Evolution Of Stiffness Research Articles

Vertical Monotonic and Cyclic Responses of a Bucket in Over-Consolidated Clay

Bucket foundations, especially multi-bucket foundations, have become an alternative for large offshore wind turbines. Vertical responses of a single bucket are critical for the serviceability design of tripod or tetrapod bucket foundations. Centrifuge tests are conducted to investigate the responses of a single bucket under monotonic and symmetric cyclic loading in over-consolidated clay. The strength of clay is obtained by cone penetration tests. The monotonic vertical capacity measured in the centrifuge tests are compared with the finite element results, with errors less than 6%. The effects of the ratio of cyclic loading amplitude to vertical capacity (ranging between 0.37 and 0.64) and the number of cycles on the accumulation of vertical displacement and evolution of stiffness are explored. Simplified functions are proposed to predict the evolutions of dimensional and dimensionless stiffness.

Fatigue performance evaluation of bistable composites at different combinations of loading and environmental conditions

Bistable composite laminates have large-scale applications in morphing and energy-harvesting structures, but their fatigue performance remains largely unexplored. This study investigates the stiffness and damage progression and evaluates bistable performance to develop protocols for long-term applications. We analyze the effects of displacement-controlled fully reversible high cycle fatigue-loading on stiffness, damage, curvature, and snap-through load in the out-of-plane loading direction at eight different combinations of parameters with frequency from 1 to 10 Hz, two boundary conditions, and temperature from 22°C to 150°C up to 3 to 10 million cycles. Stiffness and damage evolution analysis demonstrate the first two stages in out-of-plane fatigue loading. The study proposes a damage definition in terms of load adapting with two fatigue damage models: (1) Shiri Model and (2) Wu Model, while both models exhibit reasonable accuracy in predicting damage for the first two stages despite deviating at the final cycle due to assuming this cycle as the final failure cycle. Of the two models, the Shiri model provided a smaller range of model parameter values, 0.22 and 0.43, for parameters p and q, respectively, which reflects adjustability to different test conditions by maintaining a moderate range. Specimens encountered no final failure by fiber breakage and did not lose bistability for any combination. Curvature and snap-through load measurements have not substantially changed due to fatigue loading. These findings confirm application protocols with a broad range of parameters for which the laminates can operate without significant fatigue damage and maintain their bistable performance for an infinite lifetime.

Experimental Study of the Nonlinear Behaviour of Deep-Sea Mooring Polyester Fibre Ropes

Abstract Mooring ropes are essential components of ships and offshore floating structures and they are subjected to cyclic axial loads. This study investigates the evolution of the full-cycle stiffness of fibre polyester ropes under long-term static and dynamic loading. First, the static stiffness characteristics of the ropes, including the rope elongation properties at different stages, shrinkage rates, and creep coefficients after an idle period, are examined under static loads; an empirical formula for static stiffness is established. Second, the dynamic stiffness characteristics of the ropes are investigated under cyclic loads that are typical of platform production operations. The stabilities of the structure under different tensions are compared; the effects of mean tension, tension amplitude, and load cycle on the dynamic stiffness of the ropes are analysed and an empirical formula is established to predict the dynamic stiffness during the engineering design phase. The results of this study can be helpful for the rational design of deep-sea taut-leg mooring systems because they present the evolution of the full-cycle stiffness characteristics of mooring ropes.

