Nondestructive damage assessment in fiber reinforced composites with the pulsed ultrasonic polar scan

This study investigates the use of both amplitude and time-of-flight based pulsed ultrasonic polar scan (P-UPS) as a sophisticated non-destructive damage sensor for fiber reinforced composites. Focus is put on stiffness related damage phenomena, which are in general difficult to monitor nondestructively, and their associated signature in the P-UPS image. Various composite samples, with different damage states, have been inspected at multiple material spots with the P-UPS technique. The results demonstrate the capability of the P-UPS method to obtain a unique signature of the local material damage characteristics. Several indicators in the acquired P-UPS images have been identified from which the type and level of material degradation can be obtained. The P-UPS extracted characteristics are fully supported by simulations, conventional tests as well as visual inspection.     

Compression testing of continuous fiber reinforced polymer composites with out-of-plane fiber waviness and circular notches

We investigated the face-stabilized Open-Hole Compression (OHC) test method for evaluating the effects of fiber waviness on the compression strength of continuous carbon fiber reinforced polymer composites. Temporal evaluations of the load-deformation response, acoustic emissions and optical microscopy are used to understand the failure modes and damage progression in the OHC specimen. The failure modes observed are structurally correlated to matrix failure and kink zone formation leading to fiber fracture. The results show how the resin pocket plays a more critical role than the layup in influencing the initiation of damage in the composite specimens.     

Corresponding Angle Feedback in an innovative weighted transportation system

The optimal information feedback has a significant effect on many socioeconomic systems like stock market and traffic systems aiming to make full use of resources. In this Letter, we study dynamics of traffic flow with real-time information. The influence of a feedback strategy named Corresponding Angle Feedback Strategy (CAFS) is introduced, based on a two-route scenario in which dynamic information can be generated and displayed on the board to guide road users to make a choice. Our model incorporates the effects of adaptability into the cellular automaton models of traffic flow and simulation results adopting this optimal information feedback strategy have demonstrated high efficiency in controlling spatial distribution of traffic patterns compared with the other three information feedback strategies, i.e., vehicle number and flux.     

Measuring powder flowability with a modified Warren Spring cohesion tester

The measurement of powder flowability is a major concern for most industrial processes that deal with the handling of bulk solids as raw materials,intermediates,or products.The development of devices that measure the flowability of non-aerated powders has not progressed as rapidly as might have been hoped since most research activities have been based on various types of shear testers intended to aid the design of hoppers.A new flowability indicator named as weighted cohesion (WS) is established using newly...     

Alpha particle absorption and inclined incidence track parameters evaluation in plastic detectors

In this work the use of our recently constructed irradiation chamber was involved in the current experiments. The absorption of alpha particle in air has been studied through a set of experiments in which the stopping power has been measured. A comparison between the calculated values and the present experimental results is given and a good agreement has been found. Critical angle (θ_c) determination has been carried out using two different techniques, via indirect and direct measurements, under different etching conditions and at various alpha energies. An empirical fit of θ_c−h (h is the removal thickness layer) dependence has been calculated and found to work well in the studied h ranges. Also, the inclined alpha tracks parameters of energies between 1.0 and 5.0 MeV have been studied. Results can be successfully applicable in alpha autoradiography studies and detector efficiency determination for track registration in plastic recorders.     

Synthesis and characterization of 2D transition metal dichalcogenides: Recent progress from a vacuum surface science perspective

