Degradation Of Elastic Modulus Research Articles

Durability of concrete-encapsulated GFRP bars subjected to seawater immersion

Glass fiber reinforce polymer (GFRP) bars have better durability than traditional steel bars, but there are few studies on their long-term performance in actual service environments especially the marine environment. To investigate the durability of GFRP bars used in cross-sea bridges, this research delves into the performance of 132 such bars, spanning diameters of 10, 12, 16, and 25 mm, in an environment that closely mimics real-world condition. The GFRP bars, encapsulated in concrete, were submerged in seawater solution, maintained at either room temperature or 60°C, for periods spanning from 0 to 183 days. Then the tensile strength and elastic modulus degradation of GFRP bars were investigated by tensile tests. Scanning electron microscope (SEM) was also used to analyze the micro-degradation mechanism of GFRP bars. In addition, this paper compares the four existing models for predicting long-term mechanical properties of GFRP bars with the experimental data, so as to determine the formulas and methods to be used in the design. The result shows that the elastic modulus of GFRP bars after 183 days of immersion in concrete pore solution was almost unchanged while the tensile strength decreased significantly. Among them, the tensile strength of 10 mm diameter GFRP bars decreased by 41.52 %, after 183 days of immersion at high temperature (60°C). The SEM method revealed that the main cause of such phenomena was the weakening of the bond between the fibers and matrix, which led to a decrease in the interface property. Finally, by fitting and comparing the relationship between the test data and the existing prediction models for the long-term mechanical properties of GFRP bars, the residual tensile strength of GFRP bars used in the actual project of Yanzhou Bridge for 100 years was predicted according to the optimal prediction model, i.e. the Serb model, and the results showed that the residual strength of GFRP bars is able to meet the current standard requirement.

High-temperature dimensional stability and radiation behavior of high-entropy rare earth tungstate ceramics

Materials with high-temperature dimensional stability and resistance to radiation damage are receiving increasing attention in advanced nuclear energy systems. In this research, (Sm0.2Eu0.2Gd0.2Dy0.2A0.2)2W3O12-type (A = Y, Tm,Ho) high-entropy ceramics is presented. The coefficients of thermal expansion of (Sm0.2Eu0.2Gd0.2Dy0.2Y0.2)2W3O12 (HE-RE2W3O12) is 7.21 × 10−6 K−1 at 300–1100K, which is the lowest among the three synthesized high-entropy ceramics and indicates good dimensional stability at high temperatures. After irradiation with Kr+ fluence of 4.58 × 1016 ions/cm2 at 400 °C, both HE-RE2W3O12 and Gd2W3O12 maintained crystalline, but the lattice volume expansion rate of HE-RE2W3O12 (0.71 %) is much lower than that of Gd2W3O12 (1.49 %). Additionally, in the post-irradiated HE-RE2W3O12, the Raman spectra vibration peak remained unchanged and no radiation-induced segregation was observed. Nanoindentation tests displayed that the degradation of hardness and elastic modulus in HE-RE2W3O12 is rarely degrade. Overall, these results provide new insights for research on advanced nuclear materials in the future.

Flexural fatigue properties of concrete based on different replacement percentage of natural sand with manufactured sand

Compression and flexural fatigue tests were conducted on concrete specimens with varying strengths under different replacement percentages of natural river sand (R-sand) with manufactured sand (M-sand) at 0%, 30%, 70%, and 100%. The objective was to compare and analyse differences in fatigue damage of concrete across different replacement percentages. Findings were made that when the replacement percentage of M-sand was 30% and 70%, the concrete exhibited higher fatigue life, irrespective of any variations in compressive strength. The fatigue life of concrete with a compressive strength of 30 MPa could be increased by an average of 20%–30% compared to concrete with a compressive strength of 50 MPa, and this increasing trend reached its peak when the replacement percentage of M-sand was in the range of 30%–70%. In order to enhance the prediction of fatigue life for concrete subjected to varying replacement percentages of natural sand with M-sand and to elucidate the evolution process of fatigue damage in M-sand concrete, dynamic strain gauges and DIC methods were employed. This enabled determination of the cycle-varying maximum strain and residual strain during the fatigue process across different replacement percentages. The findings indicate that the addition of M-sand can significantly delay the degradation of dynamic elastic modulus (E), with this effect being enhanced by over 15%. The fatigue damage was defined based on fatigue deformation modulus; therefore, a new fatigue damage model was established. It was observed that this new model improved accuracy by up to 6.1% compared to the traditional fitted model in this test. Finally, the effects of different replacement percentages of natural sand with M-sand on the brittle transition of concrete were analysed at the microscopic level. This analysis involved a combination of microhardness testing and examination of the interfacial transition zone and hydride map of concrete under a scanning electron microscope across different replacement percentages of natural sand with M-sand.

