Tensile Stress Level Research Articles

A local stress criterion to assess the effects of hydrogen embrittlement on the fracture strength of notched tensile specimens

This work addresses the applicability of a local criterion incorporating the coupling of critical stress and a critical hydrogen concentration to predict hydrogen embrittlement effects on the fracture strength of high strength steels using notched round specimens with different notch root radii. The numerical simulations incorporating a relatively simple hydrogen transport model provide strong support for the adoption of a failure criterion in terms of achieving a critical level of tensile stress coupled to the local hydrogen concentration, which, in turn, enable the construction of a failure locus for the degraded material. For the cases analyzed here, construction of such a failure locus based on a critical combination of maximum principal stress and hydrogen concentration enabled predictions of fracture strength for hydrogen-charged tensile specimens which are in very good agreement with experimental data. Overall, the results presented here lend additional support for further developments of a local stress-based criterion to predict hydrogen embrittlement effects on the fracture strength of high strength steels.

Structural Condition Assessment and Temperature Effect Analysis of Cable–Pylon Anchorage Zone Using Long-Term SHM Data

To study the temperature effect in the anchorage zone of a long-span cable-stayed bridge, a wavelet multiresolution analysis and layered stripping method were used in this paper. Based on long-term structural health monitoring data, the signals were decomposed and reconstructed at multiple time scales, confirming temperature to be the main factor causing the stress change in the anchorage zone. The results of structural condition during operation showed that the general compressive stress level of the anchorage zone was low. However, the tensile stress level of the sidewall was high, which led to a severe concrete cracking. Excluding the influence of the seasonal temperature, the compressive stress increased slightly, the horizontal tensile stress in the upstream inner pylon wall increased, and the crack width increased gradually. By analysing the daily temperature effect on the upstream and downstream pylon walls, the regression model proposed in this study can be used to predict the daily temperature effect at any time in the diurnal cycle. The accuracy of the model is reliable within 6 days, but for the location of severe cracks, the monitoring data should be updated in real time to ensure the precision.

Influence of Current Density upon Hydrogenation on the Shape Memory Effect of Binary TiNi Alloy Single Crystals

Some results concerning the hydrogen effect at electrolytic saturation at a current density of j = 1500 and 3500 A/m2 for 3 h at room temperature on the temperature dependence of the yield stress σ0.1(T) and the shape memory effect (SME) under tension of the [011]-oriented Ti-50.55%Ni (at.%) alloy single crystals are presented. It was shown that hydrogen is in a solid solution and forms particles of titanium hydride TiH2 after hydrogenation at j = 1500 and 3500 A/m2, respectively. Both hydrogen in the solid solution and TiH2 particles led to a decrease in the Ms temperature of the onset of the forward martensitic transformation (MT) upon cooling and the Md temperature (Md is the temperature at which the stresses for the onset of the stress-induced MT are equal to the stresses for the onset of plastic flow of the high-temperature B2 phase), and increased the yield stress σ0.1 of the B2 phase at the Md temperature compared to hydrogen-free crystals. It was found that the SME under stress depends on the tensile stress level and current density. The maximum SME εSME = 10 ± 0.2% at σex = 200 MPa and εSME = 10.5 ± 0.2% at σex = 300 MPa was observed in the hydrogen-free crystals and after hydrogenation at j = 1500 A/m2, respectively, which exceeded the theoretical value of lattice deformation ε0 = 8.95% for the B2-B19′ MT in [011] orientation under tension. At j = 1500 A/m2, the physical reason for the excess of the SME of the theoretical ε0 value was due to the increase in the plasticity of B19′ martensite upon hydrogenation. At j = 3500 A/m2, εSME = 8.0 ± 0.2%, and it was less than ε0 = 8.95% for B2-B19′ MT in [011] orientation under tension. The decrease in SME after hydrogenation at j = 3500 A/m2 was associated with the interaction of two types of B19′-martensite: oriented under stress and non-oriented, formed near TiH2 particles. It was shown that the redistribution of hydrogen in the bulk of the crystals during long-term holding for 168 h at 263 K after hydrogenation at j = 1500 A/m2 increases the SME relative to crystals without long-term holding: 3.5 times at 50 MPa and 1.8 times at 100–150 MPa. After long-term holding, εSME = 9.5 ± 0.2% at 150 MPa, which exceeds the theoretical value ε0 = 8.95% for B2-B19′ MT in [011] orientation under tension.

