Amplitude Loading Research Articles

Flow and deformation due to periodic loading in a soft porous material

Soft porous materials, such as biological tissues and soils, are exposed to periodic deformations in a variety of natural and industrial contexts. The detailed flow and mechanics of these deformations have not yet been systematically investigated. Here, we fill this gap by identifying and exploring the complete parameter space associated with periodic deformations in the context of a one-dimensional model problem. We use large-deformation poroelasticity to consider a wide range of loading periods and amplitudes. We identify two distinct mechanical regimes, distinguished by whether the loading period is slow or fast relative to the characteristic poroelastic timescale. We develop analytical solutions for slow loading at any amplitude and for infinitesimal amplitude at any period. We use these analytical solutions and a full numerical solution to explore the localisation of the deformation near the permeable boundary as the period decreases and the emergence of nonlinear effects as the amplitude increases. We show that large deformations lead to asymmetry between the loading and unloading phases of each cycle in terms of the distributions of porosity and fluid flux.

Study on wear behavior and mechanism of 6007 Si3N4 full-ceramic ball bearing under different load conditions

This study investigated the effects of various load conditions on the wear characteristics of Si3N4 full-ceramic ball bearings in response to 6007 Si3N4 full-ceramic ball bearing operation-related wear problems. Using an ABLT-1A tester, the wear behavior of 6007 Si3N4 ceramic ball bearings was investigated under loads of 100, 500, and 800 N, with a rotation speed of 6000 r/min and a duration of 12,000 min. The test load, and the 100 and 500 N radial loads had an equilibrium temperature difference of about 1.91°C, while the 500 and 800 N radial loads exhibited an equilibrium temperature difference of about 2.35°C. Each group’s temperature rate difference is evident. Bearing internal clearance increased by 35.7%, 50.34%, and 56.1%, respectively, and the fault frequencies of the bearing elements appeared within the vibration signal. The surface roughness of each component of the bearing increased with the increase in the test load, while a large number of plow grooves and spalling appeared on the surface, and severe cage wear was observed. Results show that when load, temperature, clearance, runout, and frequency domain amplitude increased, the following were observed: development of rolling body surface spalling and a large area of wear spot; raceway surface crater along the direction of movement of the fur-row; obvious scratches were observed on the cage surface in the pocket hole edge melting phenomenon; the cage became the bearing system’s "weak link.” The test results are a reference value for the wear behavior of Si3N4 full-ceramic ball bearings under this test load.

Nonlinear dynamics and chaos control of circular dielectric energy generator

The current study focuses on the nonlinear dynamics of the dielectric elastomer generator (DEG). To begin with, we will go through DEG’s operating principles, parameters, materials, and deformation modes. On this basis, the dynamical modeling for the nonlinear behavior of the DEG system is described, and the second-order nonlinear ordinary differential equation representing radially symmetric motion is derived using Hamilton’s variational principle. A static applied electric field allows the DEG system to attain two equilibrium states: thick and thin. The effects of the stiffening phenomenon on the dynamics of the DEG system are investigated. The system converges to a stable equilibrium point under constant loads, and the convergence position and velocity are closely related to both the initial states. Subjected to periodic loads, certain phenomena such as quasiperiodicity and chaos are detected, and the chaotic phenomena are explored using the bifurcation diagram and 0-1 chaos test. Furthermore, parametric studies are carried out using numerical analyses to demonstrate the influence of load amplitude and external frequency on the dynamics of the DEG system. Once the DEG enters the chaotic regime, the chaotic zone is transformed into a stable manifold using established chaos control techniques such as system parameter change and parametric perturbation. However, because these techniques require changes in system design, we proposed a chaos control by using a non-linear controller, specifically, adaptive integral backstepping sliding mode control (AIBSMC) as one demonstrative candidate. The controller also enables the DEG system to follow motion along a predefined trajectory. The current work may be expected to substantially impact the future design and performance of energy harvesters.