Virtual laboratory enabled constitutive modelling of dual phase steels

Accurate and efficient characterization and modelling of deformation responses are critically essential in the development of advanced metal forming processes. This work presents a virtual laboratory enabled constitutive framework for modelling complex deformation responses of dual phase (DP) steels under complex stress states, based on multi-phase full-field crystal plasticity (CP) and advanced phenomenological modelling. In the CP based virtual testing method, kinematic hardening and degradation of elastic modulus are modelled in particular to improve the capability for describing evolving microstructure induced mechanical responses of DP steels. The workflow of this framework is completely built and numerically implemented, including (i) representative volume element (RVE) generation based on microstructure characterization, (ii) identification of multiphase CP model parameters, (iii) prediction of elastic modulus degradation, (iv) prediction of yield stresses and plastic potentials in uniaxial tension and biaxial tension with various stress ratios, and (v) prediction of stress–strain curves in reverse tension compression. Using DP780 and DP980 as case materials, the corresponding physical experiments are conducted to verify the accuracy of the virtual tests, showing a good agreement between the virtual and experimental approaches. Both experimental and virtual tests are used to calibrate advanced phenomenological constitutive models that include non-linear elasticity, anisotropic yielding, and kinematic hardening. The calibrated models are implemented into the finite element (FE) codes to predict complex deformation and springback behaviours of DP780 and DP980 sheets in U-bending processes. In comparison with forming experiments of U-channel parts, the virtually calibrated models are validated and show good performance in predicting deformation and springback behaviours, providing a high capability for process analysis. The findings support that the virtual laboratory enabled modelling approach could be a substitute for extensive, expensive, and hard-to-access physically mechanical experiments required in the model calibration for a more effective and efficient analysis of metal forming processes.

Multi-Scale Analysis of the Damage Evolution of Coal Gangue Coarse Aggregate Concrete after Freeze-Thaw Cycle Based on CT Technology.

This study utilized X-ray computed tomography (CT) technology to analyze the meso-structure of concrete at different replacement rates, using a coal gangue coarse aggregate, after experiencing various freeze-thaw cycles (F-Ts). A predictive model for the degradation of the elastic modulus of Coal Gangue coarse aggregate Concrete (CGC), based on mesoscopic damage, was established to provide an interpretation of the macroscopic mechanical behavior of CGC after F-Ts damage at a mesoscopic scale. It was found that after F-Ts, the compressive strength of concrete, with coal gangue replacement rates of 30%, 60%, and 100%, respectively, decreased by 33.76%, 34.89%, and 42.05% compared with unfrozen specimens. The results indicate that an increase in the coal gangue replacement rate exacerbates the degradation of concrete performance during the F-Ts process. Furthermore, the established predictive formula for elastic modulus degradation closely matches the experimental data, offering a reliable theoretical basis for the durability design of CGC in F-Ts environments.

