Subsequent Cyclic Loading Research Articles

Hybrid rehabilitation of 3D RC beam-column joints using UHP-HFRC and FRP wrapping: Experimental evaluation and theoretical analysis

Beam-column joints are crucial elements in reinforced concrete (RC) structures subjected to seismic loading, as they are vulnerable elements prone to significant damage during earthquakes. Due to extensive structural damage, various methods have been developed to strengthen beam-column joints. However, research on rehabilitation and repair methods has been limited, with most studies focusing on 2-D joints and overlooking the confinement effect of corner beams on the joint core. This study aims to investigate novel hybrid rehabilitation systems, including ultra-high performance hybrid fiber-reinforced concrete (UHP-HFRC) and fiber-reinforced polymer (FRP) systems, alongside embedding reinforcement into heavily damaged 3-D beam-column joints subjected to pre-cyclic loading and subsequent cyclic loading. Parameters related to hysteresis response were obtained and discussed, such as ductility, lateral stiffness, dissipated energy, equivalent hysteresis damping, and pinching ratio of the tested specimens. The experimental findings revealed that utilizing an FRP and UHP-HFRC hybrid rehabilitation system enhanced ductility, dissipated energy, and lateral stiffness of heavily damaged joints by 25 %, 73 %, and 81 %, respectively, compared to the undamaged reference specimen. Additionally, a simplified design model was developed to predict the shear resistance of beam-column joints, incorporating principles of strut-and-tie. The theoretical design model was also validated against the experimental database, demonstrating a good accuracy with MV and COV of 0.97 and 13 %, respectively.

Model-free chemomechanical interfaces: History-dependent damage under transient mass diffusion

This paper presents a data-driven framework based on distance functional for chemo-mechanical cohesive interfaces to capture transient diffusion and resulting interfacial damage evolution. The framework eliminates reliance on complex constitutive models and phenomenological assumptions, especially avoiding the coupling tangent terms with a given material database. The interfacial chemical potential and its jumps are used to expand the phase space for describing the chemical states of interfaces. Subsequently, a novel chemo-mechanical distance norm, including traction-separation pair, interfacial chemical potential-concentration pair and corresponding chemical potential jump-flux pair, is presented. Momentum conservation law for interfacial tractions and mass conservation law for interfacial concentration are enforced via Lagrange multipliers. For tracking the history-dependent state of the interface damage, an internal variable, termed interface integrity, is introduced to condition the interfacial database and manage the subsets mapping strategies, following physically motivated evolution constraints. Numerical examples are conducted to investigate the efficiency and capability of the present framework. A monotonic loading test validates a good numerical convergence relative to the number of data points. Subsequent cyclic loading simulations, compared with the reference solutions, show that the algorithm is well suitable to predict the history-dependent interface damage evolution under complex loading paths. Finally, the chemo-mechanically coupled examples capture phenomena such as interfacial swelling and interface degradation induced by transient diffusion and stress-driven diffusion. The current work provides a promising tool for understanding chemo-mechanical behaviors of interfaces within heterogeneous composites.

Effect of electromagnetic force installation method on critically reinforced interference sizes and fatigue behavior of CFRP bolted joints

AbstractThe electromagnetic force dynamic installation (DI) method of interference fit fasteners has significant advantages in improving the quality of the assembly interface of composite structure. This study aims to investigate how improving assembly interface quality may help in achieving high‐precision interference‐fit joints in composites. Single‐lap joint specimens were prepared using electromagnetic installation technique, the fatigue life and failure behaviors of the joints under different bolt‐hole clearances were measured and analyzed experimentally. In addition, a three‐dimensional finite element model was developed to predict the radial pre‐compression stresses around the hole and the progressive failure behavior of various typical damage modes in carbon fiber‐reinforced polymer induced by the interference. The results show that the critical interference range for the DI‐composite bolted structure is 1.0%–1.2%. The fatigue life of the best interference (1.0%) and the worst interference (0%, neat fit) have a difference of 36 times in amplitude. The residual stress with uniform distribution and high amplitude reduces the stress concentration of the hole wall and improves the failure evolution of the loaded hole. The contribution of this study provides vital guidance for improving the fatigue performance of CFRP bolted structures using a novel DI method.Highlights I = 1.0% is the critical damage‐enhancing interference size for dynamically installation interference fit bolts with electromagnetic forces. The residual stress amplitude introduced by the interference does not increase uniformly as the interference size increases. Excessive interference significantly reduces the uniformity and magnitude of residual stresses. Under subsequent cyclic loading, the fifth layer near the inlet and outlet of the extruded lower laminate appears to be the critical ply of extrusion deformation, and severe damage is concentrated in the critical ply of deformation. The decrease of stress amplitude and the variation of mean stress are the mechanisms of fatigue life enhancement in interference fit joints under external cyclic loading.