Evaluation of the influence of linear stress concentrators in reinforced concrete elements using the postulates of fracture mechanics

The article is devoted to the study of linear stress concentrators in reinforced concrete, which, unlike randomly located stress concentrators from a large and medium-sized concrete aggregate (granite or gravel crushed stone), can have a strategic impact on the formation of cracks in a reinforced concrete element in the presence of a sufficient level of tensile stresses. The study is one of the theoretical parts of a significant research program on the cracking of reinforced concrete elements in tension and bending. The description of the process of cracking with the help of fracture mechanics allows us to try to formulate a mathematical apparatus for taking into account stress concentrators linearly located in reinforced concrete elements and their influence on the formation of cracks, in particular, at the moment of cracking in bending elements. Field tests of specimens with artificially created stress concentrators were carried out, including the RILEM method, on the basis of which trial calculations of the cracking moment were carried out using the stress intensity factor.

Evaluation of Residual Stress in MEMS Micromirror Die Surface Mounting Process and Shock Destructive Reliability Test

Light Detection and Ranging (LiDAR) devices based on Micro-Electro-Mechanical Systems (MEMS) micromirrors have been promising sensors for automatic driving owing to the advantages of MEMS devices. The packaging plays a vital role in the reliability of MEMS micromirror but introduces the residual stress from the surface mounting process. The shock reliability of micromirrors is influenced by residual stress. In this work, the residual stress from the surface mounting process are investigated through resonant frequency tests. The frequency drift can be 718 Hz and -281 Hz under the residual stress of 107.0 MPa and -25.0 MPa respectively. As the difference of CTE rises, the residual stress proportionally increases but decreases with the thickness of the substrate. In addition, a higher curing temperature can introduce a more significant stress level. The dynamic response in shock environment under different residual stress are analyzed. Drop fall experiments verify the theoretical conclusions that the stiffness hardening effect can improve the shock level tolerance. The tensile stress levels of 41.7 MPa and 107.0 MPa are corresponding to 100 g and 200 g respectively. The influences of residual stress on frequency drift and reliability need to be considered seriously and simultaneously during the packaging design for MEMS micromirrors. This work provides a deep insight into commercial applications of MEMS-based LiDAR in autonomous driving.

Nonlinear creep characteristics of extruded Poly (vinylidene fluoride-co-hexafluoropropylene) with high ß-phase content under extreme conditions: Design, characterization, and modeling

This study is concerned with the nonlinear creep characteristics of extruded Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) dominated by β-phase polymorphs over the temperature range 40 °C − 80 °C and tensile stress levels between 5 MPa and 12.5 MPa. The measured data using the developed experimental setup as well as the dynamic mechanical analyzer invariably suggested a nonlinearity taking place at temperatures near to 60 °C, where αc-relaxation began corresponding to 100 °C above the measured glass transition temperature of PVDF-HFP. The results from accelerated testing conditions indicated the transition to tertiary creep stage at 24% to 28% strain, irrespective of test condition, referred to as critical strain range. Following the identification of Findley power law model constant at different test conditions, a generalized temperature- and stress-dependent model was developed with only 7 constants. The comparisons between the measured and modeled data conveyed the model effectiveness in describing the nonlinear creep characteristics of the polymer over the measured temperature and stress levels. Time-temperature-compliance master curves were consequently constructed using the developed generalized model, which in combination with the critical strain failure criterion, provided the essential design data including the maximum allowable ranges of stress and temperature for achievement of 20 years creep life.

