Cyclic Bending Tests Research Articles

Mandrel Bend Test of Screen-Printed Silver Conductors

Abstract Flexible electronics are electronic devices and components that can be stretched, bent, twisted, and folded without losing their functionality. Flexible electronics is conformable, lightweight, easily tailorable, and low-cost, and thus, flexible electronics is increasingly being explored in health care, internet of things, automotive, aerospace, communication, safety, security, and food-related applications. Also, flexible electronics can now support increased functionality as well as various fabrication techniques. With an increased adaptation of flexible electronics, research is being conducted to better understand the failure mechanism of flexible electronics and thus improve their reliability and service life. In this paper, a cyclic mandrel bend test has been designed and carried out on printed conductors with PET and PI substrates. With the designed test apparatus, both tensile and compressive bend tests have been performed. Using a four-wire method, the resistance change of the printed conductors with different widths has been measured in situ under tensile and compressive loading conditions using mandrels with different radii. The results have been compared among different conductor widths, bending modes, and substrate materials. Besides, in situ SEM images have been taken to understand the failure mechanisms of the printed conductors. Based on the study, it is seen that there exists a direct correlation between the mandrel diameter, the damage in the printed conductor, and thus, the resistance change with cyclic mandrel testing. Also, it is seen that the damage under compressive bending mode is significantly lower than the damage under tensile bending mode.

Multifunctional Superamphiphobic Cotton Fabrics with Highly Efficient Flame Retardancy, Self-Cleaning, and Electromagnetic Interference Shielding

Here, a facile method is reported to prepare multifunctional cotton fabrics with high flame retardancy, high electrical conductivity, superamphiphobicity, and high electromagnetic shielding. The cotton fabric surface was first modified with phytic acid (PA), which promoted dehydration and carbonization of cellulose to increase flame retardancy in the process of pyrolysis. Tannic acid (TA) and 3-aminopropyltriethoxysilane (APTES) coating with nanospheres as interlayers created hierarchical roughness that facilitated the construction of superamphiphobic surfaces and provided adhesion sites for silver nanoparticles. In addition, the TA-APTES coating improved flame retardancy because the APTES-containing silicon could form silicon carbon layers to isolate heat and oxygen. Subsequently, the surface energy of the composite cotton fabric was reduced by fluorine-containing molecules. The prepared composite cotton fabric exhibited excellent superamphiphobicity with contact angles of 160.3 and 152° for water and olive oil, respectively. The conductivity and EMI shielding efficiency of the prepared composite cotton fabric reached 629.93 S/cm and 76 dB, respectively. Importantly, the composite cotton fabric maintained a relatively stable EMI shielding efficiency even after cyclic bending and abrasion tests. Moreover, the composite cotton fabric possessed a high limiting oxygen index (LOI) of 45.3% and self-extinguishing properties with the peak heat release rate (PHHR) and total heat release (THR) reduced by 73 and 67%, respectively, than the pure cotton fabric, indicating the outstanding flame retardancy.

CYCLIC BENDING TESTS ON SEVERAL DIAMETERS CARBON STEEL PIPE FOR ORDINARY PIPING

Many buildings had stopped their service due to damage to non-structural members including ceilings, partition walls and piping after large-scale earthquakes. In particular, piping-failure-caused water leakage damaged electronic equipment and resulted in a loss of functionality of the building. Cyclic bending tests are conducted of carbon steel pipes for water supply piping with nominal diameters of 25A, 40A, 50A and 80A to evaluate their deformation capacity and strength. The results are summarized as follows.

Mechanical Properties of Aluminium/Copper Bimetallic Sheets Subjected to Cyclic Bending

The aim of this study is to evaluate the influence of the roll bonding (RB) process on the mechanical properties of Al/Cu bimetallic strip specimens. A specially designed test instrument for the cyclic bending test of strip specimens produced by RB has been proposed. The test consists of repeated pulling of a strip of sheet metal through a system of cylindrical countersamples. Samples cut along and across the sheet rolling direction were tested. The specimens were subjected to preliminary microstructural tests using an optical microscope with Nomarski contrast and scanning electron microscopy. The tests showed a slight influence of the orientation of the samples on changes to the mechanical properties of the bimetallic sheets in the cyclic bending process. For samples oriented along the sheet rolling direction, discontinuities in the transition layer were found. The samples oriented perpendicularly to the rolling direction were free of this defect.

