Loading Tests Research Articles

Application of the outlier detection method for web-based blood glucose level monitoring system

Recent advancements in biosensors have empowered individuals with diabetes to autonomously monitor their blood glucose levels through continuous glucose monitoring (CGM) sensors. Nevertheless, the data collected from these sensors may occasionally include outliers due to the inherent imperfections of the sensor devices. Consequently, the identification of these outliers is critical to determine whether blood glucose levels deviate significantly from the norm, necessitating further action. This study employs an outlier detection approach based on the 3-sigma method and the interquartile range (IQR), along with the application of the Winsorizing technique to correct the identified outliers. Additionally, a web-based system for visualizing blood glucose levels is developed, utilizing both outlier detection methods. In order to assess the system's performance, two types of testing are conducted: black box testing and load testing. The results of black box testing indicate that all test scenarios operate as anticipated. As for the load testing response times, it is observed that the 3-sigma visualization page loads an average of 606.75 milliseconds faster compared to the IQR visualization page. This study's outcomes are expected to enhance data quality, enhance the precision of analyses, and facilitate more informed decision-making by identifying and addressing extreme data points.

Rate dependent C* fracture parameter using the optimized wedge-split test geometry and vision-based automated crack tip detection

Typical monotonic load-line displacement (LLD) control and crack mouth opening displacement (CMOD) control cracking tests are limited to singular loading rates and testing temperatures. These experimental protocols are not designed to derive a fundamental fracture parameter capable of relating to the viscoelastic property and cracking behavior of asphalt concrete (AC). On the other hand, a multi-rate LLD-control monotonic fracture experiment is capable of capturing the viscoelastic cracking behavior of AC by providing the rate-dependent fundamental C* fracture parameter as an output. Crack length and crack speed parameters are combined with reaction loads to calculate the rate-dependent C* parameter. The current C* experiment protocol implements manual processing of video images and exhibits high variability. The study aims to optimize the wedge-split geometry and improve the C* fracture parameter calculation protocol by applying an automated vision-based crack detection system and recording the crack growth path, considering the horizontal and vertical movement of the crack tip with higher accuracy. The three notch lengths (5, 15, and 30 mm) were evaluated. Plant and laboratory-produced mixes with varying stiffness and fracture characteristics were included in the testing program. An optimized notch length for the test geometry was determined based on the on-specimen strain field observed using the digital image correlation (DIC) technique and crack path analysis. The optimum notch length was found to be 15 mm due to consistent strain localization at the crack tip and less crack deviation in the horizontal direction. The main contribution of this paper is the proposed vision-based crack path detection method, which enables measuring the crack path's true crack length and maximum horizontal deviation. The developed protocol and optimization of the testing geometry led to a significant improvement of the existing C* testing protocol, resulting in higher data quality and consistent results. The proposed protocol was validated using three comparable and distinctively different lab-produced mixtures. The results show that the C* experiment effectively differentiated between the mixes based on crack speed and the C* parameter.

Experimental study on seismic behavior of reinforced concrete exterior beam-column joints under varying axial load

The majority of low-cycle loading tests on reinforced concrete (RC) beam-column joints were currently conducted under constant axial loads. During earthquake shaking, the axial load on the joints fluctuates due to the overturning moment and vertical ground vibration. Particularly in exterior beam-column joints, where beam shear occurs on only one side, the range of axial load variation is significantly larger. To elucidate the impact of varying axial loads on the seismic performance of exterior beam-column joints, low-cycle loading tests were conducted on four groups of full-size beam-column subassemblage specimens. Two identical specimens from each group underwent low-cycle loading tests with constant and varying axial loads, respectively. Based on the experimental findings, this paper examines the differences in cyclic behaviors among comparison specimens, focusing on damage characteristics, joint shear strength, stiffness degradation, displacement ductility, and longitudinal bar slip. The study suggests that compared to specimens subjected to constant axial loads, the comparison specimens exhibit variations in load-carrying capacity corresponding to the fluctuations in axial load. Furthermore, varying axial loads accelerate post-peak stiffness and strength degradation, leading to reduced ductility. Additionally, specimens in one of the comparison groups exhibited different failure models when subjected to varying axial loads and constant axial loads, specifically beam failure and joint failure, respectively. Numerical simulations of the corresponding beam-column subassemblages were conducted on the Opensees platform to elucidate how the two axial loading methods affect the transformation of failure modes. A subsequent parametric study focusing on the axial compression ratio was performed to further explore the impact of varying axial loads on the seismic behavior of beam-column joints.

