Stress-strain History Research Articles

Reinforcement and toughening of the self‐piercing riveted bonded joints induced by the pre‐embedded adhesive in rivet cavities

AbstractSelf‐piercing riveted bonded (SPRB) joints are widely used in body‐in‐white (BIW) to connect dissimilar materials owing to their high fatigue and electrochemical performance. However, the strength of the SPRB joint has been found to be inferior to that of self‐piercing riveted (SPR) joints, owing to the reduction in undercut induced by the introduction of an adhesive, especially under pull‐out loading conditions. Interestingly, the mechanical strength improved when the rivet cavity was pre‐embedded with an adhesive. Before the riveting process, the rivet cavities in SPRB joints were pre‐embedded with 0.01, 0.02, and 0.03 mL of adhesive, namely them PSPRB‐V1–V3, respectively. Finite element (FE) models of SPRB and PSPRB joints were developed to analyze their formability and mechanical properties. The simulated and experimental results showed that the hydraulic pressure in the rivet cavity increased as the pre‐embedded adhesive volume in the rivet cavity increased during the riveting process, which further improved the undercut, microhardness, effective bearing area, and mechanical properties of joints. Among the prepared PSPRB joints, the PSPRB‐V3 joint exhibited the best undercut and optimal mechanical properties. Specifically, its undercut and peak force increased by 16.43% and 12.87% compared with those of SPRB joints. These findings provide important guidance for designing steel‐aluminum SPRB joints that require high‐bearing capabilities.Highlights The effect of pre‐embedded adhesive on the SPRB joint cross‐section was studied. Failure mechanisms of the SPRB and PSPRB joints were investigated via FEM. The better PSPRB joint performance over SPRB joints was elucidated. The stress–strain history was considered in the FE model of the joint failure process. Strengthening mechanism of pre‐embedded adhesive in the rivet cavity was discussed.

Fatigue analysis of spherulitic semi‐crystalline polymers: Unveiling the effects of microstructure and defect

AbstractA micromechanical model considering the spherulite structure of semi‐crystalline polymers was established in this study. The micro stress–strain histories were captured by combining the constitutive equations and multi‐axial fatigue criterion. The continuous damage theory was employed to describe the degradation of material properties during cycle loading. Based on the proposed model, the effects of microstructure features, such as grain anisotropy, defects, and crystallinity, on the fatigue performance was examined under multi‐axial loading condition. The local material degradation and damage accumulation were then focused on to understand the underlying fatigue mechanisms with various microstructures. Meanwhile, the crack initiation site was precisely predicted and discussed. This research provides theoretical support for understanding the failure mechanisms of spherulitic semi‐crystalline polymers, deepening the understanding of associated microstructural characteristics and strengthening the anti‐fatigue design of semi‐crystalline polymers.

A constitutive model for rate-dependency analysis of open hole woven composites under compression loading

The mechanical performance of composite materials under impact phenomena is of interest for several industries. Although the effect of the strain rate on composites significantly affects the behaviour of these materials, most studies describe the mechanical behaviour of CFRP laminates neglecting the strain rate dependence. This work presents a new constitutive model to numerically implement the change of in-plane properties of an plain weave CFRP (AGP193) over a wide range of strain rates (up to 500 s−1). Open hole compression tests are used to validate the model under quasi-static and dynamic loading. The importance of implementing the influence of strain rate on mechanical properties is illustrated by the maximum strength plot and the stress–strain history, both of which would be underestimated without the inclusion of strain rate in the model. The strain rates at the edge of the hole greatly exceed the average values in the sample, leading to an underestimation of the apparent strength up to 100% if the strain rate effect is not taken into account.

Research on Cold Roll Forming Process of Strips for Truss Rods for Space Construction.

