Cyclic Bending Tests Research Articles

Functional cyclic bending test for integrated inductors on flexible Kapton substrate

This work presents a method to assess the reliability of integrated inductors on a flexible substrate through a functional cyclic bending test, which consists of a full characterization of these inductors after several bending cycles. Inductors were fabricated through a simple and original technology suitable for use in the heterogeneous integration of radiofrequency and microwave applications. Failure analysis before and after tests have not found any problem related to the use of a flexible substrate. Less than 15 % of the tested inductors failed after the bending tests, indicating the good performance of the fabrication process and a robust method to evaluate the reliability of such components.

Composite design bioinspired by the mesocarp of Brazil nut (Bertholletia excelsa)

The mesocarp of Bertholletia excelsa is a rich source of inspiration for strong, stiff and damage-tolerant composites. The bioinspired composites developed here are composed of an epoxy matrix with a 3D printed polylactic acid reinforced with 30% of carbon fiber (PLA-30CF) inspired in fibers, and syntactic foam inspired by sclereids. Monotonic and cyclic four-point bending tests and compact tension fracture toughness tests were carried out assisted by digital image correlation (DIC) to evaluate flexural properties, damage tolerance, and the R-curve of the composite. Its microstructure and fracture surface were analyzed by scanning electron microscopy. The mechanical performance of the bioinspired composite is promising: density of 1.0 g cm−3, flexural apparent elastic modulus of 1.6 GPa, and flexural strength six times higher than the neat epoxy, i.e. 17 MPa. Although the PLA-30CF printed structure led to a rising R-curve, the syntactic foam needs optimization to have a synergistic effect.

Multimethod Assessment of Design, Metallurgical, and Mechanical Characteristics of Original and Counterfeit ProGlider Instruments.

A multimethod study was conducted to assess the differences between original (PG-OR) and counterfeit (PG-CF) ProGlider instruments regarding design, metallurgical features, and mechanical performance. Seventy PG-OR and PG-CF instruments (n = 35 per group) were evaluated regarding the number of spirals, helical angles, and measuring line position by stereomicroscopy, while blade symmetry, cross-section geometry, tip design, and surface were assessed by scanning electron microscopy. Energy-dispersive X-ray spectroscopy and differential scanning calorimetry were used to identify element ratio and phase transformation temperatures, while cyclic fatigue, torsional, and bending testing were employed to assess their mechanical performance. An unpaired t-test and nonparametric Mann–Whitney U test were used to compare instruments at a significance level of 5%. Similarities were observed in the number of spirals, helical angles, blade symmetry, cross-sectional geometries, and nickel–titanium ratios. Measuring lines were more reliable in the original instrument, while differences were noted in the geometry of the tips (sharper tip for the original and rounded for the counterfeit) and surface finishing with PG-CF presenting more surface irregularities. PG-OR showed significantly more time to fracture (118 s), a higher angle of rotation (440°), and a lower maximum bending load (146.3 gf) (p < 0.05) than PG-CF (p < 0.05); however, maximum torque was similar for both instruments (0.4 N.cm) (p > 0.05). Although the tested instruments had a similar design, the original ProGlider showed superior mechanical behavior. The results of counterfeit ProGlider instruments were unreliable and can be considered unsafe for glide path procedures.

Highly stretchable, robust, and conductive lab-synthesized PEDOT:PSS conductive polymer/hydroxyethyl cellulose films for on-skin health-monitoring devices

