Action Of Cyclic Loads Research Articles

Relation Between Striations Spacing and Fatigue Crack Growth Rates for Additive Manufactured Inconel 625

ABSTRACTThe principles of the mechanisms for fatigue crack propagation in metals have been established several decades ago. By definition, fatigue might be summarized as a damage process imparted to a material under the action of cyclic loads. The relation between fatigue crack growth rates (FCGRs) and formation of striation marks has been a controversial topic. Some authors stated that, in certain fatigue regimes, several fatigue cycles are required to form a single striation. The disagreement found was the motivation for the present work. This study's main goal is to provide a comparison between the striation spacing and the respective FCGR. These analyses will be performed in a Nickel superalloy, Inconel 625, obtained via directed energy deposition. Investigation of the striations spacing are compared with data obtained from FCGR tests. A good agreement between striation spacing and FCGRs was found to intermediate and large values of stress intensity factor.

Hysteretic behavior of the segmented buckling‐resistant braces with LYP160

AbstractThe goal was to evaluate the hysteretic performance of buckling‐resistant braces with low yield point steel LYP160, the monotonic tensile and cyclic loading tests of LYP160 test specimens were conducted and the cyclic constitutive relationship was obtained. According to the load–displacement curves of the specimens, the low‐yield point steel was characterized by good ductility and energy absorption ability. With consideration of the Chaboche model for the materials, the cyclic hardening parameters of low‐yield point steel were obtained. On this basis, the hysteretic properties of buckling‐resistant braces under cyclic loads were simulated and analyzed. After the analysis and comparison of buckling‐resistant braces specimens with isotropic core plate and segmented variable section core plate, it can be found that: when the conventional buckling‐resistant braces with an isotropic core plate were loaded to L/100, the lateral deformation of the buckling‐resistant brace (BRB) would reach 17 mm. Additionally, serious squeezing could be observed on the lateral restraining members. The conventional BRB would become ineffective due to the accumulation of deformation at both ends of the BRB. When the segmented buckling‐resistant brace was applied, the core plate with variable section would buckle first in the middle area, other parts could continue to consume energy thanks to the action of the limit plate. It would avoid the situation that other areas would be unable to consume energy after the core plate yields at one area first. Under the action of cyclic loads, no stiffness degradation was noted in the segmented buckling‐resistant brace. Segmented buckling‐resistant braces demonstrated superior ductility and energy dissipation capacity.

Experimental and Numerical Investigations on the Spherical Steel Bearing Capacity of New Anti-Separation Design

To solve the issues of slippage and dislodging caused by insufficient grout compactness in railway bridge spherical steel bearings, a novel kind of anti-separation spherical steel bearing is presented. Firstly, a comparative test model was meticulously designed and constructed for the bearing-stone assembly, encompassing both the traditional spherical steel bearing and the innovative anti-separation spherical steel bearing. Next, mechanical performance testing and finite element simulation analysis were completed. Finally, an empirical equation for the vertical load-strain fitting of the bearing was established. The results demonstrate that, under the maximum vertical static load, the convex spherical crown anti-separation design of the new anti-dislodging bearing is not sensitive to the bearing performance of the bearing-bedding stone combination. All indexes also meet the specifications. Under the action of cyclic load, the error of each parameter between finite element analysis and test results is less than 7%, which shows that the new anti- separation spherical bearing has higher bearing capacity and better fatigue resistance; the fitted value of the established bearing vertical load-strain calculation formula is in agreement with the test value by more than 0.95, which can predict whether the new anti-separation bearing fails.

Experimental study on seismic behavior of fabricated lightweight steel solid waste High-strength foam concrete composite wall

