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A recent contest of shear tests modelling was carried out in 2019. Teams from universities and consultancies around Europe were invited to predict the shear capacity of two reinforced concrete beams. The basics of the numerical models were to be set up according the Dutch NLFEM Guideline RTD 1016–1:2017. In the contest, two reinforced concrete beams without stirrups but having large depth (1200 mm), tested at Delft University of Technology, were selected as the modelling target. Most participants in the contest did not obtain good agreement with the test results. This paper presents a postdiction study on one of the two tests: H123. Based on this study, some adaptations are made to the recommendations of RTD 1016–1:2017 in order to approach the test results better. The intention of this contribution is to improve the existing NLFEA Guideline for practical engineering structures with uncommon reinforcement layouts.

Diagonal tension cracking is the governing failure mode for bridge girders with a thin web that are highly prestressed and contain little shear reinforcement. When assessing existing bridge girders using the Eurocode 2 [1], it often turns out that it is not possible to demonstrate sufficient resistance to diagonal tension cracking. This paper evaluates the method to determine the maximum principal tensile stresses as used in the Eurocode 2 [1] and investigates how flexural cracks affect the principle tensile stresses in the regions without flexural cracks. This paper also investigates how the tensile strength of the web is affected by the presence of compressive stresses and by the size of the area subjected to high tensile stresses. Based on the results of these investigations, an improved model is proposed to determine the resistance to diagonal tension cracking.

An all-fiber highly sensitive force sensor based on modal interferometer has been presented and demonstrated. The single-mode fiber (SMF) with coating stripped is designed into a balloon-like shape to form a modal interferometer. Due to the bent SMF, the interference occurs between the core mode and cladding modes. With variation of the force applied to the balloon-like interferometer, the bending diameter changes, which caused the wavelength shift of the modal interference. Thus the measurement of the force variation can be achieved by monitoring the wavelength shift. The performances of the interferometer with different bending diameter are experimentally investigated, and the maximum force sensitivity of 24.9 pm/ μ N can be achieved with the bending diameter 14 mm ranging from 0 μ N to 1464.12 μ N. Furthermore, the proposed fiber sensor exhibits the advantages of easy fabrication and low cost, making it a suitable candidate in the optical fiber sensing field.

A novel, simultaneous strain and temperature sensor utilizing polarization maintaining fiber (PMF) and multimode fiber (MMF) is proposed and experimentally demonstrated in this paper. The sensing head of this sensor can be obtained by splicing PMF and MMF in the structure of PMF-MMF-PMF. The extinction ratio of the transmission spectrum can be over 30 dB. The strain sensitivities of sensor by two spectrum dips can be 1.01 pm/μe and 1.27 pm/μe in the range from 0 to 2000 μe. Meanwhile, the temperature sensitivities of 49 pm/°C and 41 pm/°C can be achieved by two spectrum dips in the range from 30 °C to 70 °C. The sensitivity difference between the two spectrum dips can be used to realize dual parameters fiber sensing. This sensor exhibits the advantages of simple fabrication, compact structure and multi-purpose measuring. It may have the great potential in fields of robot arms and artificial limbs.

Interaction of ultrasonic waves with partially-closed surface-breaking cracks in concrete structures has been studied. Measurements have been conducted on a reinforced concrete beam containing various mechanical-load-induced cracks and compared with the baseline measurements at those locations. Influence of crack width, incident angle of waves with cracks, and distance from the cracks on travel time and amplitude of the waves have been investigated when the beam was unloaded. It has been observed that a measurable part of the waves propagate through the cracks due to the acoustic coupling between the crack faces, although attenuation can be relatively high. The travel time has shown a nearly independent behavior from remaining crack opening in the measured range of 0.05 mm to 3 mm. Measurements in directions orthogonal and parallel to the crack suggest that there is substantial anisotropy in the cracking zone. Furthermore, an effective width of the micro-cracking area around the cracks has been estimated from the measurements.