Lateral impact resistance of full-scale concrete-filled weathering steel tube columns

This paper conducted lateral impact tests on 10 concrete-filled weathering steel tubular (CFWST) columns and studied the effect of prestressing on their resistance to lateral impact. The results indicate that by using prestressed cantilever members, the impact resistance of the column can be significantly improved. This is mainly due to the reduction of tensile stress level in the steel tube at the fixed end of the cantilever column and of the overall plastic deformation of the member, which is achieved through the application of prestress. Furthermore, the test process was also simulated numerically. Based on verifying the correctness of the numerical simulation, the parameters of the full-scale model are analyzed. The impact resistance of the model was evaluated by considering various factors such as impact angle, impact point height ratio, prestress ratio, width-thickness ratio, slenderness ratio, and mass ratio. Based on these analyses, empirical formulas for the lateral ultimate displacement and bending moment of the cantilever members were derived. Particularly, considering the difference in yield strength between tension and compression sides, the theoretical calculation of plastic limit bending moment of concrete-filled square steel tubular members is modified. Additionally, the damage level of the component is defined, and the damage limit value of the concrete-filled steel tubular cantilever column is given based on the different damage grades. On this basis, considering the bending bearing capacity and displacement response of the component, an impact resistance design approach based on the performance of the component is proposed.

Layout optimization of steel reinforcement in concrete structure using a truss-continuum model

Owing to advancement in advanced manufacturing technology, the reinforcement design of concrete structures has become an important topic in structural engineering. Based on bi-directional evolutionary structural optimization (BESO), a new approach is developed in this study to optimize the reinforcement layout in steel-reinforced concrete (SRC) structures. This approach combines a minimum compliance objective function with a hybrid truss-continuum model. Furthermore, a modified bi-directional evolutionary structural optimization (M-BESO) method is proposed to control the level of tensile stress in concrete. To fully utilize the tensile strength of steel and the compressive strength of concrete, the optimization sensitivity of steel in a concrete–steel composite is integrated with the average normal stress of a neighboring concrete. To demonstrate the effectiveness of the proposed procedures, reinforcement layout optimizations of a simply supported beam, a corbel, and a wall with a window are conducted. Clear steel trajectories of SRC structures can be obtained using both methods. The area of critical tensile stress in concrete yielded by the M-BESO is more than 40% lower than that yielded by the uniform design and BESO. Hence, the M-BESO facilitates a fully digital workflow that can be extremely effective for improving the design of steel reinforcements in concrete structures.