A practical framework for assessing the effect of cyclic softening of clays on the lateral response of single piles

Single pile foundation has been widely used for the most offshore wind turbines to resist the cyclic loading resulted from the wind and wave. The cyclic loading can result in the degradation of the shear strength and the stiffness of clays surrounding piles. This study proposed a practical framework for assessing the effect of cyclic softening of clays on the lateral response of piles. A 3D finite difference model was developed for the pile – soil interaction under cyclic loading and then verified by full-scale monotonic and cyclic lateral load tests conducted on the single pile in clay. The cyclic softening behavior of undrained clays was described by a nonlinear dynamic softening constitutive model developed by the authors. The effect of the loading type (i.e., one-way and two-way loading) and the loading amplitude on the cyclic lateral response of piles was investigated. Moreover, a prediction function was proposed and was demonstrated to be effective in estimating the evolution of cumulative peak lateral displacement of cyclically loaded single piles in clays. The proposed practical framework provides an alternative for predicting the cyclic response of single piles in clays based on the monotonic lateral load test of piles.

Using XFEM technique to predict the crack growth in the notched plate under high loading cyclic conditions

Our study delves into analyzing fatigue damage in notched structures, an essential field in structural mechanics. We aim to understand how geometric and dimensional variations in the notches affect fatigue damage under different modes and amplitudes of cyclic loading, focussing on the behavior of steel. Our research methodology integrates experimental and numerical approaches. Experimental validation encompasses material properties, mesh selection, and boundary conditions. The finite element method is tailored to address steel’s elastoplastic behavior, employing calibrated parameters for kinematic and isotropic cyclic hardening models within the numerical realm. Specific findings elucidate the cyclic response of notched structures, tracking damage progression to critical thresholds, including the number of cycles to failure (Nf) and the critical crack length ( Lc ). Hysteresis curves vividly depict stress–strain relationships, offering insight into material behavior under cyclic loading. Implications span engineering domains, focussing on reliability and durability in the aerospace, automotive, and civil engineering sectors. Future directions include real-world applications and advanced techniques such as the Extended Finite Element Method (XFEM). In conclusion, this research enriches our understanding of fatigue damage analysis in notched structures, poised to impact safety and robust engineering design.

Bending moment capacity and failure mechanism of caisson foundations under monotonic and cyclic loading in clay

During service period, offshore wind turbines are subjected to both monotonic and cyclic loads, causing the rotation or translation of caisson foundations in the seabed. However, most of the existing studies focused on the performance of caisson foundations under monotonic static loading, and there are few studies about the effects of caisson-soil contact mode and soil strength reduction during installation in numerical simulations. This paper therefore systematically investigates the bending moment capacity and failure mechanism of caisson foundations under monotonic and cyclic loading in clay using finite element analyses. Three typical caisson-soil contact modes in different loading scenarios are considered, and the influence of soil strength condition, caisson aspect ratio on the bending moment capacity and failure mechanism of caisson foundations is explored. It is found that under monotonic loading, the bending moment capacity in the tensionless mode and the fully-bonded mode could be used as the lower and upper limit, respectively. Under cyclic loading, the fully-bonded mode always yields the highest moment capacity, while the frictionless mode and the tensionless mode produce the lowest in the case with small loading amplitude and the case with large loading amplitude, respectively. In addition, the behavior of cumulative angular displacement under combined load of wind and wave is also studied to provide insight for caisson foundation design.

