Ultra-low Cycle Fatigue Research Articles

Ultra-low cycle three-point bending fatigue of glass fabric reinforced plastic

In the present work, the static strength and ultra-low cycle fatigue (less than 100 cycles) of plain weave fiberglass reinforced plastic STEF ™ under three-point bending along the warp and the diagonal are studied. For this glass fiber reinforced plastic (GFRP), force–displacement loading diagrams are studied in combination with acoustic emission data. Digital image correlation (DIC) showed that the maximum bending strain is close to the failure strain of the tensile specimens. Bending fatigue tests showed progressive displacement (cyclic creep) and a nonmonotonic change of specimen stiffness. This is associated with two processes: straightening in warp direction and micro-damage – local delaminations and failures of the matrix and fibers. Acoustic events (piezo-sensor signals) were divided into low- and high-frequency groups. The loading in the diagonal direction is accompanied only by low-intensity, low-frequency (up to 300 kHz) events. When bending was applied in the warp direction, the acoustic image had intense low- and high-frequency events (up to 500 kHz). The presence of a high-frequency component in the frequency spectrum can be a diagnostic sign of a weakening of the material and approaching fracture, which is valuable for structural health monitoring. Under cycling with constant force amplitude, low-frequency events were repeated under loading and unloading stages, which related to friction at the fiber–matrix level. Thus, the Kaiser effect is not present for this GFRP. During unloading, acoustic events are also observed. This indicates the creep of the material under cyclic loading. The bending fatigue limit with the base of 100 cycles life was about 70% of the ultimate strength.

Latest Development in Physics-Based Modeling of Coiled Tubing Plasticity and Fatigue

Summary Coiled tubing (CT) integrity is critical for well intervention operations in the field. To monitor and manage tubing integrity, the industry has developed a number of computer models over the past decades. Among them, low-cycle fatigue (LCF) modeling plays a paramount role in safeguarding tubing integrity. LCF modeling of CT strings dates back to the 1980s. Recently, novel algorithms have contributed to developments in physics-based modeling of tubing fatigue and plasticity. When CT trips into and out of the well, it goes through bending/straightening cycles under high differential pressure. Such tough conditions lead to low- or ultralow-cycle fatigue, limiting CT useful life. The model proposed in this study is derived from a previous one and is based on rigorously derived material parameters to compute the evolution of state variables from a wide range of loading conditions. Through newly formulated plasticity and strain parameters, a physics-based damage model predicts CT fatigue life, along with diametral growth and wall thinning. The revised modeling approach gives results for CT damage accumulation, diametral growth, and wall thinning under realistic field conditions, with experimental validation. For 20 different CT alloys, it was observed that the model improved in accuracy overall by approximately 18.8% and consistency by 14.0%, for constant pressure data sets of more than 4,500 data points. The modeling results provide insights into the nonlinear nature of fatigue damage accumulation. This study allowed developing recommendations to guide future analytical modeling and experimental investigations, summarize theoretical findings in physics-based LCF modeling, and provide practical guidelines for CT string management in the field. The study provides a fundamental understanding of CT LCF and introduces novel algorithms in plasticity and damage.

Experimental investigation of failure mechanism of grid structure with bolted spherical joints

Under strong earthquakes, the grid structure members may frequently bear plastic strain so that the grid failure, which is characterized by ultra-low cyclic fatigue. In the present study, the ultra-low cycle fatigue test of three full-scale grid structures, which are under vertical cyclic large plastic strain load, is carried out. The hysteretic and skeleton curves, energy consumption capacity, ductility performance, bearing capacity degradation, and stiffness change are obtained. Also, the macroscopic morphology of rod fracture is analyzed. In addition, the corresponding recommendations are made based on the failure mechanism of the rod and the deformation characteristics of the joint. It was found that the ultra-low cycle fatigue damage of the lower chord is the main cause of grid failure. Also, the failure behavior of the rod in the structure is very similar to that in the member test. The coupling effect of ultra-low circumferential fatigue and local buckling causes the strain to concentrate in the middle of the rod. The computational analysis of such structures under strong earthquakes should consider the degradation of joint stiffness.

