Effect Of Cyclic Loading Research Articles

Fracture Toughness of Winding Carbon Plastics Based on Epoxy Matrices and Reinforced by Polysulfone Film

In this work, the fracture mechanism of winding carbon-fiber-reinforced plastics (CFRPs) based on epoxy matrices reinforced by polysulfone film was investigated. Two types of polymer matrices were used: epoxy oligomer (EO) cured by iso-methyltetrahydrophthalic anhydride (iso-MTHPA), and EO-modified polysulfone (PSU) with active diluent furfuryl glycidyl ether (FGE) cured by iso-MTHPA. At the winding stage, the reinforcing film was placed in the middle layer of the CFRP. The fracture toughness GIR of the obtained CFRP was determined by the double-cantilever beam delamination method. Additionally, the effect of cyclic loading on the fracture toughness of CFRP reinforced with polysulfone film was investigated. It was shown that heterogeneous structures arising from the dissolution of the polysulfone film in the epoxy binder during the curing process increase the fracture toughness of CFRP from 0.5 kJ/m2 to 1.2 kJ/m2. Application of cyclic loads had little effect on the fracture toughness value. As a result of this study, it was revealed that the macrocrack propagates near the reinforcement layer along the diffusion zone, which has a phase organization of the type PSU matrix–EO dispersion.

Influences of freezing and thermal cyclic loading on physical and mechanical properties of marble

In this study, marble specimens were used to investigate the effects of freezing and thermal cyclic loading on physical and mechanical properties. For this purpose, four types of loading were considered, including freezing-cooling (F–C), heating–cooling (H-C), freezing-cooling-heating (F–C-H) and heating–cooling-freezing (H-C-F). The changes observed in physical and mechanical properties of the marble were analyzed in temperature extend shifting from − 30 °C to 160 °C. To achieve this goal, two marble blocks with a dimension of 30 × 30 × 30 cm3 were selected. Then, compressive and shear wave velocities, uniaxial compressive strength (UCS), Young’s modulus (E) and Brazilian tensile strength (BTS) were measured and compared in different loading conditions. Results showed that the number of freezing-cooling cycles influenced the mechanical properties of the marble. With an increase in the loading cycles, both compressional and shear wave velocities of the marble decreased. The highest reduction in tensile strength was observed in heating–cooling cycles. That is to say, the thermal loading of rock specimens caused weakening, thereby increasing the micro-crack density in the marble specimens by means of thermally shocks. It was observed that the increase in the number of loading cycles, brings about a huge decline in the marble dynamic modulus. The damage index (D) is introduced to reflect the variation of the mechanical properties and ultrasonic wave velocities of the samples before and after the thermal cyclic loading. In all freezing and thermal cyclic loading conditions, the main and predominant failure mechanism was shear failure.

Cohesive zone approach to the cyclic response of cracked concrete beams reinforced with externally bonded FRP plate

To predict the cyclic response of concrete strengthened with FRP, this paper presents a new analytical solution, which uses the cohesive zone model (CZM). Based on the CZM, for intermediate crack-induced debonding, with intermediate flexural cracking of concrete beams repaired with FRP plate, the deteriorative effect of cyclic loading on the adhesive joints (interface) is investigated. To accomplish this aim, a cyclic damage process is coupled with a CZM to predict the cumulative damage evolution along the FRP-to-concrete interface as a function of the number of cycles. Comparing the results from the present model with those found in the literature shows that the constitutive model is able to accurately predict the overall cyclic loading response of concrete strengthened with FRP composites. The main advantage of the proposed model is that it gives the variation of the mid-span deflection, damage development, and interfacial stress distribution with increasing load cycles, and number of loading cycles. We conclude that cyclic degradation of the component materials significantly reduces the resistance and lifespan of the repaired structures.