Low-carbon metakaolin precursor for one-part and two-part geopolymer activation of demolition wastes

A critical part of civil engineering infrastructure works is to develop innovative construction materials for a low-carbon future development. This study delved into the feasibility of using metakaolin (MK) as a low-carbon precursor for one-part and two-part geopolymer stabilization of recycled demolition wastes. In this research, the effects of MK precursor were evaluated in combination with construction and demolition (C&D) waste aggregates such as crushed brick (CB), reclaimed asphalt (RAP) and recycled concrete (RCA) using one-part and two-part geopolymer activation. Various geotechnical tests were performed including compaction tests, repeated triaxial loading tests (RLT), unconfined compressive strength tests (UCS) and microstructural analysis. Results from this study indicated significant strength development of MK mixtures for both one-part and two-part geopolymer options. Furthermore, one-part geopolymer also showed better activation than two-part geopolymer under 7 days and 20℃ curing condition. Additionally, 30 % MK mixtures were optimal for road construction applications based on the obtained results. At 28 days or 40 ℃, MK mixtures performed best with RCA and CB as main aggregates, showing strength improvement of up to 250 % compared to 7 days and 20 ℃ curing condition. Resilient modulus (Mr) results indicated that the optimal mixtures experienced good resilient behavior with average Mr values ranging from 150 MPa to 200 MPa. Overall, MK precursor was found to be a suitable material for stabilizing demolition aggregates using geopolymer under various curing conditions.

Cyclic inelastic performance evaluation of locking bolt demountable shear connectors in steel-concrete composite structures

This study extends the research on the Locking Bolt Demountable Shear Connector (LBDSC), previously introduced and extensively analysed under monotonic loading through both experimental and numerical methods. The focus here shifts towards the inelastic cyclic performance of the LBDSC, a critical aspect for applications in seismic regions. The LBDSC, designed for use in composite floors or decks, such as in-situ solid slabs, profiled deck slabs, or precast slabs in buildings and bridges, features a grout-filled steel tube bolted to the top flange of steel sections. The design incorporates a unique 'locking bolt' technique to counteract the influence of construction tolerances on undesirable initial slip behaviour, significantly facilitating the deconstruction and reuse of steel sections without the need for recycling. Following the Eurocode 4 guidelines, a series of standard push out tests were conducted to explore the LBDSC's inelastic cyclic characteristics. The experimental results confirm the LBDSC's capability for high strength, high initial stiffness, and adequate slip capacity under cyclic loading, with ductile shear failure modes observed at the bolt shank and local concrete crushing above the steel flange. The cyclic loading tests reveal a larger zone of damaged concrete compared to monotonic loading conditions, yet the monotonic and cyclic force-slip curves closely align. Based on these findings, and supported by validated finite element analyses, this paper proposes theoretical monotonic and cyclic models for LBDSC under push out loading, aimed at accurately predicting the load-slip response. These models are intended to facilitate the efficient analyses, design and numerical modelling of composite structures incorporating LBDSCs, and to support the broader adoption of LBDSCs in earthquake-prone areas, promoting sustainable steel-concrete composite construction practices.

Study on hysteresis performance of four-limb CFST latticed column-box girder joints based on GA-BP neural network

This study investigates the seismic performance and energy dissipation capacity of the concrete-filled steel tube (CFST) latticed column-box girder joints. A low-cycle reciprocating load test was carried out on the CFST latticed column-box girder joints with three different connection forms. Based on the test results, a genetic algorithm (GA) is developed in combination with the back propagation neural network (BPNN) model for prediction research. Using a training set of joints with single-limb diagonal and cross-diagonal braces (training set), the development of load-displacement hysteresis curves of joints with transverse diaphragm plate (test set) under low-cycle reciprocating loads is predicted. The test results are compared with those obtained from the traditional algorithms and finite element simulations. The results on the joint hysteresis performance show that all three types of joints have a good seismic performance and energy dissipation capacity. Compared with the traditional algorithms, the GA-BP model predicts more accurate results and is therefore used to predict the load-displacement hysteresis curves of CFST latticed column-box girder joints, which is shown to be convenient and feasible.

