Cyclic Bending Tests Research Articles

CYCLIC LOADING TEST OF REINFORCED CONCRETE (RC) BEAMS INCORPORATING ALTERNATIVE CEMENTITIOUS MATERIALS

The focus of this study is to examine flexural fatigue durability of RC beams, which were made with various cementitious materials. The cementitious materials tested in this study were blast furnace slag powder, fly-ash, silica-fume, and an artificial admixture of rich SiO2 and Al2O3. Fifteen RC beams for each admixture were prepared in addition to a control mixture (without admixture). The dimensions of the RC beams without stirrups were 160 x 160 x 1200 mm. Three beams for each mixture were soaked into a water-tank until the cyclic loading test. The study conducted static and cyclic four-points bending test. The paper aims to report the effect of the alternative cementitious materials on the fatigue durability of RC beams, which were exposed to the dry and wet environmental conditions.

A facile approach to prepare a flexible and durable electrically driven cotton fabric-based heater

Flexible and durable electrically driven heaters (EDHs) was prepared by using silver nanowire/polydopamine modified cotton fabric (AgNW/PDA-C) as Joule heat generator and polydimethylsiloxane (PDMS) as stretchable matrix.The electric property of the EDHs was investigated, and it can show excellent electrical property and stability. The EDHs can be seen from the SEM morphology that cotton fabric was well protected by the PDMS and has good conductive network. The EDHs also exhibit excellent electrothermal performance under applied voltage. At low-voltages, it can quickly reach satisfactory steady-state temperatures that meet the needs of wearables. By improving the electrothermal model, it can be verified that there is a quadratic function relationship between electric power and steady-state temperature. Under cyclic voltage and bending tests, the EDHs can well maintain steady-state temperature and exhibit excellent dynamic and static electrothermal performance.

Multifunctional self-assembled BaTiO3-Au nanocomposite thin films on flexible mica substrates with tunable optical properties

Integration of multifunctional hybrid materials with hyperbolic, plasmonic and ferroelectric properties on flexible substrates is of great interest towards future flexible integrated electronic and photonic devices. In this work, the successful integration of BaTiO3-Au hybrid metamaterials on flexible mica substrates has been demonstrated using a one-step pulsed laser deposition (PLD) technique. Tunable optical properties have been achieved by controlling the geometries of the Au nanostructures between nanoparticles and nanopillars by varying the growth temperature. The overall structure and properties of the hybrid materials is stable upon cyclic bending test (more than 1000 bending cycles), evidenced by the robust physical properties and microstructural stability. Furthermore, room temperature ferroelectric property was confirmed by piezoresponse force microscopy mapping. This work demonstrates an effective approach for integrating multifunctional oxide-metal nanocomposites on flexible mica substrate, and paves a way towards future flexible integrated devices.

Wearable temperature sensors based on lanthanum-doped aluminum-oxide dielectrics operating at low-voltage and high-frequency for healthcare monitoring systems

The incorporation of lanthanum (La) into sol-gel based aluminum-oxide (Al2O3) dielectrics is investigated by analyzing the dielectric properties obtained from electrical capacitive measurements of metal-insulator-metal. The optimized La doping results in significantly suppressed variation of frequency-dependent capacitance of sol-gel based La-doped Al2O3 (LaAlO3) dielectrics even at high frequencies of ~ MHz. In addition, flexible temperature sensors with sol-gel based LaAlO3 dielectrics on polyimide substrates are demonstrated through investigations on the capacitive responses to different temperatures ranging from 30 to 200 °C at various frequencies from 100 Hz to 1 MHz. Without any buffer or passivation film, the proposed flexible temperature sensors achieved a high mechanical stability against cyclic bending tests with curvature radii from 10 to 5 mm. Finally, the feasibility of a healthcare monitoring system is demonstrated by utilizing wearable body-temperature sensor arrays consisting of sol-gel based LaAlO3 dielectrics. High-performance temperature sensor system is capable of analyzing interfacial temperature characteristics on human body between 30 and 40 °C in real-time and displaying them on the use's smartphone. We believe that this study is expected to significantly contribute to the field of wearable bioelectronics consisting of solution-processed metal-oxide dielectrics for healthcare monitoring systems.