Stretchable on-skin sensors provide multifunctional sensing in healthcare monitoring; however, it is challenging to develop cost-effective sensor materials with high stretchability, biocompatibility, mechanical robustness, and sensitivity. Stretchable conductive polymers have been extensively studied as a promising sensing material for strain sensors owing to their excellent stretchability, sensitivity, and mechanical durability. We demonstrate biocompatible on-skin sensors based on a highly conductive film consisting of a lab-synthesized polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and the biopolymer hydroxyethyl cellulose (HEC). Interface functionalization and post-treatment using 11-aminoundecanoic acid (11-AA) endow the PEDOT:PSS/HEC films with stronger bonds, high electrical conductivity, and mechanical robustness. The adhesion between PEDOT:PSS and HEC is greatly improved by the functionalization of the interface by 11-AA owing to the preferential formation of chemical bonds. Moreover, post-treatment with 11-AA enhances the electrical properties of the PEDOT:PSS film by removing the insulating PSS and changing the conformation of the PEDOT chains from benzoid to quinoid structure. Post-treatment reduces the sheet resistance of the interface-functionalized PEDOT:PSS film by 88.1% (from 4900 to 581 Ω/sq). The synergetic effects of interface modification and post-treatment using 11-AA also improve the mechanical characteristics of the PEDOT:PSS films, as demonstrated by tape attach-detach, cyclic stretching/releasing, and cyclic bending tests. Using the highly conductive and robust lab-synthesized PEDOT:PSS/HEC films, we develop highly biocompatible, stretchable, conformable on-skin sensors that can sensitively detect human motions, specifically, skin touch, finger bending, wrist bending, skin wrinkling, breathing, and walking, with excellent stretchability, excellent sensitivity, quick recovery, and conformality. We believe that the PEDOT:PSS/HEC films developed here have excellent potential for use in wearable on-skin devices that can detect various human motions owing to their outstanding performance and high stretchability, conductivity, robustness, and biocompatibility. • Highly stretchable, robust, conductive PEDOT:PSS on hydroxyethyl cellulose substrates for on-skin sensors are developed. • The interface modification and post-treatment improve the electrical and mechanical characteristics of the PEDOT:PSS films. • The PEDOT:PSS sensors are conformally integrated with human skin and successfully responded to various human activities.

Crack Initiation and Growth Behavior of HVOF Stellite-6 Coatings under Bending Loading

Stellite-6 powders were sprayed on Ni-Al bronze in order to produce coatings via high-velocity oxygen fuel (HVOF). The microstructural observations revealed the main mechanisms taking place for the substrate–coating adhesion. It was revealed that tungsten-rich particles are very active in improving the coating adhesion as well as the mechanical properties. The X-ray diffraction analysis of the coating material showed pronounced peak broadening, revealing high residual stresses related to excellent bonding to the substrate. As expected, the coating procedure led to an increase in surface hardness. The surface properties of the coatings were evaluated through cyclic three-point bending tests at different maximum loads. It was demonstrated that the main part of the fatigue life is spent in the crack initiation stage, with a short propagation stage. Obviously, this behavior decreases as the maximum cyclic stress increases. The micro-mechanisms taking place during cyclic loading were evaluated through fracture surface observations via scanning electron microscopy.

The elastoplastic flexural behaviour of corn stalks

With the purpose of quantifying and understanding the elastoplastic flexural behaviour of corn stalks, this work presents a novel three-point cyclic bending test for characterizing the biomechanical properties of controlled corn stalks. The tests have resulted the first-of-its-kind biomechanical database of corn stalks subjected in repeated bending. Moreover, an experiment-informed bending stiffness model is introduced to calculate the elastic bending angle limits, elastoplastic bending angle limits, elastic bending stiffnesses (or effective bending stiffness), and plastic ratios as well as their variabilities in the tested samples, revealing unique insights into the corn stalk flexural properties. The overall elastic bending limit decreased (from 3.4 to 1.7°) with increasing stalk size, whilst the effective bending stiffness increased (from 7.0 to 43 N m rad −1 in median value). The elastoplastic behaviour of the stalks is more consistent and independent of stalk size where in median value the elastoplastic bending limit ranged from 6.9 to 8.8° and the ratio of elastoplastic stiffness to elastic stiffness ranged from 0.40 to 0.42. Furthermore, statistical analysis of the samples and empirical results have presented insightful expressions to relate the stalk scales to responses, or distributions in properties for a population of corn stalks. The database and statistical analysis and regression expression from this work also allow for parameterizing existing numerical models for simulations of corn stalks and development of more sophisticated deformation models. • First-of-its-kind biomechanical database of corn stalks in repeated bending. • Corn stalk bending strength is dominated by the properties of the shell. • The corn stalk bending stiffness increases with increasing stalk diameter. • Experiment-informed bending properties ready for supplying discrete element modeling.

