Ultra-low Cycle Fatigue Fracture Research Articles

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.

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

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.

Study on ultra low cycle fatigue fracture behavior of Q690D high-strength steel after fire exposure

In order to investigate the fracture behavior of Q690D steel high-strength steel after exposure to elevated temperature due to the fire, standard coupon specimen, notched coupon specimen and pure shear specimen representing various stress states were tested under monotonic loading and ultra low cycle fatigue loading. For air cooling condition, the tensile test results of Q690D steel show that when exposed to the temperature above 500 °C (up to 800 °C), the strength decreases while the elongation increases. These variation trends are basically the same under different stress states, and the predictive equations are proposed to determine the residual strength and ductility after exposure to high temperature. The effect of exposed temperature on the ductility under monotonic loading is significantly greater than that on the ultra low cycle fatigue life. In addition, as compared with air cooling, water cooling can significantly reduce the deformation capacity and fatigue life of the specimens, and even lead to a failure mode close to brittle fracture. The Lode parameter enhanced cyclic void growth model (LCVGM) parameters of Q690D steel after being exposed to various temperatures and air cooling were calibrated based on the fracture failure test results, and the post-fire LCVGM of Q690D high-strength steel was established to facilitate the fracture evaluation. Finally, the LCVGM was employed to predict the post-fire ultra low cycle fatigue life of Q690D steel notched plane strain specimen, and its estimation accuracy was verified by the comparison between tests and simulations. This fracture prediction model LCVGM can be recommended to evaluate the post-fire seismic performance of Q690 high-strength steel members.

Experimental and numerical study of undersized shear panel dampers with bolted connections

Shear panel dampers have been widely utilised in steel structures to enhance structural damping and dissipate earthquake energy, and thus minimise damage to primary structures. These shear panel dampers commonly feature relatively large sizes and high loading resistance owing to their main applications in multi-storey and high-rise buildings. In contrast, this study intended to develop an undersized shear panel damper (USPD) for prefabricated low-rise structures without site welding. To investigate the cyclic behaviour and fracture characteristics, USPDs made of Q235 low-carbon steel were designed as full-scale specimens with bolted connections and then tested under cyclic shear loads. Moreover, finite element (FE) models were established by considering the material and geometric nonlinearities. The ultralow cycle fatigue (ULCF) fracture initiation life of the USPDs was further predicted based on the cyclic void growth model (CVGM) and validated against the test results. Accordingly, the USPDs exhibited plump and stable hysteresis curves before crack propagating severely and featured significant cyclic hardening behaviour, where the ultimate-to-yield resistance ratio ranged from 2.20 to 2.45. The presented FE models could simulate the cyclic behaviour of USPDs well, and the CVGM accurately predicted the ULCF fracture initiation life with the average discrepancy of 14.8%. The results demonstrate the potential applicability of USPDs, and the developed FE analysis method considering ULCF fractures provides a solution for further configuration optimisations and parametric analyses, reducing the cost of experimental studies.

Experimental program for characterization of ultra low-cycle fatigue fracture in structural steel

Structural steel components experience cyclic inelastic strains when subjected to extreme loading events such as strong earthquakes. In such cases, the steel may experience ultra-low cycle fatigue (ULCF) fracture, associated with the growth and coalescence of microscopic voids in the material. A number of ULCF criteria have been formulated and used in the literature for the analysis of fracture under cyclic loading. The calibration of such criteria requires experimental test data covering a variety of stress states. To address this need, a testing program, involving both monotonic and cyclic loading protocols, was conducted on 60 circumferentially notched coupons extracted from rectangular steel tube sections made of ASTM A1085 steel. A relatively novel approach was used to achieve a wide range of stress states by subjecting the coupons to combinations of axial and torsional loading. The experimental tests are accompanied by finite element analyses to elucidate the local stress and deformation states at the crack initiation location of each coupon. To demonstrate the importance of the experimental tests for the reliable characterization of ULCF in structural steel, an example calibration is conducted for an ULCF criterion from the literature using the experimental results.