A multi–level system for ensuring the reliability of marine mechanical systems at the stages of the life cycle

The operation of vessels in the waters of the Northern Sea Route imposes additional requirements on ship mechanical equipment in accordance with Chapter 6 of the Polar Code. The most urgent scientific tasks include the following: designing new iceclass vessels for operation in the Arctic and extending the service life of the existing Arctic fleet, which should ensure operability at low air temperatures. Forecasting the service life of marine mechanical systems, taking into account the effects of external cyclic loads and the marine corrosive environment, is a complex complex task, for which it is necessary to consider all stages of the life cycle from design to the moment of termination of their operation. In order to improve the existing methods of reliability assessment, accidents on ships operating in Polar waters were considered. A resource management program is proposed that allows you to systematize organizational and technical measures not only during operation, but also allows you to make forecasts at the design stage. A risk management program is proposed that allows you to link organizational and technical measures aimed at identifying the elements of marine mechanical systems that most need attention, thereby effectively distributing maintenance. At the stages of the life cycle, a multilevel reliability assurance system is proposed, including three levels of assessment: reliability management; resource management and risk management.

Effect of episodic pre-failure cyclic loading on whole-life geotechnical properties of soft clays

Changes in soil properties due to loading and consolidation during the life of infrastructure affect the soil response to future events. This concept is encapsulated in the whole-life geotechnical design approach which accounts for the evolution of properties such as strength, stiffness and consolidation coefficient, to improve forecasts of system response through and beyond the design life. This paper explores the changing properties of a soft clay from episodes of pre-failure cyclic loading and consolidation through a series of stress-controlled cyclic direct simple shear (DSS) tests. The scenario is relevant to offshore applications where infrastructure is subject to cyclic seasonal loading, and is particularly relevant to floating offshore wind anchoring systems as these are located in deeper water, farther from shore where soft clays are common. The results quantify the effect of cyclic stress amplitude, number of cycles per packet, and number of consolidation intervals, on the clay properties. The results show increases in undrained strength by up to 70%, stiffness by up to 50% and consolidation coefficient by a factor of up to 30, highlighting the importance of accounting for whole-life effects for reliable and efficient geotechnical design.

Degradation and Fatigue Behavior of 3D-Printed Bioresorbable Tracheal Splints.

Severe infantile tracheobronchomalacia (TBM) is often treated with invasive surgery and fixed-size implants to support the trachea during respiration. A novel 3D-printed extra-luminal splint has been developed as a flexible and bioresorbable alternative. Therefore, the goal of the present study was to use an invitro breathing simulator model to comprehensively evaluate the structural stiffness and failure modes of two sizes of a novel bioresorbable 3D-printed splint design under a range of physiological degradation conditions. Two thicknesses, 2 mm and 3 mm, of a novel 3D-printed bioresorbable splint were evaluated under two different degradation conditions, phosphate-buffered saline (PBS) and sodium hydroxide (NaOH). The splints were subjected to simulated breathing loading, involving a cyclic opening and closing of the splint by 2 mm, for a targeted duration of 7.5 to 30 million cycles. A separate new set of splints were statically soaked in their respective degradation condition for a comparative analysis of the effects of cyclic loading by the degradation medium. After successfully simulated breathing or static soaking, non-destructive tensile and compressive strengths were evaluated, and overall stiffness was calculated from destructive tensile testing. The present study indicates that the splints were more significantly degraded under simulated breathing conditions than under soaking. Cyclic simulated breathing specimens failed far earlier than the intended duration of loading. Over time, both 2 mm and 3 mm splints became increasingly more flexible when subjected to the static degradation conditions. Interestingly, there was little difference in the compressive and tensile strengths of the 2 mm and 3 mm thickness splints. The bioresorbable nature of PCL offers a valuable advantage as it eliminates the need for splint removal surgery and increases device flexibility over time with degradation. This increased flexibility is crucial because it allows for uninhibited growth and development of the infant's trachea over the intended use period of 2 years. The results of this study confirm that the splints were able to withstand tensile forces to prevent tracheal collapse. This study further supports the successful use of 3D-printed splints in the treatment of infantile TBM.