Seismic behavior of a hollow precast concrete shear wall with insulation under high axial compression ratios

A hollow precast concrete shear wall (HPCSW) with multiple vertical hollow cores that is advanced in lightweight and thermal insulation is studied in this paper. Novel tapered grouted sleeves (TGSs) are developed to connect the adjacent shear walls, which can avoid grouting defects through visible grouting processes. Seven full-size specimens with an aspect ratio of 3.3, including three solid precast shear walls, three hollow precast shear walls, and a cast-in-place (CIP) shear wall, were subjected to cyclic loading tests, which considered different and high axial compression ratios (ACRs) of 0.3, 0.45, and 0.6. The results show that the solid precast shear wall with the TGS connection displays comparable failure mode and seismic behavior with the CIP one. In contrast, the HPCSW suffers hollow core crushing under high ACRs, leading to a significant reduction in deformation and energy dissipation capacity. Finally, numerical models of HPCSW specimens with different layouts of the hollow cores were established using ABAQUS to evidence the test results and optimize the structure. The results show that the configuration of the outer hollow cores influences the seismic behavior of the HPCSW vastly. It is recommended that the concrete is partially filled into the outer hollow cores to obtain good insulation and seismic capacity simultaneously, with the filling height greater than the predicted crushing height.

From virus to viral: content analysis of HIV-related Twitter messages among young men in the U.S.

BackgroundAdolescents and young adults account for over 21% of new HIV infections in the U.S. with most new cases among young men. As an important information source for this group, social media can uniquely reveal the perspectives and communicative patterns of this key population. We identified 6,439 young male Twitter users (ages 13–24) in the U.S. using an NLP pipeline with geolocations. From their Twitter timelines, we collected 24,600 HIV-related tweets, among which the most retweeted and favorited tweets (n = 472) were analyzed through a content analysis.ResultsThree themes arose in this online viral discourse around HIV among young men: (i) othering, (ii) politics and activism, (iii) risk and wellness. Othering tweets contained stigmatizing jokes and insults alienating individuals who identify as lesbian, gay, bisexual, transgender, queer or questioning, intersex, asexual, or being elsewhere on the gender and sexuality spectrum (LGBTQIA +), and people with HIV. Politics and activism tweets discussed awareness, stigma, HIV criminalization, violence, LGBTQIA + , and women’s rights. Risk and wellness tweets discussed risk behaviors for sexually transmitted infections (STIs) (e.g., condomless sex, transactional sex, multiple sexual partners), or safer sex and preventive practices (e.g., pre-exposure prophylaxis [PrEP], condom use, achieving undetectable viral load, medication adherence, and STI testing).ConclusionThe social acceptability of high-risk sex behaviors is high among young male Twitter users. Given the double-edged nature of social media—health-promoting (e.g., awareness, health activism) as well as risk-promoting (e.g., risky behavior endorsement, identity attacks)— this population may benefit from targeted health communication intervention. Future HIV prevention efforts should counter the stigma, misinformation, and risk-promoting viral messages prevalent on social media.

Crack propagation process and lifetime prediction method of concrete-rock interface under constant loading

This study investigated the crack propagation processes and established the lifetime prediction methods of the concrete-rock interfaces under constant loading. Firstly, the constant loading tests were performed under three-point bending loading on the composite concrete-rock beams with three kinds of interfaces, i.e. natural, 4 × 4, and 7 × 7 interfaces, and under three constant load levels, i.e. initial cracking load, 80% and 97% of the peak loads. Then, the clip gauge method and digital image correlation method were employed to detect the crack lengths under constant loading. Finally, the effects of constant load levels on fracture properties of the concrete-rock interfaces were analyzed and the crack propagation processes under constant loading were discussed. The results indicated that the clip gauge method was an available and accurate method to detect the crack lengths of the concrete-rock interfaces under constant loading. Crack propagation processes and crack mouth opening processes under constant loading exhibited the three-stage feature, i.e. deceleration stage, uniform stage and acceleration stage. In addition, the logarithms of the constant loading lifetimes showed an approximate linear relationship with the logarithms of the crack mouth uniform opening rates, and a quantitative relationship was established by using the linear regression analysis to establish the lifetime prediction methods. This study proposed a practical method to predict the service life of concrete structure-bedrock interface. When the crack mouth uniform opening stage was recognized, the lifetime of concrete structure-bedrock interface in service can be predicted by substituting the crack mouth uniform opening rate into the pre-established prediction model. This study can provide the theoretical support for the safety assessment of the concrete structure-bedrock interface in service.