Laboratory Small-Strain Stiffness Measurement Using Distributed Acoustic Sensing

Abstract This article introduces a novel method for measuring the small-strain stiffness of materials at the laboratory scale using the emerging technology of distributed acoustic sensing (DAS). This setup involves modifying the traditional free-free resonant column test by wrapping the DAS fiber optic cable around the sample. Tests are conducted using both active and passive sources. Sample resonances measured by DAS demonstrate a high degree of consistency with conventional accelerometer, indicating that DAS is accurate and reliable at the kilohertz scale. To extract the sample resonant frequency from passive sources in laboratory environments (such as cooling fan noise, mixer vibrations, and manual tamping procedure), the auto-correlation method is employed to enhance the amplitude spectrum’s signal-to-noise ratio. The measured transverse resonance excited by passive sources aligns well with that measured under active sources, suggesting the potential for using nonstationary ambient vibrations for long-term monitoring. The study also reveals that impulse-like forces are more suitable for exciting distinguishable resonant frequency compared to continuous but weak ambient vibrations. In a benchmark study, the repeatability of DAS measurements is verified using fabricated samples with known and stable stiffness. The measured constrained and shear moduli are found to be consistent among DAS, accelerometer, and ground-truth values, confirming the robustness and accuracy of the DAS-based free-free resonance column test. To further demonstrate the practical application of the proposed DAS-based system, a 28-day monitoring of cement-stabilized soil is conducted. The small-strain stiffness evolution is successfully captured by DAS, with marginal differences between measurements under active and passive sources. This outcome highlights the potential quality assurance scenarios for DAS because of its distributed properties and rapid data acquisition capabilities.

Data driven prediction of fatigue residual stiffness of braided ceramic matrix composites based on Latent-ODE

A Latent-ODE method is proposed for predicting fatigue residual stiffness of braided ceramic matrix composites, combining β-Variational Auto-encoder (β-VAE) and Neural Ordinary Differential Equation (Neural ODEs). β-VAE is used to extract and separate latent features of underlying fatigue behavior and Neural ODEs is used to learn the underlying dynamics behavior corresponding to the fatigue residual stiffness evolution mechanism. Firstly, the residual stiffness of two-dimensional C/SiC ceramic matrix composites under room temperature fatigue is predicted. Based on the potential variable interpolation method, the prediction results are in good agreement with the calculation results of the phenomenological model. The fatigue residual stiffness prediction method based on partial data retraining can obtain higher-precision subsequent residual stiffness prediction results only using the initial 10% fatigue residual stiffness data. In addition, the high temperature fatigue residual stiffness of two-dimensional braided ceramic matrix composites is predicted. The law of stiffness degradation can be learned by the neural network from the high temperature data, and realizing the high precision reconstruction of high temperature fatigue residual stiffness. The method based on partial data retraining only uses the first 30% of the residual stiffness data to predict the residual stiffness.

A consistent formulation for yarn re-orientation in shear and off-axis loading of woven composites RVEs

The mechanical response of woven composites under off-axis tensile loading is strongly affected by the damage mechanism in the yarn-yarn and yarn-matrix interfaces. Due to damage in these interfaces, the yarns will be able to rotate and reorient. For plain weave and satin weaving schemes, the yarns will reorient towards the loading direction in off-axis tensile loading, resulting an increase in the apparent stiffness. The material model used to simulate the reinforcing yarns needs to handle this property as the material axes corresponding to the fiber direction should be updated during loading to reach realistic results. The key ingredient of a material model required to achieve this is a fiber frame stress rate description (Badel et al., 2008), which will be compared with a hyperelastic model (Bonet and Burton, 1998). The definition of the fiber frame is reformulated to properly conform to woven composites and the implementation of the model is extended for an implicit solver. Simulations are performed in ABAQUS/Standard with models implemented in UMAT routines. The results are compared with the standard ABAQUS implementation and are shown to retrieve a realistic stiffness evolution induced by fiber reorientations.

Characterization of interface properties for modeling the shear behavior of T-beams strengthened with ultra high-performance concrete

The application of Ultra high-performance concrete (UHPC) lateral layers on normal-strength concrete (NSC) beams substantially improves their shear performance. However, the impact of the NSC-UHPC interface properties on the shear performance has been scarcely studied and may hinder the strengthening efficiency and modify the failure mode. This study has characterized the complete shear and tensile behavior of interface specimens and evaluated the effect of anchors at the interface. Then, the complete curves of the interface allowed calibration of an interface concrete fracture (CF) model that was introduced in finite element (FE) simulations replicating the shear behavior of UHPC-strengthened T-beams tested in the same project. Parametric studies were then made with FE simulations. Results showed the FE simulations with the CF model better capture the stiffness evolution and failure mode of strengthened beams than that with the perfect bond model. The increase of UHPC layer thickness up to 80 mm and anchor spacing at the interface smaller than 300 mm considerably increased the stiffness and shear capacity of the strengthened beams. Existing dead load on beams did not impair UHPC strengthening and a substantial increase in shear capacity can still be achieved in real-size strengthened T-beams.