Experimental study and constitutive modelling of wire arc additively manufactured steel under cyclic loading

Wire arc additive manufacturing (WAAM) is an efficient and cost-effective method of metal 3D printing that is well suited to structural engineering applications. Fundamental test data on the mechanical properties of WAAM materials, especially under cyclic loading, are however lacking. To address this, an experimental study into the cyclic behaviour of WAAM steel plates has been undertaken and is reported herein. Following geometric and quasi-static mechanical characterisation, a total of 40 as-built and machined coupons were tested under different cyclic loading protocols. Key results from the tests, including the full hysteresis curves, are presented and discussed. A cyclic constitutive model allowing for a yield plateau and the degradation of elastic modulus is proposed and validated against the test data. It is shown that the examined WAAM steel exhibited cyclic hardening behaviour dependent on the strain amplitude and strain history. The elastic modulus of the WAAM material was found to decrease in the first few cycles and then remain stable with increasing accumulated plastic strain. Good energy dissipation performance was also observed, indicating the potential of WAAM steels for seismic applications.

Strain-dependent-deformation property of Gyeongju compacted bentonite buffer material for engineered barrier system

This study aims to investigate the strain-dependent-deformation property of Gyeongju bentonite buffer material. A series of unconfined compressive tests were performed with cylindrical specimens prepared at varying dry densities (ρd = 1.58 g/cm3 to 1.74 g/cm3) using cold isostatic pressing technique. It is found that as ρd increase, the unconfined compressive strength (qu), failure strain, and elastic modulus (E) of Gyeongju compacted bentonite (GCB) increases. Normalized elastic modulus (Esec/Emax) degradation curves of GCB specimens are fitted using Ramberg-Osgood model and the elastic threshold strain (εe,th) is determined through the fitted curves. The strain-dependency of E and Poisson's ratio (v) of GCB were observed. E and v were measured constant below εe,th of 0.14 %. Then, E decreases while v increases after exceeding the strain threshold. The Esec/Emax degradation curves of GCB in this study suggests wider linear range and higher linearity than those of sedimentary clay in previous study. On top of that, the influence of ρd is observed on Esec/Emax degradation curves of GCB, showing a slight increase in εe,th with increase in ρd. Furthermore, an empirical model of qu with ρd and a correlation model between qu and E are proposed for Gyeongju bentonite buffer materials.

Progressive failure characteristics and water-induced deterioration mechanism of fissured sandstone under water–rock interaction

Stability of engineered rock mass can be significantly affected by the combined action of fissures and water. In this study, uniaxial compression tests of fissured sandstone under natural state and water-chemical environments were conducted, accompanied by acoustic emission and digital image correlation to monitor the whole failure process. The results indicate that the strength and elastic modulus of fissured sandstone in the saturated state were lower than those in the natural state. The inside structural changing of dried specimen after water–chemical environments immersion had a weak influence on the strength and elastic modulus, but the peak strain and yield deformation would significantly increase. Characteristic stress determination methods were identified applicable to fissured sandstones under water–rock interactions. The σci/σf (the ratio of crack initiation stress to peak strength) could be largely degraded by the presence of water, while the impact on σcd/σf (the ratio of crack damage stress to peak strength) was weak. The structural changing could also significantly reduce σci/σf. However, only the σcd/σf ratio in the acid water environment exhibited the obvious reduction. The presence of water could also influence the crack initiation and failure mode of fissured sandstone. After a short period of immersion (45 d), the strength and elastic modulus degradation of fissured sandstone was mainly caused by the water–rock physical reaction, while the change in the peak strain was jointly affected by the water–rock chemical and water–rock mechanical reactions. The σci/σf ratio was affected by the water–rock chemical and water–rock mechanical reactions, while the σcd/σf ratio was affected only by the water–rock chemical reaction. Moreover, the water–rock mechanical reaction significantly changed the initiation mode of fissured sandstone. The water-induced deterioration mechanism of fissured sandstone was lastly revealed by these results, which could provide an important reference for evaluating the stability of water-rich engineered rock mass.

