Abstract: In tube free-bending, the connection of the bending die to the guide has a great influence on the stability of mechanical movement and the forming limit of the bent tube. The general connection of the bending die to the guide cannot realize the continuous stable movement of the bending die. Spherical connection of the bending die to the guide can guarantee a continuous changed tipping of the bending die, but has a limited minimal bending ratio (ratio of bending radius to tube outer diameter) of R/D0 ≥ 3 depending on the geometrical constrains. In this paper, a new spherical connection of the bending die to the guide was designed through geometric derivation, which can lead to a greater deflection of the bending die, to realize the bending ratio of down to R/D0 = 2.5. Based on the designed spherical connection, the bending ratio of down to R/D0 = 2.43 and R/D0 = 2.78 of brass tube were realized in the finite element simulation and bending test respectively. And with the analytical model, simulation model and experimental bending results, the forming characteristics of bent tube with small bending radii based on the spherical connection structure were investigated. The main results show that the equivalent stress, displacement of the strain neutral layer (NL) increase with the decrease of bending radius, and the wall thickness thinning in outer bend shows little differences under tight and big bending radii due to the increased axial thrust.

Abstract: A novel high-birefringence hollow-core anti-resonant THz fiber is proposed in this paper. This fiber has a simple structure which consists of only ten Topas tubes. High birefringence is achieved by introducing two large tubes. The first two resonant frequencies are 1.44 and 2.88 THz by fixing tube thickness at 0.09 mm, which makes two low-loss transmission windows exist in the frequency range from 0.8 to 3.0 THz. The lowest loss is 2.10 dB/m occurring at 1.2 THz in the first transmission window and 1.68 dB/m at 2.34 THz in the second transmission window. By optimizing the structure parameters, high birefringence above 7 × 10−4 in the frequency range from 1.0 to 1.24 THz are obtained. The highest birefringence is up to 8.7 × 10−4 at 1.04 THz. Birefringence can be further increased to the order of 10−3 by adjusting the structure parameters at the cost of loss increasing and the bandwidth decreasing. In addition, bent performance of this fiber is also discussed. In addition, this fiber can keep good performance when it is bent for x-direction. At the bend radius of 15 cm, the loss and birefringence has a more slightly change in the first transmission window than the second transmission window. The first transmission window own much better bent-insensitive characteristics.

Abstract: A novel optical fiber displacement sensor based on composite interference established within a balloon-shaped bent multimode (BSBM) fiber structure is described and experimentally demonstrated. The BSBM fiber structure is realized by bending a straight single-mode–multimode–single-mode (SMS) fiber structure into a balloon shape using a length of capillary tube to fix the shape of the structure. Owing to the bend in the multimode waveguide, the original undistorted multimode interference pattern is changed, and an extra Mach–Zehnder interferometer is effectively introduced within the multimode fiber (MMF) section at a suitable bending radius. This established composite interference greatly improves the displacement sensing performance of the SMS fiber structure. A maximum displacement sensitivity of 0.51 dB/μm over the displacement range of 0–100 μm at the operating wavelength of 1564.7 nm is achieved experimentally. Based on its easy fabrication process, low cost, and high measurement sensitivity, the sensor of this investigation could be a realistic candidate in the high-accuracy displacement measurement field.

Abstract: This paper reports the feasibility of using an ultrasonic waveguide sensor for distributed temperature measurement in two different case studies, that is: 1) skin temperature of a solid structure (pipe) and 2) fluid (water). This technique improves upon the conventional multiple thermocouples approach for multi-level temperature measurements. The range of temperatures is from room temperature to maximum utility temperature for the two case studies (85 °C for water and 200 °C for pipe). Using the interaction of the propagating ultrasonic waves in a thin rodlike waveguide with intentionally designed geometric discontinuities (bends, axisymmetric, non-axisymmetric notches, and so on), the localized information on the temperature is extracted. An ultrasonic technique for accurate temperature measurement by tracking the time-of-flight of reflected guided wave modes from appropriately spaced notch reflectors is therefore proposed here, while using the reflection from a bend is used as a reference. Using a finite element model approach, the notch size and/or the bend radius were selected in order to reduce mode conversion effects as well as to obtain uniform amplitudes of the reflected signals from these designed discontinuities. The numerical results were experimentally validated for the L(0,1) wave mode using a stainless steel waveguide sensor. This paper is of interest to industrial applications including mould cooling jacket temperature monitoring during the steel manufacturing process as well as for furnace wall temperature measurements in petrochemical industries.