Influence of repeated surface surcharge loading on tunnel displacement considering the structural characteristics of soft clay

Shanghai soft clay is a typical marine clay with specific structural characteristics. However, the effects of surface surcharge loading on structured soft clay and existing tunnels remain unclear. In this study, the effect of repeated surface surcharge loading on tunnel displacement was numerically investigated, considering the structural characteristics of Shanghai Layer 4 soft clay. An elastoplastic constitutive model (Shanghai model) that describes the mechanical properties and structural characteristics of natural clay was used to simulate the soil response. The results indicate that the first cycle of repeated surface surcharge loading had the greatest effect on the displacement of the tunnel and overlying clay; subsequent loading cycles further increased the displacement. With repeated surface surcharge loading, the displacement of the overlying clay exhibited a significant decreasing trend with increasing depth. The maximum excess pore pressure of the clay exhibited a decreasing trend with increasing loading time. The displacements of the tunnel and overlying clay were greater when the effects of the soil structure were considered. However, the initial degree of the structure had no significant effect on the accumulation of excess pore pressure when the surface surcharge loading did not reach the ultimate bearing capacity of the clay.

Physical modelling of cyclic loading-induced footing settlement with nearby pit excavation

A series of physical model tests and cyclic triaxial tests were conducted on a dry sand to investigate the effects of excavating an adjacent pit on the settlement behaviour of a footing under cyclic loading. The excavation was simulated by moving a retaining wall between loading cycles in the physical model tests. The excavation-induced stress disturbance on soil elements was modelled by reducing cell pressure between loading cycles in triaxial tests. The results indicate that nearby excavation leads to a reduction in lateral stress in ground, and therefore increases the settlement of footing in the subsequent loading cycles. However, there is no clear relationship between the settlement increment and the magnitude of wall movement, when the lateral movement of the wall is within the range of 0.1–0.37% of the wall height. The lateral excavation does not have a great impact on the influence zone of the footings under cyclic loading. An empirical model was proposed to estimate the cyclic loading-induced strain accumulation of sand with the consideration of lateral unloading effects between loading cycles. After being validated using cyclic triaxial test results, the proposed model was employed to predict cyclic loading-induced settlement of the footing before and after the excavation.

Effect of interference installation method on interfacial properties and fatigue failure behavior of bolted composite joints

A novel dynamic installation (DI) method is proposed to improve the problems of severe installation damage, small installable interference size, and low installation efficiency of traditional static installation (SI) methods for installing interference bolts in composite bolted structures. The stress distribution and CFRP damage around the hole wall due to interference fitting and subsequent cyclic loading were predicted by finite element simulations. In addition, the fatigue life of DI and SI specimens was evaluated and the fatigue failure mechanisms were discussed. The results showed that the DI method provides a visible improvement in fatigue life, which is about 4 times that of the SI interference specimens and 38 times that of the non-interference specimen. In addition, DI introduces relatively higher average residual compressive stresses than SI, especially at the inlet of the laminate, which also results in a more uniform distribution along the axial direction of DI specimens. Micromorphological analysis of the specimens after fatigue failure indicated that the uniform stress distribution and low initial installation damage at the assembly interface introduced by the DI method during interference fit prevents severe delamination and crack propagation in joints subjected to external cyclic loading, such as the SI specimen. Severe extrusion deformation of the hole wall and matrix crushing are the main failure modes of DI specimens. Therefore, it is necessary to improve the interference installation method of composite materials.