In order to actively implement the “dual carbon” goal and promote the popularization and application of green energy-saving prefabricated buildings in low-rise and multi-storey residential structures in villages and towns, a prefabricated light steel solid waste high-strength foam concrete composite wall was proposed. The wall is composed of high-strength foam concrete, cold-drawn steel wire mesh and cold-formed steel, which can realize the integration of load-bearing and energy-saving of wall components. The failure characteristics, hysteresis characteristics, bearing capacity and deformation performance, stiffness degradation and energy dissipation capacity of the wall specimens with different structures were analyzed through monotonic load and in-plane lateral cyclic loading test on three different wall specimens. The results show that under the action of cyclic load, the wall specimens with three different structures all have shear failure, and the wall specimens without cold-formed steel at the ends have low bearing capacity and poor seismic behavior. The bearing capacity and seismic behavior of the wall specimens with cold-formed steel at the ends were significantly improved, and the cold-formed steel, cold-drawn steel mesh, and high-strength foam concrete had good coordination performance. Considering the inherent characteristics of foam concrete, based on the existing specifications, the formula for calculating the shear bearing capacity of the prefabricated light steel solid waste high-strength foam concrete composite wall is given. The shear bearing capacity of wall members with different structures, different loading methods and different axial compression ratios are tested and calculated, and the calculated results of shear bearing capacity are in good agreement with the test results.

The Failure Characteristics and Energy Evolution Pattern of Compound Coal–Rock under the Action of Cyclic Loading

Based on the entire loading process of compound coal–rock, test pieces with three different coal/rock ratios (1:3, 1:1, and 3:1) have been constructed and the corresponding cyclic loading experiments have been carried out. Through the experiment, the deformation and failure characteristics of the compound coal–rock samples have been explored and the stage evolution characteristics of energy density have been subsequently analyzed. Ultimately, the relation between deformation failure and the energy evolution mechanism has been established, and thus the reasons behind rock bursts in the coal–rock compounds have been discussed. The experimental results indicate that with the increase in cyclic loading, the stress–strain curve of the compound coal–rock demonstrates a positive shift, whereas the change in the hysteretic curve from dense to sparse results in a “hysteresis expansion”. The increase in the coal body height increases the chance of brittleness failure of the compound coal–rock. The coal body, as the main controlling factor of compound coal–rock failure, generates cracks that expand to the rock body along the juncture of the coal and rock, leading to instability. The energy density evolution curve can be described by a quadratic function. The evolution process is initiated from the slow increase in input energy density and elastic energy density. A large amount of energy is stored through the rapid increase in the density mentioned above. At last, the evolution is completed by a surge in dissipated energy. The energy evolution drives the crack expansions in the compound coal–rock under load. The energy accumulation in the compound coal–rock is increased by the exploitation of the clamping effect of the thick and hard top and bottom plate. The risk of rock burst is intensified by the failure of the coal body because of the energy in the coal–rock system. The study results help to comprehend the energy evolution pattern in the surrounding rock of deep mining roadways and expand the prevention methods for impact ground pressure.

Determining performance of two-tiered GRS walls subjected to traffic cyclic loading

This study evaluates the performance of two-tiered geogrid-reinforced soil (GRS) walls subjected to traffic cyclic loading considering several influencing factors. These factors herein include the offset distance (D) of walls, the number (N), amplitude (Pmax), and frequency (f) of applied cyclic loading. Seven GRS walls with a reduced scale of 1 : 3 were prepared in the laboratory and employed to investigate their (i) vertical foundation pressures during construction, (ii) load-induced settlements, (iii) facing lateral displacements, (iv) vertical and horizontal earth pressures, and (v) geogrid strains under the action of cyclic loading. Experimental results demonstrate that GRS walls constructed in tiered configurations can effectively reduce vertical foundation pressures. The increasing D, as well as the decreasing N and Pmax, introduces a reduction to the above five mechanical and deformation properties. However, increasing f results in the decrease of wall settlements and facing lateral displacements, and in the increase of others. Performance of several empirical equations for predicting the vertical foundation pressures, location of maximum geogrid strains, and failure surfaces inside walls was examined using the experimental data obtained in this study. Comparisons were also performed to describe the deformation and failure surface modes of the walls after loading.