Ultra-low cycle fatigue fracture initiation life evaluation of thick-walled steel bridge piers with microscopic damage index under bidirectional cyclic loading

Strong earthquakes can cause ultra-low cycle fatigue (ULCF) fracture in steel bridge piers. This paper examined the fracture behavior of two thick-walled square section steel bridge piers subjecting to horizontal bidirectional cyclic loading. In addition, a microscope damage index for evaluating ULCF fracture initiation life of steel bridge piers was proposed. The findings show that ductile cracking firstly appeared at the junction between the stiffened base plate and bottom weld at the corner position of steel bridge piers under bidirectional cyclic loading, and that initial crack growth did not reduce the strength capacity. The strength capacity of steel piers began to rapidly decline as cracks propagated into the base metal. The proposed microscope damage index for ULCF fracture initiation life evaluation of steel bridge piers performs well when compared to experimental and evaluation results.

Ultra-low cycle fatigue performance of Q690 high-strength steel after exposure to elevated temperatures

High-strength steel (HSS) can reduce steel consumption compared to ordinary steel and has extensive applications in the building industry. HSS members in building structures are inevitably subjected to fire. The elevated temperature caused by fire leads to significant changes in the service properties of HSS. Therefore, research on the service properties of HSS after a fire is of great significance when evaluating whether the HSS structure can continue to serve. A series of studies on the ultra-low cycle fatigue (ULCF) performance of Q690 HSS after fire was carried out to evaluate the ability of Q690 HSS to resist strong earthquakes after fire. Q690 HSS specimens were subjected to heat treatment at 600–900 °C, and ULCF tests with different strain amplitudes were carried out on the cooled specimens. The hysteretic curves, fatigue life, and total energy dissipation density of the Q690 HSS after the fire were obtained. On this basis, the stress and strain evolution laws under the ULCF load of Q690 HSS, with different fatigue strain amplitudes and exposed temperatures, were analysed. Subsequently, the metallographic structure and ULCF fracture behaviour of Q690 HSS after fire were discussed. Based on the test results, two sets of independent prediction equations were proposed to evaluate the ULCF performance of Q690 HSS after a fire, which was in good agreement with the test results.

A stress-weighted ductile fracture model for steel subjected to Ultra Low Cycle Fatigue

A new criterion is proposed to simulate the initiation of ductile fracture due to Ultra-Low Cycle Fatigue (ULCF). Building on previous research, the new fracture criterion broadens the scope of ULCF models to account for a broader range of multi-axial stress and strain states that may be encountered in steel structures. The model formulation, supported by observations from finite element plasticity simulations of void growth and fractographic analyses of fracture surfaces, describes damage accumulation as a function of plastic strain, stress triaxiality, and the Lode stress parameter. The damage rate model is integrated over arbitrary cyclic loading histories of local strains and stresses to predict ULCF fracture. The proposed criterion (termed the Stress Weighted Ductile Fracture Model – SWDFM) is supported by a series of 66 coupon scale experiments on two grades of low-carbon structural steel (A572 and A36). These tests interrogate a range of positive and negative stress triaxiality with absolute values between 0.1 and 1.6, Lode stress parameters between 0 and 1, and monotonic and cyclic loading histories. The SWDFM criterion, which requires the calibration of only two material parameters for the materials investigated in this study, is evaluated against the experimental test data using an average-error assessment as well as a cross-validation analysis. Three other ULCF rupture criteria are similarly assessed and compared. The SWDFM is shown to accurately predict ULCF initiation over a wide range of stress states and loading histories, suggesting that the model represents well the micro-mechanical mechanisms of ULCF.