МЕТОД ПІДТВЕРДЖЕННЯ РЕСУРСНИХ ХАРАКТЕРИСТИК МЕТАЛЕВОЇ ЛОПАТІ НЕСУЧОГО ГВИНТА ВЕРТОЛЬОТА ЗА РЕЗУЛЬТАТАМИ ВИПРОБУВАНЬ

The helicopter main rotor blade is the basic product that determines the reliability and service life of the helicopter as a whole. The problem of predicting and ensuring the specified blade life is an urgent problem considered at the stage of its design. The analysis of the design, structural materials and design and technological solutions of the main rotor blade (RB) of the Mi-8 helicopter has been carried out. A brief description of the main rotor blade of the Mi-8 helicopter is presented. The analysis was carried out and a standard flight cycle (SFC) of the helicopter was developed. The type of bench equipment for carrying out bench fatigue tests of the blade has been selected and justified. The loads on the main rotor blade for the SFC are determined. To determine the fatigue life of a blade, it is necessary to know the characteristics of the stress-strain state. The calculation of the stress-strain state (SSS) of a blade by the finite element method (FEM) using the ANSYS system is presented. The characteristics of the stress-strain state of the spar of the regular and irregular parts of the rotor blade of a helicopter are determined using the ANSYS system. The use of numerical methods for calculating the characteristics of the stress-strain state can significantly reduce the time and cost of designing a blade. The paper presents the results of calculating the regular part of the main rotor blade of the Mi-8 helicopter in the hover mode in the case of its loading with aerodynamic and inertial load from rotation, as well as the force from its own weight. With the help of the ANSYS system, a finite element model of the regular part of the blade was developed, consisting of a set of beam elements of variable section, a calculation was carried out taking into account the geometric nonlinearity of the structure's behavior, and an analysis of the results obtained was carried out. To describe the response of materials to an external action, a model of an elastically deformable isotropic body was used with the assignment of the corresponding elastic constants of the material. The analysis of the calculation results includes the determination of reactions at the attachment points, the values of the maximum displacements of structural elements and stresses in dangerous sections. Dangerous sections are determined and the values of the longitudinal force and bending moment in these sections are calculated. The assessment of the static strength of the blade by the safety factor was carried out. When evaluating the static strength, the equivalent stresses according to Mises were considered as the maximum design stresses. To assess the fatigue strength, we analyzed the distribution of the main tensile stresses in the power elements over typical stress concentrators. The maximum level of the main tensile stresses in the dangerous section indicates that the blade material operates in the zone of high-cycle fatigue. A technique for calibrating strain gauge channels has been developed. The calculation of the characteristics of the rotor blade of a helicopter is based on the requirements set forth in the technical literature, regulatory documents. When performing work, the requirements of the Aviation Rules, Part 29 (AP - 29) were taken into account. These studies were the basis for the development of a method for confirming the resource characteristics of a helicopter main rotor blade based on the results of flight and bench tests.

Stress analysis of circular membrane-type MEMS microphones with piezoelectric read-out

In this study, the impact of residual tensile stress on a membrane-type piezoelectric MEMS (Micro Electromechanical Systems) microphone is investigated both theoretically by FEM (finite element method) simulations and experimentally by stress measurements, by manufacturing devices and by recording the acoustic performance. Based on these findings, a design is proposed composing of segmented electrodes which are arranged close to the silicon frame as well as on the inner part of the membrane. The MEMS microphone features an aluminum nitride layer which allows a piezoelectric read-out, and multiple metal bottom and top electrodes distributed radially and azimuthally. FEM simulations were done over a large range of different layer thicknesses, membrane radii, and tensile stress levels, which indicate the existence of a sweet-spot when targeting a maximum output signal. The data showed, that thicker membranes perform better under tensile stress than thinner membranes. Furthermore, a relation between output signal and residual stress level was established, which showed that the FEM predictions agree well with the measurement results of piezoelectric microphones. • Thicker MEMS membranes preferred over thinner counterparts when tensely pre-stressed • Finite element analyses revealed sweet spot for maximizing piezoelectric output • Design propositions for stressed closed membranes without stress relaxing measures • Microphone measurements and finite element methods results agree well

Verification of Spalling Tensile Strength of Rocks using 3D GPGPU-accelerated Hybrid FEM/DEM

The spalling test is widely applied for evaluating dynamic tensile strength of rock under high strain-rate condition. The dynamic tensile strength is indirectly measured in the spalling test by theoretically assuming the tensile stress level at the failure position of a cylindrical specimen based on the 1D stress wave propagation theory. However, the theoretical estimation method for dynamic tensile strength have not been fully validated since dynamic tensile strength was determined with insufficient understanding of the fracture process, such as crack propagation and stress distribution in the rock specimen during the spalling test, owing to observational limitations in the experiment. Thus, a proper validation method is required for the proposed method, and numerical simulation can be used to validate the proposed method by reproducing the tensile fracture process of the rock specimen during the spalling test. Thus, the dynamic spalling tensile test for rock specimens was reproduced in this study using the GPGPU-based 3D hybrid FDEM that we recently developed. The dynamic tensile fracturing process was analyzed in the simulation of spalling under various strain-rate conditions. Finally, we discussed the application of various theoretical estimation methods to the 3D spalling test.