Dynamic responses of Ca-alginate/polyacrylamide hydrogels at high strain rates

Many biological hydrogels possess remarkable mechanical performances under both quasi-static and impact loadings, yet such mechanical robustness over a wide range of strain rates has not been achieved in synthetic hydrogels due to the lack of understanding of dynamic responses at high strain rates. Here we study the dynamic responses of tough hydrogels at high strain rates by using the Ca-alginate/polyacrylamide hydrogel as a model material and a modified split Hopkinson press bar (SHPB) system. We show that the Ca-alginate/polyacrylamide hydrogel is also much tougher than the two constituents at high strain rates. The strength of Ca-alginate/polyacrylamide hydrogel at 1.2 × 104 s−1 is two orders of magnitude higher than the stress at the same strain at 0.25 s−1. We identify the fracture points by using snap rings to control the maximum compressive strains during the loading process and validate them by subsequent cyclic loading and unloading at 0.25 s−1. We formulate a five-parameter constitutive model to describe the visco-hyperelastic behaviors and achieve satisfactory agreements between theory and experiment. We further interpret the underlying mechanisms of the strain rate-dependent deformation and fracture behaviors by comparing them with the responses of the two constituents. The importance and opportunities of studying the dynamic responses of tough hydrogels at high strain rates are discussed.

The effects of excavating twin tunnels during cyclic loading on the progressive settlement of existing footing

Tunnels may be constructed in a pair undercrossing existing geotechnical structures exposed to cyclic loading. Such construction changes the stress state in the ground in different ways regarding the tunnelling sequence and would lead to large settlement or even failure to those structures under cyclic loading. Large physical model tests were carried out to examine the impact of excavating twin tunnels during cyclic loading on the progressive settlement of footings. The vertical soil stress, footing settlement and ground deformation were analysed regarding different construction sequences and spacings between twin tunnels. The results indicate that previous cyclic loading history affects the alteration of soil stress caused by tunnelling activity. Compared with excavating twin tunnels successively, excavating the tunnels in sequence leads to greater settlement caused by the subsequent cyclic loading. The change in the relative density of soil is invalid to explain the enlargement of the settlement. Finite element analysis was performed to investigate the mechanism of the effects of excavating twin tunnels during cyclic loading on the footing settlement. It was found that the increase of footing settlement in the subsequent cyclic loading and the greater settlement in the sequential tunnelling case are attributed to the greater stress ratio (q/p) in the ground.

Application of digital image correlation to derive Paris' law constants in granite specimens

This study investigates the effectiveness of Digital Image Correlation (DIC) as a powerful tool for deriving Paris' law constants in black granite specimens subjected to cyclic loading. A series of monotonic and cyclic loading tests on rectangular granite specimens yields valuable insights into geological material fatigue behavior, establishing monotonic tests as foundational references for subsequent cyclic loading experiments. Innovative DIC software application enables precise tracking and analysis of Crack Mouth Opening Displacement (CMOD) values, revealing distinct, observable patterns in crack growth during cyclic loading. A notable contribution lies in the implementation of a compliance calibration technique integrating Finite Element Method (FEM), facilitating the direct linkage between CMOD values and effective crack lengths, resulting in highly accurate Paris' law constants. With an average of 1.04 E −7 for C and 8.95 for m, these constants are essential for predicting fatigue crack propagation in granite specimens. This research underscores DIC's efficacy as a pivotal fatigue analysis tool, enhancing predictive capabilities for structural safety and durability assessments in geological materials, while emphasizing the crucial role of advanced imaging and numerical techniques in material science and structural engineering. It lays a robust foundation for future geological material studies and comprehensive structural integrity assessments.

Research on the Fatigue Crack Growth Behavior of a Zr/Ti/Steel Composite Plate with a Crack Normal to the Interface.