Experimental investigation of the effects of fatigue on caprock materials in H2 storage projects

Experimental investigation of the effects of fatigue on caprock materials in H2 storage projects

Auxetic cementitious cellular composite (ACCC) PVDF-based energy harvester

The high deformation capacity of auxetic cementitious cellular composites (ACCCs) makes them promising for strain-based energy harvesting applications in infrastructure. In this study, a novel piezoelectric energy harvester (PEH) with ACCCs and surface-mounted PVDF film based on strain-induced piezoelectric mechanisms has been designed, fabricated, and experimentally tested. Furthermore, a numerical model for simulating the energy harvesting of ACCC-PVDF system undergoing repeated mechanical loading has been established and validated against the experimental data. The mechanical behavior of ACCCs was simulated by the concrete damage plasticity model during the preloading stage, which was converted to the second-elasticity model during cyclic loading stage. Based on the mechanical responses, analytical formulas for piezoelectric effects were developed to calculate the output voltage of the PVDF film. The output voltages of the ACCCs-PVDF system under different loading amplitudes and loading frequencies were assessed. The experimental results and models of the ACCCs-PVDF energy harvester lay a solid foundation for utilizing architected cementitious composites in energy harvesting applications to supply self-power electronics in infrastructure.

Effect of rubber-sand particle size ratio on shear properties of rubber-sand mixtures under normal cyclic loading

The mechanical properties of rubber-sand mixtures are influenced by the ratio of rubber-sand particle sizes and the rubber content. Rubber-sand mixtures used as roadbed fillers are primarily subjected to normal cyclic traffic loading. Therefore, a series of direct shear tests on rubber-sand mixtures under normal cyclic loading were conducted using a large-scale direct shear apparatus. The effects of the rubber-sand particle size ratio, rubber content, initial stress, and amplitude of normal cyclic loading on the shear strength of rubber-sand mixtures were evaluated. Based on the indoor tests, discrete element models of rubber-sand mixtures with different particle size ratios were established to explore the meso-mechanical mechanism of the mixed soil during the direct shear process. The results indicate that rubber-sand mixtures with low rubber content and a particle size ratio >1 have a positive effect on the shear strength of the mixed soil. The impact of initial stress on the shear stress of the mixed soil is greater than that of the normal cyclic load amplitude. As the particle size ratio increases, the inter-particle contact force becomes larger, resulting in more noticeable group anisotropy. Additionally, the main direction of the normal contact force anisotropy in the mixture consistently aligns with the deflection direction of the force chain. Rubber particles in rubber-sand mixtures with a large particle size ratio actively participate in the contact force chain network system, facilitating load transfer and positively influencing shear strength.

Coupled stability of offshore wind monopile foundations and submarine slopes under periodic loads

Long-term cyclic loading on offshore wind monopiles will lead to a decrease in the mechanical properties of the seabed near the monopile's foundation, therefore, causing instability of the submarine slope and the monopile foundation. To capture the accumulation strain caused by high cycle numbers and plastic deformation of seabed sands, the high cycle accumulation (HCA) model is complied with Mohr-Coulomb constitutive models. The shear strength reduction (SSR) method is used to obtain the safety factor of submarine slopes. The coupled numerical model simulates a 6 MW offshore monopile foundation with a single periodic lateral load applied on the monopile at the sea level, which represents concentrated wind and wave loads in storm conditions. The results show that the plastic zones begin to form at the monopile foundation, then extends towards the slope toe and slope top, and eventually across the entire slope, leading to slope instability. In this process, the safety factor of the slope is significantly reduced with each stage of the process. It was found that both the sharp changes of load amplitude and large average load also decreases the safety factors. Moreover, at the beginning of the monopile installation, the safety factor will increase by nearly 10%, but it will drop by nearly 30% due to long-term periodic lateral loads. Therefore, these have important implications for the installation of monopile foundations for offshore wind turbines (OWTs) since the slopes we consider are within the range of slopes at planned sites for OWTs. An increase in monopile diameter and embedded depth can enhance the overall slope stability. However, an increase in embedded depth could potentially instigate more extensive seabed landslides despite enhancing overall stability.