Influence of steel fibers on the fatigue behavior of high-performance concretes under cyclic loading

Due to the advancement of high-performance concretes, the development of filigree constructions has been improved in the last decades. However, as the demand to create more filigree designs increases, the vulnerability to fatigue loads of such structures has also become a decisive factor. Various construction projects, such as wide-span bridges or wind turbines, are exposed to fatigue loads. Especially wind turbines are permanently subjected to wind and wave loads of several hundred million load cycles during their service life. At present, the fatigue behavior of high-performance concretes under cyclic loading is still unknown. In a worst-case scenario, the significantly lower ductility can lead to a sudden failure of the entire structure. In this case, the addition of steel fibers could be advantageous, as they significantly improve the ductility of concretes. However, it is still undetermined how the material fatigue is influenced by steel fibers. Hence, systematic investigations on the fatigue behavior of various high-strength concretes with steel fibers were conducted. Since the crack-bridging effect of fibers is relevant for tensile stresses, predominantly cyclic bending tests were performed on concrete beams with different steel fiber variations. To accomplish the investigations, a test setup has been developed which allows the simultaneous testing of a total of six specimens. Based on the predetermined static concrete strengths, the specimens were subjected to cyclic loads with a defined lower stress level and various upper stress levels. During these cyclic tests, the cycles-to-failure as well as the degradation within the microstructure were detected.

Bioinspired Toughening Mechanisms in a Multilayer Transparent Conductor Structure.

With increasing demands and interest in flexible and foldable devices, much effort has been devoted to the development of flexible transparent electrodes. An in-depth understanding of failure mechanisms in nanoscale structure is crucial in developing stable, flexible electronics with long-term durability. The present work investigated the mechanoelectric characteristics of transparent conductive electrodes in the form of dielectric/metal/dielectric (DMD) sandwich structures under bending, including one time and repeated cyclic bending test, and provides an explanation of their failure mechanism. We demonstrate how a thin metallic layer helps to enhance the mechanical robustness of the DMD as compared with that without, tune the mechanical properties of the cohesive layer, and improve the electrode fracture resistance. Abnormal crack propagation and toughening of multilayer DMD structures are analyzed, and its underlying mechanisms are explained. We consider the knowledge of the failure mechanisms of transparent conductive electrodes gained from the present study as a foundation for future design improvements.

Hierarchically Structured Flexible Electrode on Polyimide for Highly Sensitive and Reliable Biosignal Acquisition

Numerous wearable biomedical devices are developed for the continuous monitoring of personal health or condition. Biosignals acquisition with high sensitivity is important for designing wearable biomedical devices. A sensing electrode between the human body and wearable electronics significantly affects the sensitivity of the sensors. In this study, we fabricated hierarchically structured flexible electrodes on polyimide substrate (HSFE-PI) using micro-casting technique and gold nanoparticles electrodeposition. Polyimides provides robust and outstanding electrical characteristics, and the reliability of HSFE-PI was verified with a cyclic bending test. The integration of hierarchical structures significantly increased the surface area of the electrode by 2.06 times. We applied the HSFE-PI for electromyogram (EMG) and glucose sensing applications and achieved high sensitivity enhancement in both applications. The signal-to-noise ratio (SNR) of measured EMG signals was increased by 2.48 times, and the sensitivity of the glucose detection was increased by 1.42 times compared to the planar counterpart.

Flexible 1.3 μm photodetector fabricated with InN nanowires and graphene on overhead projector transparency sheet.

We report the first demonstration of flexible photodetectors, operating at the wavelength window of 1.3 μm, fabricated with InN nanowires (NWs) and graphene on an overhead projector transparency (OHP) sheet. The InN NWs, used as an absorption medium for the device, were formed on a Si substrate and exhibited strong emission with a peak wavelength of 1.3 μm at room temperature. They were randomly and horizontally embedded in the graphene sandwich structure functioned as a carrier channel. The photocurrent and photoresponsivity of the flexible photodetector were found to be 1.17 mA and 0.48 A W-1, respectively, at a voltage of 1 V and a light intensity of 60 mW cm-2 of a xenon lamp. The photocurrent measured when the photodetector was bent under a strain of 3% was 1.15 mA, which corresponds to 98.3% compared to that before bending. Moreover, the photocurrent and photoresponsivity of the flexible photodetector measured after the 200 cyclic-bending tests are comparable to those measured before bending.

Flexible Ni/NiOx-Based Sensor for Human Breath Detection.

