Analytical Models of Concrete Fatigue:A State-of-the-Art Review

Fatigue failure phenomena of the concrete structures under long-term low amplitude loading have attractedmore attention.Some structures,such as wind power towers,offshore platforms,and high-speed railways,may resist millions of cycles loading during their intended lives.Over the past century,analytical methods for concrete fatigue are emerging.It is concluded that models for the concrete fatigue calculation can fall into four categories:the empirical model relying on fatigue tests,fatigue crack growth model in fracture mechanics,fatigue damage evolution model based on damage mechanics and advanced machine learning model.In this paper,a detailed review of fatigue computing methodology for concrete is presented,and the characteristics of different types of fatigue models have been stated and discussed.

Fatigue strength evaluation of self-piercing riveted joints of AZ31 Mg alloy and cold-rolled steel sheets

The application of magnesium alloys to automobiles is increasing due to their superior specific strength and specific stiffness.In this study,an upper sheet of AZ31 magnesium alloy and a lower sheet of cold-rolled steel were joined by self-piercing riveting(SPR),a method commonly used to join automotive panels.A cross-shaped specimen was fabricated with a punching force of 35 kN,which exhibited the best joint strength for the SPR joint specimen geometry.Monotonic and fatigue strengths were evaluated using cross-shaped specimens at loading angles of 0°,45°,and 90°.The load amplitude corresponding to the fatigue endurance limit was assumed to be at 106 cycles,and the fatigue ratios(=fatigue endurance limit/static strength)at the loading angles of 0°,45°,and 90°are 22%,13%,and 9%,respectively.For all three loading angle specimens,fatigue cracks initiated at the triple point where the rivet shank,the upper sheet and the lower sheet are in contact with each other,with the cracks propagating through the thickness of the upper sheet and ultimately leading to fracture.The fatigue lifetimes were evaluated through the von-Mises stress,maximum principal stress,and equivalent stress intensity factor.It was found that the fatigue lifetimes could be evaluated most appropriately through the maximum principal stress.

Fatigue Crack Initiation and Propagation Dominated by Crystallographic Factors in TiB/near a-Ti

Discontinuously reinforced titanium matrix composites(DRTMCs)with a network structure have been extensively researched due to their superior combination of strength and ductility.However,their fatigue performance has remained unknown.In order to elucidate the fatigue behavior of DRTMCs,a tension-tension fatigue test was performed on a TiB/nearα-Ti composite with network structure.The results showed that the variability of fatigue lifetime increased as the stress level decreased.Fractography analysis indicated that fatigue crack initiation was associated with facet formation,while the subsequent propagation was hindered by the network structure comprising TiB whiskers and silicides.Crystallographic characterization further revealed that facets formed due to a combination of shear and normal stress.The reduction in fatigue lifetime was attributed to an increase in the effective slip length,which was influenced by the orientation of grains near the crack-initiation sites toward basal slip in the life-limiting specimen.Quasi in situ observation suggested that the crack initiation was facilitated by both basal and prismatic slip ofα-Ti as well as fracture of TiBw.Crack propagation was found to be associated with basal and prismatic slip systems with high Schmid factors,regardless of whether the crack was intergranular or intragranular.

A new model for life prediction of GH4133 under TMF conditions

Thermal mechanical cyclic strain tests were carried out under in-phase andout-of-phase conditions on a Nickel-base Superalloy GH4133 in the temperature range of 571-823 degC. Based on analyzing the present models of TMF (thermal mechanical fatigue) life prediction, a newmodel for predicting nickel-base superalloy TMF lifetime was proposed. TMF life of superalloy GH4133was calculated accurately based on the new model. Experimental TMF life has been compared with thecalculated results and all results fall in the scatter band of 1.5. The calculating results showthat the new model is not only simple, but also precise. This model will play great roles as lifeprediction of the metal materials and the engineering components subjected to non-isothermal serviceconditions.

A fatigue damage-cumulative model in peridynamics

While the present structural integrity evaluation method is based on the philosophy of assumed similitude, Fatigue and Damage Tolerance(F&DT) evaluations for next generation of air-vehicles require high-fidelity physical models within cyberspace. To serve the needs of F&DT evaluation in digital twin paradigm, a fatigue damage-cumulative model within peridynamic framework is proposed in this paper. Based on the concept of fatigue element block and damage accumulation law in form of Coffin-Manson relationship, the proposed model applies to both fatigue crack initiation and fatigue crack growth;fatigue crack growth rates under constant-amplitude and simple variable-amplitude block loading cases can be well predicted for three common structural materials without inputs of Paris law parameters. Additionally, the proposed model can also be easily extended to a probabilistic version;for verification, multiple-site-damage problems are simulated and the statistic nature of fatigue process in experiments can be well captured. In the end, main features of the proposed model are summarized, and distinctions from the other models are discussed. There may be a potential for the peridynamic damage-cumulative model proposed in this work to numerically predict fatigue problems in digital twin paradigm for future generations of aerospace vehicles.

Thermal analysis and cooling structure design of the primary collimator in CSNS/RCS

The rapid cycling synchrotron （RCS） of the China Spallation Neutron Source （CSNS） is a high intensity proton ring with beam power of 100 kW. In order to control the residual activation to meet the requirements of hands-on maintenance, a two-stage collimation system has been designed for the RCS. The collimation system consists of one primary collimator made of thin metal to scatter the beam and four secondary collimators as absorbers. Thermal analysis is an important aspect in evaluating the reliability of the collimation system. The calculation of the temperature distribution and thermal stress of the primary collimator with different materials is carried out by using ANSYS code. In order to control the temperature rise and thermal stress of the primary collimator to a reasonable level, an air cooling structure is intended to be used. The mechanical design of the cooling structure is presented, and the cooling efficiency with different chin numbers and wind velocity is also analyzed. Finally, the fatigue lifetime of the collimator under thermal shocks is estimated.

Oxidation-induced damage of an uncoated and coated nickel-based superalloy under simulated gas environment

Turbine blades and vans operated in an aggressive gas environment usually suffer from combined oxidation and cycle loading effects. The surface oxide layer will result in premature failure and lead to a significant reduction in the service lifetime. The effects of prior oxidation-induced damage under a simulated combustion-gas environment on the fatigue lifetime of the directionally solidified（DS） nickel-based superalloy DZ125 with and without an oxidation-resistant coating were presented. The fatigue lifetime of uncoated samples is adversely affected by prior oxidation exposure. The deterioration of fatigue lifetime in uncoated samples is associated with surface microstructural degradation, which occurs during prior exposure. However,the presence of MCrAlY coating is beneficial for the sample＇s lifetime under high stress. Further scanning electron microscopy（SEM） analysis demonstrates that the coating does not contribute to the initiation mode of fatigue cracks.