The current work reveals the influence of loading parameters on the crack growth behavior of a Zr/Ti/steel composite plate with a crack normal to the interface by using an experiment and the finite element method. The Chaboche model was first used to study cyclic plastic evolution in composite materials. The results reveal that an increase in Fmax, Fm, and Fa can promote da/dN; meanwhile, an increase in R will reduce da/dN. The plastic strain accumulation results indicate that Fm mainly contributes to the tensile strain and compressive stress after the first cycle. Additionally, Fa increases the stress range and compression stress and greatly improves the plastic strain accumulation degree in subsequent loading cycles. The Fmax can significantly increase the stress amplitude and plastic strain accumulation level. When R increases, the plastic strain accumulation increases a little, but the stress amplitude and compression stress decrease greatly. Furthermore, it is also found that the elastic-plastic mismatch also affects the plastic evolution, that is, strengthening or weakening the effect of the loading parameters.

Assessment of rail grinding maintenance surface quality and damage propagation in subsequent loading cycles for premium rail grades

Rail samples were tested under various scenarios as part of a complete analysis of the rail grinding process. Two main group of tests were performed which assessed the following: 1) preventive and corrective maintenance on fresh rail samples and 2) post-grinding tribological performance of the ground samples. The results allowed further knowledge to be acquired with regards to the performance and effectiveness of the grinding process and its effect on the surface quality of the rail samples. Results indicated a correlation between White Etching Layer and the formation of cracks and defects. Additionally, the harder grades were found to retain larger quantities of White Etching Layer upon completion of the rolling/sliding testing due to the hardness gradient between the White Etching Layer and bulk material, promoting the formation of cracks.

Experimental Study on Cumulative Deformation of Pile Group in Saturated Clay under Horizontal Cyclic Loading

In order to investigate the cumulative deformation of the pile group in saturated clay under horizontal cyclic loading, a series of 1g model tests were conducted using the self-made loading equipment in this paper. Firstly, the loading equipment and testing procedure are introduced. Then, the cumulative deformation of the pile group, the dynamic response of the soil, and the bending moment of the pile shaft under horizontal cyclic loading are studied. Finally, the horizontal cyclic stiffness of the pile group is analyzed based on the experimental results. It can be found that the cumulative displacement, the rotation angle of the bearing platform, the pile shaft bending moment, and the pore water pressure can attain 90% of the peak values within the first 1000 cycles, and the growth rate slows down in subsequent loading cycles. Moreover, the bending moment of each pile increases with the burial depth and gradually decreases after the peak values. Notably, the horizontal cyclic stiffness of the pile group grows with the cycle loading times and decreases with the loading amplitude.

The limit value of plasticity coefficient of reinforcement under seismic loads

Plastic deformations develop in bending reinforced elements under seismic loads. In calculation of reinforced concrete constructions by spectral method they are taken into account only at the stage of seismic loads determination by introduction of reducing coefficients. But the development of plastic strains in bending elements leads to appearance of cracks, which don’t close after changing the internal forces sign. Due to it the work height of a section decreases, and the strength of elements reduces during subsequent loading cycles. The experimental evaluation of the influence of plastic strains on the stress-strain state of beams under low-cycle alternating effects of high intensity has been carried out. The first group of beams, considered as reference, was loaded with a monotonously increasing load until complete destruction. The other groups of beams were loaded by the monotonically increasing load up to the specified value of plastic deformations was reached. Then they were unloaded and loaded by the force of the opposite sign up to destruction. The stress-strain state of normal sections at all stages of loading was studied. It has been discovered that when changing the sign of load the through cracks are formed which close later if the plastic deformations of reinforcement are not large. The limit value of plasticity coefficient in the first semi cycle of loading preventing the formation of non-closing through cracks when changing the sign of internal forces has been determined.