Layered transition metal dichalcogenides (TMDs) are a diverse group of materials whose properties vary from semiconducting to metallic with a variety of many body phenomena, ranging from charge density wave (CDW), superconductivity, to Mott-insulators. Recent interest in topologically protected states revealed also that some TMDs host bulk Dirac- or Wyle-semimetallic states and their corresponding surface states. In this review, we focus on the synthesis of TMDs by vacuum processes, such as molecular beam epitaxy (MBE). After an introduction of these preparation methods and categorize the basic electronic properties of TMDs, we address the characterization of vacuum synthesized materials in their ultrathin limit-mainly as a single monolayer material. Scanning tunneling microscopy and angle resolved photoemission spectroscopy has revealed detailed information on how monolayers differ in their properties from multi-layer and bulk materials. The status of monolayer properties is given for the TMDs, where data are available. Distinct modifications of monolayer properties compared to their bulk counterparts are highlighted. This includes the well-known transition from indirect to direct band gap in semiconducting group VI-B TMDs as the material-thickness is reduced to a single molecular layer. In addition, we discuss the new or modified CDW states in monolayer VSe₂ and TiTe₂, a Mott-insulating state in monolayer 1T-TaSe₂, and the monolayer specific 2D topological insulator 1T′-WTe₂, which gives rise to a quantum spin Hall insulator. New structural phases, that do not exist in the bulk, may be synthesized in the monolayer by MBE. These phases have special properties, including the Mott insulator 1T-NbSe₂, the 2D topological insulators of 1T′-MoTe₂, and the CDW material 1T-VTe₂. After discussing the pure TMDs, we report the properties of nanostructured or modified TMDs. Edges and mirror twin grain boundaries (MTBs) in 2D materials are 1D structures. In group VI-B semiconductors, these 1D structures may be metallic and their properties obey Tomonaga Luttinger quantum liquid behavior. Formation of Mo-rich MTBs in Mo-dichalcogenides and self-intercalation in between TMD-layers are discussed as potential compositional variants that may occur during MBE synthesis of TMDs or may be induced intentionally during post-growth modifications. In addition to compositional modifications, phase switching and control, in particular between the 1H and 1T (or 1T′) phases, is a recurring theme in TMDs. Methods of phase control by tuning growth conditions or by post-growth modifications, e.g. by electron doping, are discussed. The properties of heterostructures of TMD monolayers are also introduced, with a focus on lateral electronic modifications in the moiré-structures of group VI-B TMDs. The lateral potential induced in the moiré structures forms the basis of the currently debated moiré-excitons. Finally, we review a few cases of molecular adsorption on nanostructured monolayer TMDs. This review is intended to present a comprehensive overview of vacuum studies of fundamental materials' properties of TMDs and should complement the investigations on TMDs prepared by exfoliation or chemical vapor deposition and their applications.     

Theoretical study on instantaneous linewidth of Fourier-domain mode-locked fiber lasers

We have numerically simulated the operation of the Fourier-domain mode-locked (FDML) fiber laser based on the wavelength reconstruction method instead of numerical solving the nonlinear Schrödinger equation. We studied the influences of the filter bandwidth and the relative time delay caused by the fiber chromatic dispersion on the instantaneous linewidth of the FDML fiber laser. The results show that the instantaneous linewidth broadens as the filter bandwidth and the relative time delay increase. When the filter has the bandwidth of 0.02 nm, the narrowest and broadest instantaneous linewidths are 0.024 and 0.042 nm, respectively. We give an understanding for the oscillation of the instantaneous linewidth of FDML. The presented result can be used to evaluate the performance achievable in the FDML fiber lasers.     

High-power, continuous-wave Ti:sapphire laser pumped by fiber-laser green source at 532 nm

We report the first experimental demonstration of efficient and high-power operation of a Ti:sapphire laser pumped by a simple, compact, continuous-wave (cw) fiber-laser-based green source. The pump radiation is obtained by direct single-pass second-harmonic-generation (SHG) of a 33 W, cw Yb-fiber laser in a 30-mm-long MgO:sPPLT crystal, providing 11 W of single-frequency green power at 532 nm in TEM₀₀ spatial profile with power and frequency stability better than 3.3% and 32 MHz, respectively, over 1 h. The Ti:sapphire laser is continuously tunable across 743-970 nm and can deliver an output power up to 2.7 W with a slope efficiency as high as 32.8% under optimum output coupling of 20%. The laser output has a TEM₀₀ spatial profile with M²<1.44 across the tuning range and exhibits a peak-to-peak power fluctuation below 5.1% over 1 h.