Study on seismic behavior of corroded reinforced concrete walls in flexural-shear failure

In this study, artificial climate-accelerated corrosion tests and pseudostatic loading tests were conducted on six reinforced concrete (RC) shear walls subjected to flexural-shear failure. The effects of the corrosion degree and axial compression ratio parameters on the seismic performance of RC shear walls were investigated. The corrosion conditions (corrosive cracks and corroded steel bars), failure process, bearing ability, deformation capacity and energy consumption capacity were compared. The test results showed that chlorine salt erosion induced severe deterioration of the properties of the RC shear wall. The internal reinforcement of the specimen was severely corroded near the protective layer. The failure mode of the severely corroded specimens changed from the original flexural-shear failure mode to the bending failure mode. Rebar corrosion markedly decreased the bearing ability, deformability and energy dissipation capability of the specimens. Based on the limited test data in this study, the existing equations for calculating the parameters of the characteristic points were modified, and equations for calculating the parameters of the skeleton curve of the corroded RC shear wall were proposed. The cyclic corrosion degradation index was utilized to define the degradation of various mechanical properties of the specimen (yield load, hardening stiffness, reloading stiffness, peak load, unloading stiffness, etc.). With cyclic loading, and considering the pinching characteristics, a restoring force model of a corroded RC shear wall was established based on the Ibarra-Medina-Krawinkler (IMK) model. The comparison illustrated that the built restoring force model was in good agreement with the skeleton curve and hysteresis rules and could accurately reflect the hysteresis performance of corroded RC shear walls to a certain extent.

Fractional viscoelastic constitutive modelling of real-time strain response for asphalt pavement composites subjected to simulating wheel loadings

ABSTRACT This paper proposed a replacement of the Kelvin–Voigt model with fractional basic elements, and the model constructed with this method was used to describe the viscoelastic strain response of asphalt pavement materials under the simulated wheel loading. Similarities between Abel element and Kelvin–Voigt model were explored, and strain response analytical solutions of Burgers and 1S1A1D models under intermittent have sine loading were derived. Then, different intermittent loading tests of asphalt binder and mixture were carried out. The results showed that both the Abel element and the Kelvin–Voigt model could represent the delayed viscoelastic behaviours accurately, and the strain response represented by them could be fully recovered during the unloading stage. Furthermore, parameters of the Abel element and Kelvin–Voigt model showed a high degree of correlation, and the 1S1A1D model exhibited the best effects for asphalt binders, particularly during the unloading period, with correlation coefficients mostly above 0.999. Moreover, when expressing the strain response of asphalt mixture under simulated wheel loading, the 1S1A1D model outperformed the Burgers model in terms of peak strain value, residual strain and changing trends. This study demonstrated that the fractional constitutive model was advantageous in representing the strain response of asphalt pavement composites.

HIV-1 pretreatment and acquired antiretroviral drug resistance before tenofovir/ /lamivudine /dolutegravir (TLD) roll-out in Mozambique