The effects of undercrossing tunnelling on the settlement of footings subjected to cyclic loading

In urban areas, due to the expansion of underground systems and space limitation, many tunnels will be constructed beneath existing tunnels, roads or railways which may be subjected to traffic loading. Undercrossing tunnelling disturbs the stress state in soils and may affect the long-term settlement behaviour of existing structures under cyclic loading. A series of physical model tests were performed to investigate the effects of undercrossing tunnelling on the settlement behaviour of existing footings subjected to cyclic loading. The undercrossing tunnelling induced volume loss is simulated using a self-designed device in steps between loading cycles. The variation of footing settlement and soil stresses in the ground are presented and discussed under different cyclic loading amplitudes, volume losses and footing positions relative to the new tunnel. The results indicate that including volume loss between loading cycles enlarges the footing settlement during the subsequent cyclic loading, even if the footing is out of tunnelling induced settlement profile, and the greater the volume loss, the greater the enlargement. The enlargement decreases as the footing gets farther away from the new tunnel. The enlargement is attributed to the change of relative density and/or the change of stress state in the soil induced by tunnelling.

Pre-cyclic-loading-enhanced Stage-1b stress corrosion crack growth of pipeline steels

The influence of prior cyclic loading on Stage-1b stress corrosion cracking (SCC) growth of X52 pipeline steels was investigated. Results showed that Stage-1b SCC growth was observed for pre-cyclically-loaded specimens under subsequent cyclic loading at the maximum stress intensity factor of 16 MPa m0.5 rather than those without pre-cyclic-loading. Large magnitude of prior stress cycles favored the SCC crack growth and its transition. Intergranular attack by static corrosion contributed marginally to the Stage-1b crack growth. Prior cyclic loading plus heat treatment facilitated strain-aging-assisted SCC crack growth. Removal of the plastically deformed regions before re-coating could reduce the cracking susceptibility.

The role of prior creep duration on the acoustic emission characteristics of rock salt under cyclic loading

This study aimed to investigate the acoustic emission (AE) behavior of rock salt under combined creep and fatigue. The loading path was in a stress-controlled mode and contained a constant stress load with different durations, followed by cyclic loading. The AE counts, peak frequency, amplitude, b-value, and RA (rise time/amplitude)-AF (average frequency) distribution were systematically analyzed. The results show that the AE counts gradually increased during loading and existed sporadically within the prior creep stage and frequently in the subsequent cyclic loading phase. The peak frequency exhibited a zonal distribution and was divided into low, medium, and high parts. A longer prior creep duration induced a wider frequency distribution range, a higher average amplitude, and a reduction in b-value. According to the RA-AF distribution, the tensile cracks constituted more than 90% of the total cracks, and the prior creep duration decreased the shear crack propagation compared with the cases without prior creep. The results indicate that prior creep increases the crack dimensions and damage severity and makes the salt sample prone to fracture in the subsequent cyclic loading.

Mechanism for superior fatigue performance of warm laser shock peened IN718 superalloy after high-temperature ageing

The high cycle fatigue properties of IN718 alloy treated by warm laser shock processing (WLSP) were studied after high-temperature ageing at both room temperature and 600 ℃. The strengthening and toughening mechanism of WLSP on the hardened surface layer of IN718 alloy was elucidated to reveal the highly stable strengthening effects at high temperature. It was found that the thermally assisted surface hardening techniques had more obvious advantages in maintaining fatigue resistance and prolonging fatigue life of the alloy with the hardened surface layer during long-term service at elevated temperature, comparing with the conventional laser shock peening (LSP). After WLSP treatment, complex structures of γ″ phase/high-density dislocation with stacking faults (SFs) and nano-sized twins in γ″ phases surrounded by the more stable and higher density of geometrically necessary dislocations (GNDs), formed in the hardened surface layer. Therefore, a remarkable strengthening effect in the hardened surface layer was achieved due to the production of more stable hetero-deformation induced (HDI) stress. Meanwhile, the substructure evolution, compressive residual stress reduction and γ″ phase growth in the hardened layer of WLSP alloy were also inhibited during high-temperature ageing, owing to the existence of the special structure, contributing stability of microstructure and strengthening effect under conditions of high-temperature ageing and subsequent cyclic loading at high temperature. Thus, both the fatigue crack initiation resistance and initiation time of WLSP alloy can be improved in the surface of specimen. This is the main reason why after high-temperature ageing the fatigue resistance of WLSP sample can be kept stably and the median fatigue strength can be improved at both room temperature and 600 ℃.