Multiple periodic motions of a two degrees-of-freedom carbon fiber reinforced polymer laminated cylindrical shell

Carbon fiber reinforced polymer is a composite material, which is widely used in various engineering fields due to its excellent properties. We systematically discuss the influence of axial load amplitude parameters on the multiple periodic motions of carbon fiber reinforced polymer laminated cylindrical shell model. Based on the Melnikov vector function, the bifurcation regions of periodic orbits are obtained. It is found that the system has at most four periodic orbits under parameters conditions. Moreover, the phase portraits of periodic orbits are given by numerical simulation. The results offer an idea for parameter control of shell structure.

Nonlinear response of laterally cyclic loaded pile in cohesionless soil based on conical strain wedge model

The response to environmental lateral cyclic loads is an important design consideration for offshore piles installed in sand. This paper proposes a novel conical strain wedge (CSW) model to simulate the pile-cohesionless soil interaction under lateral cyclic loads. The CSW model enhances the depiction of passive compression soil zone resistance and failure pattern compared to the conventional strain wedge (SW). The developed model is incorporated in a simplified and efficient method to calculate the nonlinear pile response to lateral cyclic loads. The performance of the developed theoretical model is validated by comparing its predictions with observations from several experimental studies reported in the literature. The validated CSW model and its solutions are then employed to elucidate the performance characteristics of the pile-soil system under cyclic loading. The results revealed that: (1) the degradation of soil stiffness is more pronounced at the onset of cyclic loading and the rate of increase in pile response due to degradation diminishes with the increase in the number of loading cycles; (2) as the cyclic load amplitude increases, the increase in pile displacement becomes increasingly severe; (3) the number of loading cycles has greater influence on the response of piles with lower flexural stiffness; and (4) the effect of soil shear strength on the pile displacement becomes significant as the number of loading cycles increases, whereas its effect on the pile maximum bending moment diminishes gradually.

Development of ultra-low cycle fatigue life prediction model for structural steel considering the effects of surface roughness, loading frequency, and loading amplitude

Abrasive blast cleaning is often done for steel structures before applying various protective coatings, which produces rough surfaces with changes in fatigue properties. This problem has been addressed in the low- and high-cycle fatigue regimes; however, the effect of surface roughness in combination with different loading parameters on the ultra-low cycle fatigue (ULCF) life has not been reported thus far. To this aim, a total of 59 ULCF tests on designed specimens of SM400 steel with five levels of surface roughness were performed under various loading frequencies and displacement amplitudes. The analysis of experimental results indicates a substantial reduction in fatigue life with an increase in the surface roughness and loading amplitude and a decrease in the loading frequency. Additionally, the strength degradation, dissipation energy, and load–displacement curves are discussed in detail. With the use of experimental data, a new life prediction model characterizing the combined effects of surface roughness, loading frequency, and loading amplitude on the ULCF life is proposed. Moreover, the proposed model is validated by predicting the fatigue life under variable and constant loading amplitude patterns. Comparison between experimental and theoretical results shows that the proposed model accurately estimates the ULCF life within an error band of ±15%, with a reasonable selection of model parameters.

Preparation of cement modified by multi-walled carbon nanotubes and investigation of its piezoelectric property

This study focuses on investigating the properties of a cement-based piezoelectric composite modified with 0.5 % Multi-Walled Carbon Nanotubes (MWCNTs) under varying stress conditions. Through calibration methods, a critical point named G was identified at around 60.4 MPa. Below this point, the force-electric response displayed a linear relationship, characterized by an average sensitivity of 117 pC/N. Conversely, above this point, non-linear behavior surfaced due to internal damage within the piezoelectric ceramic, leading to diminished piezoelectric properties. The frequency response characteristics highlighted efficient conversion of pressure signals to electrical signals with minimal delay, showcasing stability across diverse cyclic loading amplitudes. In addition, a dynamic compression experiment revealed sensitivity to strain rate and the notable contribution of domain switching to total strain. Moreover, an analysis of the stress–strain curve under impact load provided insights into the composite's non-linear growth and its response to strain rate variations.

