Number Of Fatigue Loading Cycles Research Articles

Experimental and Theoretical Study on the Fatigue Crack Propagation in Stud Shear Connectors.

Steel-concrete composite girder bridges are subjected to reciprocal cyclic loading from vehicles, and the stud shear connectors are the key components for transmitting shear forces. Thus, it is necessary to study the fatigue performance of the stud shear connectors. At present, there are few studies on the fatigue crack propagation process of studs, and the variation curve of the crack depth of studs with the number of fatigue loading cycles is not clear. In this study, the degradation law of fatigue properties and the fatigue crack propagation law of stud shear connectors in steel-concrete composite structures are examined under fatigue loading. The fatigue properties, i.e., failure mode, the dynamic slip-fatigue number curve, cross-sectional characteristics, and the residual bearing capacity of the stud specimens, are first systematically studied through ten standard push-out specimen tests. The test results show that the relative value of the fatigue crack extension area increases, while the relative value of the residual bearing capacity of the studs decreases approximately linearly. Then, the expression of the relationship between the fatigue crack depth and the residual load-bearing capacity of the stud is proposed, based on the fatigue crack theory of fracture mechanics. Finally, combined with the ABAQUS and FRANC3D software, a fatigue crack propagation finite element analysis (FEA) model of the stud is established. The FEA results showed that the trends in the number of cyclic loads and the fatigue crack depth of studs are basically the same for the simulation curve, test curve and theoretical calculation curve.

Limited Stress Surface Model for Bending and Torsion Fatigue Loading with the Mean Load Value.

In this study, a linear model of the transformation of the stress amplitude due to the mean value was used. The coefficient of the material sensitivity to cycle asymmetry with consideration of the dependence of this coefficient on the number of fatigue loading cycles is also used. A three-parameter surface model of limited stresses is proposed in this paper. The model is verified using the results of fatigue tests for cyclic bending and torsion under mean loads. The tests are performed for two types of alloy steels—S355J0 and S355J2G1W. Comparison of the allowable stress amplitudes obtained experimentally with those predicted using the proposed model shows errors of no more than 18%, with the area of the surface with the largest error being relatively small.

Research on fatigue crack propagation process of fracturing pumphead

Fracturing pumpheads are a key and crucial component in fracturing technology. Fatigue crack failure greatly decreases the pumphead working life and operational efficiency. This paper describes an analytical model for a five-cylinder fracturing pumphead using the submodel method with the re-meshing technique to analyze its fatigue crack propagation process. Changes in the crack face, a-KI curve (crack length a and stress intensity factor KI), and a-N curve (number of fatigue loading cycles N) during crack propagation are considered. The results show that the crack failure of the fracturing pumphead results primarily from mode-I cracks. During crack propagation, the curvature of the crack front gradually decreases due to the higher stress intensity factor KI at the pressured inner surfaces of the pumphead. The stress distribution on the pumphead greatly influences the change law for the a-KI curve. When the crack tip localizes in the high stress zone, KI increases with the crack length a; when the crack tip localizes in the low stress zone, KI decreases with the crack length a. The step structure of the pumphead also influences changes to the a-KI curve. Around the step structure, a wave peak appears in the a-KI curve, but the influence is localised and its effect on the a-N curve is not significant.

Self-Sensing Carbon Nanotube-Cement Composite Material for Structural Health Monitoring of Pavements

Abstract Self-sensing materials are capable of sensing the occurring strains and damage states present within. Smart concrete pavement made with cement-based self-sensing materials could be used for in situ monitoring of both the structural integrity as well as traffic characteristics. This article studies the electrical resistivity and piezoresistivity of cement enriched with multiwall carbon nanotubes (CNTs) under cyclic loading. The effects of fatigue damage and environmental factors, such as temperature, water content, and freeze-thaw cycles, on the resistivity and piezoresistivity were investigated. The test results show that the resistivity increases with the number of fatigue loading cycles. On the other hand, the resistivity decreases as the water content increases. In contrast, the piezoresistivity increases first and then decreases. The temperature has a significant yet offsetting effect on the electrical resistivity as well as the piezoresistivity. As the temperature increases, the electrical resistivity decreases, whereas the piezoresistivity increases. The major cause of the change in electrical resistivity and piezoresistivity was the change of tunneling conduction under different conditions. Moreover, the freeze-thaw cycles induce damage to the CNT-cement composite material. Therefore, the data collected by self-sensing materials must be calibrated for the water content and temperature before interpretation.