Experimental investigation on cyclic behaviour of wire arc additively manufactured carbon steel

AbstractWire arc additive manufacturing (WAAM) is a metal 3D printing technique that has attracted significant interest from the construction industry for its ability to efficiently and rapidly build large‐scale component. However, there is currently limited information on the cyclic performance of WAAM metal materials. To address this gap, an experimental investigation was conducted to study the cyclic behaviour of WAAM carbon steel. The investigation involved the testing of forty specimens with two nominal thicknesses and two surface finishes under ten cyclic loading protocols. Following this, the failure modes, cyclic performance, and elastic modulus degradation were analysed. The results indicate that WAAM carbon steel exhibits excellent ductility under cyclic loading. The phenomenon of Bauschinger effect, cyclic hardening and elastic modulus degradation were observed in the experiment. The cyclic performance of WAAM carbon steel was found to depend on strain amplitude and strain history.

Aging deterioration of mechanical properties on coal-rock combinations considering hydro-chemical corrosion

Coal-rock masses located in water-rich environments, such as heavy water mines, and water-isolated coal-rock pillars installed underground, are consistently subjected to erosion from acidic groundwater. In this study, we employed various techniques including acoustic emission (AE), digital image correlation (DIC), scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), and X-ray diffraction (XRD) to investigate the evolution of mechanical properties and AE damage characteristics of coal-rock combinations under water-chemical corrosion. Moreover, we analyzed the microscopic deterioration mechanism. Our results reveal that as the acidity of the solution increases, the cumulative energy and AE count of the specimen decrease to varying degrees. The AE ringing counting process can be categorized into stable, active, and sharp development stages. Moreover, as the pH value of the solution decreases, the AE b-value follows a “V" type variation. The dynamic b-values demonstrate localized sudden drops, indicating the occurrence of large-scale cracks. The internal cracks within the coal-rock combination transform from shear cracks to tensile cracks, corresponding to a transition in failure mode from shear failure to cleavage failure. Furthermore, a detailed analysis of the deterioration mechanism of the coal-rock combination under water-rock interaction was conducted. Under on-site groundwater conditions (pH = 6.24), the overall degradation of uniaxial compressive strength (UCS) and elastic modulus reached 13.37% and 28.73%, respectively. The physical, chemical, and mechanical effects of the water-chemical solution induce microstructural changes within the coal-rock combination. Consequently, the total percentage of macropores (>1 μm) in the sandstone section increased by 8.85%, reaching 35.3%, thereby further deteriorating its mechanical properties. In contrast, the proportion of macropores in the coal section only increased by 0.95%. Therefore, water-rock action has a more severe damaging impact on the sandstone section compared to the coal section of the combination.

A damage-coupled viscoplastic constitutive model considering tension-compression asymmetry for modeling low cycle fatigue mechanical behaviors of Inconel 617 at elevated temperature

Design and analysis based on the advanced constitutive models capable of simulating the complex mechanical behaviors of materials would improve the operational reliability and safety of high-temperature engineering components that experience thermo-mechanical fatigue loading in aerospace and power industries. Inspired by the experimental observations of a set of low cycle fatigue tests of nickel-based superalloy Inconel 617 at 700 °C, a damage-coupled unified viscoplastic constitutive model considering tension-compression asymmetry is proposed. For a better description of the hysteresis loop shape, the cyclic hardening behavior is modeled by the kinematic hardening rule. The damage is measured by the degradation of elastic modulus, and a low cycle fatigue damage model is employed to describe the cyclic softening behavior and predict the fatigue life. Due to the similarity in the evolution of damage and compressive hardening ratio, a compressive hardening factor, which is a function of damage, is incorporated into the kinematic hardening model to simulate the tension-compression asymmetry, i.e. the mean stress evolution. The strain-range dependence of cyclic hardening and tension-compression asymmetry is also considered. The simulation results show that the proposed model can effectively model the low cycle fatigue mechanical behaviors of the material at elevated temperatures.