Numerical investigation on the suction caisson response considering seabed trenches and cyclic loading

As anchors of floating offshore structures, suction caissons endure cyclic loads transmitted by mooring chains. The moving chains repeatedly cut into the seabed, triggering the formation of trench, which further reduces the stability of suction caisson. However, the coupling effects of cyclic loading and trenches on the movement of suction caisson remain unexplored. To address this issue, finite element analyses were conducted to investigate the coupling effect of cyclic loading and trench on the movement of rigid suction in clay, employing a bounding surface constitutive model. The influences of cyclic amplitude, mean load, trench depth and width on the anchor behaviour are analysed. The results show that movement at the fixed loading point on the rigid caisson nonlinearly increases with the amplitude (qam) and mean (qme) of cyclic loading on a trenched seabed. This increase becomes pronounced when qam exceeds 0.15, and qme surpasses 0.35 the caisson's bearing capacity (qref). Additionally, trench dimensions significantly influence the caisson's movement. Specifically, trench depth has complex influence on the caisson's movement pattern, and increasing trench width results in larger movement, especially beyond a certain width. Considering the seabed trenches and cyclic loading, these findings offer valuable insight into the suction caisson movement.

Effect of cyclic loading on microstructure and plastic deformation in heat-treated Co–28Cr–6Mo alloy fabricated via laser powder bed fusion: An in situ synchrotron X-ray diffraction study

We present a systematic microstructure-oriented plasticity investigation of an additively manufactured cobalt-based alloy aiming to relate microstructural changes to the fatigue resistance. A load-controlled cyclic test in the low-cycle fatigue regime was utilized to study mechanical response and microstructural changes in the alloy after reverse phase transformation heat treatment. Deformation-induced FCC → HCP phase transformation was followed using in situ synchrotron X-ray diffraction combined with ex situ electron backscatter diffraction and scanning electron microscopy. Scanning transmission electron microscopy provided experimental evidence of the presence of precipitates in the solution annealed sample. It was found, that a stepwise increase in plastic deformation is attributed to nucleation and growth of different martensitic crystallographic variants. These insights into plasticity and microstructural changes suggest that the reverse phase transformation heat treatment is an effective approach for improving the fatigue resistance of low stacking fault energy alloys, that are susceptible to deformation-induced martensitic transformation.

Cyclic loading effects on the heat-generation performance of carbon nanotube-embedded cement composites

Abstract This study investigates the heat-generation stability of carbon nanotube (CNT)/cement composites after exposing to cyclic loading conditions. The specimens were fabricated with varying CNT contents and levels of fly ash replacement. Results showed that increasing CNT content reduced electrical resistivity, while the impact on the electrical characteristics was found to be insubstantial, even though a considerable portion of fly ash was replaced. In addition, the electrical resistivity of the specimens after exposed to cyclic loading increased. Electrical heating tests revealed both negative and positive temperature coefficient effects depending on the applied voltages. Higher CNT contents improved the heat-generation capability, but the heating capability decreased after exposed to the cyclic loadings which is deduced from the damage of CNT networks during cyclic loadings. In this regard, the authors concluded that the heat-generation stability can be significantly affected by the applied loadings. Thus, the future research will be conducted to improve the heat-generation stability of the cement-based electrical heating systems as exposed to artificial deteriorations.

Full-scale in-situ tests on a displacement cast in situ energy pile: Effects of cyclic thermal loads under different mechanical load levels on pile stress and strain

Numerous full-scale in situ tests have been conducted to assess the effect of thermal cycles on the pile response. However, those studies investigated the response of only precast and cast in-situ energy piles, with limited focus on the impact of the applied mechanical load on the pile response. This study presents the results of a field test conducted on a new type of energy pile, i.e. a displacement cast in-situ energy pile in multilayered soft soils, subjected to different fixed mechanical loads while undergoing simultaneous thermal cycles. Four tests were carried out, each corresponding to various axial loads ranging from 0 % to 60 % of the pile’s estimated bearing capacity. After applying the axial load on the pile head (0 %, 30 %, 40 %, or 60 % of the bearing capacity), the pile was subjected to up to ten thermal cycles. The highest magnitudes of thermal axial strains were observed near the pile top due to the lowest restraint provided by the made ground layer in all tests. Under zero (0 %) mechanical load, the thermal axial strains near the pile head were elastic and recoverable, while residual strain was observed near the toe. Under reasonable working mechanical loads (30 %, 40 %, or 60 %) residual strains were observed near both the pile head and the toe, with higher residual strains observed under higher mechanical loads. The results indicate that the cyclic thermal loadings could induce an increase in the compressive stress in the energy pile, attributed to the drag-down effects of the surrounding soil. The compressive stress induced by drag-down effects counteracts thermally induced tensile stress and thus leads to an insignificant effect on the energy pile during cooling. A limited impact of the shaft capacity was observed and was mainly attributed to the drag-down of the surrounding soil and thermal creep along the pile-soil interface.