Bending reliability of screen-printed vias for a flexible energy module

The future of printed electronics involves advancements not only related to full system integration, but also lean process manufacturing. A critical aspect of this progress is developed in this study, which evaluates a highly flexible screen printed through-hole-via using silver microparticle inks, for applications in energy harvesting and storage modules. The printed vias’ fabrication and durability are evaluated by means of a double sided screen-printing method and repetitive (cyclic) bending tests. Vias, through 125-µm-thick PET foil, were laser cut (nominally 50, 100, 150, and 200 µm diameters) then filled, and simultaneously connected to adjacent vias by screen printing. To investigate the use of the printed via in a monolithic energy module, the vias were used for the fabrication of a flexible printed supercapacitor containing aqueous electrolyte and carbon electrodes. The results indicate that the lower viscosity silver ink (DuPont 5064H) fills the via less effectively than the higher viscosity ink (Asahi LS411AW), and as the via size increases (≥150 µm diameter) via walls are coated rather than filled. Conversely, the more viscous ink fills the via thoroughly and exhibited a 100% yield (1010 vias; 100 µm nominal via diameter) with the two-step direct screen-printing method. The 10-mm radius bending test showed no signs of via specific breakdown after 30,000 cycles. The results indicate that this via filling process is likely roll-to-roll compatible to enable multi-layered printed electronic devices.

Bending Properties of Carbon Nanotube/Polymer Composites with Various Aspect Ratios and Filler Contents.

The key characteristics of bending sensors are piezoresistive effect, hysteresis, and durability. In this study, to investigate the influence of the aspect ratio and contents of multi-walled nanotubes (MWNTs) on the properties of bending sensors, MWNT/polydimethylsiloxane (PDMS) composites were fabricated with various aspect ratios and filler contents. The MWNTs were uniformly dispersed in the composites using the three-roll milling method. By increasing the bending angle gradually, the sensitivity of each composite was analyzed. Furthermore, discontinuous cyclic bending tests were conducted to investigate the piezoresistive effect and hysteresis. In addition, stable repeatability of the composites was confirmed through continuous cyclic bending tests. As a result, optimal aspect ratios and filler contents have been presented for application in bending sensors of MWNT composites.

Development of a multi-stage semi-circle bending cyclic test to evaluate the cracking resistance of asphalt mixtures

This study aimed to develop, evaluate, and validate a new cyclic cracking assessment test that has advantages over the available monotonic and cyclic cracking assessment tests. Also, it addresses the major concerns related to the implementation of the Balanced Mix Design (BMD) such as long specimen preparation and testing time and the validity of cracking performance assessment tests. The laboratory evaluation findings showed that the proposed test and derived performance indicators were sensitive to the composition of mixes, had variability lower than other cyclic tests, and similar to the one obtained from the monotonic performance indicators. Besides, it correlated well with the observed field cracking performance of sixteen field projects. The derived indicators were able to distinguish between different field cracking performance groups and develop a pass/fail performance cracking assessment threshold.

Exchange Bias in a La0.67Sr0.33MnO3/NiO Heterointerface Integrated on a Flexible Mica Substrate.

Flexible electronics integrating spintronics are of great potential in the areas of lightweight and flexible personal electronics. The integration of ferromagnetic and other functional oxides on flexible mica substrates is crucial for the proposed computer technology. In this work, we demonstrate the successful integration of a ferromagnetic-antiferromagnetic nanocomposite of La0.67Sr0.33MnO3 (LSMO)/NiO with unique perpendicular exchange bias properties on a flexible mica substrate. Utilization of multiple sets of buffer layers has been attempted to overcome the large mismatch between the film and the substrate and to achieve high-quality nanocomposite growth on mica. Exchange bias of ∼200 and ∼140 Oe for the applied magnetic field perpendicular and parallel to the film surface, respectively, has been achieved and attributed to the strongly coupled vertical ferromagnetic/antiferromagnetic interfaces. Such nanocomposite thin films exhibit excellent structural robustness and reliability under a cyclic bending test. This work demonstrates the enormous potential of integrating complex two-phase multifunctional oxides on mica for future flexible wearable personal devices.