Quantitative evaluation of crack damage under a variable temperature environment based on a Gaussian mixture model

During the operation of high-speed trains, components are prone to fatigue crack damage under the action of cyclic loading and extreme temperature environments. Temperature change has become an important factor for monitoring the health of high-speed train structures. A new fatigue crack size quantification method under a variable temperature environment based on a Gaussian mixture model (GMM) is presented in this paper. A series of damage indexes are proposed to characterize the interaction mechanism between the signal and crack under temperature change. Moreover, multidimensional damage indexes extracted by the fusion of dimensionality reduction technology are used to establish a complete working condition baseline GMM database. The service temperature of the structure is determined according to the maximum similarity criterion between the baseline GMMs and detection GMMs. Furthermore, the quantitative crack length detection model is established at each temperature. To validate the effectiveness of the method, fatigue crack experiments in a variable temperature environment are carried out. The verification results show that this method can detect fatigue crack growth at different temperatures.

Deterioration characteristics of coal gangue concrete under the combined action of cyclic loading and freeze-thaw cycles

Coal gangue is a kind of solid waste. Adding coal gangue as aggregate into concrete can realize the resource utilization of solid waste. The deterioration law of coal gangue concrete under the combined action of cyclic loading and freeze-thaw was studied by macro and micro methods. The mass loss, relative dynamic elastic modulus loss, and compressive strength loss of specimens under the combined action were analyzed. The microscopic morphology and pore structure deterioration law of gangue concrete were investigated by scanning electron microscopy and mercury compression test. The quantitative relationship between the pore structure and compressive strength damage was established. The results show that with the increase of cyclic loading and freeze-thaw cycles, the mass of the specimen first increases and then decreases. The relative dynamic elastic modulus and compressive strength gradually decrease. The larger fractal dimension indicates the higher complexity of the pore structure. The fractal dimension decreases with the increase of cyclic loading times and freeze-thawing, implying that the pore structure gradually deteriorates under the combined action. The regression effect between compressive strength damage and composite pore structure parameters and fractal dimension is evident, describing the quantitative relationship between compressive strength damage and pore structure under the combined action of cyclic loading and freeze-thaw cycles.

АВТОРСЬКА МЕТОДИКА ВИЗНАЧЕННЯ НЕСУЧОЇ ЗДАТНОСТІ ПОХИЛИХ ПЕРЕРІЗІВ БАЛКОВИХ КОНСТРУКЦІЙ ЗА ДІЇ ПОВТОРНИХ НАВАНТАЖЕНЬ

Analysis of the results of both our own tests and studies of other scientists shows that the destruction of structures under the action of repeated loading occurs at lower stresses than the destruction of structures under the action of short-term static loads. Namely, the bearing capacity of beams under the action of cyclic loads is on average 10% lower compared to the bearing capacity of their "twins" under static forces, that must be taken into account when calculating and designing structures. In this regard, the author's method for calculating inclined sections of reinforced concrete beam structures under low-cycle and multi-cycle loads is presented in the paper. Repeated loads are proposed to be taken into account by calculating the fatigue strengths of the beam’s materials, namely the compressed zone of concrete above the critical inclined crack, the longitudinal reinforcement at the intersection with the inclined crack, as well as the anchoring of the longitudinal reinforcement. In particular, the design lines of endurance of concrete and reinforcement are considered. In practical calculations, the change in stresses that occurs due to the development of vibrocreep deformations of compressed concrete at complex stress state is taken into account by the functions of stress accumulation in concrete, longitudinal and transverse reinforcement. These functions of deformations of concrete vibrocreep are calculated according to the theory of vibrocreep. According to the results of the research, the corresponding author's expressions and dependencies are derived and presented, which allow determining the limit forces of structural element with a given shear span. In addition, it is proposed to introduce the coefficient kcyc into the calculation, which takes into account the more destructive effect of a multi-cycle load compared to a low-cycle repeated one. Thus, kcyc is the ratio of the breaking shear force to the minimum design ultimate endurance force of the near support sections of the specimens-beams, calculated using the characteristic values of the strength of concrete and reinforcement, that is, the prismatic strength of concrete and the yield strength of reinforcement.

Study on Microstructure and Fatigue Properties of FGH96 Nickel-Based Superalloy.