We developed a simple methodology to fabricate an Ni/NiOx-based flexible breath sensor by a single-step laser digital patterning process of solution-processed NiOx thin-film deposited using NiOx nanoparticle ink. Laser-induced reductive sintering phenomenon enables for the generation of three parts of Ni electrodes and two narrow NiOx-sensing channels in between, defined on a single layer on a thin flexible polymer substrate. The Ni/NiOx-based breath sensor efficiently detects human breath at a relatively low operating temperature (50 °C) with fast response/recovery times (1.4 s/1.7 s) and excellent repeatability. The mechanism of the gas-sensing ability enhancement of the sensor was investigated by X-ray photoelectron spectroscopy analysis. Furthermore, by decoupling of the temperature effect from the breathing gas, the response of the sensor due to the temperature alone and due to the chemical components in the breathing gas could be separately evaluated. Finally, bending and cyclic bending tests (10,000 cycles) demonstrated the superior mechanical stability of the flexible breath sensor.

Enabling shape memory effect wires for acting like superelastic wires in terms of showing recentering capacity in mortar beams

Superelastic shape memory alloy (SMA) wires embedded in mortar beams show a recentering capacity, however, shape memory effect wires can provide a prestressing effect after inducing phase transformation. In this study our goal is to empower shape memory effect wires to show recentering capacity by designing a new wire configuration. These wires will thus be able to achieve both a prestressing effect and a recentering capacity in mortar beams. To this end, various shapes of martensitic SMA wires, including as-received, crimped, dog bone, and anchoring reinforced crimped wires, were employed to examine the potential of displaying recentering capacity while preserving the prestressing force in mortar beams. Several monotonic and cyclic three-point bending tests were also conducted to investigate the crack-closing effect, as well as the load bearing of the specimens. To evaluate the effect of temperature and heating procedure, two different methods, namely, a hot air gun and electric current heating, were used to induce phase transformation in shape memory effect wires. The results generally show that for two types of as-received and dog bone wires, the displacement recovery ratio have been reduced about 65% and 74%, respectively, however, for the crimped wire and anchoring reinforced crimped wire, this ratio is almost the same. More importantly, when the results of displacement recovery ratio of crimped wires heated by electrical heating are compared with the available experimental data for superelastic SMA bars, proves the great ability of SME wires to fulfill recentering capacity.

The effects of locking inserts and overtorque on the mechanical properties of a large fragment locking compression plate

PurposeThe study was to determine the effect of locking hole inserts and their insertion torque on the fatigue life of a large fragment Locking Compression Plate (LCP) under bending forces.MethodsFatigue strength of the LCP was examined using cyclic three-point bend testing at 80% yield strength of the construct. Locking hole inserts were used in 2, 4, and 6-hole of a 12-hole plate to simulate three different working lengths. Within each working length, plates were tested without locking inserts serving as the control group. In the experimental groups, inserts were tightened to manufacturer recommendations (4 Nm) and using overtorque (8 Nm).ResultsSignificantly fewer cycles to failure were observed in control groups versus the locking hole insert groups for all working lengths (2-hole: 4 Nm p = 0.003, 8 Nm p = 0.003; 4-hole: 4 Nm p = 0.02, 8 Nm p < 0.001; 6-hole: 4 Nm p = 0.004, 8 Nm p < 0.001). There was a statistically significant increase in fatigue strength when using overtorque in the 4-hole (p = 0.04) and 6-hole (p = 0.01) defect groups. This was not shown in the 2-hole defect group (p = 0.99).ConclusionsBy placing locking inserts in the empty locking regions of Combi holes along the working length, the number of cycles to failure was increased. Tightening inserts to twice the recommended insertion torque further increased cycles to failure in longer working length models. A longer fatigue life has the potential to decease the incidence of plate failure especially in the setting of delayed union due to poor intrinsic healing capacity, fractures in the geriatric population, osteoporosis and periprosthetic fractures.