Damage evolution and acoustic emission characteristics of sandstone under different true triaxial cyclic loading and unloading modes

In the process of deep coal mining, the cyclic mining disturbance behavior produces significant cyclic loading and unloading effects, to ensure safe production in the mining area, it is necessary to study the damage characteristics and acoustic emission characteristics of the rock body under different cyclic loading and unloading effects. This study developed a TAWZ-5000/3000 rock true triaxial (disturbance) test system and used it to carry out true triaxial cyclic loading and unloading tests in different modes, with acoustic emission (AE) monitoring of the deformation and damage processes. The test results proved that a large number of cycles weakens the rock’s ability to control deformation, and this weakening ability is also related to the types of cyclic loading and unloading modes. The hysteresis loop characteristics and mechanical parameters show obvious path dependence. Based on the damage equivalence method, it is found that the absolute damage parameter increases with the increase of the number of cycles in the same mode, and the accumulation rate of the cumulative damage parameter is faster; the cumulative damage parameter in the equal amplitude cyclic loading and unloading (EAC) mode is always higher than that in the other two modes. The normalized AE ring count rate in the rock samples under cyclic loading and unloading has a significant relationship with the damage percentage, and the crack extension scale function (b-value) shows the “steady increase – strong fluctuation − significant decline” trend. By analyzing the AE characteristic parameters, namely rise time/amplitude (RA) and ringing count/duration (AF), it is found that the sandstones in EAC, graded cyclic loading and unloading (GC), and stepped cyclic loading and unloading (SC) modes are mainly damaged by tension-shear and shear, respectively. By analyzing the values of the acoustic emission parameters RA-AF, it is found that the sandstone is mainly damaged by tensile shear, tensile tension, and shear under EAC, GC, and SC modes, respectively, corresponding to the rock samples’ damage patterns. The study results have important guiding significance for preventing and controlling disasters in deep mines.

Fatigue-thermal damage characteristics of red sandstone with a hole under high-temperature cyclic loading coupling and microstructural degradation

In underground coal gasification (UCG) projects, deeply buried subterranean projects are significantly influenced by high temperatures, alongside disruptive loads such as excavation, blasting, and drilling. Therefore, it is essential to study the fatigue behaviors of rocks under the coupled effects of thermal elevation and cyclic loads. This study performed uniaxial compression and cyclic loading-unloading tests on red sandstone specimens with cavities. The high-temperature fatigue behaviors of specimens were explored from various aspects, encompassing stress, deformation, energy, damage, and the microstructural variations of the specimens after high-temperature procedures were assessed by utilizing scanning electron microscopy (SEM). At temperatures below 600 °C, SEM images indicate a contraction of microcracks in the specimens, whereas temperatures above this threshold lead to their expansion. Notably, the total, elastic, and dissipated energy density of the specimens exhibit a curve of second degree with the upper limit stress. Additionally, elastic and dissipated energy densities linearly correlate with input energy density. Furthermore, the coupled damage and axial strain of the specimens positively correlate with the temperature. The damage mode of thermal fatigue is attributed to the accumulation and penetration of transgranular fractures within the red sandstone.