Ultra Low-Cycle Fatigue Behavior Comparison between Additively Manufactured and Rolled 17-4 PH (AISI 630) Stainless Steels

This study investigates the mechanical behavior of additively manufactured (AM) 17-4 PH (AISI 630) stainless steels and compares their behavior to traditionally produced wrought counterparts. The goal of this study is to understand the key parameters influencing AM 17-4 PH steel fatigue life under ULCF conditions and to develop simple predictive models for fatigue-life estimation in AM 17-4 steel components. In this study, both AM and traditionally produced (wrought) material samples are fatigue tested under fully reversed (R = −1) strain controlled (2–4% strain) loading and characterized using micro-hardness, x-ray diffraction, and fractography methods. Results indicate decreased fatigue life for AM specimens as compared to wrought 17-4 PH specimens due to fabrication porosity and un-melted particle defect regions which provide a mechanism for internal fracture initiation. Heat treatment processes performed in this work, to both the AM and wrought specimens, had no observable effect on ULCF behavior. Result comparisons with an existing fatigue prediction model (the Coffin–Manson universal slopes equation) demonstrated consistent over-prediction of fatigue life at applied strain amplitudes greater than 3%, likely due to inherent AM fabrication defects. An alternative empirical ULCF capacity equation is proposed herein to aid future fatigue estimations in AM 17-4 PH stainless steel components.

A model for ultra low cycle fatigue damage prediction of structural steel

Ultra low cycle fatigue (ULCF) damage of structural steel is an important failure mode in earthquakes. A modified continuum damage mechanics (CDM) model was proposed to predict the fatigue life for ULCF damage. To this end, the functional form of the reduction coefficient considering the effect of plastic strain was constructed. The constant of function form was fitted by ULCF tests and genetic algorithm. Finally, the accuracy of the modified CDM model was verified by ULCF tests. Results showed that the modified CDM model could reduce the relative error between the predicted and tested fatigue life.

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.

Experimental evaluation and numerical simulation of low-yield-point steel shear panel dampers

Shear panel dampers are considered as a type of reliable passive energy dissipation device for seismic resistant structures with a damage-controlling concept. It has been revealed that the shear bulking of core plates and the premature failure of boundary plates are the main concerns for a shear panel damper that result in insufficient energy dissipation and undesirable ultra-low cycle fatigue performance. This study utilizes square steel tubes serving as out-of-plane stiffeners for the core plates made of a low-yield-point steel with a nominal yield stress of 225 MPa (named LYP225 steel). Meanwhile, a reduced flange plate section is adopted to mitigate the end fracture. An experimental study is carried out on three full-scale damper specimens with different flange plates and loading protocols. The test results show that the square tubes well prevent the bulking of the core plates and all the specimens exhibit plump hysteretic response and satisfactory energy dissipation capacity. The reduced plate sections reduce the likelihood of crack propagation occurring at the flange end, and thereby improve the ultra-low cycle fatigue performance. In order to develop the numerical model of shear panel dampers, a combined-hardening constitutive model with a memory surface is applied to represent the plasticity of LYP225 steel. Based on a plasticity analysis, a simplified calibration procedure is proposed associated with the three-dimensional constitutive model, where the plasticity parameters of LYP225 steel are determined using only the monotonic tensile test data. Eventually, the numerical results of uniaxial specimens and damper specimens are in good agreement with the test results, which validates the effectiveness of the constitutive model and the proposed calibration method.