Experimental study on the mechanical and self-sensing behaviors of prestressed carbon fiber–reinforced polymer reinforced concrete composite structures

A type of self-sensing prestressed carbon fiber–reinforced polymer reinforced concrete composite structure was proposed and studied, composed of reinforced concrete beam, prestressed carbon fiber–reinforced polymer plate, and long-gauge fiber Bragg grating sensors. The carbon fiber–reinforced polymer plate was prestressed and bonded to the bottom of the reinforced concrete beam. Two types of anchorage systems were compared and studied. The long-gauge fiber Bragg grating sensors were used as active elements for the self-sensing of mechanical responses, which were installed on the tensile rebars, carbon fiber–reinforced polymer plates, and concrete. A series of static and fatigue 4-point flexural experiments were carried out to study the bending and fatigue performances of the composite structures. After a prescribed number of fatigue loading cycles, monotonic flexural bending was performed to investigate the deterioration of properties. The results showed that the long-gauge fiber Bragg grating sensor is valid for the mechanical response sensing of the proposed structures. The compatibility of the prestressed carbon fiber–reinforced polymer plate and concrete in the pure bending zone is excellent even under fatigue loading. The load-carrying capacities were improved by more than 30% due to the application of prestressed carbon fiber–reinforced polymer plates. The stiffness was also improved remarkably and generally decreased with the accumulation of fatigue cycles linearly.

Mechanistic Assessment of Fatigue Performance and Cost Analysis of Pavement Overlays: Comparison between Conventional Hot Mixed Asphalt, Asphalt Rubber, and Polymer-Modified Mixtures

Abstract Fatigue cracking, which is commonly associated with repeated traffic loading, is considered one of the major distresses occurring in asphalt pavements because of the low tensile strength of hot mixed asphalt (HMA). Studies have shown that introducing modifiers, such as rubber and polymers, increases the tensile strength of HMA, prolonging the fatigue life of pavement. Although research in the past has studied the effect of added rubber and polymers on the long-term fatigue performance of newly constructed pavement, few studies have shown the effect of those modifiers on the fatigue life of an asphaltic overlay and their associated cost-effectiveness. In this study, the long-term fatigue performance of asphalt overlay was constructed utilizing three different mixtures, conventional HMA, asphalt rubber (AR), and polymer-modified mixtures, which were evaluated utilizing 3D Move Analysis software. In addition, cost-effectiveness analysis was performed. Eighteen 3D Move Analysis scenarios were run including two different overlay thicknesses, three different vehicle speeds, and three different overlay mixtures. The tensile strain at the bottom of the overlay was determined and the number of fatigue loading cycles to failure (Nf) was calculated based on strain-Nf relationships of the three asphalt mixtures. Based on these analyses, use of modified mixtures as an overlay resulted in a significant increase in the service life of the overlay. It was demonstrated that AR and polymer-modified mixtures had better fatigue life and were more cost-effective than the conventional HMA mixtures.

Service Life Prediction Method for Concrete and Prestressed Concrete Bridges under Chloride-Aggressive Environments