Cumulative effects of cyclic train loading on strains in reinforced soils around tunnels

Metro tunnels often experience uneven settlement of the soil beneath them during operation, a problem that is especially pronounced in soft soil layers such as silt. This uneven settlement negatively affects the operational safety of subways, prompting engineers to use soil reinforcement techniques to mitigate ground deformation. However, there is a lack of research on the cumulative deformation of reinforced soils under cyclic train loading. In this study, dynamic triaxial tests were conducted to obtain the deformation parameters of silty reinforced soils under cyclic loading. The principal dynamic relationship was summarized, and the effect of loading frequency on soil deformation was analyzed. The results indicated that as the loading frequency increased, the cumulative deformation of the soil decreased. Using the dynamic constitutive relationship derived from the tests, the finite element method was employed to model the interaction system between the load, tunnel, and strata, as well as the dynamic modulus attenuation behavior of the reinforced soil layer. This approach was used to investigate the cumulative deformation characteristics of reinforced soils under cyclic loading. The findings indicated that the dynamic modulus of the reinforced soil decayed rapidly at the beginning of the loading period, leading to an accelerated increase in the cumulative deformation. Additionally, the cumulative deformation was measured at different train speeds, revealing that when the speeds exceeded 80 km/h, the cumulative strain of the soil increased gradually with speed.

Articular cartilage fatigue causes frequency-dependent softening and crack extension

Soft biological polymers, such as articular cartilage, possess exceptional fracture and fatigue resistance, offering inspiration for the development of novel materials. However, we lack a detailed understanding of changes in cartilage material behavior and of crack propagation following cyclic compressive loading. We investigated the structure and mechanical behavior of cartilage as a function of loading frequency and number of cycles. Microcracks were initiated in cartilage samples using microindentation, then cracks were extended under cyclic compression. Thickness, apparent stiffness, energy dissipation, phase angle, and crack length were measured to determine the effects of cyclic loading at two frequencies (1 Hz and 5 Hz). To capture the fatigue-induced material response (thickness, stiffness, energy dissipation, and phase angle), material properties were compared between pre-and-post diagnostic tests. The findings indicate that irreversible structural damage (reduced thickness), cartilage softening (reduced apparent stiffness), and reduced energy dissipation (including phase angle) increased with an increase in the number of cycles. Higher frequency loading resulted in less reduction in energy dissipation, phase angle, and thickness change. Crack lengths, quantified through brightfield imaging, increased with number of cycles and frequency. This study sheds light on the complex response of cartilage under cyclic loading resulting in softening, structural damage, and altered dynamic behavior. The findings provide better understanding of failure mechanisms in cartilage and thus may help in diagnosis and treatment of osteoarthritis.

A comparative study of low cyclic loading effects on plastic strain, nonlinear damping, and strength softening in fiber-reinforced recycled aggregate concrete

The incorporation of fibers significantly enhances the properties and structural performance of recycled aggregate concrete. This study introduces Fiber-Reinforced Recycled Aggregate Concrete (FRAC) by integrating steel fibers (SF) and polypropylene fibers (PPF) into the recycled aggregate concrete (RAC) matrix. A comprehensive series of cyclic loading tests, including reload and unload sequences, were conducted to meticulously analyze the deterioration of strength and energy dissipation capabilities of SF/PPF-reinforced RAC. The research examines the effects of fiber characteristics, volume ratios, lengths, and diameters on strength degradation after the peak stress along the stress-strain curve. Degradation models based on plastic strain were proposed for both SF-reinforced RAC and PPF-reinforced RAC. The study further investigates the fluctuation of strain energy in FRAC under low cyclic loading, revealing the correlation between plastic strain energy and equivalent viscous damping ratio. A nonlinear viscous damping model that accounts for the influence of the reinforcing index (RI) is proposed. This model effectively predicts the degradation of strength, the evolution of plastic strain, and the dynamics of nonlinear viscous damping in composite materials with varying fiber content. Ultimately, this research provides essential technical insights for the practical engineering application of RAC.

Development of improved finite element formulations for pile group behavior analysis under cyclic loading