A fully sustainable, self-poled, bio-waste based piezoelectric nanogenerator: electricity generation from pomelo fruit membrane

A self-powered system is very much essential aspect in the recent trend to improve the working efficiency of the portable and wearable devices. Here, we have reported a fully sustainable, self-poled, bio-compatible, and bio-waste based piezoelectric energy harvester which has been made of Pomelo Fruit Membrane (PFM). PFM based piezoelectric generator (PFMBPEG) could generate ~ 6.4 V output voltage and ~ 7.44 μA output current directly, only by finger tapping on the device and registers a power density of ~ 12 μW cm−2 whereas, the same piezoelectric generator can generate ~ 15 V output voltage, 130 μA output current, and power density of ~ 487.5 μW cm−2 by using a full wave rectifier. The sensitivity and energy harvesting competence of the generator have also been assessed by attaching this nanogenerator into various parts of human body (as energy sources) such as wrist, elbow, finger, throat, jaws, leg and putting the device into ultrasonic bath and in every case, it could successfully generate voltage. Therefore, this bio-waste based energy harvester can be used as a power source for the different potable and wearable electronic goods where a small amount of energy is required, specifically in the biomedical applications (i.e., health monitoring, power source for the implantable devices and so on). Finally, mechanical stability the developed piezoelectric generator has been evaluated by cyclic bending test and it has been observed that there is no significant deformation of the PFM film even after 100 cycles.

Setup for studying the kinetics of crack growth in cyclic bending tests

The goal of the study is analysis of the features of fatigue cyclic fracture of steels. An installation has been designed to induce fatigue cracks and to study the kinetics of fatigue crack development. Crack growth is recorded by the method of potential difference. The data on the crack growth kinetics were processed on a computer using LGraph2 programs and Excel spreadsheets. When studying the kinetics of the fatigue crack development, the electrodes were soldered to the edges of the initial notch of the sample and time dependence of the potential difference was recorded on a computer during crack growth. To interpret the experimental data, a calibration chart in the coordinates «potential difference (U) – the crack length (Lcr)» constructed on the basis of the millivoltmeter readings was used, with due regard for the size of the sample section, current flow and length of the fatigue crack. Cyclic loading of the sample resulted in a stepwise character of the crack growth: first occurred zone of plastic deformation of the metal is then followed by accumulation of stresses of a certain size, their relaxation in the form of a crack and sudden crack growth. An abrupt crack growth is clearly visible on a graph of the fatigue crack growth rate obtained upon computer processing of experimental data. Using a graphical editor KOMPAS, a graph was constructed which characterized the growth of the fatigue crack against the number of cycles of fatigue tests for bending. The experimental setup provides the possibility of fatigue crack formation for impact tests, determination of the work of crack propagation, as well as studying of the kinetics of crack development and computer processing of experimental data.

Critical Buckling Strains in Thick Cold-Formed Circular-Hollow Sections under Cyclic Loading

AbstractContrary to the large dataset of test results exploring the monotonic bending response of steel tubes, the corresponding dataset of cyclic bending tests remains very small. Seven compact an...

Flexible powder electroluminescent device on transparent electrode of printed invisible silver-grid laminated with conductive polymer

Invisible Ag-grid transparent electrodes at a line width of the Ag-grid of 5 μm, spacing between the grid lines of 150 μm and a line thickness of 0.6 μm have been prepared on a 50 μm thick flexible polyethylene naphthalate (PEN) film substrate by a conventional gravure offset printing using our newly developed Ag nanoparticle ink. Furthermore, the Ag-grid electrode was laminated with a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer. The Ag-grid laminated with PEDOT:PSS (Ag-grid/PEDOT) transparent electrode was applied to a commercial alternating-current powder electroluminescent device (ACPELD) as an ITO replacement. The as-prepared Ag-grid/PEDOT-based ACPELD shows uniform luminescence with a luminance of 292 cd m−2 at 150 V0-p and 2 kHz. Repeated bending durability of the fabricated ACPELD was evaluated by a cyclic bending test at a bending radius of 1.5 mm up to 20 000 bending cycles. Luminance of the Ag-grid/PEDOT-based ACPELD is stable throughout the bending cycles compared to that of an ITO-based ACPELD. Luminance of the 20 000 cycle-bended Ag-grid/PEDOT-based ACPELD maintains 97% of that of the as-prepared. These results demonstrate that the Ag-grid/PEDOT on PEN-based ACPELDs develop stable luminance characteristics throughout the bending cycles.