Abstract: The bending radius (R) of the bending component is defined by the deflection (U) of the bending die in the tube free-bending process. The U-R relationship is the key factor to obtain the precise geometry size of the complex bending tubular components. Therefore, this study aims at proposing a new method to predict the U-R relationship for the arbitrary power hardening aluminum alloy (Al alloy) circular tube based on the U-R relationship of the reference material and the sensitivity analysis of material parameters, which may reduce many experimental works. In the current study, AA1100 alloy was set as the reference material, and the effects of each material parameter on the U-R relationship were investigated by carrying out the deformation and sensitivity analysis of the FEA simulation results. The results show that the bending radius increases with the decrease of elastic modulus (E), density (ρ), and strain-hardening exponent (n) and the increase of strength coefficient (K) and initial yield stress (σs), where σs has the greatest influence on the U-R relationship. Moreover, the U-R relationship prediction method for arbitrary power hardening Al alloy circular tube was presented based on the sensitivity analysis of the reference material. Finally, the bending tests of the AA6061-T6 tubes were carried out to prove the accuracy of the U-R relationship prediction method. The bending results show that the experimental U-R relationship of the AA6061-T6 tube was consistent with the predicted value, and the prediction method had good applicability to power hardening Al alloy circular tube.

Abstract: A novel design of segmented cladding fiber (SCF) with resonant ring is proposed in this paper. A high leakage loss ratio (>100) between the high-order modes (HOMs) and the fundamental mode can be achieved at wavelength 1.064 μm. The HOMs suppression is better than the traditional SCF. The numerical results show that the effectively single-mode operation with a mode area of 790 μm2 can be achieved at a bend radius of 15 cm. Besides, the fiber performance is insensitive to the bending orientation at the ranging of [−180°, 180°]. This fiber design shows great potential in developing compact high-power fiber lasers and amplifiers.

Abstract: A flexible fan-out wafer-level packaging (FOWLP) process for heterogeneous integration of high performance dies in a flexible and biocompatible elastomeric package (FlexTrateTM) was used to assemble >600 dies with co-planarity and tilt <1µm, average die-shift of 3.28 µm with ? < 2.23 µm. We have also engineered a novel corrugated topography of a stress buffer layer for metal interconnects on FlexTrateTM to mitigate the buckling phenomenon of metal films deposited on elastomeric substrates. Corrugated interconnects were then tested for their mechanical bending reliability and have shown less than 0.4% change in resistance after bending at 1 mm radius for 1,000 cycles. Finally, we demonstrate integration of an array of 25 dielets interconnected in a daisy chain configuration at 40 µm interconnect pitch.

Abstract: Bend-forming is an efficient and economical method to manufacture curved sandwich panels, and the major concerns for fabricating precise products are forming characteristics and springback prediction. In this paper, numerical simulation, analytical approach, and forming experiments were conducted to assess the multi-point cylindrical bend-forming process of bi-directional trapezoidal sandwich panel. Analysis was performed on the deformation characteristics, regularity, and forming defects of sandwich panel in the early stage. And afterwards, equivalent elastic constants of the core layer were deduced by the semi-analytic approach combined with finite element method (FEM). On this basis, a theoretical model was established to calculate bending moments and predict springback within formable range. The results indicate that the stress of face sheet in the welded area was obviously lower than that in the suspended area. The core cell was primarily deformed by changing the angle between the inclined plane and the platform, and the deformation of the cell layer mainly occurred in the transition surface area. The dimple and straight plane effect are the most common forming defects during the bending process. The main factors that affect the forming defects are face sheet thickness and bending radius. The springback amount of the sandwich panel approximates the equivalent thickness plate. Thus, it is easy to control the forming precision for small springback, and the springback ratio increases with the increase of bending curvature radius. The springback in theoretical model and experiment has an error of less than 1.0 mm, verifying the accuracy of the theoretical prediction model.

