameter on the ULCF fracture index was analyzed. The results indicate that all three indexes of CVGM can accurately predict the triggering time, crack location, and crack propagation of ULCF fracture after buckling. The failure mechanism of PCFST bridge piers experiencing post-buckling fracture is also revealed, i.e., under repeated lateral loads, the plastic hinge region at the pier bottom bulges, causing an expansion of the plastic region. Meanwhile, the cumulative rate of ULCF damage rapidly increases during the expansion process and slows down after the bulge reaches the maximum stable value until the cumulative damage value meets the fracture condition. In addition, the results of the parametric analyses indicate that the model with high ductility can undergo a greater number of cycles before localized instability failure, and thus is more likely to suffer ULCF fracture first.

Study on ultra-low cycle fatigue of steel slit dampers

Steel slit dampers are typically made by cutting a number of slits in a steel plate. With the existence of the slits, the steel strips between them would yield under earthquake action and dissipate seismic energy through plastic deformation. Ultra-low cycle fatigue (ULCF) fracture of the steel strips will significantly reduce the energy dissipation capacity of the slit damper. In this research, ULCF tests were conducted on uniform, taped and hourglass-shaped steel strip (USS, TSS and HSS) specimens, which are fundamental elements of steel slit dampers. During the ULCF tests, symmetric cyclic loadings with constant amplitude were applied on the steel strip specimens. All USS, TSS and HSS specimens fractured due to ULCF but at different sections. The ULCF life and energy dissipation capacity of these specimens increase significantly in that order. Finite element (FE) analysis was carried out to simulate the ULCF tests on the steel strip specimens, based on which the ULCF fracture of these specimens was predicted with the cyclic void growth model (CVGM). The simulated hysteretic curves and the predicted ULCF lives agree well with the test results, validating the applicability of the FE model and the CVGM for the ULCF analysis of steel slit dampers. ULCF analysis was then conducted on USS, TSS and HSS specimens subjected to symmetric cyclic loadings with variable amplitudes and asymmetric cyclic loadings with constant amplitude to confirm the superior ULCF performance of the HSS specimen. Beneficial effects of additional axial compression and deviation of cyclic loadings from the damper plane on the ULCF of steel slit dampers were also investigated through numerical analysis.

Cyclic Void Growth Model Parameter Calibration of Q460D Steel and ER55-G Welds after Exposure to High Temperatures

When high-strength steel is heated to high temperatures and then cooled naturally, its ductility decreases. In earthquake-prone areas, it is necessary to evaluate the ultra-low cycle fatigue fracture (ULCF) behavior of high-strength steel structures after a fire if these structures are used continuously. However, the ULCF fracture model of high-strength steel subjected to high temperatures followed by natural cooling has not been deeply studied. In view of this, twelve notched, round bar specimens fabricated from Q460D steel and ER55-G welds were heated to 900 °C followed by natural cooling and then cyclic loading experiments and finite element analyses (FEA) were performed on these specimens. The fracture deformation obtained from the experiments was used in the FEA to calibrate the damage degradation parameter of a Cyclic Void Growth Model (CVGM) of Q460D steel and ER55-G welds under this condition. The calibrated values were 0.30 and 0.20, respectively. The calibrated CVGM was employed to predict the number of cycles and the force and displacement at the fracture moment of the notched round bar specimens. The predicted results aligned closely with the experimental results, indicating that CVGM is effective in predicting the fracture of Q460D steel and ER55-G welds following exposure to 900 °C and subsequent natural cooling.

New ultra-low cycle fatigue model for metal alloys

Ultra-low cycle fatigue failure is an important ultimate limit state of the structures. Based on the ductile fracture model proposed by the authors, the Coffin-Manson model, and the Miner law, a new model was proposed in this study. The strategy of the proposed model is intended for the prediction of ultra-low cycle fatigue fracture onset covering a wide range of stress triaxiality and being suitable for the variable amplitude straining. Three kinds of metal alloys, Q345qC steel, G20Mn5QT cast steel, and 2024-T351 aluminum alloy, were used to verify the model. The results showed that the proposed model advanced the original Coffin-Manson model greatly. More importantly, the verification demonstrates that the proposed model can predict the ultra-low cycle fatigue fracture initiation over a wide range of stress triaxiality and under variable amplitude straining. Besides, compared with the ULCF-2021 model, the proposed model has a relatively lower average error.