In this study, using synchrotron radiation X-ray imaging, the microstructure, tensile properties, and fatigue properties of FGH96 nickel-based superalloy were tested, and the fatigue damage mechanism was analyzed. An analysis of the experimental results shows that the alloy structure is dense without voids or other defects. It was observed that the primary γ′ phase is distributed on the grain boundary in a chain shape, and the secondary γ′ phase is found inside the crystal grains. The X-ray diffraction (XRD) pattern indicates that no other phases were seen except for the γ and γ′ phases. The tensile strength of the alloy is 1570 MPa and the elongation is 12.1%. Using data fitting and calculation, it was found that the fatigue strength of the alloy under the condition of 5 × 106 cycles is 620.33 MPa. A fatigue fracture has the characteristics of secondary crack, cleavage step, fatigue stripe, tire indentation, and dimple. The fracture is a mix of cleavage fracture and ductile fracture. Through a three-dimensional reconstruction of the alloy synchrotron radiation imaging area, it was found that the internal defects are small and mostly distributed at the edge of the sample. The dimple morphology is formed by cavity aggregation and cavity germination resulting from defects in the material itself, fracture of the second-phase particles, and separation of the second-phase particles from the matrix interface. By analyzing the damage mechanism of fatigue fractures, it is concluded that the cleavage step is formed by the intersection of cleavage planes formed by branch cracks, with the main crack of the confluence extending forward to form a cleavage fracture. The crack propagation path was also analyzed, and under the action of cyclic load and tip passivation, the crack shows Z-shaped propagation.

Investigation the influence of miniaturized RFID tag sensor on coupling effect

Purpose The skin and skeleton of aircraft are connected by adhesives or rivets to bear and transfer aerodynamic load. It is easy for crack and fracture damage to occur under the action of cyclic load, thus reducing aircraft bearing capacity/integrity and causing serious security risks. Therefore, it is particularly important that passive wireless radio frequency identification (RFID) sensors be used for the health monitoring of aircraft skin in its whole life cycle. This paper aims to investigate the influence of miniaturization on the coupling effect between RFID tag sensors. Design/methodology/approach Two groups of crack sensing systems based on RFID tags were designed. Gain and mutual impedance of sensor tags were analyzed via mode analysis. The reliability of crack detection of both sensing systems was compared using a preset experimental scheme. Findings Miniaturized antennas can reduce edge influence and the coupling effect. Gain and mutual impedance decrease with the increase in distance between dual tags. Backscatter power shows a decreasing trend and threshold power to activate tags in reader antenna increases. Results show that the miniaturization of size is more suitable for the application of multiple sensors. Originality/value By comparing two groups of sensing systems, the consistency of crack detection sensitivity is better when small tags are placed in parallel, which provides a theoretical basis for the application of small, passive and densely distributed crack sensors in the future.

Numerical simulation of fatigue crack growth life of single side notched specimen

Under the action of cyclic load, fatigue fracture failure often occurs in many engineering structures during their services. Fatigue cracks in engineering structures begin to propagate under cyclic loading, and eventually lead to the failure of engineering structures. The study of fatigue crack is of great significance for evaluating the service life of engineering structures under cyclic loading. The prediction of fatigue crack growth life and path is the key problem of fatigue crack research. In this paper, the numerical simulation of fatigue crack growth of single side notched specimen is carried out by using the extended finite element method. According to the calculated stress intensity factor and the generalized Paris formula, the fatigue crack growth rate of single side notched specimen is obtained and the fatigue crack growth life is predicted. The results show that this method can effectively predict the fatigue crack growth life.

Corrosion Fatigue Damages of Rebars under Loading in Time.

Nowadays, a relatively small number of studies concern the study of corrosion processes in reinforced concrete structures under load. Additionally, rather little research has been carried out concerning changes in the stress–strain state parameters of structures under the simultaneous action of aggressive environment and load. This issue requires additional experimental and theoretical investigation. Determination of mechanical properties, fatigue characteristics and susceptibility to corrosion cracking was performed on samples of reinforcing St3GPF steel. The chemical composition of steel was determined by structural analysis. The spectral method for the determination of alloying elements and impurities in steels is based on the excitation of iron atoms and admixtures by electric discharge, decomposition of radiation into a spectrum, followed by its registration on photoplate with the use of electrograph. Experimental tests of samples in an aggressive environment under the action of statically applied tensile force showed that corrosion damage has little effect on the strength characteristics. At the same time, the decrease in area reduction and the decrease in strain were recorded. Additionally, the action of cyclic loads in an aggressive environment leads to a significant reduction in the fatigue limit to values from 20 to 24% of the yield strength of the original samples, which is 2–3 times lower than the fatigue limit of undamaged samples.