GMDH-Kalman Filter prediction of high-cycle fatigue life of drilled industrial composites: A hybrid machine learning with limited data

In industrial composites applications, drilling is one of the most common operations and complex processes during the final assembly, which can generate undesirable damage to the manufactured part. Data collection from a given composite’s fatigue life is often costly and time-consuming. To address this challenge, the current case study aims to adapt a hybrid machine learning framework to predict the fatigue life of the drilled Glass Fiber Reinforced Polymer composite laminates (with both unidirectional and woven lay-ups) under a limited and noisy data assumption. Composite specimens were drilled at various cutting speeds and feed rates. The size of the delamination around the hole was scanned by a microscopic camera. Cyclic three-point bending tests were conducted, and results indicated that the drilling-induced delamination size and the composite lay-up affect the specimens’ fatigue lives. The latter were then modeled in two steps. In the first step, an offline deterministic model was established using the group method of data handling along with a singular value decomposition. Pareto multi-objective optimization was applied to prevent overfitting. In the second step, the Kalman filter was employed to update the polynomial of the deterministic model based on minimizing the mean and variance of error between the actual and modeled data. Results showed an excellent learning reliability, with a correlation coefficient of 97.6% and 96.5% in predicting the fatigue life of unidirectional and woven composite laminates, respectively. A sensitivity analysis was performed and indicated that the fatigue life of the samples has been more affected by the drilling feed rate, compared to the cutting speed.

Line‐Shaped Laser Lithography for Efficiently Fabricating Flexible Transparent Electrodes with Hierarchical Metal Grids across λ/10 to Microscale

AbstractHierarchical metal grids are proven to be a promising solution for achieving high‐performance flexible transparent electrodes (FTEs) due to their significantly enhanced conductivity without noticeably sacrificing transparency. This work develops a one‐step mask‐free line‐shaped laser lithography by separated pulse laser ablation to efficiently fabricate large‐area FTEs composed of hierarchical metal grids, namely microscale grids interconnected with aligned nanowire arrays. The linewidth of aligned wires is highly controllable from the nanometer scale far beyond the diffraction limit (<λ/10) to the micrometer scale. This work experimentally studies the overall performance of FTEs to provide a guideline for selecting the layout and feature sizes of hierarchical metal grids. Hierarchical metal grids with 50 nm nanowires aligned inside the microscale grids show outstanding optoelectronic properties and mechanical stability, with sheet resistance of 4.6 Ω sq−1, transmittance of 82.9%, and a tiny increase in sheet resistance less than 4% after 1000 cycles of bending tests. These results prove that the line‐shaped laser lithography technique is a facile, low‐cost, and high‐throughput fabrication method for high‐performance FTEs.

Deep neural network-based structural health monitoring technique for real-time crack detection and localization using strain gauge sensors

Structural health monitoring (SHM) techniques often require a large number of sensors to evaluate and monitor the structural health. In this paper, we propose a deep neural network (DNN)-based SHM method for accurate crack detection and localization in real time using a small number of strain gauge sensors and confirm its feasibility based on experimental data. The proposed method combines a DNN model with principal component analysis (PCA) to predict the strain field based on the local strains measured by strain gauge sensors located rather sparsely. We demonstrate the potential of the proposed technique via a cyclic 4-point bending test performed on a composite material specimen without cracks and seven specimens with different lengths of cracks. A dataset containing local strains measured with 12 strain gauge sensors and strain field measured with a digital image correlation (DIC) device was prepared. The strain field dataset from DIC is converted to a smaller dimension latent space with a few eigen basis via PCA, and a DNN model is trained to predict principal component values of each image with 12 strain gauge sensor measurements as input. The proposed method turns out to accurately predict the strain field for all specimens considered in the study.