Towards three dimensional aspects of plasticity-induced crack closure: A finite element simulation

The mutual interactions between intrinsic (damage) and extrinsic (shielding) mechanisms are essential for understanding fatigue crack growth in ductile materials. In the latter case, plasticity-induced crack closure is the dominating retardation mechanism in the Paris regime. The transition between plane strain and plane stress states leads to a locally different fracture surface contact, which is difficult to access during experiments. In this work, plasticity-induced crack closure under constant amplitude loading of an AlCu4Mg (AA2024) aluminium sheet material is studied from a three-dimensional perspective. An elastic–plastic 3D finite element model of an M(T)160 specimen with a bilinear isotropic hardening model is used to study the evolution of plasticity during fatigue crack growth. Cyclic crack propagation is simulated with the releasing-constraint method.In the results, plasticity-induced crack closure is present up to a load ratio R = 0.3. Detailed investigations of the contact pressure distributions indicate a strong dependence on the stress intensity factors and the sheet thicknesses. The contact pressure distributions on the fracture surface can be divided into three characteristic regions. Near the plastic zone, plastic energy is still induced during the unloading process after the crack is already closed. However, the crack surface contact does not affect the shape of the plastic zone. Additionally, further plastic energy accumulates in the plastic wake region during crack closure within subsequent load cycles, described here as cyclic plastic wake. In summary, the finite element model can reproduce the major features of fatigue crack closure like Kop or Kcl and the three-dimensional and load-dependent evolution of plasticity-induced crack closure.

Simulation of crack propagation through voxel-based, heterogeneous structures based on eigenerosion and finite cells

This paper presents an algorithm for the efficient simulation of ductile crack propagation through heterogeneous structures, as e.g. metallic microstructures, which are given as voxel data. These kinds of simulations are required for e.g., the numerical investigation of wear mechanisms at small length scales, which is still a challenging task in engineering. The basic idea of the proposed algorithm is to combine the advantages of the Finite Cell Method allowing for a convenient integration of heterogeneous finite element problems with the eigenerosion approach to still enable the mesh-independent simulation of crack propagation. The major component is to switch from finite subcells to finite elements wherever the crack progresses, thereby automatically adaptively refining at the crack tip by managing the newly appearing nodes as hanging nodes. Technically relevant problems of crack propagation at the microscale are mostly linked with sub-critical crack growth where the crack moves fast and stepwise with subsequent load cycles. Therefore, inertia may become important which is why dynamics are taken into account by spreading the mass of the eroded elements to the nodes to avoid a loss in mass resulting from the erosion procedure. Furthermore, a certain treatment for the finite cell decomposition is considered in order to ensure efficiency and accuracy. The numerical framework as well as the voxel decomposition techniques are analyzed in detail in different three-dimensional numerical examples to show the performance of the proposed approach.

Experimental study on uniaxial ratchetting of VHB 4910 dielectric elastomer

In this work, a series of uniaxial cyclic tests of VHB 4910 dielectric elastomer were carried out to investigate the uniaxial ratchetting and its dependence on the stress level, stress rate, peak/valley stress hold and loading history. The results show that in the uniaxial stress-controlled cyclic tests, VHB 4910 produces obvious ratchetting, and the higher the applied stress level and the lower the stress rate, the more remarkable the ratchetting is. Compared with the experimental results of pure-shear cyclic deformation, the apparent modulus of VHB 4910 during uniaxial cyclic deformation is lower and the dissipation is smaller. In addition, the uniaxial ratchetting of VHB 4910 is also affected by the peak/valley stress hold and loading history. The peak stress hold will promote the generation of ratchetting, but the valley stress hold will hinder the increase of ratchetting strain. The cyclic loading history with a higher mean stress will inhibit the development of ratchetting in the subsequent cyclic loading case with lower mean stress.