AbstractThe effect of cyclic loading is an essential factor leading to progressive soil strength degradation. Therefore, a comprehensive analysis of the pile‐soil system behavior under cyclic loading is required to ensure the stability of pile group. There is room for improvement in the inherent constraint of the conventional numerical model in terms of approximating the soil resistance distribution along the pile by point loads at element nodes, necessitating a specific element that integrates considerations of pile group effect and cyclic loading within a unified framework. This study aims to develop a newly specific type of element for efficiently predicting nonlinear behavior within the pile‐soil system, addressing simulations involving nonlinear pile‐soil interaction, pile group effect, and cyclic loading. Modified element formulations based on soil stiffness matrices and soil resistance vectors specifically address pile group effect and consider parameters that influence pile behavior under cyclic lateral loading. The numerical solution procedure with Newton‐Raphson iteration allows the calculation of pile responses in geometric and material nonlinear analyses. The validation of the proposed method includes several examples, comparing it with existing numerical solutions and experimental tests of single piles and pile groups under cyclic loading. These comparisons further support the consistency of the proposed method with measured data and validate its accuracy in considering group effect and cyclic loading. The parametric study illustrates the ability of the proposed method to capture cyclic loading parameters while considering the influence of the number and magnitude of load cycles, the cyclic load direction, and the installation methods.

Numerical Investigation of Load Transfer in Geosynthetic-Reinforced Embankments Over Cavities: Effects of Opening Process and Cyclic Loading

Numerical Investigation of Load Transfer in Geosynthetic-Reinforced Embankments Over Cavities: Effects of Opening Process and Cyclic Loading

Deformation behavior of plastic zone at crack tip of 304 stainless steel

AbstractIn this paper, the deformation behavior of plastic zone at crack tip during crack propagation of 304 stainless steel was studied. Firstly, the fatigue crack propagation tests of 304 stainless steel material were carried out. Combined with digital image correlation (DIC) technology, the strain field data of plastic zone at crack tip, the size of plastic zone, and the strain evolution law of crack tip region at different positions were obtained. Then, combined with the theoretical model and the numerical model, the theoretical, numerical, and experimental results were compared to verify the accuracy of the model. Finally, the effects of cyclic loading, specimen thickness, and weld position on plastic zone at crack tip were analyzed. The research results provide a reference for accurately predicting the fatigue crack growth of 304 stainless steel.

Shear and interface shear fatigue of semi‐precast slabs with lattice girders under cyclic loading

AbstractMore and more often, semi‐precast reinforced concrete slabs with lattice girders are employed for industrial buildings or bridges, where they are exposed to high cycle fatigue loading. Despite research in recent years that has led to the formulation of an S–N curve for lattice girders, the effects of cyclic loading on semi‐precast slabs have not yet been sufficiently clarified. Moreover, research has so far been limited to single‐span slabs. The effects of continuous slab systems, which are mainly realized in buildings and bridges, cannot be considered in the calculation of fatigue resistance yet. To improve the fatigue design concept for shear and interface shear, theoretical and experimental investigations were conducted at the Institute of Structural Concrete, RWTH Aachen University. In addition to the fatigue behavior of 16 tests under cyclic loading, particular attention is paid to the increase in shear and interface shear resistance at the inner support of continuous slabs. Furthermore, the influences of the support detailing were investigated. The findings illustrate further potential for optimization of the design under cyclic loading.

Experimental and Numerical Investigation of Impact Behaviour of Anchored CFRP-to-Concrete Bonding Interface

In the last 20 years, Carbon Fiber Reinforced Polymers (CFRP) have become a common alternative to strengthening and retrofitting. The performance of the strengthened specimens is closely related to the bond slip between the CFRP strips and the adhesion surface. Therefore, researchers have begun to concentrate on the various anchoring methods to delay the debonding of the CFRP from the surface of strengthened specimens. When the literature review of the adhesion behavior between the anchored CFRP strips and the concrete surface is examined, it has been shown that the investigations are mostly done under the effect of static or reversible cyclic loads. In the literature review, there is not any research related to the adhesion behavior between anchored CFRP strips and the concrete surface under the effect of sudden dynamic impulsive impact loading. Therefore, the variables examined in the experimental study are the anchor type used in the CFRP strips, CFRP strip adhesion length, and the number of anchors placed on the strip. As a result of the experiments, the acceleration-time, displacement–time curves of the beams, and the strain–time curves along the CFRP strips were interpreted. In addition, simulation models of the test specimens were created with ABAQUS software, and non-linear finite element analyzes were performed. Finally, by comparing numerical results with experimental ones, the effects of variables such as bond length, applied anchor types and numbers on the impact behavior of the beam and the strain distributions along the CFRP strips were interpreted.