In this paper, a new technology for on-orbit cold forming of space truss rods is proposed. For the cold roll forming process of asymmetric cross sections of thin strips, the effects of roll gap and roll spacing on the forming of asymmetric cross sections of strips were investigated using ABAQUS simulation + experiments. The study shows the following. When forming a strip with a specific asymmetric cross section, the stresses are mainly concentrated in corners 2/4/6, with the largest strain value in corner 2. With increasing forming passes, when the roll gap is 0.3 mm, the maximum equivalent strain values are 0.09, 0.24, 0.64 sequentially. Roll gaps of 0.4 mm and 0.5 mm equivalent strain change amplitude are relatively similar, and their maximum equivalent strain values are approximately 0.07,0.15, 0.44. From the analysis of the stress-strain history of the characteristic nodes in corners 2/4/6, it can be seen that the stress and strain changes in the deformation process mainly occur at the moment of interaction between the upper and lower rollers, where the stress type of node 55786 shows two tensile types and one compressive type, the stress type of nodes 48594 and 15928 shows two compressive and one tensile type, and the strain of the three nodes is in accordance with the characteristics of plane strain. When the roll gap is about 0.4 mm, the forming of the strip is relatively good. With increased roll spacing, the strip in the longitudinal stress peak through the rollers shows a small incremental trend, but the peak stresses are 380 Mpa or so. When the roll spacing is 120 mm, the longitudinal strain fluctuation of the strip is the most serious, followed by the roll spacing at 100 mm, and the minimum at 140 mm. Combined with the fluctuation in strip edges under different roll spacings, manufacturing cost and volume and other factors, a roll spacing of 100 mm is more reasonable. It is experimentally verified that when the roll gap is 0.4 mm and the roll spacing is 100 mm, the strip is successfully prepared in accordance with the cross-section requirements. When the rolling gap is 0.3 mm, due to stress-strain concentration, the strip is prone to edge waves in forming. The top of corner 2 of the flange triangular region is susceptible to intermittent tear defects, and the crack extension mechanism is mainly based on the cleavage fracture + ductile fracture.

Understanding the behaviour of a new robotic device for next-generation site investigation: A 3D DEM Exploration

Accelerating advancements in robotic engineering provide an opportunity for developing economical and efficient site characterisation devices, particularly for critical offshore applications. A new intelligent robotic device, ROBOCONE (Figure 1), is being developed for site characterisation and design of offshore structures, which would be capable of mimicking the load histories of the soil inside the ground at a desired depth [1]. The obtained soil constitutive properties would support an efficient design of offshore structures, considering “whole life” stress-strain histories. ROBOCONE could apply lateral, shear, and torsional modes (Figure 1(a)) of loading on the surrounding soil, operated remotely from the ground. In the pursuit of developing such a device, a prior understanding of its behaviour is essential. The lateral loading static and cyclic responses of offshore foundations are critical aspects of their design. The present work highlights the lateral module (P-Y Module) behaviour of the ROBOCONE device (Figure 1(a)) under various stresses, using 3D Discrete Element Modelling (DEM). The P-Y module moves horizontally at a desired depth in the ground and this lateral motion is numerically simulated using commercial DEM software, Particle Flow Code (PFC3D) [2].
 The proposed dimensions of the P-Y module are 200mm long and 54mm diameter (DRC). Present DEM model uses rigid walls as P-Y module placed in a rigid monodisperse 3D granular assembly with grain diameter of around 1/3rd DRC [3]. The Hertz contact model micro-parameters for grain-grain (E50 value of 27.4±2.2MPa) and grain-wall contacts are shown in Table 1. A 10 times higher shear modulus of grain-wall contact than the grain-grain contacts is sufficient to have stiffer response. The servo mechanism in PFC enables the model to reach desired average stresses. The primary objective of this work is to understand the ROBOCONE P-Y module behaviour and thus the lateral soil reaction curves without the influence of boundary. For the current testing mode, where the module moves laterally in the soil, the ratio of the moving object to soil bed dimensions for no boundary effects is unprecedented. A simple and innovative method of using periodic boundaries is proposed in the present study. Conventionally, periodic boundaries replicate the representative volume element infinitely at the boundaries. Due to the presence of ROBOCONE in the centre of the model, the required extent reduces to half in X and Y directions, as schematically shown in Figure 1b. This paper discusses the model side to ROBOCONE diameter, L/DRC, (e.g., 9, 18, 30) influence on the P-Y curves (at a given stress level). The effects of random particle generations for different models in PFC software can be minimized using same central core zone of contacts with ROBOCONE for each simulation, while the extent of model side is increased. ROBOCONE wished-in-place rather than driving/jacking in the model reduces the DEM computation costs. Limited amount of lateral displacement of the ROBOCONE (around 2.5mm) is sufficient to establish the effectiveness of the proposed modelling. Optimum loading rate (the lateral velocity of P-Y module) of around 0.8mm/s, which can maintain the inertial number lower than the threshold value of 7.9x10-5[4] further helps the computational efficiency.
 Once the optimum L/DRC value is obtained, the quantitative outcomes from these simulations are the lateral soil reaction curves at different stress levels and also the bending moments on the ROBOCONE P-Y Module. Though the ROBOCONE model in these simulations is a rigid wall and doesn’t mobilize in bending, the moments calculated from the magnitude and locations of each contact force on the P-Y module provide an estimate of bending stresses generated on the module. These results provide a basis for developing an understanding of the physical capacity of the robotic device in sustaining the lateral forces. Figure 1(c) shows preliminary results of soil reaction curve from the P-Y module at 500kPa stress level for L/DRC value of 9. The changes in the contact forces during the P-Y module motion can be qualitatively assessed through the changes in the force chain network (Figure 1(d)). Figure 1(e) shows a representative plot of bending moments calculated at different lateral movement stages of the P-Y module, and the maximum value at a desired displacement of the P-Y module suggests the physical design requirements of the ROBOCONE. The results from these DEM simulations would directly inform the design of the robotic device for ground investigation and hence improve the economical offshore foundation designs.