Comparison of axial behavior of RC columns using alkali-activated slag-based concrete and OPC concrete during and after fire

As no study on fire behavior of RC columns using alkali-activated concrete has been reported so far, this paper presents an experimental program comparatively investigating the thermal response and structural behavior of reinforced columns using alkali-activated slag-based concrete (AAC) and Portland cement-based concrete (PCC) during and after subjecting to the standard fire curve, with concrete strength and fire duration as test parameters. The experimental and analytical studies showed that compared with PCC columns, AAC columns had slightly lower temperature field but faster increase of axial deformation during fire. AAC and PCC columns using normal-strength concrete had equivalent residual axial capacity after exposure to fire. However, AAC columns using high-strength concrete retained greater load-bearing capacity than their PCC counterparts due to the spalling of high-strength PCC columns. AAC columns had lower axial stiffness than PCC columns at ambient temperature due to lower elastic modulus of AAC, but they had similar axial stiffness after exposure to fire of 2 h since PCC underwent more degradation of elastic modulus under high temperature. Furthermore, as concrete strength and fire duration increased, the axial contraction under heating and axial capacity loss after heating of AAC columns increased. A numerical method based on finite difference was used to predict the temperature field and corresponding residual axial capacity of RC columns after the fire.

Approximating Dynamic Elastic Modulus of Concrete for an Old Aqueduct Using Dynamic Tests and BP Neural Network

Monitoring the degradation of the dynamic elastic modulus (Ed) of concrete is of great importance to track the durability deterioration for hydraulic concrete structures. For the aqueduct under investigation in this study, the dynamic elastic modulus of bent frames and moment frame supports (Ed-frame), the dynamic elastic modulus of arch trusses (Ed-arch) and the shear stiffnesses of the asphaltic bearings of U-shaped flumes (Kflume) are the main parameters to define the dynamic behavior of the structure, which have direct correlation with its vibrational characteristics and thus practicably can be estimated by a BP (back-propagation) neural network using modal frequencies as inputs. Since it is impossible to obtain sufficient experimental field data to train the network, a full-scale 3D FE model of the entire aqueduct is created, and modal analyses under different combinations of Kflume, Ed-arch and Ed-frame are conducted to generate the analytical dataset for the network. After the network’s architecture is refined by the cross-validation process and its modeling accuracy verified by the test procedure, the primary modal frequencies of the aqueduct obtained from in situ dynamic tests are put into the network to obtain the final approximations for Kflume, Ed-arch and Ed-frame, which sets an evaluation baseline of the general concrete Ed status for the aqueduct and indicates that the makeshift asphaltic bearings of U-shaped flumes basically can be treated as a three-directional hinge in the FE model. It is also found that more inputs of modal frequencies can improve the prediction accuracy of the BP neural network.

Stiffness degradation analysis of reinforced concrete beams under corrosion-fatigue coupling action based on experimental research

In order to study the effect of corrosion-fatigue coupling action on the stiffness degradation of reinforced concrete beams, a novel corrosion-fatigue coupling experimental device is designed, eleven reinforced concrete T-shaped test beams are made, in which a set of static load test, then seven sets of simple fatigue tests under different stress amplitudes and three sets of corrosion-fatigue coupling tests under different stress amplitudes are conducted respectively. The test phenomenon, load-deflection relationship, and fatigue life of the simple fatigue test beams and the corrosion-fatigue coupling test beams are compared. The test results show that the deflection development of the corrosion-fatigue coupling test beams includes three stages of sudden increase-stable-sudden increase, as the same as the simple fatigue beams. The deflection growth rate of the corrosion-fatigue coupling beams in the second stage is greater than that of the simple fatigue beams. Fatigue cyclic loading significantly increases the residual deflection of the test beams compared to the static load test beam. Then a new calculation method of the midspan deflection of the simple fatigue beam and the corrosion-fatigue coupling beam is developed according to the measured deflection data and the fatigue life test results. In this method the influence factors such as fatigue loading times, fatigue stress amplitude, concrete elastic modulus degradation, steel corrosion and other factors are systematically considered, and this method is verified by experimental data.