Mechanical fatigue testing in silico: Dynamic evolution of material properties of nanoscale biological particles

Biological particles have evolved to possess mechanical characteristics necessary to carry out their functions. We developed a computational approach to "fatigue testing in silico", in which constant-amplitude cyclic loading is applied to a particle to explore its mechanobiology. We used this approach to describe dynamic evolution of nanomaterial properties and low-cycle fatigue in the thin spherical encapsulin shell, thick spherical Cowpea Chlorotic Mottle Virus (CCMV) capsid, and thick cylindrical microtubule (MT) fragment over 20 cycles of deformation. Changing structures and force-deformation curves enabled us to describe their damage-dependent biomechanics (strength, deformability, stiffness), thermodynamics (released and dissipated energies, enthalpy, and entropy) and material properties (toughness). Thick CCMV and MT particles experience material fatigue due to slow recovery and damage accumulation over 3-5 loading cycles; thin encapsulin shells show little fatigue due to rapid remodeling and limited damage. The results obtained challenge the existing paradigm: damage in biological particles is partially reversible owing to particle's partial recovery; fatigue crack may or may not grow with each loading cycle and may heal; and particles adapt to deformation amplitude and frequency to minimize the energy dissipated. Using crack size to quantitate damage is problematic as several cracks might form simultaneously in a particle. Dynamic evolution of strength, deformability, and stiffness, can be predicted by analyzing the cycle number (N) dependent damage, [Formula: see text] , where α is a power law and Nf is fatigue life. Fatigue testing in silico can now be used to explore damage-induced changes in the material properties of other biological particles. STATEMENT OF SIGNIFICANCE: Biological particles possess mechanical characteristics necessary to perform their functions. We developed "fatigue testing in silico" approach, which employes Langevin Dynamics simulations of constant-amplitude cyclic loading of nanoscale biological particles, to explore dynamic evolution of the mechanical, energetic, and material properties of the thin and thick spherical particles of encapsulin and Cowpea Chlorotic Mottle Virus, and the microtubule filament fragment. Our study of damage growth and fatigue development challenge the existing paradigm. Damage in biological particles is partially reversible as fatigue crack might heal with each loading cycle. Particles adapt to deformation amplitude and frequency to minimize energy dissipation. The evolution of strength, deformability, and stiffness, can be accurately predicted by analyzing the damage growth in particle structure.

Potential contribution of steel slag fillers to asphalt mastic in terms of microwave heating efficiency, electromagnetic mechanisms and fatigue durability

ABSTRACT This paper investigates the potential contribution mechanisms of steel slag fillers in asphalt mastic, covering asphalt-filler interaction mechanisms, microwave heating efficiency, electromagnetic absorption mechanisms, and fatigue durability. The physico-chemical microscopic mechanisms of the steel slag filler, the limestone filler and the interactions between the fillers and the styrene-butadiene-styrene (SBS)-modified binders were characterized. The microwave-heating efficiency of steel slag-based asphalt mastic (SBS-SS) and limestone-based asphalt mastic (SBS-LS) was quantified. The dielectric behaviour of the fillers and the corresponding mastic was further characterized to reveal the electromagnetic mechanisms associated with the microwave heating technology. The stiffness evolution and the fatigue durability of SBS-based bitumen under the effectiveness of steel slag and limestone fillers were assessed by the viscoelastic continuum damage theory model and dissipative energy approaches. The steel slag fillers exhibited a greater potential to promote microwave heat efficiency than the natural stone fillers due to higher ferric oxide compositions. SBS-SS mastics have an overall greater dielectric constant and loss factor than those of SBS-LS mastics, resulting in superior electric field energy storage and conversion capabilities. The addition of steel slag fillers magnifies the load sensitivity of the bitumen and weakens the fatigue impedance and fatigue life of asphalt mastic composites.