Abstract: We present the design, fabrication and characterization of long-range surface plasmon polariton waveguide arrays with materials, mainly silicones, carefully selected with the aim to be used as mechanically flexible single-mode optical interconnections, the so-called “plasmonic arc” working at 1.55µm. The fabricated plasmonic arcs show a TM/TE polarization ratio of ~25 dB. By using the cut-back method, the straight propagation loss at 1.55µm is estimated to 0.5-1 dB/mm and coupling loss to ~1-2 dB/facet after dicing. In the free-standing S-curved configuration, the bending loss of single cladding plasmonic arc is 2.2-2.8 dB/90° at bending radius 2.5 mm. For double cladding plasmonic arcs, it is decreased to 0.7-1.7 dB/90° for the same radius. The coupling loss with single-mode glass PCB waveguides is estimated to be 1.7 dB/interface in the best condition.

Abstract: We propose a bend-loss free, high-resolution, fiber-optic distributed sensor by using a new sensing fiber namely Ge-doped-core photonic crystal fiber (PCF) in optical frequency domain reflectometer (OFDR). PCF is fabricated with high concentration of Ge-doping in the small core leading to large refractive index difference between core and air-silica cladding. We achieved negligible macrobending loss in our Ge-doped-core PCF for a bend radius of curvature down to 1 mm in experiment and the results are validated by numerical simulation. By using OFDR, with a bend radius of 1 mm, the strain and temperature sensitivities in Ge-doped-core PCF are measured to be 0.138 GHz/ $\mu \varepsilon $ and 1.46 GHz/°C, respectively, with 5 cm spatial resolution.

Abstract: Flexible electronics in wearable applications may be subjected to flexing, bending, stretching in addition to exposure to temperature and humidity. Presently, there is a general lack of test protocols for the reliability assessment and survivability assurance of the flexible substrates. Flexing and bending in operation may be accrued under stresses of daily motion. Flexible substrates often use serpentine patterns to accommodate high stretch in the neighborhood of 25-100 percent. Flexible copper traces subjected to cyclic mechanical bending result in stretching of the outside layers and simultaneously compressing the inner layers. Cyclic bending may also result in formation of wrinkles on the flexible substrate causing delamination. Meaningful accelerated test protocols and acceleration transforms relating test-performance to operational reliability are needed. In this paper, a new test protocol has been developed for replicating the stresses of daily motion for flexible substrates. The conventional-fabricated flexible circuits have been studied. Test coupons have been designed to include the common trace geometries encountered in flexible electronics applications. The effect of cyclic mechanical bending, exposure to human body temperature has been studied and analyzed using Flex-PCBs. Flex-PCBs have been subjected to cyclic bending and the failure modes were analyzed as a function of bend radius, bend angle and number of cycles to failure. Extremely tight bend radius and bend angle larger than 90° have been studied. Further, in order to monitor the resistance of copper traces and develop life prediction model, Data Acquisition unit has been used. This prognostic health monitoring technique captures the increase in resistance of copper traces with the growth in fatigue due to cyclic mechanical bending. The study addresses the need for life prediction models of flexible copper traces on flexible polyimide substrate.

Abstract: In this paper, an asymmetric large-mode-area photonic crystal fiber (LMA-PCF) with low bending loss at a smaller bending radius is designed. The finite-element method with a perfectly matched layer boundary is used to analyze the performance of the PCF. To achieve LMA-PCF with low bending loss, the air holes with double lattice constants and different sizes at the core are designed. Numerical results show that this structure can achieve low bending loss and LMA with a smaller bending radius at the wavelength of 1.55 μm. The effective mode area of the fundamental mode is larger than 1000 μm2 when the bending radius is ≥10 cm. The bending loss of the fundamental mode is just 0.0113 dB/m, and the difference between the fundamental and high-order modes of the bending loss is larger than 103 when the bending radius is 10 cm. Simulation results show this novel PCF can achieve LMA and have effective single-mode operation when the bending orientation angle ranges in ±110°. This novel photonic crystal has potential application in high-power fiber lasers.