A rapid method to predict biaxial fatigue life of automotive wheels using proper orthogonal decomposition and radial basis function algorithm

This paper presents a rapid method for predicting the biaxial fatigue life of automotive wheels using a combination of proper orthogonal decomposition and radial basis function algorithm. Currently, numerical simulations of biaxial fatigue tests are being developed to evaluate wheel performance. However, these simulations are computationally expensive due to the need to simulate multiple discrete loading cases within a given biaxial spectrum. To address this issue, we propose a novel approach that utilizes proper orthogonal decomposition and radial basis function algorithm to improve computational efficiency. By leveraging high-fidelity simulation results from a small number of loading cases, a reduced order model is constructed to accurately predict the tire-rim interface force fields required for wheel strength calculations. The reduced order model significantly reduces the computational time by 65.4% for simulating all loading cases, while maintaining a maximum predicted error of less than 2% compared to the high-fidelity model. Subsequently, the predicted interface forces are mapped onto the rim surface for strength calculation, and the wheel fatigue life is determined using the Brown-Miller multiaxial damage criterion. Comparative analysis with experimental results demonstrates the desirable accuracy of our method in simulating the stress-strain history, crack initiation position, and minimum fatigue life of the wheel. Overall, the proposed method offers a powerful tool for the rapid fatigue analysis of spectrum-loaded wheels, providing an efficient and accurate means of predicting biaxial fatigue life.

Experimental Investigation on the Post-liquefaction Behavior of Sands in Simple Shear Conditions

Abstract Experimental evidence shows that earthquake induced liquefaction can occur more than once in sandy soils. Moreover, despite an increase in soil density caused by the dissipation of the excess pore pressure induced by earthquakes, the liquefaction resistance of soils that have experienced liquefaction may be lower than that of virgin soils. This paper offers insight into this topic starting from the analysis of the undrained monotonic behavior of post-liquefied sands by means of tests performed with a simple shear cell equipped with flexible boundaries, which maintains a constant diameter to guarantee the “K0-condition.” The control system of cyclic, reconsolidation, and monotonic phases is described in detail. The experimental results show that neither the relative density, effective confining stress, cyclic stress ratio, nor the direction of shear strain play important roles in the monotonic behavior of post-liquefied soils. Moreover, the comparison between the monotonic response of virgin and post-liquefied soils (prepared by moist tamping technique) shows that it is not affected by the stress–strain history experienced by soils. It can be explained through a microstructural interpretation. According to which, the initial soil fabric generated with the moist tamping method and that formed during liquefaction remain almost unchanged because of the rotation of principal stress directions occurring during simple shear tests. A further confirmation is given by the results of tests performed on specimens prepared by air pluviation method.