Unveiling the Silent Danger of Childhood Obesity: Non-Invasive Biomarkers Such as Carotid Intima-Media Thickness, Arterial Stiffness Surrogate Markers, and Blood Pressure Are Useful in Detecting Early Vascular Alterations in Obese Children.

Obese children present a higher cardio-metabolic risk. Measuring vascular biomarkers that assess the evolution of arterial stiffness, subclinical atherosclerosis, and hypertension in such patients could be helpful in the long term. We studied 84 children, aged from 6 to 18 years: 50 obese subjects, versus 34 of normal weight. Clinical examination involved: BMI, waist circumference, waist-to-height ratio, and detection of the presence of acanthosis nigricans and irregular menstrual cycles (the latter in adolescent girls). The carotid intima-media thickness (CIMT) was measured with the Aixplorer MACH 30 echography device. The pulse wave velocity (PWV), augmentation index (AIx), and peripheral and central blood pressures (i.e., SBP, DBP, cSBP, cDBP, and cPP) were acquired through a Mobil-O-Graph device. Obese subjects underwent body composition analysis with a Tanita BC-418. Blood tests were: HOMA-IR, lipid panel, uric acid, and 25-OH vitamin D. All vascular biomarkers presented increased values in obese subjects versus controls. The following cut-off values were significant in detecting obesity: for PWV > 4.6 m/s, cSBP > 106 mmHg for the <12-year-olds, PWV > 4.5 m/s and cSBP > 115 mmHg for the 12-15-year-olds, and PWV > 5 m/s, cSBP > 123 mmHg for the >15-year-olds. AIx is higher in obese children, regardless of their insulin resistance status. Waist circumference and waist-to-height ratio correlate to all vascular parameters. HOMA-IR is an independent predictor for all vascular parameters except CIMT. Cut-off values for PWV of >4.8 m/s, SBP > 125 mmHg, and a cSBP > 117 mmHg predicted the presence of acanthosis nigricans. Obese girls with irregular menses displayed significantly higher PWV, SBP, and DPB. Elevated levels of uric acid, LDL-c, non-LDL-c, triglycerides, and transaminases, and low levels of HDL-c and 25-OH vitamin D correlated with higher arterial stiffness and CIMT values. We conclude that CIMT and the markers of arterial stiffness are useful in the early detection of vascular damage in obese children.

Stiffness evolution of jet grouting columns performed under port caissons using PS suspension logging

AbstractJet grouting is one of the ground improvement techniques that are currently more widely used for upgrading gravity quays, especially to increase their berth depth. Irregular columns are formed because of a rotating high‐pressure injection of a cementitious fluid in the ground that mixes with the natural soil. Despite its popularity, there is hardly any field experimentation on the stiffness gain of the mixture, especially under the base of port caissons. As fresh columns behave like a dense fluid with an excess of pore pressure, the evolution of mixture stiffness, especially at early ages after injection, is a key aspect when programming working windows for ground improvement, drill locations, sequencing and timing. Otherwise, crane rails, pavement and other services are at risk of undergoing undesirable local movements. One of the upgrading programs of the Port Authority of Valencia, Spain, is the increase of the berth depth of its quays, many engineered as concrete caissons. The concern about the temporary local loss of the foundation stiffness during the execution of each jet grouting column prompted research to find an in situ procedure capable of measuring the stiffness evolution of the jet grouting columns at early ages. This paper presents a successful procedure using PS suspension logging, expanding its scope of geotechnical engineering applicability in an innovative way. The use of the technique for the intended purpose and the execution process itself stands out as a novelty, with the insertion of the borehole casing pipe through the axis of the recently executed column. The key outcomes of this research are: (1) the novelty of studying the evolution of the stiffness in the early ages of the jet grouting columns executed under port caissons, where it is not possible to execute test columns; and (2) to propose an adequate and reliable method at the beginning of the works to plan the injection sequence of the next columns of jet grouting.