Ultra-low cycle fatigue test study of bolted spherical joints in steel grid structures

During a catastrophic earthquake, bending and failure were exhibited in the bolted spherical joints, and these phenomena have obvious ultra-low cycle fatigue (ULCF) failure characteristics. To study the failure characteristics, hysteresis behavior, stiffness degradation, and energy consumption of bolted spherical joints, the ULCF test of bolted spherical joints specimens was carried out. The experimental study showed that the high-strength bolts experienced an ULCF failure process of bending, crack initiation, crack propagation, and instantaneous failure, and the fatigue life of the specimens was less than 20 times. The pinch phenomenon in the hysteresis curve of the specimen was significant, indicating that its energy consumption was poor. The energy-damage model of the specimens under cyclic displacement was derived based on the cumulative energy consumption. A damage assessment method for the specimen was proposed based on the energy-damage model and angle change characteristics of the specimen. This method provides a reference for damage assessment and repair design of bolted spherical joint grid structures after an earthquake.

Initiation and growth of fatigue cracks in sheets with U-shaped notches in the first and mixed modes of fracture

Initiation and growth of fatigue cracks in sheets with U-shaped notches in the first and mixed modes of fracture

Performance Assessment of Retrofitted Drilled Connections Subjected to Ultra-Low Cycle Fatigue Loading

ABSTRACT In this research, ultra-low cycle fatigue in retrofitted connections with R-RBS1 and R-RBS2 drilling patterns has been investigated using experimental and numerical methods. The connection was retrofitted with the RBS1 pattern, with a 1% increase in seismic capacity, which failed in the 6% drift angle cycle. While the connection was retrofitted with the R-RBS2 pattern failed in the 5% drift angle cycle. In both connections retrofitted with R-RBS1 and R-RBS2 patterns, the place of damage caused by ultra-low cycle fatigue was in the drilling area.

A computational tool for the design of rigid origami structures based on a lumped damage model

As it is well known, origami structures have broad application prospects in aerospace, medical, architectural, robotics, material science, and other engineering fields. However, the comprehension and modeling of the collapse process of this kind of structure are still in the process of development. This paper investigates one specific failure mode: ultra-low cycle fatigue under large displacements. Origami structures are represented as sets of elastic panels with small strains connected by inelastic (plastic-damaged) hinge lines with large rotations. A novel experimental study of the behavior of the Waterbomb origami structure subjected to cyclic loadings with constant amplitudes is first presented. Then, a new model of damage evolution for origami structures is proposed. Finally, parametric studies are described on the influence of several design parameters (initial folding height, number of sides of regular polygons, and length ratio) on structural collapse. The computational tools proposed in this paper can then be used for the optimum design of origami structures and the structural assessment during the two phases of loading: first, the deployment stage, and then service loads on the unfolded solid.

Influence of corrosion on ultra-low cycle fatigue performance of steel butt-welded joints with various welding methods

Preferential corrosion of welded joints in steel structures has been observed in a wide range of scenarios and should receive enough attention. The current work focuses on the corrosion and ultra-low cycle fatigue (ULCF) behavior of low-alloy steel weld joints. Firstly, 1600 h neutral salt spray (NSS) tests were conducted on welded joints made by different welding methods, and then topography scanning of corrosion pits was carried out. It was found that the welded region is more susceptible to corrosion. Dynamic polarization curves of the welded joints were measured, and the results show that the self-corrosion potential of the welded region is lower than that of the base metal (BM) region, while the corrosion current density is higher than that of the BM region. Subsequently, cyclic tests were carried out on the corroded weld joints. The results show that corrosion can cause a maximum reduction of 11.79% in yield load and 20.87% in ultimate load of the welded joints. Corrosion has slight influence on the energy dissipation capacity of welded joints, but the maximum reductions in ULCF life could reach 77.31% due to corrosion. Fractography analysis further shows that secondary cracks, map cracks, and intergranular brittle fracture, apart from more common fatigue fracture, ductile fracture, and quasi-cleavage fracture, appear on the fracture surfaces. Supplementary numerical simulation was conducted to enable a better understanding of the failure processes. In the simulation, a geometric modeling method using topography scanning data of corrosion surface was proposed. By using the proposed modeling approach, together with Chaboche cyclic plasticity constitutive model and cyclic void growth model (CVGM) for capturing fracture behavior, the predicted results show an acceptable agreement with the experimental ones.