BackgroundThe World Health Organization (WHO) recommends that HIV treatment scale-up is accompanied by a robust assessment of drug resistance emergence and transmission. The WHO HIV Drug Resistance (HIVDR) monitoring and surveillance strategy includes HIVDR testing in adults both initiating and receiving antiretroviral therapy (ART). Due to limited information about HIVDR in Mozambique, we conducted two nationally representative surveys of adults initiating and receiving first-line ART regimes to better inform the HIV program.MethodsWe carried out a cross-sectional study between March 2017 and December 2019. Adults (older than 15 years) living with HIV (PLHIV) initiating ART or receiving first-line ART for between 9-15 months at 25 health facilities across all eleven provinces in Mozambique were included. Genotypic HIVDR was assessed on dried blood spots (DBS) when viral loads were ≥ 1000 copies/ml. Genotypic resistance for non-nucleoside reverse transcriptase inhibitors (NNRTIs), nucleoside reverse transcriptase inhibitors (NRTIs), and protease inhibitors (PIs) was determined using the Stanford HIV database algorithm 9.5 and calibrated population resistance tool 8.1.ResultsOf 828 participants -enrolled, viral load (VL) testing was performed on 408 initiators and 409 ART experienced. Unsuppressed VL was found in 68.1% 419 initiators and 18.8% (77/409) of the ART experienced. Of the 278 initiators and 70 ART experienced who underwent sequencing, 51.7% (144/278) and 75.7% (53/70) were sequenced successfully. Among the new initiators, pretreatment drug resistance (PDR) for NNRTI and PI was found in 16.0% (23/144) and 1.4% (2/144) of the participants, respectively. Acquired drug resistance (ADR) was found in 56.5% (30/53) of the ART-experienced participants of whom 24.5% (13/53) were resistant to both NRTI and NNRTI.ConclusionHigh rates of PDR and ADR for NNRTI and ADR for NRTI were observed in our study. These findings support the replacement of NNRTIs with dolutegravir (DTG) but high levels of NRTI resistance in highly treatment-experienced individuals still require attention when transitioning to new regimens. Moreover, the study underlines the need for routine VL testing and HIVDR surveillance to improve treatment management strategies.

Cyclic loading experiments and numerical analysis of laterally loaded piles constructed in marine soil

This study aims to thoroughly analyze the lateral loads that impact tubular steel piles, which are extensively employed in the construction of coastal structures. The ASTM D 3966 standard was followed for conducting field tests on test piles installed at the Mersin International Port. The “time-displacement” and “load-displacement” curves were obtained from the cyclic loading test of the laterally loaded piles at the construction site. The finite elements models of the tests conducted on the construction site with the same parameters was built to perform numerical analysis. At the end of the analysis, it was determined that the numerical model's results were highly consistent with those obtained from the field test. After verifying the field experiments with numerical models, a parametric study was conducted. Parametric studies were conducted to compare the lateral displacements of tubular steel piles with variations in pile diameter, wall thickness, and load application height effect. The relationship between pile head displacements and these parameters appears to be almost linear. It is noteworthy that a change of approximately 10 percent in these parameters shows a correlation with changes of up to 3 percent in deformations.

Seismic Performance of Precast Concrete Exterior Beam-Column Joint with Disc Spring Devices

ABSTRACT To improve the seismic performance of precast concrete reinforced frame structures which are widely used worldwide, and enable a certain repairable function for the precast structure after an earthquake, this paper proposes a new type of precast concrete frame joint using a full grouting sleeve connection with a disc spring device. The disc spring device was connected in series to the longitudinal reinforcement located in the plastic hinge zone at the beam end to provide a restoring force and protect the concrete. Six exterior beam-column joints consisting of one cast-in-situ beam-column joint and five precast concrete frame joints, using a fully grouted sleeve connection equipped with or without a disc spring device, were designed and fabricated. The failure process, hysteretic characteristics, and energy dissipation capacity of the joint specimens were analyzed based on reversed cyclic loading test. The results showed that the energy dissipation and seismic performance of the newly precast concrete joints equipped with the disc spring device were better than those of the precast concrete frame joints connected by ordinary grouting sleeves without a disc spring device. The disc spring device proposed in this study plays a similar role as a damper in the precast joint during loading and has the characteristics of concentrated damage as it transfers part of the damage in the joint core area to the built-in disc spring area at the beam end. Additionally, the deformation of the precast joints under a lateral seismic load was theoretically calculated, the calculated results were in good agreement with the experimental results.

Nonlinear behavior of existing pre-tensioned concrete beams: Experimental study and finite element modeling with the constitutive Serial-Parallel rule of mixtures