An ultrafast high-sensitivity refractive index (RI) and temperature-sensing system based on an optoelectronic oscillator (OEO) is proposed and demonstrated in this paper. A Fabry-Perot fiber Bragg grating (FP-FBG), which combines a gap with two FBGs in a silica V-shaped slot and characterizes a narrow notch in the reflection spectrum, is incorporated in the OEO to implement a microwave photonic filter and perform oscillating frequency selection. A microwave signal is generated by the OEO, whose oscillating frequency is determined by the center frequency of the FP-FBG notch, which varies with the surrounding environments. The RI or the temperature change can be accordingly measured by monitoring the frequency shift of the microwave signal using an electrical spectrum analyzer or a digital signal processor. An experiment is performed to verify the proposal. An RI sensitivity of 413.8 MHz/0.001RIU and a temperature sensitivity of 2516 MHz/°C are successively demonstrated.

We experimentally demonstrate a switchable narrow linewidth single-longitudinal-mode (SLM) erbium-doped fiber (EDF) ring laser with LP 11 transverse mode output. The laser is based on a mode selective all-fiber fused coupler which is composed of a single-mode fiber (SMF) and a two-mode fiber (TMF). By controlling the polarization state of the output light, the laser can provide narrow linewidth SLM output with LP 11 transverse mode at two specific wavelengths, which correspond to two transmission peaks of the chirped moire fiber grating (CMFBG). The 20 dB linewidth of the fiber laser for each wavelength is approximately 7.2 and 6.4 kHz.

By incorporating a phase-shifted fiber Bragg grating (PS-FBG) inscribed in few-mode fiber (FMF) named FM-PS-FBG for the first time, a novel frequency-switchable optoelectronic oscillator (OEO) to perform simultaneous strain and temperature measurement is proposed and experimentally demonstrated. In the OEO loop, two switchable oscillation frequencies are determined by the reflected bands of FM-PS-FBG based on different mode couplings and corresponding notches. Due to the characteristics of modes, two OEO-generated microwave signals exhibit different strain and temperature sensitivities. Therefore, through monitoring the frequency shifts of microwave signals, the simultaneous strain and temperature sensing can be realized by the proposed OEO sensor with high-speed and high-resolution interrogation.

A compact graphene monolayer on substrate nano-structure to realize nanofocusing and slow light effect is analytically and numerically investigated in this letter. By creating a chemical potential well in the desired positions via chemical doping or external gating, the normalized electric field enhancement factor here reaches over 300 where the mid-infrared (MIR) light is slowed down to over as high as 1/7000 times the speed in vacuum at the same time. The spot size is in 1 nm magnitude. Furthermore, when the chemical potential along the graphene sheet is properly designed, multi-focusing can be successfully controlled. The incident wave can also be trapped at the corresponding positions if it contains multiple frequency components. This structure may find applications for energy focusing on nano chips or lenses in the MIR regime beyond the diffraction limit.

We experimentally demonstrate a narrow linewidth single-longitudinal-mode erbium-doped fiber ring laser with an output of tunable second-order optical vortex beams (OVB). The laser is based on a mode selective all-fiber fused coupler, which is composed of a single-mode fiber (SMF) and a few-mode fiber (FMF). The fused SMF–FMF coupler inserted in the cavity of the laser acts as a mode converter, which converts LP01 mode to LP21 mode group. The tunable OVB is produced by combing two even LP21 modes with orthogonal polarization directions. The tunability of second order OVB in the fiber laser is experimentally realized by adjusting a polarizer at the output end of the fiber. The 20 dB linewidth of the fiber laser is approximate 7.2 kHz.

A highly sensitive curvature sensor based on asymmetrical twin core fiber (TCF) and multimode fiber (MMF) is proposed and experimentally demonstrated. By applying the coupled-mode theory and equivalent refractive index model, we theoretically analyze the uncoupled feature of the TCF and the relationship between peak wavelength and the curvature. Two segments of MMF used as beam splitter and combiner are embedded on the two ends of the TCF, and the extinction ratio of the comb transmission spectrum is about 15 dB. The experimental result shows that the curvature sensitivity of the sensor can be achieved as high as 103.35 nm/m−1 ranging from 0.24 m−1 to 0.6 m−1, and the strain sensitivity is up to −4.01 pm/μe in the range from 0 μeto 1400 μe. The simultaneous detection of the curvature and strain can be realized. The temperature sensitivity is 0.431 nm/°C in the range from 40 °C to 70 °C. This fiber sensor exhibits the advantages of low cost, easy and repeated fabrication, and high sensitivity.