Continuum damage mechanics based ductile fatigue-fracture prediction in buckling steel braces

A Continuum Damage Mechanics criterion (the Stress Weighted Ductile Fracture Model - SWDFM) is used to predict Ultra Low Cycle Fatigue fracture in buckling steel braces. The focus is to examine the efficacy of this approach for predicting fracture in large-scale structural components with localization, when the constitutive and fracture parameters have been calibrated only at the coupon scale. A dataset consisting of 11 two-thirds scale, cyclically loaded, hollow brace specimens, and corresponding material coupon specimens is utilized for assessment of the fracture prediction approach. In these tests, fracture occurs either at the center where cyclic local buckling amplifies the strains, or at the gusset-plate connections. Particle Swarm Optimization is used to calibrate material constitutive parameters based on load-deformation response of the coupon tests, whereas the SWDFM parameters are calibrated using their fracture response. These parameters are applied within Continuum Finite Element simulations of the brace tests. The results indicate that the approach predicts fracture in a satisfactory manner. The error in fracture predictions is most correlated with the error in predicting the event of localization (i.e., local buckling). There does not appear to be a strong relationship between the error in the constitutive model calibration (based on the coupon tests) and the prediction of either local buckling or fracture. This suggests that in themselves, the small-scale coupon specimens are unable to calibrate the constitutive model with the required accuracy to capture localization. Commentary is provided for calibration and application of the CDM-based approach to predict fracture.

Ultra-low cycle fatigue properties and fracture mechanism of corroded structural steel

ABSTRACT Ultra-low cycle fatigue (ULCF) tests of corroded steel specimens were conducted to illustrate the effects of corrosion on the ULCF properties and fracture mechanism of structural steel. Fatigue crack initiation and propagation process and fracture morphology were observed respectively. The results indicate that the ULCF fracture life of corroded specimens decreases more with increasing surface roughness. The crack-initiation life of corroded specimens is less than half of the fracture life. With corrosion degree increases, the area of the ULCF crack propagation zone expands observably, where it gets harder to find dimples, indicating a transition from ductile to brittle fracture mode.

Experimental and numerical study on ultra low cycle fatigue fracture of X steel tubular joints with CHS braces to SHS chord

Steel tubular joints in steel structures may suffer from ultra low cycle fatigue fracture under strong seismic action. This paper aims to investigate the ultra low cycle fatigue behavior of X steel tubular joints with CHS braces to SHS chord by means of experiments and numerical simulation. Firstly, three X-joints with different constructions were tested. The experimental results indicate that the X-joint with fillet weld or penetration weld exhibits punching failure mode, and the weld forms have no obvious effect on the deformation ability and fatigue life of the joint. As for the X-joint with internal stiffener in the chord, it shows tensile fracture failure characteristic and has low deformation capacity, and there is no obvious surface crack before sudden fracture failure. Afterwards, the fracture failure of the X-joints were numerically simulated by using the cyclic void growth model. It shows that the crack growth rate is greatly underestimated by the model, and the predicted fatigue life of the X-joints was obviously longer than that of the tests, thus the evaluation on fatigue life of X-joints tended to be unsafe. A Lode parameter enhanced cyclic void growth model (referred to as LCVGM) was thus developed to consider shear effect on the fracture prediction, and the LCVGM parameters of base metal and weld metal were determined based on the experimental results of the pure shear specimens. Finally, the LCVGM was applied to simulate the ultra low cycle fatigue fracture of the X-joints, and it is shown that the model can well predict the whole fracture process and fatigue life of the joints.

Improved Ultra-Low Cycle Fatigue Fracture Models for Structural Steels Considering the Dependence of Cyclic Damage Degradation Parameters on Stress Triaxiality

The ultra-low cycle fatigue (ULCF) fracture initiation caused by cyclic large plastic strain in structural steels is often the governing limit state in steel structures when subjected to strong earthquake actions. Based on the author’s previous work, this paper presents the improved cyclic void growth model (CVGM) and degraded significant plastic strain (DSPS) model considering the dependence of cyclic damage degradation parameters on stress triaxiality. To this end, tests on circular notched specimens and coupon specimens made of Q345qC steel were conducted, and scanning electron microscopy studies were performed on fracture surfaces of specimens, analysis results of which show that the ULCF fracture of Q345qC steel exhibits the typical behaviour of “void nucleation, growth, and coalescence.” The cyclic damage degradation parameters of CVGM and DSPS model were calibrated at different stress triaxialities based on experimental results of specimens and complementary finite element analysis, and empirical formulas were subsequently established between cyclic damage degradation parameters and stress triaxiality. Finally, detailed finite element analysis results demonstrate that the improved CVGM and DSPS model can predict ULCF fracture behaviour with higher accuracy in comparison with original models.