Biomechanical Properties of Novel Lateral Hole Pedicle Screws and Solid Pedicle Screws: A Comparative Study in the Beagle Dogs.

Although pedicle screws are widely used to reconstruct the stability of the spine, screw loosening is a common complication after spine surgery. The main objective of this study was to investigate whether the application of the hollow lateral hole structure had the potential to improve the stability of the pedicle screw by comparing the biomechanical properties of the novel lateral hole pedicle screws (LHPSs) with those of the solid pedicle screws (SPSs) in beagle dogs. The cancellous bone of the distal femur, proximal femur, distal tibia, and proximal tibia were chosen as implantation sites in beagle dogs. In each of 12 dogs, four LHPSs, and four SPSs were implanted into both lower limbs. At 1, 2, and 3 months after surgery, four dogs were randomly sampled and sacrificed. The LHPS group and SPS group were subdivided into four subgroups according to the length of their duration of implantation (0, 1, 2, 3 months). The biomechanical properties of both pedicle screws were evaluated by pull-out and the cyclic bending tests. The results of the study showed that no significant difference was found between LHPSs (276.62 ± 50.11 N) and SPSs (282.47 ± 42.98 N) in pull-out tests at time 0 (P > 0.05). At the same time point after implantations, LHPSs exhibited significantly higher maximal pullout strength than SPSs (month 1: 360.51 ± 25.63 vs 325.87 ± 28.11 N; month 2: 416.59 ± 23.78 vs 362.12 ± 29.27 N; month 3: 447.05 ± 38.26 vs 376.63 ± 32.36 N) (P < 0.05). Moreover, compared with SPSs, LHPSs withstood more loading cycles (month 2: 592 ± 21 vs 534 ± 48 times; month 3: 596 ± 10 vs 543 ± 59 times), and exhibiting less displacement before loosening at month 2 (1.70 ± 0.17 vs 1.96 ± 0.10 mm) and 3 (1.69 ± 0.19 vs 1.92 ± 0.14 mm) (P < 0.05), but no significant difference in time 0 and month 1 (P > 0.05). The pedicle screw with the hollow lateral hole structure could allow bone to grow into the inner architecture, which improved biomechanical properties by extending the contact area between screw and bone tissue after implantation into the cancellous bone. It indicated that LHPS could reduce loosening of the pedicle screws in long term after surgery.

Advanced structural health monitoring in carbon fiber-reinforced plastic using real-time self-sensing data and convolutional neural network architectures

In this study, advanced structural health monitoring (SHM) using a non-destructive self-sensing methodology was proposed for large-sized carbon fiber-reinforced plastic (CFRP). Cyclic point bending tests were performed on three types of CFRPs. The damage severity identification and localization were classified and investigated using four different convolutional neural network (CNN) architectures. Electrical resistance images were used to train each CNN architecture for damage analysis. An optimized CNN architecture for the damage analysis of CFRPs using electrical resistance data was proposed and compared with traditional damage analysis CNN architectures. The applicability of the proposed SHM methodology was verified by analyzing unseen damage in the CFRPs. This study addresses the limitations of previous self-sensing methods by reducing the number of electrodes, which reduces data complexity and increases the sensible area of CFRPs. Thus, this study successfully designed an efficient SHM methodology with a high accuracy of over 90 % for analyzing CFRP damage, including the severity and location, regardless of the type of carbon fiber and stacking sequence of composite structures that showed high applicability.

High-Resolution Femtosecond Laser-Induced Carbon and Ag Hybrid Structure for Bend Sensing.

Miniaturized resistance-based portable bending sensors have been widely used for human health monitoring in recent years. Their sensitivities are defined by their resistance variations (ΔR/R), which strongly rely on the conductivity and minimum line width of the sensing unit. Laser-induced carbonization is a fast and simple method to fabricate porous-sensing structures. However, the fabrication resolution of conductive and deformation-sensitive structures is limited by the thermal effect of commonly used laser sources. With the assistance of femtosecond laser temporal shaping, plasma ejection confinement, and silver nitrate doping, the sheet resistance of the sensing structure was improved from 15 to 0.0004 Ω/□. A thin line with a lateral resolution of 6.5 μm is fabricated as the sensing unit. The fFabricated structures are characterized by electron microscopy, Raman spectroscopy, energy-dispersive spectroscopy, X-ray scattering, and time-resolved images. The strain sensor demonstrates a ΔR/R of 25.8% with a rising edge of 109 ms in the cyclic bending test. The sensor is further applied for detecting human pulse and finger bending.