On the failure mechanisms of Cr-coated 316 stainless steel in bending fatigue tests

Fully-reversed cyclic bending tests were conducted on uncoated and Cr-coated 316 stainless steel specimens. At high stresses, the uncoated specimens outperformed the Cr-coated ones, while the trend reversed at low stresses. For Cr-coated specimens subjected to high stress amplitude loading, experimental observations showed crack initiation at the Cr/substrate interface due to shear steps formed from dislocation activities in the substrate—a conclusion supported by accompanying finite element analysis. Interfacial crack initiation was not observed under low stress amplitudes. This difference in fatigue crack initiation mechanisms induced an opposite trend in fatigue life between high and low stress amplitudes.

Solution‐Processable Transparent Organic Molecular Nanoadhesives for Exceptionally Durable Nanowire Electrodes

AbstractSilver (Ag) nanowires (NWs) are promising building blocks for fabrication of flexible transparent electrodes, but their poor adhesion to polymeric substrates causes delamination of NWs from a substrate during repeated deformation, which leads to degradation in electrical performance. A new and simple approach is developed to dramatically improve mechanical, chemical, and thermal stability of a Ag NW electrode by hybridizing a NW network film with an organic molecular nanoadhesive layer without sacrificing its inherent excellent optoelectrical properties. It is discovered that some of pyridine derivatives such as 4‐dimethylaminopyridine can be solution‐processed and annealed to form a transparent layer with high adhesion to polymeric substrates such as poly(ethylene terephthalate). In addition to high optical transparency and electrical conductivity, the NW‐nanoadhesive hybrid film exhibits low surface roughness, reduced optical haze, and excellent adhesion to the substrate without delamination against 100 cycles of tape detachment and 10 000 cycles of bending tests at 1 mm of bending radius. Furthermore, the hybrid film exhibits good air‐oxidation and thermal stability over 24 h at 100 °C. The hybrid electrodes show good performance in applications for flexible touch panel and thin film heater.

Bending reliability of transparent electrode of printed invisible silver-grid/PEDOT:PSS on flexible epoxy film substrate for powder electroluminescent device

Invisible Ag-grid transparent electrode at a grid line width of 5 μm, spacing between the grid lines of 200 μm and a grid line thickness of 0.6 μm have been prepared on a 50 μm-thick flexible epoxy film substrate by a conventional gravure offset printing using our developed Ag nanoparticle ink. Furthermore, the Ag-grid were laminated with a poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layer. Bending durabilities of the Ag-grid laminated with PEDOT:PSS (Ag-grid/PEDOT:PSS) transparent electrode were investigated via bending tests. The Ag-grid/PEDOT:PSS transparent electrode on the epoxy film substrate shows no noticeable resistance change throughout 20,000 outer bending cycles at a bending radius of 1.0 mm. The Ag-grid/PEDOT:PSS transparent electrode was applied to a commercial alternating-current powder electroluminescent device (ACPELD). Repeated bending durability of the fabricated ACPELD was investigated via a cyclic bending test. Luminance of the fabricated ACPELD is stable throughout the 30,000 outer bending cycles at a bending radius of 1.0 mm. The transparent electrode-based ACPELD develops also excellent bending durability.

Experimental study into the location of the neutral axis in fiber‐reinforced concrete prisms

AbstractThe results of an extensive experimental program into the location of the neutral axis in cracked fiber‐reinforced concrete (FRC) are reported. In total, 74 prisms are tested in a monotonic (51 specimens) and cyclic (23 specimens) bending test in accordance with EN 14651. In the program, 13 different FRCs are considered with 9 different fiber types and dosages. A normal strength concrete with residual strength values between 1b and 2d, cf. Model Code 2010 (MC10), is considered. The deformation profile in the cracked section is measured by side‐mounted linear variable displacement transducers on the prisms, and the position of the neutral axis is calculated. The experimental results are compared against predictions of the MC10. Based on a simplified stress diagram, the MC10 predicts that the location of the neutral axis depends on the residual strength of the FRC at the serviceability limit state: increasing the strength results in a larger compressive zone and vice versa. However, the results obtained within this study do not confirm this relation. The cyclic results indicate that the neutral axis shifts upward during unloading and subsequent reloading decreases the neutral axis again to preunloading levels. Finally, for all reported FRC types, the neutral axis lies very high in the cracked section, even at very small crack mouth opening displacement values.