Mechanical behaviour and fracture evolution of coal specimens containing an over‐excavated hole: Experimental study and numerical modelling

AbstractUtilising a series of mechanically over‐excavated cavities along borehole is a novel technique for enhancing the permeability of soft coal seams and, consequently, gas drainage. The evolution of cracks induced by a wide range of pressure‐relief around an over‐excavated hole is intrinsically complex. In this study, the mechanical behaviour and crack evolution of the specimens containing an over‐excavated hole under uniaxial compression loading were studied by experimental and 3D numerical simulation. The results indicated that the peak strength and elastic modulus of the specimens gradually decrease with increasing cavity diameter and length, which is also verified by the numerical simulation. The inclusion of cylindrical cavities in over‐excavated holes results in reduced crack initiation stress and a greater degradation of peak stress and elastic modulus, despite having an equivalent volume to the ellipsoidal cavity. This is likely attributed to the difference in stress concentration between the cylindrical and ellipsoidal cavities. The crack propagation process can be classified into four stages based on the acoustic emission (AE) event counts, initial crack compaction, stable crack propagation, unstable crack propagation and post‐peak failure stage. The two AE indices, rise angle and average frequency value, demonstrated that the failure is dominated by tensile crack and gradually transformed to shear crack. Stress redistribution is essential in the initiation and propagation of cracks. Tensile stress concentration leads to cracks forming at the top and bottom of the hole, which propagate in the direction of loading. Compressive stress concentration results in shear cracks forming at the left and right sides of the hole, which propagate diagonally. The failure pattern of the specimen is ultimately determined by a combination of tensile and mixed crack propagation. The experimental and numerical results contribute to a deeper understanding of the crack evolution mechanism of coal seams with over‐excavated holes.

Response of reinforced concrete structural members exposed to high temperature fire and retrofitting of fire damaged structural members adopting R.C jacketing

When fire breaks out in a building, the most important criteria is the retention of stability of the fire damaged structure against partial or total collapse. An actual case study has been presented in the current work on the response of structural elements present in a 12 storey RC multi storied building exposed to high temperature during a fire event taken place in it. The rise of the temperature in the structural elements in the event of a fire exposure, causes physio-chemical changes in the materials the structural members are made up of, namely, concrete and reinforcing steel, leading towards the degradation of material stiffness, elastic modulus and decrease in the load carrying capacity of such structural members. In the present work, two compartments at the periphery of the building are exposed to high temperatures of 300 °C, 500 °C and 700 °C, and the structural analysis is carried out with dead load, live load and temperature. Structural response of the structural members is assessed, as well as maximum displacements, internal forces developed in them as a consequence of exposure to high temperature has been recorded. As per observation, a combination of temperature and gravity loads(DL&LL), increases the probability of the structure to undergo a higher degree of damage, as the restrained structural members due to the effect of temperature, undergo nonlinear deformations (buckling of columns and bowing action of beams). Reinforced concrete columns and beams that might have undergone a repairable degree of damage due to different temperatures have been retrofitted adopting the RC jacketing method. It is observed from the results of the present case study that, by adopting RC jacketing to retrofit temperature damaged structural members, present in the structure, an enhancement in the stiffness and overall stability of the structure is observed to further withstand externally applied loads.

Experimental study on fatigue damage of self-compacting concrete of CRTS III slab track

To study fatigue performances and establish the fatigue damage model of self-compacting concrete (SCC) of China Railway Track System (CRTS) III slab track under different stress levels, static and fatigue tests were conducted. In the static test, the flexural strength of SCC was obtained. In the fatigue test, fifteen specimens were divided into five groups. The maximum stress levels were 0.65, 0.70, 0.75, 0.80, and 0.85 for each group, and the minimum stress level was 0.10 for every group. The S-N curve of SCC is obtained and shows that the fatigue life of SCC decreases with the increase of the maximum stress level. The strain evolution and elastic modulus degradation can be divided into 'Fast-Slow-Fast' three stages. With the maximum stress level increases, the proportion of elastic modulus degradation increases and decreases in the first and third stages, respectively. The fatigue damage model derived from the improved elastic modulus method and the Chaboche fatigue damage model can reflect the damage evolution of SCC under different stress levels. Moreover, the proposed model can provide theoretical guidance for the further numerical simulation of SCC for ballastless tracks under the fatigue load.