Ultra-low cycle fatigue evaluation method for unstiffened steel piers using fiber model

The safety of steel bridge piers is threatened by ultra-low cycle fatigue (ULCF) damage, which dominates the failure mode in those with relatively compact section under strong seismic action. Existing methods for calculating such ULCF damage face the dilemma between accuracy and efficiency. To this end, this paper proposes a simplified evaluation method based on fiber model to expediently evaluate the ULCF damage of unstiffened steel piers, evenly with sound precision. Specifically, the average stress triaxiality is assumed as 0.63 for steel piers, and it is validated by cyclic loading experiments with the modified Coffin-Masson formula and solid element sub-model. The disparity between local plastic strain range in fiber and solid models is compensated by introducing a magnified coefficient. The analyzed results indicate the slenderness ratio, axial compressive ratio, especially the flange thickness and the width-to-thickness ratio, strongly influence the strain concentration effect occurring in the pier bottom. In addition, the proposed close equation for the magnified coefficient fits well with the discrete calculated points. The 3.05% mean absolute error is realized on the predicting half cycles when applying this method in random cyclic loading. Its favorable applicability in practical engineering is also verified with considering different ground motion types.

Seismic Analysis Model of Partially Concrete-Filled Steel Piers Considering Ultralow-Cycle Fatigue Crack Propagation

ABSTRACT Seismic performance of partially concrete-filled steel tube (PCFST) bridge piers is controlled by the coupled failure of ultralow-cycle fatigue (ULCF) and local instability of steel plates. An empirical formula was established for the plastic fracture displacement on average stress triaxiality of the crack intiation process, and the two-stage prediction model for the ULCF cracking of steel was improved. Furthermore, an empirical formula was established for the plastic strain amplification factor on the element size and the stress triaxiality of the coarse model. A complete modeling method was proposed for the seismic performance analysis model of PCFST piers considering ULCF cracking.

Laboratory and ultra-low cycle fatigue evaluation of the seismic performance of BFP connection with different amounts of pre-tension in bolts

Bolted Flange Plate Moment Connections (BFP) are one of the most practical connections, used as prequalified connections in special steel moment frames. Using three full-scale BFP connections designed according to AISC, this paper has explored the effects of bolt pre-tension levels experimentally under SAC cyclic loading protocol. The bolt pre-tension level has been defined as α coefficient to indicate the pre-tensioning level in three specimens. The bolts of the first specimen were not pre-tensioned, called snug-tightened bolts, and considered as reference connections. The bolt pre-tension levels of the second and third specimens were established in accordance with the minimum and allowable limits of AISC design code and ultimate strength of the bolt, called pre-tensioned and fully pre-tensioned, respectively. The moment capacity, total energy absorption, and strain variation of the bolts have been investigated. Meanwhile, using the ultra-low cycle fatigue failure index obtained from experimental specimens, the capacity of modeled BFP connections in finite elements has been compared with the laboratory samples in terms of failure location and connection capacity. The experimental results revealed that final resistance in BFP connections occurs in the cycle of 7% of the drift angle of SAC protocol loading, and the location of the connection failure is around the farthest hole from the column, which is consistent with the failure results from ultra-low cycle fatigue in finite element modeling for BFP connections. The measure of moment resistance in the pre-tensioned connection (Both connections were minimum pre-tensioned according to the regulation and ultimate strength of the bolt) has increased by about 50% compared to the without pre-tensioned connection. The amount of energy absorption of the pre-tensioned connection with the ultimate strength of the bolt has increased by 176% of the energy absorption capacity of the connection without pre-tensioned and 14% of the energy absorption capacity of the connection with the minimum allowable pre-tensioning of the regulation. The extent of energy absorption in the three connections was equal to 38, 92, and 105 KJ.

Study of Ultralow-Cycle Fatigue of Iron-Based SMA in Triaxial Stress States

Study of Ultralow-Cycle Fatigue of Iron-Based SMA in Triaxial Stress States

A cyclic GTN model for ultra-low cycle fatigue analysis of structural steels

Ultra-low cycle fatigue (ULCF) is a critical concern in the seismic design of steel structures due to its adverse effects on the ductility and energy dissipation capacity of steel connections. The Gurson-Tvergaard-Needleman (GTN) model, a well-accepted micromechanical fracture model, cannot be applied directly to ULCF analysis, as it requires proper estimation of the effects of cyclic loadings on the ductile fracture of materials. In this paper, a cyclic GTN (C-GTN) model was proposed for the ULCF prediction of materials, in which the evolution of microvoids during ULCF loadings was addressed for tensile and compressive half load cycles, respectively, and the effect of the cumulative plastic strains on the material’s resistance to ductile fracture was considered by the reduction of the critical void volume fraction for void coalescence. Then, a VUMAT subroutine was programmed for the C-GTN model, in which the cyclic hardening behavior of materials was simulated with the Voce-Chaboche model. Finally, the C-GTN model was calibrated for the G20Mn5QT cast steel based on previous tests on notched round bar specimens of the material. The applicability of the C-GTN model and the VUMAT subroutine were validated in the ULCF analysis of double-hole plate specimens of the G20Mn5QT cast steel.