Behaviour of high strength steel butt joints exposed to arctic low temperatures

The present study investigated the tensile behaviour of high strength steel (HSS) butt joints exposed to arctic low temperatures. Two HSS grades, namely Q890 and Q960, with thicknesses of 6 mm and 8 mm, were employed. Single-V-groove butt joint specimens were fabricated by means of robotic gas metal arc welding using 1.2 mm-diameter ER100S-G and ER120S-G feedstock wires. The single-V-groove and butt joint tensile specimens were designed according to the Structural Welding Code-Steel AWS D1.1. Metallographic observations and Vickers hardness tests were performed on five series of HSS butt joint. Moreover, a total of 25 butt joint specimens were tested under uniaxial tension in a liquid nitrogen cooling chamber, with temperatures ranging from -75 °C to 25 °C. The stress-strain histories, tensile strengths and failure modes of the HSS butt joints exposed to arctic low temperatures are fully reported. The influence of low temperature exposure, parent steel grade and weld material selection on the tensile behaviour of the HSS butt joints is discussed. Based on the test results, recommendations for weld material matching for Q890 and Q960 HSS butt joints are provided. Additionally, prediction equations for HSS butt joint strengths exposed to arctic low temperatures are proposed using the best subset regression analysis. The findings from this study contribute to a better understanding of the behaviour of HSS butt joints under arctic low temperatures, and can be used to inform design and fabrication practices of HSS structures in cold environments.

Anisotropic plasticity and fracture modelling of cold rolled AA5754

The effect of the anisotropy, the loading rate and stress state on the ductile fracture of AA5754 cold-rolled sheets has been investigated. The plasticity has been analysed experimentally testing biaxial and uniaxial tensile specimens machined at several orientations with respect to the rolling direction and calibrating the Yld2000-3D yield criterion. The strain rate dependency of the material has been evaluated testing notched tension specimens at different loading rates. The resulting force–displacement curves, showing negative loading rate sensitivity, have been employed to identify the work hardening constants of a modified isotropic rate-dependent law through inverse modelling. Finite element simulations of the experiments on central hole, notched, shear and biaxial specimens conducted at several loading rates and orientations, monitored with three-dimensional digital image correlation, have been performed to obtain the Cauchy stress and equivalent plastic strain histories from the critical elements located where fracture is observed in such experiments. The anisotropic Hosford-Coulomb fracture initiation criterion has been modified to accommodate the loading rate effects giving reasonable failure predictions.

Modelling of ductile failure in materials with heterogeneous particle structure using localization analysis

In this study, we evaluate the use of strain localization analysis based on the imperfection band approach to model the effect of heterogeneous particle distribution on the ductile failure of structural metallic materials. To this end, tensile tests are performed on smooth and notched samples made of aluminium alloy AA6110 with an equiaxed grain structure and an inhomogeneous distribution of the constituent particles. The constituent particles are assumed to be the main contributors to the ductile failure of the alloy. By varying the heat treatment of the alloy, three different materials are considered with different strength, work hardening, and ductility, while the grain structure and constituent particle distribution are unaltered. Finite element simulations of the tension tests are conducted using both metal and porous plasticity models and the stress–strain histories of the elements in the minimum section of the specimen are used in strain localization analyses. In these analyses, ductile failure is assumed to occur when the strain rate inside a thin, planar imperfection band becomes infinite. The imperfection band is modelled with porous plasticity, using either a higher initial porosity than in the bulk material or by adding void nucleation. It is found that the ductility of the materials is most accurately captured by modelling the imperfection band by stress-enhanced nucleation.

Multi-stage creep behavior of frozen granular soils: experimental evidence and constitutive modeling

The significance of ground freezing is becoming ever more germane as the design of new urban tunneling systems requires more complex geometries and higher bearing capacities, which are limited with conventional construction methods. Ground freezing is an advanced construction technique to make the water-saturated subsoil impermeable and temporarily increase its strength and stiffness. This study reports experimental investigations consisting of single-stage and multi-stage creep tests under uniaxial loading. The comparison of the different loading types reveals the influence of the stress–strain history on the rate- and temperature-dependent behavior of frozen granular soils. We extend the constitutive model for frozen soils proposed by Cudmani et al. (2022, Géotechnique, doi:10.1680/jgeot.21.00012) to consider stepwise loading and creep by coupling creep time with stress–strain history. Moreover, we simulate element tests and compare the simulations with our own experimental data as well as data from the literature to achieve the first step in validating the extended model. The good agreement of the numerical and experimental results confirms the constitutive model’s ability to capture the main features of the complex mechanical behavior of frozen granular soils for single-stage as well as multi-stage loading under constant temperatures.