Numerical Simulation Study on Vibration Reduction Effect of Flexible Cutting-Tooth Unit

Under the conditions of drilling in gravel-bearing and heterogeneous stratas, the movement and force of the PDC bit during drilling are highly unstable. Irregular impact loads often cause fatigue failures such as tooth fracture, tooth breakage and delamination of the composite sheet. Dynamic impact load is the main cause of fatigue failure of cutting-tooth, which seriously affects the rock-breaking performance of PDC bits. This paper proposes a flexible cutting tooth unit consisting of a central tooth, an elastic element, and a base. The technical concept of flexible-cutting rock breaking is to reduce the impact load amplitude suffered during the cutting process to a certain threshold range by setting elastic elements or reducing the support stiffness of the cutting tooth, so as to inhibit the expansion of micro defects caused by the impact dynamic load of cutting teeth and prolong the service life of drill bits. The finite-element models of flexible cutting teeth for rock cutting were established. The influence of cutting depth, front rake angle and stiffness of elastic elements on the cushioning and vibration-absorption effect of flexible cutting was analysed. The results show that flexible cutting can reduce the peak and average value of von Mises stress at the cutting edge. Under different cutting-depth conditions, the decline rates were 21.45–49.02% and 9.42–17.8%, respectively. Then, under different front-rake-angle conditions, the decline rates were 20.51–24.02% and 9.41–17.8%, respectively. There is a suitable stiffness of the elastic element, which makes the peak and average value of von Mises stress at the cutting edge of the flexible cutting-tooth unit perform better and the effect of improving the uneven stress distribution at the cutting edge better. Flexible cutting technology can effectively improve the adaptability of the PDC bit in heterogeneous formations and reduce the occurrence of abnormal failure of cutting teeth. The research results of this paper can provide theoretical support for the drilling speed of PDC bits in hard formations.

Fatigue Assessment of Cable-Girder Anchorage Zone in a Low Ambient Temperature Environment Based on Extended Finite Element Method

The fatigue safety of cable-girder anchorage structures in cable-stayed bridges under long-term service has attracted much attention. For bridges located in seasonally cold regions, the effect of low-temperature environments should be considered when evaluating fatigue performance. Using the Heilongjiang Bridge in China as a case study, a room-temperature fatigue test with a numerical simulation that considers the low-temperature effect on both load effect and fatigue resistance was proposed. A fatigue test with increased testing load amplitude was performed on a 1:3.75 ratio specimen. After 3.2 million loading cycles and using an acoustic emission technique, no fatigue crack was observed in the anchorage structure. The extended finite element method was then adopted to analyze the anchorage zone’s fatigue crack initiation position and propagation path. Finally, based on the fatigue characteristics of bridge steel, the fatigue resistance to the crack propagation of the vulnerable area was evaluated under three different service conditions. The results show that the fatigue performance of the anchorage zone at low temperatures is sufficient. Moreover, this paper provides a more widely applicable and cost-effective approach for the fatigue evaluation of steel bridges.

Numerical Simulation of Fatigue Behavior of Four Ti2AlNb Alloy Structural Parts

Abstract In aerospace engineering, many titanium alloy structures are subjected to fatigue loads and thus fail. Based on the Seeger fatigue life theory and the improved Lemaitre damage evolution theory, the fatigue behavior of four Ti2AlNb alloys is investigated. First, finite element models of four structural parts are established by ABAQUS software. Meanwhile, the fatigue life of four Ti2AlNb alloys is predicted by referring to the damage model parameters determined by previous work. Under the same initial conditions, the average errors of the predicted fatigue lives of the four structural parts are 20.1, 19.8, 20.9, and 19.5 %, respectively. The effects of load amplitude, temperature, and structural characteristics on the fatigue properties of Ti2AlNb alloy structural parts are studied. The stability of the two fatigue life simulation methods is analyzed. By comparing fatigue data of Ti2AlNb structural parts from various literature, the rationality of the simulated data is confirmed. Finally, the application of the Ti2AlNb structural fatigue database to machine learning is illustrated. These results provide a numerical simulation method for evaluating the fatigue life of various Ti2AlNb alloy aviation structural parts.