Physico-mechanical properties and brittle to plastic transition of a thermally cracked marble under uniaxial tension

Thermal stress induces severe changes in rock microstructures, resulting in the variations of physical and mechanical properties. To study thermal effects on rock physical properties and tensile behaviors, monotonic and cyclic tension tests were conducted on marble samples heated at different temperatures. The results show that marble samples are weakened by thermal damage, showing a decrease in tensile strength and P-wave velocity, and an increase in porosity. The changing trends of Young's modulus and failure strain with temperature appear to be scattering and complicated probably depending on the deformation types (i.e., nonlinear elastic deformation and plastic deformation). Nonlinearity is a fundamental feature of tensile stress versus strain curves, which is enhanced when the sample is heated at higher temperature. Even under low tensile stress levels, permanent damages are always observed, highlighting a hysteresis in the stress-strain curves obtained from cyclic tension tests. A transition from the nonlinear elastic deformation to the plastic deformation occurs at approximately 300 - 400°C. A crack density based constitutive model is introduced for the nonlinear elastic deformation (25 - 300°C). A modified Voce exponential function is recommended to capture the tensile stress and strain relationships for the plastic deformation (400 - 700°C).

Heat control and design-related effects on the properties and welding stresses in WAAM components of high-strength structural steels

Commercial high-strength filler metals for wire arc additive manufacturing (WAAM) are already available. However, widespread industrial use is currently limited due to a lack of quantitative knowledge and guidelines regarding welding stresses and component safety during manufacture and operation for WAAM structures. In a joint research project, the process- and material-related as well as design influences associated with residual stress formation and the risk of cold cracking are being investigated. For this purpose, reference specimens are welded fully automated with defined dimensions and systematic variation of heat control using a special, high-strength WAAM filler metal (yield strength > 790 MPa). Heat control is varied by means of heat input (200–650 kJ/m) and interlayer temperature (100–300 °C). The ∆t8/5 cooling times correspond with the recommendations of filler metal producers (approx. 5–20 s). For this purpose, additional thermo-physical forming simulations using a dilatometer allowed the complex heat cycles to be reproduced and the resulting ultimate tensile strength of the weld metal to be determined. Welding parameters and AM geometry are correlated with the resulting microstructure, hardness, and residual stress state. High heat input leads to a lower tensile stress in the component and may cause unfavorable microstructure and mechanical properties. However, a sufficiently low interlayer temperature is likely to be suitable for obtaining adequate properties at a reduced tensile stress level when welding with high heat input. The component design affects heat dissipation conditions and the intensity of restraint during welding and has a significant influence on the residual stress. These complex interactions are analyzed within this investigation. The aim is to provide easily applicable processing recommendations and standard specifications for an economical, appropriate, and crack-safe WAAM of high-strength steels.

Improving fatigue performance of metal parts with up-facing inclined surfaces produced by laser powder bed fusion and in-situ laser remelting

• Three-point bending fatigue life significantly improved by in-situ laser remelting. • In-situ laser remelting proved to be applicable to two different materials. • No microstructural defects were observed in remolten zone or interface with bulk. It is well known that the relatively poor surface quality of metal parts produced by Laser Powder Bed Fusion (LPBF) compared to conventionally manufactured parts has a negative effect on their fatigue life. To enhance fatigue life, time consuming post-processing (e.g. machining, polishing) is typically applied to improve the surface quality. Laser remelting is a promising technique for surface quality improvement, applicable to variable shapes. However, laser remelting is traditionally only used as post-process since surfaces inclined with respect to the powder bed are covered with loose powder during building. Recently, the authors have developed a dual laser technique, which enables improving the quality of up-facing inclined surfaces during building. The technique consists of two steps: (1) selectively removing powder from inclined surfaces by laser-induced shock waves, (2) laser remelting newly exposed surfaces. This paper shows a clear improvement in three-point bending fatigue of such treated samples from maraging steel 18Ni-300 and titanium alloy Ti-6Al-4V, compared to as-built state. This performance was quantified by the number of load cycles until failure at a fixed maximum tensile stress level. The improved performance could be linked to reduced surface roughness, total notch depth, critical stress concentration factor and a lower density of surface asperities. The remolten layer exhibited no visible defects and a decreased microhardness, which could beneficially impact the crack nucleation. This technique shows large potential for reducing the time and cost of metal parts produced by LPBF with enhanced fatigue performance by limiting or avoiding the need for surface post-processing.