Development of ultra-low cycle fatigue life prediction model for structural steel considering the effects of surface roughness, loading frequency, and loading amplitude

Abrasive blast cleaning is often done for steel structures before applying various protective coatings, which produces rough surfaces with changes in fatigue properties. This problem has been addressed in the low- and high-cycle fatigue regimes; however, the effect of surface roughness in combination with different loading parameters on the ultra-low cycle fatigue (ULCF) life has not been reported thus far. To this aim, a total of 59 ULCF tests on designed specimens of SM400 steel with five levels of surface roughness were performed under various loading frequencies and displacement amplitudes. The analysis of experimental results indicates a substantial reduction in fatigue life with an increase in the surface roughness and loading amplitude and a decrease in the loading frequency. Additionally, the strength degradation, dissipation energy, and load–displacement curves are discussed in detail. With the use of experimental data, a new life prediction model characterizing the combined effects of surface roughness, loading frequency, and loading amplitude on the ULCF life is proposed. Moreover, the proposed model is validated by predicting the fatigue life under variable and constant loading amplitude patterns. Comparison between experimental and theoretical results shows that the proposed model accurately estimates the ULCF life within an error band of ±15%, with a reasonable selection of model parameters.

Influence of loading frequency on ULCF life of structural steel under variable surface roughness

AbstractThis paper investigates the effects of a wide range of loading frequencies ranging from 0.005 to 5 Hz and blasting induced surface roughness ranging from 20 to 100 μm on the ultra‐low cycle fatigue (ULCF) life of structural steel. The combined effect of these parameters on ULCF life has not been investigated so far. To this aim, comprehensive ULCF tests were performed on steel specimens under a constant displacement‐controlled environment until the failure occurred. The experimental results manifest that the surface roughness and loading frequency are the governing parameters affecting the ULCF life. The test material exhibits cyclic softening behavior under all loading conditions. An increase in loading frequency from 0.005 to 5 Hz causes a significant increase of 33% and 27% in ULCF life for 20 and 100 μm rough specimens, respectively. However, the effect of frequency on ULCF life becomes less significant at higher values of surface roughness. Moreover, an increase in surface roughness from 20 to 100 μm causes a significant reduction of about 45% and 47% in ULCF life at the loading frequencies of 0.005 and 5Hz, respectively. Furthermore, a correlation between fatigue life and loading frequency as a function of surface roughness is derived by the power law relationship.

Study on ultra‐low cycle fatigue fracture of thick‐walled steel bridge piers

AbstractSteel structures with thick‐walled section tend to have local fracture before the occurrence of instability under strong earthquakes, which is termed as ultra‐low cycle fatigue (ULCF). In this study, a series of specimens made with structural steel Q345qC for bridge in China were firstly tested to investigate the ULCF fracture behaviour in material level. Based on the test data, three models for ULCF fracture were calibrated. Subsequently, the fracture behaviour of thick‐walled square section steel bridge piers were also investigated in component level. Moreover, the finite element global and submodels of steel bridge piers were established, and ULCF fracture initiation life was predicted by calibrated models and validated against test results. The research results showed that the crack firstly occurred at the bottom of piers under cyclic oblique loading, which was at the junction between base plate and bottom weld, and then, cracks propagated along the bottom weld metal which did not cause a decrease in strength capacity. Finally, the strength capacity of steel bridge piers began to rapidly declined as cracks propagated into base metal. The presented finite element global model can accurately simulate the cyclic behaviour of steel bridge piers, and the submodel has the ability of capturing the local stress and plastic strain. The fracture models accurately predicted ULCF fracture initiation life of steel bridge piers with the average discrepancy of 7.14%. The developed finite element analysis method considering ULCF fracture provides a solution for the further parametric analysis on ULCF fracture performance of steel bridge piers.