Cyclic behavior of FeCoCrNiMn high entropy alloy coatings produced through cold spray

FeCoCrNiMn high entropy alloy powders were employed to produce coatings on carbon steel through high pressure cold spray (at 1100 °C and 7 MPa in temperature and pressure respectively). X-ray diffraction of the sprayed material revealed a dense coating with the retention of the original crystallographic structure. Once splatted, particles revealed high flattening ratio with consequent excellent adhesion of the coating to the substrate. Residual stresses and hardness increased through the thickness of the coating. The cyclic behavior of the coatings was evaluated through bending tests. The cyclic bending tests were performed by linearly increase the maximum deformation at a fixed number of loading cycles. Other bending tests were conducted at fixed maximum stroke at the same deformation levels indicated for the previous test up to the coating fracture. For ach 100 cycles the test was stopped in order to observe the coating surface to gain the possibility of evaluating the aspect of the superficial fracture. The fracture surface of the coatings was observed through scanning electron microscopy in order to describe the cracking features related to the different loading conditions imposed to the coatings.

High-Throughput Manufacturing of Flexible Thermoelectric Generators for Low- to Medium-Temperature Applications Based on Nano-Silver Bonding

The melting point of the commercial soldering materials is a limiting factor for the functionality of the thermoelectric generators (TEGs) in medium-temperature ranges higher than 200 °C. Hence, using a proper bonding method which could be formed at lower temperatures and withstand higher temperatures is inevitable to unlock the full potential of the commercial thermoelectric materials. Nano-silver bonding is known as the “low-temperature joining technique” due to its lower processing temperature compared with the other conventional bonding approaches. This study is focused on presenting a fabrication process of flexible TEGs using nano-silver (Ag) paste bonding. The pellets of thermoelectric materials were prepared by wire cutting of bismuth telluride ingots in desired sizes. The electrical interconnects between the thermoelectric legs are nano-silver paste traces printed by a digitally control dispenser printer. Reliability of the thermoelectric module was evaluated from room temperature up to 300 °C as well as conducting the cyclic bending test. The proposed fabrication technique is similar to the flexible hybrid electronics (FHE) concept, which is scalable for high-throughput manufacturing, such as roll-to-roll or sheet-to-sheet printing.

Repeated bending durability evaluation of ZnO and Al-doped ZnO thin films grown on cyclo-olefin polymer for flexible oxide device applications

ZnO and Al-doped ZnO (AZO) thin films were fabricated on cyclo-olefin polymer (COP) substrates with different thicknesses for flexible device applications, and their bending strength was evaluated using surface observations and two-terminal resistance measurements. The fabricated films were subjected to repeated bending endurance tests up to 10 000 cycles. As COP substrate thickness was decreased, surface distortion decreased, two-terminal resistance fluctuations decreased, cracks in SiO2 buffer layer suppressed, and bending strength improved. When COP substrate thickness was 50 μm, variation in two-terminal resistance was negligible, and an oxide thin film device with durability of 10 000 cycles was successfully fabricated. Crystallinity evaluation using X-ray diffraction and mechanical characterization using nanoindentation were performed. This study discusses ZnO and AZO thin films for flexible device applications in relation to their surface conditions, electrical properties, crystallinity, and mechanical properties in cyclic bending tests. The results indicate that these thin films are useful materials for flexible devices.