Abstract: In this investigation, the attention is focused on the minimum bending radii of 2196-T8511 and 2099-T83 Al-Li alloy extrusions. To predict the failure of Al-Li alloys, sheet and extrusion stretch bending tests are developed, carried out and simulated using finite element model. The theoretical minimum bending radius is introduced to derive a safe lower limit for the bending radius which can serve as a guideline for tool and product design. Stretch bending tests of Al-Li alloys are performed using the three-point bending test and displacement-controlled stretch bending test at room temperature. The finite element model incorporates three-dimensional solid elements and ductile damage modeling. The experimental results show that Al-Li alloy extrusions in stretch bending show three types of failures, occurring at the unbent region near the entrance of the jaws, at the region below the exit of the die and within the region in contact with the die, respectively. Comparison between predicted values and experimen...

Abstract: We report a low loss U-bend waveguide for realization of GaAs-based gain elements employed in hybrid photonic integration. This architecture allows us to position the input and output ports of the gain waveguide on the same facet and thus alleviates the geometrical constrains in hybrid integration, i.e., the need for precise alignment with silicon photonic waveguides on both ends of the III–V chip. As an exemplary demonstration, we report the loss and gain characteristics of GaInNAs/GaAs U-bend waveguides operating at 1.3 μm. In particular, we demonstrate a bending loss as low as 1.1 dB for an 83 μm bending radius. Efficient laser diode operation is also demonstrated.

Abstract: A novel segmented cladding fiber structure is proposed in this paper for large-mode area properties. In this structure, a parabolic-profile core is surrounded by segmented cladding. It is called parabolic-profile core segmented-cladding fiber. The novel fiber has a better single-mode operation than segmented-cladding fiber. A large effective area of 980 µm2 is achieved. It is found that the effective-mode area does not vary with the changing of the bending radius. The bending performance does not change with varying the bending orientation.

Abstract: A novel structure of modified multi-trench fiber (MTF) with characteristics of bend-resistance and large mode-area is proposed. In this structure, each low refractive-index trench of traditional MTF is broken by two gaps up and down. Numerical investigations show that the mode field area of 840 μm2 can be achieved with effective single-mode (SM) operation when the bending radius is 15 cm. Moreover, the high order mode (HOM) suppression of the proposed design is better than that of standard MTF. The SM operation property can be enhanced with the decreases of bending radius. The proposed design shows great potential in high power fiber lasers with compact structure.

Abstract: The understanding of how bending modifies the dispersion of optical fibers, in particular, the zero-dispersion wavelength (λ0), is essential in the development of compact nonlinear optical devices such as parametric amplifiers, wavelength converters, soliton lasers and frequency comb generators. Typically, substantial variations in the parametric gain and/or conversion efficiency are significant for changes in λ0 of ~0.1 nm, which occur for variations on the bending radius (Rb) of 1 cm or less. Measuring λ0 as a function of bending radius (Rb) is challenging, as it requires detecting changes < 0.1 nm and in short fibers. By using a method based on four-wave mixing (FWM) generated by an incoherent-pump with relatively broad spectrum and a weak laser, we report measurements of λ0 as a function of Rb in a dispersion-shifted fiber with <0.1 nm accuracy on λ0. This method is sensitive enough to measure small variations in λ0 of ~0.04 nm in very short fibers (~20 m). We observe that λ0 increases by 12 nm when Rb is decreased from 10 cm to 1 cm, and a change of 1 nm is obtained for Rb = 3 cm. We also present numerical simulations of the bent fiber that are in good agreement with our measurements, and help us to explain the observations and to predict how high-order dispersion is modified with bending. This study can provide insights for dispersion engineering, in which bending could be used as a tuning, equalization, or tailoring mechanism for λ0, which can be used in the development of compact nonlinear optical devices based on fibers or other bent-waveguide structures.

Abstract: A low-loss bent waveguide with a Clothoid shape whose width is adjusted along the curvature is demonstrated. For a bend radius of 1.6 μm, the loss is reduced by 0.34 dB to less than 0.04 dB for a single S-shape bend when compared to a standard circular waveguide bend.