Research of the endurance of epoxy composite materials under cyclic compression

The widespread use of polymer composite materials is largely constrained by the complexity of predicting the characteristics of their stress-strain state under cyclic types of loading. Due to the poor knowledge of the behavior of polymer composite materials under this kind of loading. At present, it is rather difficult to predict the durability of polymer composite materials under static types of loading, and the available methods for materials of a crystalline structure and polymers are not always applicable to them. Insufficient theoretical and experimental data on the assessment of the stress-strain state of polymer concretes under prolonged loading. Under long-term action of cyclic loads, changes in the structure of polymer composite materials occur due to local self-heating at the tops of growing submicrocracks and the associated change in the elastic-hesteretic properties of the material. In this case, the energy of fatigue manifests itself, when the simultaneous multifactorial cyclic action of natural and force stresses leads to an effect that exceeds the total action of certain types of stresses. And, if such processes have been sufficiently studied for wood, a natural polymer material, then for artificial conglomerates, such as polymer composites, these issues require careful study. Studies of the endurance of epoxy polymer concretes were carried out on samples of prisms 100x100x400 mm according to the method developed by the authors. The formula was used to determine the minimum number of samples required to build endurance lines. The value of the cycle asymmetry coefficient was taken equal to 0.6. The value of the maximum cyclic loading was taken from 0.62 Rв with a gradual decrease. The values of the endurance limit of an epoxy composite material under cyclic loading Rв,pul = 0.43 Rв or 93.72 MPa were obtained, which makes it possible to judge the sufficient durability of this material.

Elaboration of the Equipment and Technology for Cutting Rolled Round Bars and Pipes with the Cyclic Circular Bending Method

A brief overview of methods for cutting round bars and pipes is presented, their main advantages, disadvantages and field of application are considered. It is proposed to use the effect of fatigue fracture for round bars and pipes breaking. The task is to study this process under the action of cyclic loads. The possibility of applying the known provisions of calculating theory of strength analysis for low-cycle fatigue for this process is tested. A comparative assessment of theoretical and experimental data is given and their significant difference is established. New calculated dependences are proposed, which are in good agreement with the experimental data. A theoretical assessment of the productivity of the process of round bars and pipes breaking by the method of cyclic circular bending was carried out using new results and various options for the design of a device for breaking were considered with an analysis of the features of their operation. Based on the results of the analysis, a variant of the device for the implementation of a continuous process of breaking the pointed ends of thick-walled fuel pipes during drawing in coils and a diagram of the mechanism for breaking the ends of the pipe with its supply from the coil were selected. Samples of pipes obtained by brittle cyclic bending are presented.

Experimental investigation results of composite RC beams under cyclic loading

This paper presents the results of the study of composite beam elements under the action of cyclic loads. The experimental results demonstrated that with a decrease in the level of applied load, with its frequency unchanged, the “viability” of the experimental composite beams increased (the number of cycles before collapse); under cyclic effects, shear deformations and contact openings are characterized by nonlinear functions.

Fatigue Topology Optimization Design Based on Distortion Energy Theory and Independent Continuous Mapping Method

Fatigue failure is a common failure mode under the action of cyclic loads in engineering applications, which often occurs with no obvious signal. The maximum structural stress is far below the allowable stress when the structures are damaged. Aiming at the lightweight structure, fatigue topology optimization design is investigated to avoid the occurrence of fatigue failure in the structural conceptual design beforehand. Firstly, the fatigue life is expressed by topology variables and the fatigue life filter function. The continuum fatigue optimization model is established with the independent continuous mapping (ICM) method. Secondly, fatigue life constraints are transformed to distortion energy constraints explicitly by taking advantage of the distortion energy theory. Thirdly, the optimization formulation is solved by the dual sequence quadratic programming (DSQP). And the design scheme of lightweight structure considering the fatigue characteristics is obtained. Finally, numerical examples illustrate the practicality and effectiveness of the fatigue optimization method. This method further expands the theoretical application of the ICM method and provides a novel approach for the fatigue optimization problem.