Compression-After-Impact analysis of carbon/epoxy and glass/epoxy hybrid composite laminate with different ply orientation sequences

This article is a continuation study of the LVI and CAI response of the plain carbon fiber composite laminates and explores the experimental & numerical study of the post-impact damage propagation in carbon/epoxy and glass/epoxy hybrid laminate under compression. LVI (Low-Velocity Impact) tests are first carried out at three different energy levels, and the same specimens are put under a displacement-controlled compression test. The damage initiation and propagation in both impact tests and compression tests are studied using X-ray computed tomography. The results matched well with the results obtained from the finite element model, simulated using ABAQUS/explicit. The effect of the changing impact energy and the influence of the stacking orientation on post-impact damage propagation is studied. The strain maps during the compression tests are studied through Digital Image Correlation (DIC) technique based on which the stress–strain histories are plotted by using well-calibrated strain values. Upon completing the study, it is observed that the CAI (Compression-After-Impact) strength depends on the factors like properties of the constituent material, ply orientations and stacking sequence of the laminate. It is observed that even when the damage is barely visible on the surface of the specimen, compressive strength has been reduced drastically. The observed mode of damage propagation in CAI testing is buckling of the sub-laminates in the vicinity of the damage zone formed by the impact event. The size of the impact damage zone depends on the impact energy, which actually governs the CAI strength.

Numerical simulation for the tensile failure of randomly oriented short fiber reinforced plastics based on a viscoelastic entropy damage criterion

In this study, we conduct a numerical simulation of the tensile failure of randomly oriented short fiber reinforced plastics (SFRP) by integrating an entropy damage criterion. The Mori-Tanaka theory is used as the model for the composite. The constitutive equation of the resin is defined by a conventional viscoelastic model with a power law considering entropy damage. The dissipated energy is calculated using this constitutive equation and the stress-strain history. Dividing the dissipated energy by the absolute temperature yields an entropy value, which is related to material damage that results in a decrease in the elastic moduli. This deteriorating behavior of the resin is integrated into the Mori-Tanaka theory. The tensile failure simulation is implemented at nine angles ranging from 5° to 85° in 10° increments to determine the relationship between the tensile strengths and off-axis angles. These results are introduced into the layer wise method (LWM) and superimposed to obtain the tensile strength of randomly arranged short fiber reinforced plastics. The calculations are performed by varying the strain rate. The numerical results are in good agreement with the experimental results. Thus, the originality of this study is the introduction of a viscoelastic entropy damage criterion into a conventional micromechanical model.

Deformation of the European Plate (58-0 Ma): Evidence from Calcite Twinning Strains

We present a data set of calcite twinning strain results (n = 209 samples; 9919 measured calcite twins) from the internal Alpine nappes northwestward across the Alps and Alpine foreland to the older extensional margin along the Atlantic coast in Ireland. Along the coast of Northern Ireland, Cretaceous chalks and Tertiary basalts are cross-cut by calcite veins and offset by calcite-filled normal and strike-slip faults. Both Irish sample suites (n = 16 with four U-Pb vein calcite ages between 70–42 Ma) record a sub-horizontal SW-NE shortening strain with vertical extension and no strain overprint. This sub-horizontal shortening is parallel to the margin of the opening of the Atlantic Ocean (~58 Ma), and this penetrative fabric is only observed ~100 km inboard of the margin to the southeast. The younger, collisional Alpine orogen (~40 Ma) imparted a stress–strain regime dominated by SE-NW sub-horizontal shortening ~1200 km northwest from the Alps preserved in Mesozoic limestones and calcite veins (n = 32) in France, Germany and Britain. This layer-parallel shortening strain (−3.4%, 5% negative expected values) is preserved across the foreland in the plane of Alpine thrust shortening (SE-NW) along with numerous outcrop-scale contractional structures (i.e., folds, thrust faults). Calcite veins were observed in the Alpine foreland in numerous orientations and include both a SE-NW layer-parallel shortening fabric (n = 11) and a sub-vertical NE-SW vein-parallel shortening fabric (n = 4). Alpine foreland strains are compared with twinning strains from the frontal Jura Mountains (n = 9; layer-parallel shortening), the Molasse basin (n = 26; layer-parallel and layer-normal shortening), Pre-Alp nappes (n = 39; layer-parallel and layer-normal shortening), Helvetic and Penninic nappes (Penninic klippe; n = 46; layer-parallel and layer-normal shortening plus four striated U-Pb calcite vein ages ~24 Ma) and calcsilicates from the internal Tauern window (n = 4; layer-normal shortening). We provide a chronology of the stress–strain history of the European plate from 58 Ma through the Alpine orogen.