Fatigue strength of tubular welded connections in offshore floating platforms

AbstractThe work reported in the present paper is part of a European research program aimed at developing a hybrid tension‐leg floating platform, suitable for combined offshore wind and wave energy exploitation. Eight (8) fatigue tests on welded tubular joint specimens are performed, which represent the full penetration brace‐to‐cylindrical column welded connections of the project prototype structure. The large‐diameter columns are reinforced internally with a grid of longitudinal and transverse stiffeners fillet welded to the shell. The specimens are approximately 1/6‐scale physical models made of S355 grade steel, and subjected to cyclic loading under constant load amplitude histories. The brace‐to‐cylinder welds are semiautomatic, and high‐frequency mechanical impact treatment is applied in four specimens. The experiments are supported by post‐fracture metallographic examination and finite element analyses. The results indicate two locations for fatigue crack initiation: brace‐to‐cylinder crown and cylinder‐to‐stiffener connection.

Assessment of repaired reinforced concrete dapped-end beams exhibiting bond deterioration under static and cyclic loading

An experimental investigation has been conducted to scrutinize the behavior of reinforced concrete dapped end beams repaired with different configuration of CFRP sheet under static and cyclic loading due to induced cracks from previous examination. The static load was adopted to determine the amplitude of cyclic loading at an increment of 15% until failure. It was demonstrated that retrofitting increased load capacity and ductility significantly; approximately 28% increase in capacity and 1.5 times higher ductility over the un-retrofitted beam under static loading, and approximately 80% higher capacity with three times increase in ductility and a change in crack orientation under cyclic loading. Nonetheless, the reentrant corner's significant concrete spalling is a major concern. It is thus essential to focus on the dapped section for monitoring and maintenance of this beam.

Experimental investigations into nonlinear dynamic behaviours of triply periodical minimal surface structures

Natural frequency, damping ratio and dynamic stiffness are fundamental to the performance of structures subjected to dynamic loads. Triply Periodic Minimal Surface (TPMS) composite structures, celebrated for their superior energy absorption capacity and specific strength, represent some of the most promising meta-structures. However, their dynamic properties are yet to be fully understood, thereby hindering their practical applications within civil engineering domains. Damping properties, crucial to vibration reduction, remain particularly elusive; previous studies have not successfully established a connection between these properties and the force excitation amplitude in TPMS structures. This study aims to compare the damping properties of different TPMS structures and to investigate their potential for adoption within civil engineering fields. Three types of TPMSs, including Schoen I-graph-wrapped package surface (IWP), Schwarz primitive (Primitive) and Schoen Gyroid (Gyroid), have been adopted to design solid and sheetal TPMS composite structures with identical relative density (50%) to compare their damping properties under varying excitation amplitudes. These TPMS structures are manufactured from photosensitive resin (UV resin) using stereolithography 3D printing technology. Owing to the lack of previous research into the damping properties and dynamic stiffness of TPMS structures as support structures, we have conducted single-freedom modal tests to determine the modal parameters, including natural frequency and damping ratios. Our novel results indicate that the natural frequency and damping ratio of the TPMS structures vary with the excitation amplitude. Additionally, the dynamic stiffness of TPMS structures reveals a similar decreasing trend to frequency when load amplitude escalates. As the excitation amplitude increases, the TPMS structures demonstrate softening and are capable of offering a higher vibration damping coefficient. Notably, the SS-Gyroid structure exhibits higher stiffness and damping ratio compared to other TPMS structures. These fresh insights pave the way for the integration of 3D printed smart TPMS structures within civil engineering, particularly for vibration reduction and efficient material usage. A judicious TPMS structure design can provide relatively high stiffness and damping ratio without excessive material use.