Seismic performance of concrete-filled SHS column-to-beam connections with slip-critical blind bolts

This paper investigates the use of slip-critical blind bolts to connect I-beams to concrete-filled steel square hollow section (SHS) columns. The strength and stiffness of the resulting joints are determined experimentally for the purpose of classifying them according to the Eurocode. Their suitability for use in special moment frames is also assessed through cyclic bending tests. Three types of beam sections are tested, being a compact welded section, a reduced beam (flange) section, and a reduced beam section with concrete slab at the top. All tested joints are full strength according to the Eurocode, allowing the connected beams to reach their respective plastic moment capacities. In addition, they are rigid for braced and unbraced frames, except for the reduced beam section specimen, which are semi-rigid only for unbraced frames according to the Eurocode. However, all specimens have sufficient ductility to be used in special moment frames, with no pinching effect in their hysteretic moment-rotation curves. Their initial rotational stiffness is dominated by the stiffness of the column flange in bending, which can be conservatively estimated using the formulation presented in this paper.

High-performance and ultraflexible PEDOT/silver nanowires/graphene films for electrochromic applications.

We fabricate a kind of flexible electrochromic (EC) film by spraying the mixed dispersion of silver nanowires (AgNWs) and poly (3, 4-ethylenedioxythiophene) (PEDOT) on the graphene (GR) electrode. AgNWs are embedded in the PEDOT nanoparticles, forming an interlaced conductive network and double electron transport channels; therefore, the disadvantages of poor conductive GR electrodes have been remedied effectively. The subsequent GR-based PEDOT/AgNWs composite films have revealed remarkable optical contrast (63%), high coloration efficiency (${182.8}\;{{\rm cm}^{2}}\;{{\rm C}^{ - 1}}$182.8cm2C-1), and good cycle stability (keeping the optical contrast about 60% after switching cycles of 16,000 s). In addition, the GR-based composite films can keep good EC performance after 2000 cycles of bending tests, while those of the ITO/PET-based composite films decrease dramatically. The ultraflexible GR-based PEDOT/AgNWs composite films with excellent EC properties present an opportunity for fabricating large-area flexible EC devices.

Assessment of Fatigue Induced Pad Cratering With a Universal Expression of Printed Circuit Board Fatigue Resistance

Abstract Repeated loading is an important reason to cause pad cratering fatigue failure in ball grid array (BGA) device in printed circuit board (PCB) assembly. For industry application, the board level drop test is commonly applied to evaluate the pad cratering fatigue strength under the repetitive drop loading. Although this testing method is consistent with the actual service condition of BGA-PCB assembly, it is extremely time consuming in the testing operation and expensive in costs. Another fatigue evaluation testing method for BGA-PCB assembly is the board level cyclic bending test. Compare with the board level drop test, this testing method can be handled by universal testing machine automatically without manual operation during the testing process. In consequence, the cyclic bending test has the merits of simple, fast, and low costs, and it is always desirable to evaluate the repeated drop life of pad cratering with cyclic bending test. This research proposes a correlation between the cyclic bending and repetitive drop test in BGA-PCB assemblies. With assistance of finite element method, the equivalent cyclic bending testing conditions of drop tests are developed. The experimental validation is also conducted to prove accuracy of the correlation. From the analysis of finite element method and experiments, both cyclic bending tests and repetitive drop tests agree with the same strain–number of cycle (S–N) curve. This means the S–N curve can be treated as a generalized failure criterion of fatigue induced pad cratering. The conclusion is crucial for reliability design phases to prevent the pad cratering fatigue failure.

Flexible pH sensors based on printed nanocomposites of single-wall carbon nanotubes and Nafion

High-performance flexible pH sensors based on nanocomposites of single-wall carbon nanotubes (SWCNTs) and Nafion, fabricated by screen printing on flexible substrates in ambient air are demonstrated. The electrical characteristics and thus the sensing performance of the pH sensors monitoring the change in resistance of printed nanocomposite films in a wide range of pH from 1 to 12 were controlled by the number of printed layers in multi-patterns. The operational stability of the flexible pH sensors was nearly unchanged after 200 cycles of bending tests having a curvature radius of 5 mm, which is essential for intergration of flexible pH sensors into wearable nanoelectronics. The sensing mechanism of the chemiresistive pH sensors based on nanocomposites is investigated in terms of chemical reaction. The hydrogen or hydroxide ions in the pH solution interact with the carbonyl and CH bonds bonds of the nanocomposite film, which induces or withdraws positively charged carriers, respectively. In addition, a real-time pH sensor system integrated into a drone application with wireless communication and data transmitting modules is demonstrated for the simultaneous visualization of processed data and remote detection of targets on the user's smartphone based on a designed Android algorithm.