Mass transport properties of recycled aggregate concrete under coupling the action of chloride salt attack and uniaxial tensile loading

Water and chloride permeability are two important parameters for the evaluation of recycled aggregate concrete (RAC) durability. In service, the tensile loading plays an extremely important role in the generation and propagation of microcracks inside the RAC. Thus, aiming at understanding the mass transport properties of RAC under the coupled action of chloride salt attack and uniaxial sustained tensile loading, an improved experimental set-up was self-designed. And then a series of tests involving the effects of different uniaxial sustained tensile stress levels (10%, 30% and 50% of ultimate tensile stress, namely λt = 0.1, 0.3 and 0.5) and different replacement ratios of recycled coarse aggregate (RCA) (0, 30%, 50% and 100% mass replacement of normal coarse aggregate, namely R0, R30, R50 and R100) on the transport properties of RAC were conducted, and their coupling effects were further analyzed. Generally, the results indicate that both the sorptivity and chloride diffusion coefficient of RAC increase with the increase of RCA replacement ratio and tensile stress level. The synergistic effect of RCAs and tensile stress significantly accelerates the process of water and chloride invasion into concrete. The initial sorptivity of R100 specimens at λt = 0.5 was about 4.44 times that of R0 specimens without loading. Moreover, the chloride diffusion coefficient exhibits a linear relationship with the RCA content, while it has a quadratic relationship with the tensile stress level.

Influence of initial tensile stress on mechanical properties of calcium silicate hydrate under various strain rates by molecular dynamics simulation

Molecular dynamics (MD) was introduced to explore mechanical properties of calcium silicate hydrate (CSH) under different initial tensile stress levels and strain rates. It shows the strain rates have obvious enhancement on the basic mechanical parameters. The dynamic tensile strength will not decrease until the initial tensile stress level reaches about 76%.The enhancement mechanisms are proclaimed by the interaction potential function and the development of chemical bonds. The constitutive model for CSH considering the initial tensile stress levels is proposed. Our observations can provide atomic insights into the influence of loading history on the dynamic tensile strength of cement-based materials.

Creep-fatigue properties of austenitic cast iron D5S with tension and compression dwell: A dislocation density-based crystal plasticity study

To predict and better understand the creep-fatigue behaviour of austenitic cast iron D5S under tension and compression dwell at 800∘C, a physics-based crystal plasticity model that describes the complex rate- and temperature-dependent deformation of the material as a function of the dislocation density is implemented. In addition to the tension and compression dwell direction, the effect of three different dwell times (30, 180 and 600 s) on the creep-fatigue properties is investigated. The dislocation density-based crystal plasticity simulations are compared to experimental tests from a prior work. While relaxation tests and low-cycle fatigue (LCF) tests without dwell assist in systematically identifying the material parameters, creep-fatigue (CF) data is used to validate the predictions. The virtual testing is performed on a large-scale representation of the actual test specimen with a polycrystalline structure. To analyse the fatigue damage mechanism, small-scale predictions are also conducted using a micromechanical unit cell approach. Here, a single graphite nodule frequently found in the material is embedded into the austenitic matrix. In the present work, a close agreement is achieved between the predicted CF behaviour and the experimental results. Consistent with the experimental findings, the simulation results show that the addition of compression dwell leads to an uplift of the overall tensile stress level, which significantly reduces the fatigue life of the material. The unit cell studies demonstrate that during this uplift, a strong localisation of stresses and strains arises at the graphite/matrix interface, triggering the nucleation and growth of cavities and/or debonding.