We propose and demonstrate an asymmetrical Fabry–Perot interferometric (AFPI) fiber sensor for simultaneous measurement of the curvature and temperature. The AFPI fiber sensor is comprised of a low reflective fiber Bragg grating with narrow bandwidth and a no-core fiber with broadband Fresnel reflection. The extinction ratio measured in the experiment is defined as the difference between the amplitude of the maximum peak and its left adjacent dip, which is about 6 dB. With the variation of curvature and temperature, the maximum amplitude peak wavelength of the AFPI and the extinction ratio show different responses, thus the simultaneous measurement of curvature and temperature can be achieved. From the experimental results, sensitivities of the proposed sensor for the curvature and the temperature are about 9.57 dB/m $^{-1} $ (ranging from 0.8844 to 1.0995 m $^{-1} $ for the measured extinction ratio) and 10.2 pm/°C (ranging from 10 °C to 60 °C), respectively. Such fiber sensor is simple, low cost, and compact.

In this paper, a novel few-mode large-mode-area fiber is proposed. This type of fiber consists of 11 conventional cores and 8 air-hole cores circularly arranged around the center core. Few-mode condition equal to strict dual-mode here is available by appropriate adjusting on corepitch, relative refractive index difference and core radius. Large effective area of fundamental mode around 1403.561 μm2 could be achieved by optimization of structural parameters. Bending loss less than 10 − 3 dB / m is realized when effective area is over 1400 μm2.

In this paper, we proposed a new kind of quasi-37-cores fiber structure consisting of 15 conventional cores and 22 air-hole cores. Strict dual-mode condition is systematically analyzed about three structure parameters, core-to-core pitch, relative core diameter, and difference of refractive index between core and cladding. Large effective area about 1910.98 μ m 2 with low bending loss less than 10 − 3 dB / m is achieved while keeping strict dual-mode. This structured fiber has a great potential in high power fiber lasers and amplifiers.

For reinforced concrete (RC) slabs without shear reinforcement, shear and punching can be the governing failure mode at the ultimate limit state if subjected to large concentrated loads. Sh...

This work reports a model and Monte Carlo simulations of excluded volume mediated interactions between colloids and topographically patterned substrates in the presence of thermosensitive depletants. The model is matched to experiments to yield density, free energy, and potential energy landscapes that quantitatively capture particle microstructures varying from immobilized non-close packed configurations to random fluid states. A numerical model of local excluded volume affects is developed to enable computation of local depletion attraction in the presence of arbitrary geometries. Our findings demonstrate a quantitative modeling method to interpret and predict how surface patterns mediate local depletion interactions, which enables the design of colloidal based materials and devices.

The paper presents a large experimental research program carried out at Delft University of Technology supported by the Dutch Ministry of Infrastructure and the Environment. The research evaluates the shear capacity of RC beam/slab strips at transition of the failure modes between flexural and shear (defined as v bd ). With 106 tests, the values of v bd were obtained for specimens with 7 different configurations covering the variables that are usually found in the existing solid bridge deck slabs. The results are compared with the related expressions suggested by several available design provisions. Suggestions are made to further improve these expressions.

This paper presents a new concept to evaluate the shear capacity of reinforced concrete beams without shear reinforcement having flexural shear failure. Based on experimental observations, it is proposed that the opening of the critical inclined crack can be considered as the lower bound for the shear capacity of a structural member. It is also proposed that the unstable opening of the critical inclined crack is triggered when the shear displacement in an existing flexural crack reaches a critical value Δcr. Thus the critical shear displacement is used as a failure criterion, and based on that a new shear evaluation method is proposed. The method shows agreeable accuracy when compared with test results in literature.