Eco‐Friendly and Particle‐Free Copper Ionic Aqueous Precursor for In Situ Low Temperature Photothermal Synthesizing and Patterning of Highly Conductive Copper Microstructures on Flexible Substrate

Liquid precursor‐based laser‐induced reductive patterning techniques enable fast and economical fabrication of flexible electronics. Herein, an eco‐friendly and particle‐free copper ionic solution for laser‐induced reductive patterning of copper microlines on a low glass transition temperature and transparent flexible substrate is successfully developed. The solvent used is water and the reducing agent is l‐ascorbic acid. A continuous wave 640 nm laser is used as the heat source to initiate the reductive reaction and to deposit copper microlines on the substrate surface. The resistivity of the copper microlines fabricated with optimized laser parameters is remarkably low at 4.7 μΩ cm. To further improve the electrical conductivity and to enhance mechanical durability, the fabricated microlines are annealed using the same laser after the copper ionic liquid precursor is removed. The microstructures and mechanical robustness of the copper microlines, as well as the adhesion between the copper microlines and the substrate, are investigated by cyclic bending test, pull‐off test and scanning electron microscope analyses. Up to 39% reduction of electrical resistance and up to 77% enhancement of mechanical durability after a 1000 cycle bending test can be achieved by laser postprocessing.

Performance of FRCM composites and FRCM-strengthened RC beams subjected to anodic polarization and cyclic loading

Fabric reinforced cementitious matrix (FRCM) systems can provide structural strengthening (SS) of aging reinforced concrete (RC) structures. Furthermore, impressed current cathodic protection (ICCP) has been integrated with FRCM systems to develop ICCP-SS intervention systems to help improve the long-term performance of FRCM-strengthened RC structures. However, there is a lack of research regarding the fatigue performance of FRCM composites and FRCM-strengthened RC members. This study investigated the effects of fabric layers in carbon-FRCM, anodic polarization in the process of ICCP, and cyclic loading conditions on the fatigue performance of FRCM composites in tension and FRCM-strengthened RC beams in flexure. Cyclic tensile loading tests of FRCM specimens and cyclic four-point bending tests of beam specimens were conducted. Cyclic tensile test results showed that the fatigue life performance of FRCM was superior to that of steel rebar and inferior to that of carbon fiber reinforced polymer (CFRP, a composite that commonly has an epoxy resin matrix). The anodic polarization of ICCP degraded the fatigue life of carbon-FRCM at a given stress level and fatigue strength after two million loading cycles. The cyclic bending test results indicated that the fatigue performance of the beams was largely dependent on the number of fabric layers in the FRCM and slightly influenced by the anodic polarization in the ICCP-SS. A new curve of the steel stress range versus the number of cycles (S-N curve) for FRCM-strengthened beams was obtained in this study and provided satisfactory fatigue life predictions.

Experimental Study On the Critical Current of CORC Cable Under Cyclic Bending-straightening

To develop the application potential of conductor on round core (CORC) cables in the next generation of fusion magnets and accelerator magnets, fully automatic mechanized winding technology is required for the long CORC cables manufacturing. Meanwhile, the manufacturing and usage of the CORC cable are both limited by the strain sensitivity of the wound ReBCO tape on the CORC cable, because the processes may cause the superconducting layer of the ReBCO tape damaged when the strain limit is exceeded. The strains can arise from the manufacturing, bending, cooling, and operation of the cable and magnet, especially for the needed cyclic bending-straightening procedure for a long and multilayer cable winding layer by layer. Thus in this manuscript, a 1-m-long 12-layer CORC cable was manufactured to study the influence of cyclic bending on the critical current carrying performance of the CORC cable. It is consisted of 28 ReBCO tapes from Fujikura with 4 mm width and 0.05mm substrate thickness. The central core has an outer diameter of 3.85 mm. The cyclic bending-straightening experiment was conducted with a bending diameter of 900 mm and 700 mm, the Ic was tested at 77 K and self-field. After the cyclic bending test, the sample was bended to a diameter of 95 mm, which is planned to be used for the following insert solenoid coil, and has been verified no influence on the critical current carrying performance if bended without cyclic bending-straightening treatment. The results show that the required diameter for the reel experiencing cyclic bending-straightening procedure for ReBCO cable winding is suggested to be larger than 700 mm, especially for the equipment used for multilayer cables winding.