Abstract The behavior of concrete and prestressed concrete bridges under chloride-aggressive environments during their service life is influenced not only by load (especially fatigue forces) but also by the corrosion environment. At present, the research of working performance and life prediction of bridges mainly focuses on the role of force or takes a single parameter as failure criterion, such as chloride ion concentration and steel bar depassivation. In this article, the service life prediction model of concrete and prestressed concrete bridges subjected to chloride-aggressive environments and fatigue is established, and the interaction of environmental effects and fatigue is taken into full account based on the mechanism of heat and mass transfer in porous media, the metal corrosion theory, and the classical mechanics theory. Taking depassivation as a demarcation point, the calculation method for the service life of concrete and prestressed concrete bridges is developed by analyzing the service life evolution process and determining the failure criterion. The service life of concrete bridges in coastal areas is calculated by the self-compiled program with MATLAB software (MathWorks, Natick, MA), and the results show good agreement with those in the project example. According to the results of the service life calculation with different increments of the number of fatigue loading cycles, ΔN, the increment of the number of fatigue loading cycles ΔN has an influence of no more than 5 % on the service life. This indicates that the calculation is converged and stable and that the calculation results are reliable. The service life of bridges is shortened with the increase of surface chloride concentration of concrete, daily vehicle traffic volume and load level, and the decrease of the concrete cover thickness. The study results are referred to predict the service life of concrete and prestressed concrete bridges under chloride-aggressive environments.

Fracture analysis of monolithic CAD-CAM crowns.

The aim of this study was to compare the integrity of zirconia, lithium disilicate, and zirconia-reinforced lithium silicate CAD-CAM crowns after being subjected to cyclic loading and then subjected to static loading until fracture. Zirconia (Zirkonzahn), lithium disilicate (LDS, Ivoclar Vivadent AG), and zirconia-reinforced lithium silicate glass ceramic (ZLS) (Vita Suprinity, Vita Zahnfabrik) monolithic crowns were milled (n = 6). The crowns were bonded using composite resin cements and subjected to cyclic loading under wet conditions. Three specimens from each group were loaded for 10 000 cycles, and the other three specimens were loaded for 50 000 cycles with 250 N. Specimens were subjected to loading until fracture. Load-to-fracture values were analyzed with 2-way (ANOVA) and Tukey-Kramer post hoc test (α = 0.05). Specimens from each group were examined using an SEM. Mean load-to-fracture values among materials were significantly different from each other (P < 0.05). No significant effect of the number of cycles was found on the load-to-fracture values of crowns (P > 0.05). Load-to-fracture values of zirconia were higher than those of LDS, which were higher than those of ZLS. The number of fatigue loading cycles did not affect the load-to-fracture of the tested crowns for a given material. More research needs to be conducted before considering the routine use of ZLS for molars in patients with high risk of parafunctional habits.

Temporal effect of in vivo tendon fatigue loading on the apoptotic response explained in the context of number of fatigue loading cycles and initial damage parameters.

Accumulation of damage is a leading factor in the development of tendinopathy. Apoptosis has been implicated in tendinopathy, but the biological mechanisms responsible for initiation and progression of these injuries are poorly understood. We assessed the relationship between initial induced damage and apoptotic activity 3 and 7 days after fatigue loading. We hypothesized that greater apoptotic activity (i) will be associated with greater induced damage and higher number of fatigue loading cycles, and (ii) will be higher at 7 than at 3 days after loading. Left patellar tendons were fatigue loaded for either 100 or 7,200 cycles. Diagnostic tests were applied before and after fatigue loading to determine the effect of fatigue loading on hysteresis, elongation, and loading and unloading stiffness (damage parameters). Cleaved Caspase-3 staining was used to identify and calculate the percent apoptosis in the patellar tendon. While no difference in apoptotic activity occurred between the 100 and 7,200 cycle groups, greater apoptotic activity was associated with greater induced damage. Apoptotic activity was higher at 7 than 3 days after loading. We expect that the decreasing number of healthy cells that can repair the induced damage in the tendon predispose it to further injury.