Mechanical properties and microstructure of layered cemented paste backfill under triaxial cyclic loading and unloading

In this paper, an experimental investigation was conducted to study mechanical properties and microstructural characteristic of layered cemented paste backfill (CPB) under triaxial cyclic loading and unloading condition. A number of CPB specimens were prepared with different number of layers (1, 2, 3, and 4). Four confining pressures (0, 0.2, 0.5, and 0.8 MPa) and three cycles were designed. The results indicated that cyclic loading and unloading had obvious influence on the mechanical properties of the CPB, which was closely related to the number of layers and confining pressure. Compared with the conventional triaxial compression test results, when the confining pressure is 0 and 0.2 MPa, the cyclic loading and unloading can increase the uniaxial compressive strength (UCS) of CPB, and when the confining pressure is 0.8 MPa, it decreased the UCS. When the confining pressure is 0.5 MPa, it increased the strength of the CPB 1 and 2 layers, but decreased the strength of CPB with 3 and 4 layers. The change of UCS had a positive linear correlation with the change of secant modulus of unloading curve caused by the cycle loading and unloading. The results of SEM and CT scanning showed that there were more pores in the post filling layers due to the difference of curing time which were compacted easily under small confining pressure. Under the action of cyclic loading and unloading, the CPB was repeatedly compacted, resulting in the increase of UCS. With the increase of confining pressure, the difficulty of pore compaction increases gradually and stress concentration was easy to form around the pores, which resulting in local fatigue damage and reduce the strength under cyclic loading and unloading.

Bond of bundled strands under static and cyclic load and freezing-thawing effect

The bond behavior of prestressing strands is of great importance for the capacity of precast prestressed concrete structures. There are cases in design and manufacturing practice where strands need to be bundled. However, the current design codes have no design guidelines for application of bundles of strands in precast prestressed concrete structures and there is a lack of information about their bond behavior. Therefore, the investigation of bond behavior of two bundled three-wire strands is presented in this article. Twenty three hinged beam specimens were tested to study unidirectional constant amplitude cyclic load (different amplitude and number of load cycles) influence as well as freezing and thawing effect on bond properties of two bundled three-wire strands. The three mechanisms: adhesion, friction, and several mechanical interlocking actions were found to be present at the strand concrete interface at different slip values. The results from the experiments showed that cyclic load did not negatively influence the bond strength of two bundled strands. Additionally, combined action of cyclic loading, freezing and thawing resulted in a decrease of bond strength. Based on experimental results the new static and cyclic bond parameters for two bundled three-wire strands are proposed on the basis of τ-s and S-N models provided in Model Code 2010.

Experimental assessment and damage modelling of hybrid timber beam-to-steel column connections under cyclic loads

This paper presents an experimental and numerical study on the behaviour of timber beam-to-steel column connections under cyclic loads. Special attention is given to the accumulation of damage in the timber components and to its simulation. To this end, the fundamental nonlinear cyclic response of three specimens involving different configurations of top and seat angle connections with long bolts is examined. The experimental set-up, connection details and material properties are introduced first, followed by a detailed account of the testing procedure and results. The experimental outcomes enable a direct comparative assessment of the connection strength, hysteretic response, joint ductility, failure mode and energy dissipation capacity. In addition, finite element analyses employing a newly proposed damage-plasticity constitutive model of wood are presented together with a detailed description of the adopted modelling approach. The numerical output of these simulations at the local level, expressed in terms of strains and damage indices, is discussed and compared against the experimental measurements of local damage in the wood obtained through Digital Image Correlation (DIC) techniques. It is demonstrated that the connections under consideration are able to sustain their bending capacity without a significant deterioration in their stiffness or strength even up to large levels of deformation and several repetitions of loading cycles. Besides, the results and discussion presented in this paper support the conventional definition of global failure as a post-peak strength reduction higher than 20% of the capacity but so long as the strength measurements are obtained from the stabilized envelope curves of the specimens. The applicability of damage mechanics concepts to provide a reliable prediction of crack zones and damage accumulation in timber structures under the action of cyclic loads is also highlighted.Finally, general recommendations for avoiding brittle failure modes in this type of connection are given.