The performance of existing prestressed concrete (PC) beams and their time-dependent behavior have been concerns for bridge maintenance services. In this paper, laboratory tests of three PC void slab beams taken away from an obsolete bridge were conducted and an alternative more efficient finite element (FE) method was developed. In preparing the three beam specimens, two retained their original reinforced concrete (RC) overlay and one chiseled off its RC overlay. Through the four-point loading tests, the load response plots of the beams from elastic to plastic failure were carefully documented, including the strains along the mid-span section, the strains of longitudinal rebars and strands, and the mid-span deflection. The test results showed that no interface slip was observed between the RC overlay and the top of void slab during the whole loading process, and the yield load of the test beams with the overlay was approximately 45 % higher than that without the overlay. Regarding the proposed nonlinear numerical formulation, the modelling process avoided the treatment of complex reinforcement and prestressing tendon layouts. The prestressed concrete was simulated as a composite material whose behavior is depicted by a constitutive serial-parallel rule of mixtures (SP-RoM). The generalized Maxwell model and generalized Kelvin model were applied to calculate the time-dependent losses of prestress. All the developments have been implemented within the open-source FE code Kratos-Multiphysics environment and are fully available. Compared with the test outcomes, it shows that the numerical results are in good agreement with the solutions in terms of stiffness, load-deflection curves, and damage evolution. The differences between the experimental and numerical values of ultimate load for the specimens with and without RC overlay were all within 10 %. Furthermore, the proposed nonlinear models can also predict the time-dependent prestress losses of existing PC beams.

Experimental research on PPF anti-cracking effect in scale model of hollow rigid frame bridge

The hollow rigid frame bridge has many advantages, such as being lightweight, having strong spanning ability, and having slight deflection in span. This type of bridge has a broad potential for application in regions of many canyons and hills. While there has been less research into the application of this type of bridge, some of the bridges in early service have cracking problems. To satisfy the demand for cracking resistance of the Yunnanzhuang large-span hollow rigid frame bridge, our group proposed using Polypropylene Fibers (PPF) to enhance the strength of concrete against cracking. After analyzing the structure of the whole bridge, we gave the critical sections of the bridge and carried out a series of tests to give the PPF dosage that applies to the project. This paper focuses on the use of reduced-size bridge model tests to investigate the effect of PPF on the cracking resistance of hollow rigid frame bridges under bending.In this paper, the similarity criterion between the actual bridge and the model was calculated by the method of magnitude analysis, according to which scale-down model beams of the upper chord S4 segment and mid-span jointing segment of Yunnanzhuang Bridge were designed under the geometry of 1:5. We respectively made these models using the same C55 concrete as the actual bridge and Polypropylene Fibre Reinforced Concrete (PPFRC) with 0.3 % volumetric admixture of PPF in the same mix ratio, and three-point bending static load tests were performed on these models. To make up for the insufficiency of test groups, we performed a finite element analysis using ABAQUS and the CDP constitution, which led us to some reliable conclusions.Considering that concrete materials have size effect, directly using the results of model tests to predict the cracking stresses of the actual bridge is not accurate. In this paper, some theories on size effects were examined. Finally, the Size Effect Model (SEM), Boundary Effects Model (BEM), and Weibull's brittle strength theory were selected to correct the test results.The results show that PPF can increase the cracking load of the test beams, ranging from 11.22 % to 13.49 %. After using selected size effect theories to predict the cracking stresses in actual bridges, the results show that PPF can enhance the cracking strength of C55 concrete, with an increase ranging from 9.15 % to 18.75 %. Combined with the results of the structural analysis of the whole bridge, the critical section has a cracking stress between 2.6 MPa and 3.5 MPa when using C55 concrete, which has a certain risk of cracking. However, when using PPFRC, the cracking stress ranges from 3.1 MPa to 4.2 MPa, with no risk of cracking in most unfavorable cases, which can satisfy the demand for concrete cracking resistance of Yunnanzhuang Bridge.

Mechanical properties of high-strength concrete (HSC) under projectile penetration

With the advancement of weapon systems, high-strength concrete (HSC) has been extensively utilized in protective engineering. To investigate the damage characteristics of HSC under impact resistance, this study first conducted Split Hopkinson Pressure Bar (SHPB) tests on HSC, along with cyclic loading and unloading tests. Based on the test data of SHPB and cyclic loading and unloading, the original Holmquist-Johnson-Cook (HJC) model was adjusted accordingly. Additionally, tungsten alloy bullet penetration tests on concrete plates with different initial velocities were performed to measure the depth of penetration (DOP) and crater area corresponding to each velocity. A numerical model was then established using Abaqus software. The test results and numerical simulations were compared and analyzed, demonstrating that the predicted results closely matched the test, with a DOP error margin of less than 10%. This confirmed the feasibility and accuracy of the modified model and its parameters. Finally, the results of numerical simulation were used to analyze the mechanical properties of HSC during impact resistance.