Low cycle fracture behavior of steel members with circular hollow section subjected to eccentric load

This study aims to investigate the effect of eccentric load on the seismic and fracture behavior of Q235B steel members with circular hollow section (CHS). To this end, uniaxial tension and ultra-low cycle fatigue tests on both un-notched and notched Q235B steel coupon specimens were conducted first to evaluate the material’s micromechanical fracture behavior. Next, 7 full-scale Q235B CHS steel members with different global slenderness were tested under cyclic axial loads with varying eccentricities. It is observed that: (1) the increase of the eccentricity of the axial load negatively affects the seismic and fracture resistance of CHS steel members, resulting in the deterioration of the strength, stiffness, energy dissipation, and ductility; (2) Large eccentricities lead to the reduction in the fracture life of Q235B CHS steel members; and (3) High global slenderness results in the reduction in both compression bearing capacity and energy dissipation of CHS steel members. In addition to the experimental program, finite element analyses are performed to simulate the seismic and fracture behavior of CHS steel members. The results from the finite element analyses are in good agreement with those from the experiment.

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.

A proposal on ultra-low cycle fatigue damage evaluation of structural steels

The ductile crack initiation observed in structural steels subjected to ultra-low cycle fatigue (ULCF) loading often leads to the catastrophic failure in steel structures. In the existing ULCF damage models, there is always a contradiction between the cost of model parameters calibration and prediction accuracy. In order to solve the contradiction, a one-parameter model for ULCF damage evaluation of structural steels was proposed based on the concept that ULCF damage consists of ductile damage in tensile loading and cyclic damage under cyclic plastic stain loading, and a simple and practical parameter calibration method only using tensile results of coupon specimens is developed. Tests and finite element analysis were carried out on circular notched specimens to verify the prediction accuracy of proposed model. The results show that predicted results are in good agreement with test results, indicating a good performance of the proposed model for ULCF damage evaluation of structural steels.

Earthquake-induced damage updating for remaining-life assessment of steel frame substructure systems

This paper presents a framework combining theoretical, numerical, and experimental approaches for evaluating the remaining-life of high-rise steel buildings based on earthquake-induced fatigue damage propagation in local beam-to-column connections. A mesh-independent finite element model is developed and implemented using the concept of lumped damage mechanics to estimate the fatigue-induced life loss associated with the extent of crack propagation until complete rupture failure occurred in beam-to-column connection substructure of high-rises. In this framework, a damage state variable is introduced at each end of beam elements and can be linked to the extension of the fatigue fracture in real frame building joints. Then, the damage evolution law is developed by extending the conventional Manson–Coffin law and a new crack-driving parameter to quantify the extent of damage states and predict the remaining life of the associated components in high-rises buildings. The proposed model is validated by a damage index measured from the local wireless sensors in substructure tests of ductile steel beam-to-column connections subjected to ultra-low cycle fatigue loadings. The residual resistance can be quantified by updating the damage curve with the identified damaged conditions.

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.

Effect of Corroded Surface Morphology on Ultra-Low Cycle Fatigue of Steel Bridge Piers.

Corrosion is a common form of durability degradation of steel bridges. Corrosion morphology affects stress distribution under cyclic loads and causes strain concentrations in pits, thus affecting the mechanical properties of steel structures, including ultra-low cycle fatigue (ULCF). To precisely simulate corrosion morphology and investigate the ULCF failure mechanism of corroded steel piers, a sculpting method was applied to mesh units using three-dimensional surface morphology data of corroded steel specimens. Moreover, the ULCF crack-initiation life was numerically predicted using the finite element model based on the cyclic void growth model (CVGM). The cumulative equivalent plastic strain, cyclic void growth index, and critical void growth index of corroded steel piers with different corroded morphologies were compared. Results reveal that, regardless of whether the pier is corroded, fatigue cracks tend to initiate at the weld toe at corners when exposed to cyclic loads under an oblique direction at the pier top. Additionally, the ULCF crack-initiation life in a corroded pier is less than that in an uncorroded pier, and it is significantly affected by a reduction in the pier wall thickness. Corrosion pits affect the position of ULCF crack initiation in a steel pier and cracks tend to initiate at the bottom of pits with large depth-to-diameter ratios. In the case of minor corrosion, the corrosion morphology affects the seismic performance of piers to a small extent.