Abstract: The invention discloses a bend safe speed calculation method based on vehicle and road collaboration. The method comprises the steps as follows: firstly, the cut-in speed of a vehicle at a bend, the geometrical characteristic of the bend and other information are acquired by use of an OBU (on-board unit) and an RSU (roadside unit) in a vehicle and road collaboration system, driving styles of different drivers are reflected by replacing the bend radius with the driver running radius, and a bend speed model self-adaptive to the driving styles of the drivers is established; then, accident forms such as sideslip, rollover, rear-end collision and the like of the vehicle are considered, the optimal bend speed of an autonomous vehicle is decided in the aspects of yaw stability, sideslip stability, car following safety, driving comfort and the like, and finally, expected speed planning is provided for an active control system of the vehicle. Compared with existing bend safe speed calculation methods, the bend speed calculation method has the advantages that multiple accident forms such as sideslip, rollover, rear-end collision and the like are considered comprehensively, considered factorsare more comprehensive, and the speed planning of the autonomous vehicle can be supported.

Abstract: We have proposed a germanium doped core fiber design for large mode area single-mode applications. The designed fiber effective index, dispersion and bend loss of the fundamental mode and next higher order modes have been calculated using the numerical method. The fiber exhibits a high differential loss between the fundamental mode and higher order modes. Therefore, the designed fiber structure effectively suppresses the higher order modes and retains only the first mode (or fundamental mode) in the core region. Our simulation results demonstrate that, a low loss of 0.1dB/m is achieved for fundamental mode at 1060nm wavelength with 10cm bend radius, along with it also exhibits a high loss of 4.8 dB/m to first higher order mode. The fiber shows a large mode area of 831.4 μm2 at 1060nm wavelength. The proposed paper further explores the fiber properties such as dispersion and fabrication tolerances. Our design shows a dispersion of 39 ps/km-nm at 1060nm, and also the structure shows a less dispersion variation over a wavelength band of 400nm. The fiber reduces the fabrication difficulties as compared to the other designed fibers. We fabricated the present fiber using the renowned vapor axial deposition technique. In this method, we can achieve the large diameter preforms and also the method decreases the tolerances when dealing with glasses.

Abstract: A refractive index sensor based on a semicircular bent fiber is presented. The interference occurs between the cladding mode excited in the bending region and the core mode. The experimental results show that the resonant dip wavelength decreases linearly with the increase of the refractive index of the surrounding environment and the sensitivity of the sensor increases linearly with the increasing of the bending radius. A high sensitivity of 1031 nm per refractive index unit is obtained over the refractive index range of 1.3324 to 1.3435 by using a bent fiber with a bending radius of 500 μm.

Abstract: The bending limit or minimum bending radius of sheet metal is conventionally measured in a wiping (swing arm) or in a vee bend test and reported as the minimum radius of the tool over which the sheet can be bent without fracture. Frequently the material kinks while bending so that the actual inner bend radius of the sheet metal is smaller than the tool radius giving rise to inaccuracy in these methods. It has been shown in the previous studies that conventional bend test methods may under-estimate formability in bending dominated processes such as roll forming. A new test procedure is proposed here to improve understanding and measurement of fracture in bending and roll forming. In this study, conventional wiping test and vee bend test have been performed on martensitic steel to determine the minimum bend radius. In addition, the vee bend test is performed in an Erichsen sheet metal tester equipped with the GOM Aramis system to enable strain measurement on the outer surface during bending. The strain measurement before the onset of fracture is then used to determine the minimum bend radius. To compare this result with a technological process, a vee channel is roll formed and in-situ strain measurement carried out with the Vialux Autogrid system. The strain distribution at fracture in the roll forming process is compared with that predicted by the conventional bending tests and by the improved process. It is shown that for this forming operation and material, the improved procedure gives a more accurate prediction of fracture.

Abstract: Inertial lift is a fluid phenomena exploited in microfluidic devices to separate particles/cells based on their size. Whilst it has been studied extensively for spherical particles suspended in flow through straight ducts, typically of rectangular shape, many applications involve ducts that are curved. This paper explores the estimation of focusing behaviour in curved ducts by simply adding inertial lift forces computed for a straight duct (having the same cross-section) to the drag forces within the cross-sectional plane which are generated by the secondary motion of the fluid flow through the curved duct. We examine the specific case of a curved rectangular duct with height `, width 2` and bend radius R in which a neutrally buoyant particle with radius a is suspended. The simple force model is appropriate when R is large and the flow rate is low such that the Dean number is small. The magnitude of the secondary flow drag relative to the inertial lift force scales with κ = `4/(a3R) and the dominant focusing behaviour is found to approximately collapse onto a single curve when plotted against κ, particularly when κ≤ 30.