Cracking Behaviour of Alkali-Activated Aluminosilicate Beams Reinforced with Glass and Basalt Fibre-Reinforced Polymer Bars under Cyclic Load

Cement is an essential material for concrete, which is mostly used worldwide second to the consumption of water. Due to the emission of CO2 into the atmosphere, the alternative material of geopolymer concrete was used. In this research work, silica and alumina content such as ground granulated blast furnace slag (GGBS), fly ash, and triggered by alkali activator solutions were used in geopolymer concrete. Due to the dwindling of river sand, alternative material of manufactured sand (M-Sand) was considered. To avoid corrosion problems in reinforced concrete structures, glass fibre reinforced polymer (GFRP) and basalt fibre-reinforced polymer (BFRP) bars were used as an alternative material for steel reinforcement in this work. As per the code, IS: 10262, the concrete mix design of M30 grade has arrived for the control mix and the same proportion was adopted for geopolymer concrete. Six beams of geopolymer and a concrete control beam of 100 × 160 × 1700 mm were cast and examined under a four-point cyclic load. Cyclic load results were compared with static load under ambient curing. Residual deflection, moment capacity, energy dissipation, and stress–strain behaviour results were compared and discussed. A sudden shear and premature failure were observed in FRP beams under static and cyclic bending tests.

Toward flexible X-tronics: Transparent conductors with enhanced room-temperature ferromagnetism using Sn–Mn-codoped In2O3/mica thin films

Herein, flexible transparent conductors (TCs) with enhanced room-temperature ferromagnetism were fabricated via growing an Sn–Mn-codoped In2O3 (ITMO) film on a flexible mica substrate. The ITMO/mica film shows an average transmittance (Ta) of 81.9% in the visible region and a sheet resistance (Rs) of 58 Ω, which are comparable to those of the flexible Sn-doped In2O3 (ITO) and Mn-doped In2O3 (IMO) films in the previous reports. Moreover, the ITMO/mica film exhibits a room-temperature ferromagnetism with a saturation magnetization (Ms) of 29.1 emu/cc and a coercive field (Hc) of 430 Oe. The ferromagnetism in ITMO/mica film is much stronger than that in the previously reported IMO/mica films. The charge transport measurements in conjunction with bending tests indicate that the ITMO/mica film possesses large critical bending strains (ϵcr) and thus a robust mechanical durability. The optical and magnetic properties of the ITMO/mica film are also well maintained during the bending cyclic tests. The demonstration of the flexible multifunctional ITMO/mica films with remarkable performance in various aspects paves a route toward future flexible X-tronics.

Flexible miniaturized thin film thermocouples arrays for high temperature applications

Thin film thermocouples (TFTCs) are critical for surface temperature measurements of hot components in high temperature environments. In this paper, miniaturized Pt/Pt-10%Rh thin film thermocouple arrays were fabricated on flexible Hastelloy substrates by magnetron sputtering. The morphology and microstructure of Al2O3/Y2O3 insulating films were investigated by a field-emission scanning electron microscope with an energy dispersive X-ray spectroscopy system. The thermoelectric performance of the TFTCs arrays was investigated by cyclic calibration and bending tests. During the whole calibration process, the average maximum temperature differences (ΔT′ ) of measuring points 2, 3, and 4 are 637 °C, 617 °C, and 574 °C, respectively, which differ from the maximum temperature difference (ΔT max ) of measuring point 1 by -13 °C, -34 °C, and -76 °C, respectively. The calibration results show that each single measuring point has good repeatability and stability during the five calibration processes, and the temperature difference between multiple measuring points is obvious, stable and reliable. The thermoelectric properties of the thin film thermocouple arrays barely change after 10,000 cyclic bending tests.

Spatial-Interleaving Graphene Supercapacitor with High Area Energy Density and Mechanical Flexibility.