A study of ultra-low cycle fatigue failure based on a fracture strain energy model

Ductile failure under ultra-low cycle fatigue (ULCF) loading is often the limit service state of a nuclear structure. In the paper, ULCF tests for a C-Mn-Si steel are carried out. It is observed that the fracture displacement decreases as the number of fatigue cycles increases. An energy-based model, which considers the effects of both stress triaxiality and cyclic loading on the critical fracture strain energy, is proposed. Coupled with the new model, finite element analyses of the round notched specimens can predict the equivalent plastic strain and the number of fatigue cycles to fracture within a scatter band of 1.5.

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.

Damage evaluation and ultra-low-cycle fatigue analysis of high-rise steel frame with mesoscopic fracture models

In this study, a straightforward method for evaluating damage and ultra-low-cycle fatigue (ULCF) life of high-rise steel frame structures under strong seismic loading at the material level was newly proposed on basis of inter-story drift measurement and mesoscopic fracture models. A mesoscopic ULCF model for cyclic loading was improved by considering both effects of strain amplitude and loading sequence, which was implemented into finite element software. Since elastic loading and unloading processes do not affect damage accumulation, four filtering methods for inter-story displacement data were proposed to improve computing efficiency without impeding simulation accuracy. An improved fracture model was integrated into ABAQUS using a user-defined subroutine, and a hybrid finite element analysis using both shell and beam elements was conducted, where filtered displacement was employed as input data. Experimental results of a shaking table test on a 1/3-scale 18-story steel frame conducted at E-Defense were employed to prove validity of the analysis method and fracture models. Comparison results show that the improved mesoscopic ULCF model can evaluate crack initiation of steel structures with acceptable accuracy.

Evaluating ultra low cycle fatigue based on ductile fracture model in double core BRBs

While lots of efforts have been made to develop accurate numerical models for prediction of ultra-low cycle fatigue (ULCF), it still needs more studies. As it is crucial to exactly define the mechanical behavior of steel for entire range up to failure, this study first examines the required theories and presents a simple single-parameter model to predict the crack initiation. After selecting appropriate behavior models, to ensure the proper performance in numerical solution of the problem, the cyclic performance of steel coupons and two types of buckling restrained braces (BRBs) are validated comparing their local and global behaviors to experimental ones. The double core BRB consisting of two parallel steel cores is then introduced and its cyclic behavior is evaluated. Its advantages include easy connection to gusset plates, increased brace moment of inertia around the weak axis, high stability of connections under local buckling, and low impact of unpredictable defects. Finally, hysteresis curves, energy absorption rate, crack initiation and evolution, and cumulative ductility are discussed in this research.

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.

Simulation of ultra-low cycle fatigue cracking of coiled tubing steel based on cohesive zone model

An optimal fatigue cohesive zone model (FCZM) was developed for coiled tubing (CT) steel through experiments and finite element calculation. First, aiming at the characteristics of small diameter and thin wall of CT, arc-shaped specimen with single-edge crack was designed and processed. Ultra-low cycle fatigue (U-LCF) tests controlled by displacement were carried out to obtain the variation of both force and crack size with the number of cycles, as well as crack propagation rates corresponding to different crack sizes. Second, FCZM was established by introducing damage into the Park-Paulino-Roesler (PPR) model. Based on the measured time histories of extreme force, the optimal cohesive parameters that described the U-LCF behavior of the CT steel were determined by means of inverse analysis. Third, the optimal FCZM was used to simulate the other U-LCF processes of the CT specimen. The predicted results agreed well with the measured ones. On the one hand, a method of determining the optimal fatigue cohesive parameters is developed based on the time-history of parameter extreme in this paper. On the other hand, an FCZM is provided for life prediction of CT.