Long-Term Response of Sand Subjected to Repetitive Simple Shear Loading: Shakedown, Ratcheting, and Terminal Void Ratio

Low-amplitude repetitive drained loading may hinder the long-term performance of engineered and natural systems. This study examines the volumetric and shear response of a uniform quarzitic sand subjected to repetitive drained simple shear loading under constant vertical stress while tracking the evolution of the secant stiffness and the small-strain shear modulus. We explore the effects of initial density, initial shear stress and cyclic shear stress amplitude to identify criteria that can be used to anticipate asymptotic volumetric and shear states. We analyze experimental results in reference to the sand response under monotonic simple shear loading. All specimens evolved toward some asymptotic terminal void ratio eT when subjected to simple shear cycles. Contractive specimens exhibited unceasing shear strain accumulation and ratcheting when the normalized shear stress exceeded τ*=(τo+Δτ)/τult>0.85; on the other hand, dense-dilative specimens exhibited ratcheting only when the normalized shear stress exceeded τ*=(τo+Δτ)/τult>1.25. The small-strain Gmax and the secant Gpp shear moduli increased during repetitive shear cycles to reflect early fabric changes followed by abrasion/fretting among enduring contacts. Results obtained in this study allow us to propose simple guidelines to predict the asymptotic shear and volumetric response of uniform sands subjected to repetitive simple shear loading for first-order engineering analyses.

A proposed approach for accurate fatigue quantification of the base asphalt binders coupling nonlinear viscoelastic effects in S-VECD theoretical framework

The nonlinear viscoelasticity (NLVE) effects at high loading amplitudes are neglected in the stiffness evolution and fatigue quantification for asphalt binders. This study is to determine the applicability of the simplified viscoelastic continuum damage (S-VECD) model considering NLVE effects to the fatigue test protocols of asphalt binders at high strain levels, and to identify the real damage degradation and fatigue life quantification. An equivalent non-linear complex modulus (|G*|NLVE) of the NLVE-based S-VECD model was proposed and quantified by incremental stress sweep testing. A refined fatigue failure criterion GRNL was proposed in the NLVE model. The effectiveness of NLVE on the released pseudo-strain energy (PSE) and failure criteria was also verified. |G*|NLVE values in NLVE region are lower than |G*|LVE values in LVE region. The linear viscoelasticity (LVE)-based S-VECD model overestimates the damage effectiveness on the stiffness degradation. A significant difference exists between the NLVE and LVE models in the damage characteristic curves of the asphalt binders. The actual damage quantified by the NLVE model is lower than that of the LVE model. For both types of binder, the GRNL parameter remains well linear with the fatigue life (Nf). The linear relationship between the released PSE shrinkage values (ΔΣWrR) and the reduced damage (ΔS) is clear when comparing the LVE and NLVE models. The NLVE model predicted a higher fatigue life for asphalt binders than that predicted by the LVE model. The fatigue life determined by the NLVE model exhibits a linear functionality to that quantified by the LVE model, which further validates the applicability and validity of the NLVE model for the diagnosis of fatigue measurement. In contrast, the NLVE-based S-VECD model predicts the fatigue life of asphalt binders under practical service conditions more closely to the mechanical response under actual traffic loads.

Effect of stress ratio on the tension–tension fatigue behavior of tungsten carbide nanoparticles toughened GFRP

AbstractThe fatigue behavior of woven ply (0/90°) glass fiber reinforced plastic (GFRP) laminates filled with 2 wt.% of tungsten carbide nano‐powder at different stress ratios, 0.1 and 0.3, was investigated. Specimens were subjected to constant amplitude fatigue loading for three stress levels, 80%, 70%, and 60%, of ultimate tensile strength at each stress ratio for both neat GFRP and tungsten carbide modified GFRP (GFRP‐WC). The fatigue life was enhanced at all stress levels in neat GFRP composites with an increase in stress ratio from 0.1 to 0.3. In addition, incorporation of nanoparticles also increases fatigue life at all stress levels for both stress ratios. The addition of nanoparticles acts as a barrier to crack propagation, which leads to a significant effect on the evolution of fatigue stiffness, cyclic creep strain, and dissipated energy during cyclic loading at both stress ratios.