Load-Measuring Pot Bearing with Built-In Load Cell —Part II: Fatigue Performance and Experimental Temperature Correction

This paper presents the results of fatigue performance tests performed up to 10 million cycles on a load-measuring pot bearing with built-in load cell to verify its field applicability and proposes an empirical temperature correction formula. In Part I of this work, various measurement performances of the load-measuring pot bearing were evaluated through static and dynamic loading tests. Bridge bearings are subjected to the effect of fatigue caused by the repeated application of moving loads and exposed to harsh site conditions including cold and hot weathers differently to laboratory conditions. Accordingly, the durability of the load-measuring pot bearing with built-in load cell shall be secured and the environmental effects like temperature shall be minimized for its application on field. This study conducted fatigue tests up to 10 million cycles on a load-measuring pot bearing with the capacity of 1000 kN to examine eventual degradation of the measurement accuracy with respect to the number of fatigue loading cycles. In addition, the experimental temperature correction procedure is proposed to obtain the temperature correction formula enabling to correct the effect of temperature on the load measurement.

Residual Strength Degradation of CoSb3 Skutterudite Compounds Under Low-Cycle Fatigue Loading

This paper reports an experimental and theoretical study on the residual strength degradation of CoSb3-based skutterudites subjected to compression–compression low-cycle fatigue loading. Based on the experimental results and findings of other researchers, a residual strength degradation model using a normal distribution function is proposed. The model can predict the residual strength at a given number of fatigue loading cycles with given probability. Lastly, the validity of the model is confirmed by experimental results for CoSb3.

Molecular response of the patellar tendon to fatigue loading explained in the context of the initial induced damage and number of fatigue loading cycles

Accumulation of sub-rupture fatigue damage has been implicated in the development of tendinopathy. We previously developed an in vivo model of damage accumulation using the rat patellar tendon. Our model allows us to control the input loading parameters to induce fatigue damage in the tendon. Despite this precise control, the resulting induced damage could vary among animals because of differences in size or strength among their patellar tendons. In this study, we used number of applied cycles and initial (day-0) parameters that are indicative of induced damage to assess the molecular response 7 days after fatigue loading. We hypothesized that day-0 hysteresis, elongation, and stiffness of the loading and unloading load-displacement curves would be predictive of the 7-day molecular response. Results showed correlations between the 7-day molecular response and both day-0 elongation and unloading stiffness. Additionally, loading resulted in upregulation of several extracellular matrix genes that suggest adaptation; however, several of these genes (Col-I, -XII, MMP 2, and TIMP 3) shut down after a high level of damage was induced. We showed that evaluating the 7-day molecular profile in light of day-0 elongation provides important insight that is lost from comparing number of fatigue loading cycles only. Our data showed that loading generally results in an adaptive response. However, the tendon's ability to effectively respond deteriorates as greater damage is induced.

Punching Shear Strength Considering Cumulative Damage for FRP Strengthened Bridge Decks under Fatigue Loading

This study investigates the punching shear strength of concrete bridge decks strengthened with various fiber reinforced polymer (FRP) materials, carbon fiber sheet (CFS), glass fiber sheet (GFS) and carbon fiber grid (CFG). This study performed fatigue loading tests on the strengthened bridge decks with different fatigue loading levels. Based on the experimental results, a damage index was determined considering the damage mechanics and was applied to the plastic punching shear strength model for the evaluation of the punching shear strength with respect to the number of fatigue loading cycles. The developed model seems to successfully estimate the punching shear strength of damaged bridge decks with sufficient reliability. It is anticipated, therefore, that the developed model may help improving the design of strengthening of damaged bridge deck panels.

Fractographic analysis of the character of fracture of steel 10G2S1 welds subjected to low-temperature fatigue

1. We found that the metal of welds obtained with UONI 13/55 electrodes and welds of steel 10G2S1 have a high resistance to fatigue crack growth at low temperatures. 2. Embrittlement of the weld metal with decreasing temperature is due to a certain degree to the formation of a coarse-grained structure during welding. 3. An increase in impact-test temperature and increase in the number of fatigue-loading cycles produces a tendency toward grain-boundary embrittlement in the region of the coarsegrained structure appearing during welding, this tendency in turn being associated with a reduction in impact toughness. 4. Both a reduction in impact-test temperature and an increase in the number of load cycles lead to twinning, which shows up in the fractures in the form of tongues on the cleavage facets. This is evidence of the partial substitution of twinning for slip as the fracture mechanism at lower temperatures.