Perfectly ordered states are targets in diverse molecular to microscale systems involving, for example, atomic clusters, protein folding, protein crystallization, nanoparticle superlattices, and colloidal crystals. However, there is no obvious approach to control the assembly of perfectly ordered global free energy minimum structures; near-equilibrium assembly is impractically slow, and faster out-of-equilibrium processes generally terminate in defective states. Here, we demonstrate the rapid and robust assembly of perfect crystals by navigating kinetic bottlenecks using closed-loop control of electric field mediated crystallization of colloidal particles. An optimal policy is computed with dynamic programming using a reaction coordinate based dynamic model. By tracking real-time stochastic particle configurations and adjusting applied fields via feedback, the evolution of unassembled particles is guided through polycrystalline states into single domain crystals. This approach to controlling the assembly of a target structure is based on general principles that make it applicable to a broad range of processes from nano- to microscales (where tuning a global thermodynamic variable yields temporal control over thermal sampling of different states via their relative free energies).

In this paper we propose and experimentally demonstrate an optical fiber structure sensor based on a Mach–Zehnder interferometer for pressure measurement. The fiber sensor is composed of a single-mode-no-core-single-mode structure, a section of capillary pure silica tube and refractive index matching fluid (RIMF). As the pressure decreases, the sealed air in the tube expands and the liquid level of the RIMF increases, which causes a wavelength shift of the interferometer. The measurement of the pressure variation can thus be achieved by monitoring the wavelength shift. The experimental results agree well with the numerical simulation, and a maximum pressure sensitivity of 266.6 nm is achieved experimentally. Furthermore, the proposed fiber sensor has the potential to obtain higher sensitivity by enlarging the length of the air cavity.

We propose a new approach to realize switchable mode operation in a few-mode erbium-doped fiber laser. The ring fiber laser structure is constructed with a core-offset splicing between single-mode fiber and dual-mode fiber. Stable operating on the fundamental mode laser and second-order mode laser individually or simultaneously is realized by appropriately adjusting the state of the polarization controller and bending status of the few-mode fiber Bragg grating. The narrow 3 dB linewidth less than 0.02 nm and high optical signal to noise ratio more than 42 dB are obtained for both modes in either separate laser or simultaneous laser operating conditions.

We propose and experimentally demonstrate a compact Mach–Zehnder interferometer (MZI)-based no-core fiber hollow-core fiber no-core fiber (NHN) structure. Two segments of the no-core fiber embedded on two ends of the hollow-core fiber are employed as beam splitter and combiner, which realize an MZI to measure temperature and strain. With the variation of the temperature and strain, the measurement is achieved by monitoring the transmission spectrum shift of the MZI. The results show that the proposed sensor has a temperature sensitivity of 30.92 pm/°C in the range from 30°C to 90°C, and strain sensitivity of 0.652 pm/μϵ in the range from 0 to 700 μϵ, respectively. The NHN structure exhibits the advantages of compactness, easy fabrication, and low cost, which shows the potential sensing applications of the proposed fiber structure.

The ability to assemble nano- and micro- sized colloidal components into highly ordered configurations is often cited as the basis for developing advanced materials. However, the dynamics of stochastic grain boundary formation and motion have not been quantified, which limits the ability to control and anneal polycrystallinity in colloidal based materials. Here we use optical microscopy, Brownian Dynamic simulations, and a new dynamic analysis to study grain boundary motion in quasi-2D colloidal bicrystals formed within inhomogeneous AC electric fields. We introduce "low-dimensional" models using reaction coordinates for condensation and global order that capture first passage times between critical configurations at each applied voltage. The resulting models reveal that equal sized domains at a maximum misorientation angle show relaxation dominated by friction limited grain boundary diffusion; and in contrast, asymmetrically sized domains with less misorientation display much faster grain boundary migration due to significant thermodynamic driving forces. By quantifying such dynamics vs. compression (voltage), kinetic bottlenecks associated with slow grain boundary relaxation are understood, which can be used to guide the temporal assembly of defect-free single domain colloidal crystals.