Predicting the mechanical properties of E-glass fiber-reinforced polymer bars after exposure to elevated temperature

This paper presents the experimental investigations on the tensile, flexural, and shear performance of E-glass fiber-reinforced polymer (GFRP) bars after exposure to elevated temperatures from ambient to 500 ℃. Two types of GFRP bars with nominal diameters of 12 mm and 16 mm were directly exposed to several target temperatures for 0.5 h, and their mechanical properties were tested after cooling. The experimental results showed that the GFRP bars gradually loosened, and their surface color darkened with the increasing exposure temperature. After experiencing high temperatures, the bars’ tension, bending, and shear failure modes remained unchanged. However, their mechanical strengths showed a phased decline in three typical temperature ranges of 20 to 120 ℃, 120 to 270 ℃, and 270 to 420 ℃. In addition, no significant tensile and flexural elastic modulus degradation was observed. The measured strengths were compared with the results of several existing prediction models. A simplified model was proposed to predict the residual strength of GFRP bars after high-temperature exposure. Based on fire safety requirements, a fire-resisting reduction factor, CF, was proposed to determine the design strength of GFRP bars used in concrete slabs. For GFRP bar-reinforced concrete slabs with a cover thickness of 60 mm, the suggested CF for tensile strength and the other two strengths should be 0.65 and 0.75, respectively, to achieve structural safety after 2-h fire disasters.

A Quantitative Representation of Damage and Failure Response of Three-Dimensional Textile SiC/SiC Ceramics Matrix Composites Subjected to Flexural Loading

Abstract In the present work, the microstructure deformation and synergetic damage evolution of a three-dimensional textile SiC/SiC ceramic-matrix composite under flexural loading are investigated by in situ digital image correlation at ambient temperatures. The correlations between microstructure evolution and macro-mechanical degradation of 3D textile composites under flexural loading are established based on the experimental results. In addition, by establishing continuum damage mechanics and a thermodynamic framework with synergetic effects of microstructures, a flexural loading-induced damage evolution model is developed to reveal the relationship between the energy release rate and elastic modulus degradation. The proposed model can be used to predict the flexural stress–strain curves of 3D textile SiC/SiC composites to further improve the design and assessment of new textile architectures with specific mechanical properties.

A new look at the resistance of recycled aggregate concrete (RAC) to the external sulfate attacks: The influence of the multiple mesoscopic material phases

Recycled aggregate concrete (RAC) prepared with recycled coarse aggregate (RCA) has more complex material phases on the meso scale compared to natural aggregate concrete (NAC), including the old virgin aggregate, the new/old mortar, and the three types of interfacial transition zone (ITZ). These mesoscopic material phases would influence the RAC to external sulfate attacks in different ways. This paper provides a new look at the performance of RAC subjected to external sulfate attacks by considering such influence. The quality and quantity of the multiple mesoscopic phases were changed by taking varying water-to-binder ratios (i.e., w/b) and volumetric fractions of RCA (i.e., VRCA), respectively. The changes in properties of both the material phases on the meso scale and concrete on the macro scale subjected to external sulfate attacks were experimentally investigated. The old mortar and ITZ phases (regardless of the ITZ type) were identified as the weaker phases against external sulfate attacks as their microhardness deteriorated faster than that of the new mortar. Improving the quality of the new mortar/new ITZ phases by taking a lower w/b can greatly enhance RAC's resistance to external sulfate attacks: the ingress of sulfate ions was retarded, meanwhile the degradation in compressive strength and dynamic elastic modulus were postponed obviously. Increasing the volumetric fraction of old virgin aggregates by taking a higher VRCA can also help RAC sustain better under sulfate attacks, even though it was accompanied by the simultaneously increasing quantity of the old mortar and ITZ which were identified as the weaker phases against sulfate attacks. Basically, changing the quality of the multiple mesoscopic material phases had more remarkable effects on RAC's sulfate resistance than changing the relative quantity ratios. This paper thereby suggests that RAC's sulfate resistance should be better enhanced by the strengthening or removal of the old mortar phase, or by the improvement of the new mortar and ITZ phases, rather than adjusting their quantity ratios in concrete.