Optimizing the Voce–Chaboche Model Parameters for Fatigue Life Estimation of Welded Joints in High-Strength Marine Structures

This work studies the Voce–Chaboche (V–C) material model parameter optimization for high-strength steel welded joints subjected to cyclic loading. The model parameters of each material zone in a S690 steel butt-welded joint were determined using an optimization algorithm based on the Newton trust region (NTR) method and an accumulated true strain parameter. The model parameters were fitted to stress–strain histories from uniaxial strain-controlled cyclic tests. To validate the model, fully-reversed variable amplitude fatigue experiments were performed under load control. The experimental results were then compared to numerical results from a finite element analysis. When the elastic modulus is optimized as a V–C parameter, the results indicate that the V–C model slightly underestimates the strain range, leading to conservative fatigue life estimates. However, the results can be improved by using an elastic modulus obtained experimentally. In this case, the resulting material model slightly overestimates the strain range, leading to a non-conservative, but more accurate, fatigue life estimation. It can be concluded that the NTR-based accumulated true strain approach successfully determined the V–C model parameters for different material zones in the welded joint, and closely estimated the strain range and the fatigue life for a variable amplitude load history.

Evolution of Lüders banding under axial loading and reverse loading

Low slenderness steel rods that exhibit Lüders banding are used to examine the evolution of localized deformation when a specimen is unloaded and reverse loaded. Special attention is given to how a specimen behaves when the initial loading is interrupted with part of it Lüders deformed and the rest still elastic. Digital image correlation revealed that both parts unload elastically, but on reverse loading the previously elastic zone yields and develops Lüders strain of the opposite sign. By contrast, the plastically deformed zone follows the usual Bauschinger rounding. Thus, the specimen behaves as a structure with its overall response being a combination of the two distinctly different stress-strain histories, and depends on the length of each zone. The experiments are simulated using finite element analysis coupled to a custom combined isotropic-kinematic hardening plasticity model with a partially softening branch spanning the Lüders stress plateau. The analysis reproduces the measured responses and evolution of localized deformation quite accurately demonstrating the fidelity of the constitutive model adopted. The results also demonstrate that the propagation of the Lüders zone has a more complex three-dimensional profile than that reported for thin strips. Overall, the study provides a guide on how Lüders banding can be treated in structures that undergo loading and reverse loading.

Multiaxial thermo-mechanical fatigue life prediction based on notch local stress-strain estimation considering temperature change

The modified incremental multiaxial Neuber rule by considering the temperature change was firstly proposed, which combined with the Chaboche viscoplastic constitutive model to approximately compute the notch local stress and strain under multiaxial thermo-mechanical loading. Based on the proposed method, a life prediction method that can take into account fatigue, oxidation, and creep damage was developed, which can quickly predict notch fatigue failure lifetime under variable amplitude multiaxial thermo-mechanical loading. The proposed method was verified by the coupled thermal-structural nonlinear FEA and the experimental life results for the tenon joint structure specimens under eight thermo-mechanical loading paths. The comparison of computed and analyzed stress-strain history showed that a good agreement can be obtained. Comparing with experimental life data, all predicted lives fall in a 2-factor scatter band.