The booming portable electronics market has raised huge demands for the development of supercapacitors with mechanical flexibility and high power density in the finite area; however, this is still unsatisfied by the currently thickness-confined sandwich design or the in-plane interdigital configuration with limited mechanical features. Here, a spatial-interleaving supercapacitor (SI-SC) is first designed and constructed, in which the graphene microelectrodes are reversely stacked layer by layer within a three-dimensional (3D) space. Because each microelectrode matches well with four counter microelectrodes and all 3D spatial-interleaving microelectrodes have narrow interspaces that maintain the efficient ions transport in the whole device, this SI-SC has a prominent liner capacitance increase along with the device thickness. As a result, the high specific areal capacitance of 36.46 mF cm-2 and 5.34 μWh cm-2 energy density is achieved on the 100 μm thick device. Especially, the microelectrodes in each layer are interdigitated, ensuring the outstanding mechanical flexibility of SI-SC, with ∼98.7% performance retention after 104 cycles of bending tests, realizing the excellent integration of high area energy density and mechanical flexibility in the finite area. Furthermore, the SI-SC units can be easily integrated into wearable electronics to power wristwatches, light-emitting diodes (LEDs), calculators, and so on for practical applications.

Failure initiation and progression of sandwich beams under cyclic bending tests

This study presents experimental results and analysis of failure of sandwich beams with steel face sheets and polyurethane core tested in three-point bending. Firstly, a series of experiences are conducted in order to obtain the mechanical properties of the metallic face sheets and the foam core. To model the foam behavior, quasi-cyclic tests were performed by increasing the punch stroke. An exponential model is proposed describing the variation of the elastic modulus as a function of the strain. Secondly, the quasi-static and quasi-cyclic bending tests are carried out leading to failure of the sandwich beam. The results of the quasi-cyclic bending show that the behavior of the sandwich beam is similar to the foam behavior in quasi-cyclic compression. Thirdly, a Finite Element Analysis (FEA) is performed in order to predict the observed foam fracture in bending test using a shear damage criterion for the foam core. The comparisons show a satisfactory agreement between the experimental results and the FEA predictions.

Mechanically Flexible Thermoelectric Hybrid Thin Films by Introduction of PEDOT:PSS in Nanoporous Ca3Co4O9.

Nanoporous Ca3Co4O9 exhibits high thermoelectric properties and low thermal conductivity and can be made mechanically flexible by nanostructural design. To improve the mechanical flexibility with retained thermoelectric properties near room temperature, however, it is desirable to incorporate an organic filler in this nanoporous inorganic matrix material. Here, double-layer nanoporous Ca3Co4O9/PEDOT:PSS thin films were synthesized by spin-coating PEDOT:PSS into the nanopores. The obtained hybrid films exhibit high Seebeck coefficient (∼+130 μV/K) and thermoelectric power factor (0.75 μW cm–1 K–2) at room temperature with no deterioration in electrical properties after cyclic bending tests (98% preservation of electrical conductivity after 1000 cycles bending to a bending radius of 3 mm). Compared with the nanoporous Ca3Co4O9 thin film, the mechanical flexibility of the hybrid film can be effectively improved after hybrid with PEDOT:PSS with only a slight decrease of the thermoelectric properties.

Low Cost and Facile Fabrication of a Micro‐Mold with High Aspect Ratio for Transparent Electrodes with Metal Mesh Using Micro‐Scale 3D Printing

AbstractTransparent electrodes (TEs) with metal mesh are regarded as a substitute for traditional indium tin oxide (ITO) due to their excellent optoelectronic properties. The manufacture of metal mesh based on micro‐molds will be a low‐cost and high‐efficiency method, but the cost‐effective fabrication of micro‐molds with a high aspect ratio (AR) currently faces challenges. Here, a polymer micro‐mold with high AR based on an electric‐field‐driven (EFD) micro‐scale 3D printing and molding process is proposed for the mass production of TEs with metal meshes. The final fabricated flexible transparent electrode (FTE) based on polymer micro‐mold with high AR exhibits superior optoelectronic properties with a figure of merit (FOM) of 1800, as well as excellent mechanical stability with a slight increase in the sheet resistance (Rs) during cyclic bending, scratching, torsion, and adhesion tests. Furthermore, the fabricated rigid TE based on polymer micro‐mold shows remarkable performance and stability with a FOM of 2500, a negligible increase in the Rs under harsh working conditions, and a robust heating cycle. Whether used for the manufacture of FTEs or rigid TEs, the polymer micro‐mold shows good service life. This strategy provides support for the efficient and environmentally friendly mass production of high‐performance TEs.