Singly TASE Operators for the Numerical Solution of Stiff Differential Equations by Explicit Runge–Kutta Schemes

In this paper new explicit integrators for numerical solution of stiff evolution equations are proposed. As shown by Bassenne, Fu and Mani in (J Comput Phys 424:109847, 2021), the action on the original vector field of the stiff equations of an appropriate time-accurate and highly-stable explicit (TASE) linear operator, allows us to use explicit Runge–Kutta (RK) schemes with these modified equations so that the resulting algorithm becomes stable for the original stiff equations. Here a new family of TASE operators is considered. The new operators, called Singly TASE, have the advantage over the TASE operators of Bassenne et al. that the action on the vector field depends on the powers of the inverse of only one matrix, which can be computationally more simple, without loosing stability properties. A complete study of the linear stability properties of k–stage, kth–order explicit RK schemes under the action of Singly TASE operators of the same order is carried out for k le 4. For orders two, three and four, particular schemes that are nearly strongly A–stable and therefore suitable for stiff problems are devised. Further, explicit RK schemes with orders three and four that can be implemented with only two storage locations under the action of Singly TASE operators of the same order are discussed. A particular implementation of the classical four–stage fourth–order RK scheme with two Singly TASE operators is presented. A set of numerical experiments has been conducted to demonstrate the performance of the new schemes by comparing with previous RKTASE and other established methods. The main conclusion is that the new integrators provide a very simple solver for stiff systems with good stability properties and avoids the difficulties of using implicit algorithms.

Levels of Procollagen Type I C-Terminal Pro-Peptide and Galectin-3, Arterial Stiffness Measured By Pulse Wave Velocity, and Cardiovascular Morbidity and Mortality in 44 Patients 2 Years After Kidney Transplantation.

BACKGROUND Cardiovascular (CV) mortality remains high despite the improvement of kidney function after kidney transplantation. In heart failure (HF), high concentrations of biomarkers of fibrosis, related to cardiac and/or vascular impairment, are associated with CV outcomes, but their significance in kidney transplantation is still unclear. Our aim was to investigate the association of procollagen type I C-terminal pro-peptide (PICP) and galectin-3 (Gal-3), markers of fibrosis, with arterial stiffness measured by pulse wave velocity (PWV) and CV morbi-mortality in kidney transplantation recipients from the prospective monocenter TRANSARTE study (Transplantation and Arteries), which compared the evolution of arterial stiffness in transplanted patients and patients remained on dialysis. MATERIAL AND METHODS PICP and Gal-3 were measured at 2 years after transplantation in 44 kidney transplantation patients. Spearman's rank-order correlation analysis was conducted to assess the relationship between biomarkers and PWV. Association of biomarkers with CV morbi-mortality was evaluated using Cox regression analysis adjusted for age, renal function, and PWV. RESULTS There was no significant correlation between PWV and PICP (r=-0.16, P=0.3) or Gal-3 (r=0.03, P=0.85). Gal-3, after adjusting for key prognostic factors, including PWV, was significantly associated with CV morbi-mortality [HR (95% CI)=4.30 (1.01-18.22), P=0.048], whereas PICP was not significantly associated with outcome. CONCLUSIONS In multivariable adjusted analysis, elevated Gal-3 concentrations were associated with CV morbi-mortality in kidney transplantation patients, whereas PICP was not. As Gal-3 was not related to PWV, other sources of fibrosis (eg, cardiac fibrosis) may be underlying the prognostic value of Gal-3 in kidney transplantation.

Fatigue behaviour analysis of aluminium alloy riveted single-shear lap joints

Analysing the dynamic response, the fatigue failure process of aluminium alloy riveted single-shear lap joints is quantitatively described. Firstly, numerical analysis of hysteretic loops based on MATLAB is proposed, and cyclic relative displacement amplitude, stiffness and dissipated energy are calculated. Secondly, the service behaviour corresponding to the evolution of relative displacement amplitude and dissipated energy is analysed and verified by finite element simulation. Finally, based on the evolution of cyclic compression stiffness, the cycles corresponding to crack initiation, as well as slow and rapid propagation, are calculated and verified by observing crack growth propagation through interruption tests.