A modified Coffin-Manson model for ultra-low cycle fatigue fracture of structural steels considering the effect of stress triaxiality

The ductile crack initiation of structural steels due to ultra-low cycle fatigue (ULCF) loading often governs the limit state in steel structures. This paper proposes a modified Coffin-Manson model for ULCF fracture of structural steels considering the effect of stress triaxiality. To this end, tests on coupon specimens and circular notched specimens made of Chinese Q345qC steel were conducted to form different stress triaxiality states. Based on test results and corresponding finite element analysis, the parameters of modified Coffin-Manson model were determined, and empirical formulas were established between model parameters and stress triaxiality. Finally, detailed finite element analysis was conducted on circular notched specimen tests to validate the proposed model. The results show that the proposed model can be used to predict ULCF fracture of structural steels at different stress triaxialities with high accuracy.

Improvement of Cyclic Void Growth Model for Ultra-Low Cycle Fatigue Prediction of Steel Bridge Piers.

The cyclic void growth model (CVGM) is a micro-mechanical fracture model that has been used to assess ultra-low cycle fatigue (ULCF) of steel structures in recent years. However, owing to the stress triaxiality range and contingency of experimental results, low goodness of fit is sometimes obtained when calibrating the model damage degradation parameter, resulting in poor prediction. In order to improve the prediction accuracy of the CVGM model, a model parameter calibration method is proposed. In the research presented in this paper, tests were conducted on circular notched specimens that provided different magnitudes of stress triaxiality. The comparative analysis was carried out between experimental results and predicted results. The results indicate that the number of cycles and the equivalent plastic strain to ULCF fracture initiation by the CVGM model calibrated by the proposed method agree well with the experimental results. The proposed parameter calibration method greatly improves prediction accuracy compared to the previous method.

Predicting ultimate shear capacities of shear connectors under monotonic and cyclic loadings

For steel‒plate concrete composite structures, shear connectors, including studs and tie-bars, are critical in resistance of interfacial shear between steel plate and concrete core. The previous push-out experimental results indicated that the primary factor resulting in significant decreases in ultimate shear capacities of shear connectors under cyclic loading was the ultra‒low cycle fatigue fracture of welded joint between shear connector and steel flange in push-out specimen. In this paper, the finite element models of push-out specimens carried out by the authors and a new simplified fracture criterion based on Xue-Wierzbicki fracture criterion were developed for predicting the ultimate shear capacities of shear connectors under monotonic and cyclic loadings. The numerical simulation results validated the developed finite element models, compared with the experimental results. Meanwhile, the parameters in the new simplified fracture criterions for studs and tie-bars were calibrated through the push-out experimental results, respectively. Finally, the ultimate shear capacities of studs and tie-bars at different level of axial tension under monotonic and cyclic loadings were predicted based on the calibrated fracture criterions and the developed finite element models. It indicated that the decreasing ratios of ultimate shear capacities of tie-bars under monotonic and cyclic loadings were less than those of studs at the same ratio of axial tension to specified tension capacity of shear connector. In cyclic loading condition, the decreasing ratios of ultimate shear capacities of studs were almost equal to the corresponding ratios of axial tensions to specified tension capacities.

Ultra-low Cycle Fatigue Fracture of High-Strength Steel Q460C and Its Weld

AbstractIn order to investigate the ultra-low cycle fatigue (UCLF) fracture in welded connections of high-strength steel (HSS) under earthquakes, this paper studies micromechanical fracture models ...