Effects of laminate stacking sequence on the strength properties of aluminum alloy–carbon fiber-reinforced plastic dissimilar single-lap adhesive joints

Here, two single-lap adhesive joints (SLJs) of dissimilar materials were subjected to static and cyclic loading tests. A2017 aluminum alloy was used as an adherend for one, and carbon fiber reinforced plastic (CFRP) was used as the adherend for the other. Four types of orthogonal laminated CFRPs with different laminate stacking sequences were used as adherends to investigate the effect of adherend stiffness on the strength properties of the joints. Furthermore, the results of the finite element analysis of the dissimilar SLJs revealed that when the tensile load was applied to them, the out-of-plane deformation asymmetry increased with increasing difference in stiffness between both adherends. This asymmetry affected the peel and shear stress distributions. Furthermore, the experiments revealed that the static tensile strength of the SLJs increased with increasing stiffness of the CFRP adherend. Additionally, fracture simulation using cohesive-zone modeling (CZM) revealed that the SLJs with higher CFRP stiffness exhibited higher strength, qualitatively agreeing with the experimental results. CZM analysis and adhesion strain measurements during the tests indicated that failure occurred at the A2017 adherend–adhesive interface. In contrast, no differences were observed between the fatigue strengths of the different types of adherends in the short-life region, with a number of cycles to failure (Nf) being ≤ 2 × 105. However, in the long-life region, beyond Nf = 2 × 105, the SLJ bearing the unidirectional CFRP adherend exhibited lower fatigue strength than the others. The anodizing process on A2017 was found to improve fatigue strength by a factor of two or more.

Seismic retrofit of intact and damaged RC columns using prefabricated steel cage-reinforced UHPC jackets and NSM GFRP bars

The use of high-performance materials, such as ultra-high-performance concrete (UHPC) and fiber-reinforced polymer (FRP), in the seismic retrofitting and strengthening of reinforced concrete (RC) columns has attracted wide attention. In this study, a novel prefabricated steel cage-reinforced UHPC jacket (PSRUJ) was proposed to improve the retrofitting efficiency of conventional cast-in-place UHPC jackets and combine the advantages of UHPC and steel jackets. PSRUJ was further combined with the near surface-mounted (NSM) technique to develop a new method for retrofitting the seismic performance of RC columns in buildings. To investigate the effectiveness of the proposed retrofitting method, cyclic loading tests were performed on one intact RC column retrofitted with PSRUJ and two damaged RC columns retrofitted with PSRUJ and the combination of PSRUJ and NSM glass FRP (GFRP) bars. After PSRUJ retrofitting, the load-carrying capacity, deformation capacity, and stiffness of the intact and damaged RC columns increased by (30∼46)%, (54∼74)%, and (24∼40)%, respectively. The inclusion of NSM GFRP bars further increased the load-carrying capacity but reduced the residual drift ratio while having little effect on the ultimate drift ratio of the retrofitted columns. Further, a damage index was proposed to assess the damage degree of core normal concrete and develop the peak strength and strain models for PSRUJ-confined damaged concrete. Finally, a flexural strength model was proposed for damaged RC columns retrofitted with PSRUJ and NSM bars.

Recoverable and plastic strains generated by forward and reverse martensitic transformations under external stress in NiTi SMA wires

Although superelastic NiTi shape memory alloy wire displays very high resistance to plastic deformation in austenite and martensite phases, incremental plastic strains are recorded whenever the cubic to monoclinic martensitic transformation (MT) proceeds under external stress leading to functional fatigue degradation. Therefore, special closed-loop thermomechanical loading tests were performed to shed light on the mechanism by which the incremental plastic strain are generated. These tests revealed that both forward and reverse MTs occurring above certain stress thresholds generate plastic strains specific for the [temperature, stress] conditions under which the MTs occurred. While the forward MT upon cooling does not produce plastic strain or permanent lattice defects up to 500 MPa stress, the reverse MT upon heating starts to generate them from 100 MPa. While plastic strain generated by the forward MT merely elongates the wire, plastic strain generated by the reverse MT also reduces the recoverable strain. Since the reverse MT upon heating generates plastic strains at lower external stresses than the forward MT upon cooling, it is largely responsible for cyclic instability of NiTi actuators. The characteristic thresholds and magnitudes of plastic strains generated by the forward and reverse MTs define the functional fatigue limits for specific NiTi wires.