The normal impact stiffness of a debris-flow flexible barrier

This paper proposes a normal oriented impact stiffness of a three-supporting cable flexible barrier under a small pretension stress to estimate the structural load behaviour, and employs two categories of small-scale debris flows (coarse and fine) to explore the stiffness evolution through physical model experiments with high-speed photography and load sensing. Results suggest that the particle-structure contact is essential to the normal load effect. Coarse debris flow performs more frequent particle-structure contact and exerts evident momentum flux, while fine debris flows with few physical collisions impart much smaller one. The middle-sited cable that receives only tensile force from vertical equivalent cable-net joint system exhibits indirect load behaviour. The bottom-sited cable shows high load feedback due to the sum of direct contact of debris flow and tensile forces. The relationship between impact loads and maximum cable deflections can be explained by power functions according to quasi-static theory. The impact stiffness is not just affected by the particle-structure contact but by the flow inertia and particle collision effect. Savage number Nsav and Bagnold number Nbag manage to depict the dynamical effects on the normal stiffness Di. Experiments indicate that Nsav has positive linear correlation with the nondimensionalization of Di, whilst Nbag has positive power correlation with the nondimensionalization of Di. This idea is an alternative scope for the study on flow-structure interaction and may contribute to the parameter identification in numerical simulation of the debris flow-structure interaction and the optimization of the design standardization.

The high cycle fatigue testing of High‐Performance Concretes using high frequency excitation

AbstractThe effect of fatigue failure in brittle materials like (ultra) High Performance Concrete (UHPC) due to cyclic loading causes unexpected failure that consequently results in heavy costs in marine and civil structures. To characterize the effect of fatigue, cyclic loading tests are performed, and “the number of cycles to failure” are experimentally determined. One problem with these kinds of tests is that such experimental investigations are potentially expensive, i.e., time‐consuming process since the number of loading cycles could be extremely high. Further, within the different damage phases of the cycling tests, one has no access to the small‐scale, i.e., microscopical evolution of (micro‐)cracks. Additionally, a full characterization of the small‐strain stiffness evolution of the material is challenging. The goal of the research investigation is to combine a (large amplitude) High Cycle Fatigue experiment with a (low amplitude) Dynamic Mechanical Analysis (DMA). Using a setup based on the piezoelectric actuator, the (rate‐dependent) mechanical properties of the material in tangential space, and the failure modes of the material will be examined accurately. The excitation frequency is between 0.01 Hz to 1000 Hz which allows for reducing the experimental investigation time to failure. Further, it allows investigating the effect of frequency on the number of cycles to failure. Firstly, experimental results for HPC and berea sandstone samples will be presented. Harmonic experimental data include (direct) strain measurements in axial and circumferential directions as well as forces in axial directions. In addition, the resulting complex Young's modulus and evolving damage‐like “history” of HPC and berea sandstone specimens will be shown.

Numerical analysis of size effect on the deformation behavior and damage evolution mechanism of segmental tunnel lining rings

This article investigates the size effect of segmental tunnel linings on the deformation behavior and the damage evolution mechanism using the Finite Element Method (FEM). A series of models with different diameters are established under the same loading condition to control the variable. The bolts and the rebar apply the elastic-plastic model. The Concrete Damage Plasticity model has been considered for the concrete lining. The vertical convergence deformation, internal force of these models, and damage to the segment body are selected for comparison. The number and location of the yielded bolts are explored for the analysis of the overall structure stiffness and damage evolution mechanism. In terms of plastic hinge theory, an extended discussion is carried out to explain the differences in damage evolution mechanisms in different-sized linings. The results show the large-diameter lining rings have a relatively greater increasing rate of convergence deformation. Furthermore, fewer plastic hinges emerged in large-scale lining rings before the structures reached their ultimate strength. According to the experimental results, we can know that the failure of large-diameter has the characteristic of more brittle damage than that of small-diameter one and these large-size structures are prone to become unstable geometry.