Research on the Method of Temporary Prestressing to Regulate the Stress in the Section during the Construction of the Main Arch Ring of the Cantilever Cast Arch Bridge

To control the level of tensile stress in the section of reinforced concrete arch cantilever construction, a construction control method of temporary prestressing reinforcement is proposed in this study. The influence law of temporary prestressing on the arch ring section stress is revealed based on the engineering background (Shatuo Special Bridge in Guizhou). Meanwhile, a temporary prestressing test is carried out on the ring section of the main arch to verify the effectiveness of the method. The results show that the prestressing configuration during the construction of the main arch ring of the suspension arch bridge has obvious influence on the control of tensile stress. The method can also effectively improve the force uniformity of the buckle cables, reduce the variation range of cable forces during construction, and improve the safety of arch ring construction.

Effects of PWHT on the Residual Stress and Microstructure of Bisalloy 80 Steel Welds

Quenched and tempered (Q & T) steels have numerous applications, particularly in the defence industry with welding as the main fabrication route. Since welding imparts stresses due to thermal gradients development during welding, plus the fact that the Q & T fabricated structures are expected to function in a complex loading environment, it is critically important to relax the welding stresses before exposing the parts to service conditions. The present study reports on the generated residual stresses when Bisalloy 80 is welded by pulsed gas metal arc welding (GMAW-P) and verifies the effects of post-weld heat treatment (PWHT) on the microstructural changes, removal or reduction of residual stresses and the resulting mechanical properties of the welded Q & T steel joints. Neutron diffraction was utilized to measure the residual stresses in the as-welded and after PWHT of the Bisalloy 80 steel weldments. High levels of tensile residual stresses reaching to the yield strength of the weld metal were present (642 ± 24 MPa) in the as-welded joints but were substantially reduced after PWHT (145 MPa ± 21 MPa, which is ~23% of the yield strength of the weld metal). PWHT led to microstructural changes in different regions of the parent and weld metals, including the formation of coarsened polygonal ferrite grains and bainitic ferrite laths. This finding is in line with hardness measurements, where hardness reductions were evident in the heat-affected zone (HAZ) and the weld metal (WM) of the heat-treated specimens.

Carbonation Behavior of Engineered Cementitious Composites under Coupled Sustained Flexural Load and Accelerated Carbonation.

Engineered cementitious composites (ECCs) belong to a broad class of fibre-reinforced concrete. They incorporate synthetic polyvinyl alcohol (PVA) fibres, cement, fly ash and fine aggregates, and are designed to have a tensile strain capacity typically beyond 3%. This paper presents an investigation on the carbonation behaviour of engineered cementitious composites (ECCs) under coupled sustained flexural load and accelerated carbonation. The carbonation depth under a sustained stress level of 0, 0.075, 0.15, 0.3 and 0.6 relative to flexural strength was measured after 7, 14 and 28 days of accelerated carbonation. Thermogravimetric analysis, mercury intrusion porosimetry and microhardness measurements were carried out to show the coupled influence of sustained flexural load and accelerated carbonation on the changes of the mineral phases, porosity, pore size distribution and microhardness along the carbonation profile. A modified carbonation depth model that can be used to consider the coupled effect of flexural tensile stress and carbonation time was proposed. The results show that an exponential relationship can be observed between stress influence coefficient and flexural tensile stress level in the carbonation depth model of ECC, which is different when using plain concrete. Areas with a higher carbonation degree have greater microhardness, even under a large sustained load level, as the carbonation process refines the pore structure and the fibre bridges the crack effectively.