Highly aligned and densified carbon nanotube films with superior thermal conductivity and mechanical strength

Flexible thermal conducting materials with the combination of excellent thermal conductivity and mechanical strength are highly desired for many practical applications, but meeting this strict requirement remains as a big challenge. Herein, facile preparation of highly aligned carbon nanotube (CNT) films is demonstrated, which involves the continuous production of a hollow cylindrical CNT assembly, followed by in-situ alignment of CNTs through optimizing the winding rate and a further stretching-pressing process. Benefiting from the resultant excellent CNT alignment and high density (1.59 ± 0.05 g cm−3), the CNT film simultaneously has a high thermal conductivity of 558.06–700.15 W m−1 K−1, electrical conductivity of 4870 ± 150 S cm−1, mechanical strength of 2.74–2.95 GPa, and Young's modulus of 55.2–58.9 GPa. The combination of such high thermal and mechanical properties is outstanding for carbon-based thermal conducting materials. Furthermore, the CNT material exhibits negligible structural and mechanical decay even after 500 cycles of bending test. Our study provides an effective strategy toward developing new thermal conducting materials for applications under harsh conditions.

Three-dimensional printing of locally bendable short carbon fiber reinforced polymer composites

Locally bendable solid plates were manufactured in a single 3D-printing operation, using a single material, i.e., short carbon fiber reinforced plastic (CFRP). The locally bendable CFRP plates included solid and bendable parts, which were connected seamlessly using double-stepped lap configuration. A parallel cross shape structure and 100% infill structure was adopted for the bendable and solid parts, respectively. The bendability could be controlled by varying the girder angle of the parallel cross shape structure. The bending stiffness was reduced to nearly 98% compared to that of the solid plate. The cyclic bending tests indicated that the locally bendable CFRP plate underwent reversible bending deformation. The bending stiffness decreased by approximately 8–14%. However, visible damage was not observed even after 100 cycles of bending deformation.

Polyimide photodevices without a substrate by electron-beam irradiation

Despite the insulating properties of polyimide (PI), this study implemented an optoelectronic device comprising only PI. To realize suitable photocharacteristics of the PI photodetector, the PI-coated glass substrate was irradiated with an electron beam to transform the optical and electrical characteristics of PI from lower light absorption and insulation to relatively higher light absorption and conductive properties; the substrate was then removed. With the optimization of the electron-beam irradiation treatment conditions, detectivity of 6.38 × 1010 Jones was exhibited for the PI photodetector at 1 V under laser illumination at a wavelength of 450 nm. Furthermore, the mechanical characteristics of the free-standing PI photodetector under cyclic bending tests suggested a reduction of less than 25% after 1,000 cycles of bending. This demonstrates an innovative approach to the design of stable, flexible PI photodevices through elimination of a separate substrate.

Impact of organic inter-layer dielectric for improvement in mechanical flexibility of self-aligned coplanar in-Ga-Zn-O thin-film transistor

A novel device structure has been suggested to introduce organic inter-layer dielectric (ILD) for improving mechanical flexibility of self-aligned (SA) coplanar In-Ga-Zn-O (IGZO) thin-film transistor (TFT). The SA coplanar TFTs fabricated on poly(ethylene naphthalate) (PEN) substrate showed a high saturation mobility of 11.4 cm 2 /V∙s, low threshold voltage of 0.26 V, steep subthreshold swing of 115 mV/dec, and high on/off current ratio (~10 9 ) at flat state. In the proposed structure, all-oxide gate stacks were defined into an island configuration embedded in the organic ILD, and hence, the mechanical stress applied to the active region could be effectively relieved. To investigate the feasibility of a new device concept, the mechanical properties of the fabricated TFTs were characterized. When the PEN thickness was varied 25 and 50 μm, the critical radius of curvature (R C ) values were estimated to be 1 and 3 mm, respectively, which were superior to those for conventional oxide devices. In addition, the mechanical durability during the cyclic bending test at R C of 5 mm was significantly improved from 20,000 to 70,000 cycles when the PEN film thickness was reduced from 50 to 25 μm. The effects of PEN thickness on the mechanical stability of the SA coplanar TFTs were quantitatively discussed from a view point of surface strain. As a result, robust mechanical bendability of proposed device was confirmed even under harsh deformation conditions. • We fabricated flexible SA coplanar IGZO TFTs employing Zeocoat™ ILD on PEN film. • Effects of the island configuration on mechanical flexibility were investigated. • The device structure using organic ILD is favorable design for stress relaxation. • The PEN film thickness has marked impacts on strain induced on device plane. • Reduction of PEN thickness is effective for enhancing the mechanical durability.