Damage of non-steam-cured UHPC under axial compression with and without short-term sustained loading history

The damage of non-steam-cured ultra-high performance concrete (UHPC) under short-term sustained compression was experimentally investigated. The UHPC specimens were tested under monotonic loading, cyclic loading without a sustained loading history, or cyclic loading with a sustained loading history. The influence of the sustained stress levels and loading age of UHPC was elucidated. The stress-strain history of the specimens was measured. The effect of damage due to the short-term sustained loading history on the mechanical properties of UHPC was analyzed. The fraction of the sustained strains contributing to the damage was calibrated. The micro-cracking propagation within the UHPC is imagined based on non-destructive testing using an ultrasonic wave method and by assessing the nominal Poisson’s ratio. The interface between steel fibers and matrix was observed by Scanning Electron Microscope (SEM). The damage occurred for the UHPC specimens under short-term sustained compression when the sustained stress was higher than 0.7fc′. Both the compressive strength and the elastic modulus of the specimens were reduced. When the sustained stress was greater than or equal to 0.8fc′, the elastic modulus of the UHPC specimens measured during the reloading phase was considerably reduced. The specimen group subjected to a sustained stress of 0.9fc′ with a duration of 128 s experienced a reduction of about 30% in elastic modulus. The damage is statistically independent of the loading age of the UHPC. Damage is characterized by micro-cracking propagation within the UHPC specimens. Microstructural analysis of the interface between steel fibers and matrix shows that under high sustained stress, the bond between the steel fibers and the concrete matrix was impaired by the creep effect and could not prevent the propagation of micro-cracks.

Finite element numerical simulation of a cable‐stayed bridge construction through the progressive cantilever method

AbstractCable‐stayed structures have become popular worldwide as one of the main alternatives for bridge designs with spans from 200 to 1000 m. Due to the dimensions of the structural elements that typically compose such bridges and their nonlinear structural behavior, it is usually employed in the designing phases an analysis that considers the executive process to predict the stay‐cables prestress and the deck elevation. Other relevant factor to consider in numerical simulations is the materials viscous behavior since they have an important role in deflection responses. This study simulates numerically, using the Finite Element Method (FEM), the phases of a prestressed concrete cable‐stayed bridge construction by the progressive cantilever method. The ANSYS software, version 19.2, is customized in its USERMAT3D subroutine, which is used to introduce the viscoelastic models for creep and shrinkage, and the cracking of concrete into the main software. The FEM model simulates the pylon, deck, and stay‐cables of the bridge. The vertical displacements of the deck and the history of forces in the stay‐cables are compared with the measurements taken during a real bridge construction showing good agreement. Horizontal displacements of the pylon and the stress–strain history in the concrete with the construction sequence are also presented, showing good agreement as well even with the added viscous effects.

The Role of Temperature in the Stress–Strain Evolution of Alpine Rock-Slopes: Thermo-Mechanical Modelling of the Cimaganda Rockslide

AbstractIn this work, the thermo-mechanical stress–strain history of an Alpine slope is analyzed, with particular focus on the historical Cimaganda large landslide (Sondrio Province, Italy), which mobilized an estimated volume of 7.5 mm3 of rock material. Accurate geomorphological and geomechanical characterization involving field surveys and laboratory testing was carried out, leading to the development of a conceptual model of the slope. A thermo-mechanical semi-coupled approach was developed, considering both glacial debuttressing and thermo-mechanical effects due to gradual exposure of the slope to atmospheric conditions and paleo-temperature redistribution resulting from the Last Glacial Maximum deglaciation. A 2D distinct-element numerical approach was adopted, supported by a 2D finite-element analysis to simulate heat diffusion over the Valley cross-section. Modelling results allow to simulate the general evolution of the Cimaganda rock-slope and to highlight the significance of thermal processes in preparing rock-slope instabilities. While the mechanical effect of ice thickness reduction alone brings about moderate rock mass damage, the introduction of temperature couplings results in a substantial increase of damage, representing a significant factor controlling the stress–strain evolution of the slope. Simulated displacement and the development of a deep region of shear strain localization at a depth roughly corresponding to that of the detected Cimaganda sliding surface, allow to highlight the significance of temperature influence in preparing the rock-slope to instability.