Pulse Shaping Technique Research Articles

Effect of preheating and cooling of the powder bed by laser pulse shaping on the microstructure of the TiC based cermets

For the fabrication of brittle materials like cermets through Selective Laser Melting (SLM), the pulse shaping technique has proven to be an effective and optimal way to produce cermet with no cracks. The purpose of this work is to investigate the effect of laser peak in pulse shaping as a preheating and cooling approach for the melt pool of TiC − Fe based cermet using the pulse shaping technique. Several samples were fabricated with variations in laser power and exposure time refereed as pre-pulse and post-pulse before and after the laser peak power (LPP). Microstructural and phase analyses were performed on the fabricated SLM sample using SEM and XRD to study the effect of pulse shaping with variation in exposure time and therefore energy density. Mechanical properties like microhardness were calculated and hence the effect of hardness and energy density were studied. Results indicated that preheating and cooling through laser pulse during pulse shaping has a beneficial effect on the fabrication of the crack-free brittle cermet.

Temperature dependent dynamic compressive response of PA66-GF30 composite under constant strain rate multiaxial loading

This paper studies the uniaxial and multiaxial dynamic compressive behaviour and failure of PA66-GF30 (30 wt % glass fibre reinforced polyamide 66), a typical light weight and high strength composite increasingly used in automobiles including the electric cars in applications from ambient temperature to elevated temperatures up to 90 °C. Likewise, the constitutive relation of PA66-GF30 is characterized from quasi-static to high strain rates. Constant high strain rate loading is achieved by pulse shaping technique on a bespoke split Hopkinson bar. The stress-strain relations are pressure, strain rate and temperature dependent. Effects of strain rate and temperature are found to be decoupled on the pressure sensitivity of PA66-GF30. Beyond maximum stress, micro crack has already formed in dynamically deformed PA66-GF30, which corresponds to macro strain localization monitored by high speed photography and digital image correlation techniques. The PA66-GF30 with confinement shows adiabatic shear failure, with fibres coated by severe shear matrix facets and evenly distributed filaments. This is different from the unconfined PA66-GF30 which shows fibres pull out in the fractured matrix. A modified Drucker-Prager model is proposed to describe the pressure dependent compressive strength of PA66-GF30 over various strain rates and temperatures.

Temporal shaping of narrow-band picosecond pulses via noncolinear sum-frequency mixing of dispersion-controlled pulses

A long-sought-after goal for photocathode electron sources has been to improve performance by temporally shaping the incident excitation laser pulse. The narrow bandwidth, short wavelength, and picosecond pulse duration make it challenging to employ conventional spectral pulse shaping techniques. We present a novel and efficient intensity-envelope shaping technique achieved during nonlinear upconversion through opposite-chirp sum-frequency mixing. We also present a numerical case study of the linac coherent light source-II photoinjector where transverse electron emittance is improved by at least 25%.

Additive manufacturing of TiC-based cermet with stainless steel as a binder material

In this work, TiC-based cermet was fabricated via Selective Laser Melting using chromium ferritic steel as a binder material. Due to their brittle characteristics, cermets are challenging materials to be fabricated by additive manufacturing. Taking into account health care, economic importance, and supply risks, toxic metals such as Ni, Co, and W should be reduced or replaced with other green-based binder metals. Non-toxic materials such as TiC as the matrix and Fe-based alloys as the binder material could be a possible solution is to replace these toxic materials based cermets. The laser pulse shaping technique has been adapted for the fabrication of crack-free cermet with different laser-processed parameters. By shaping the laser pulses with different laser cycles, laser energy is delivered to a single layer in the form of controlled and defined pulses. This controlled delivery of energy can be used to regulate the melt pool characteristics including the temperature of the melt pool. The pulse shaping technique helps to reduce or eliminate the process defects such as cracks making it the most suitable method for the fabrication of TiC-Fe-based cermets. The laser process parameters that influence the microstructure morphology and mechanical properties such as hardness and fracture toughness have been investigated in detail. The results pave the way for the fabrication of novel cermets without cracks using selective laser melting.

Selective Laser Melting of TiC-Fe via Laser Pulse Shaping: Microstructure and Mechanical Properties.

In the present study, TiC-Fe cermets were fabricated through selective laser melting (SLM) for the first time employing pulse wave using a pulse shaping technique and regular laser pulse wave. Two samples were fabricated each with adapting pulse shaping technique and regular laser pulse wave with varied laser peak power and exposure time to obtain an optimized parameter. The pulse shaping technique proves to be an optimal method for fabrication of the TiC-Fe-based cermet. The effect of the laser peak power and pulse shaping on the microstructure development was investigated through scanning electron microscopy and X-ray diffraction analysis. Two-phased microstructures revealed the distribution of TiC and Fe. A maximum hardness and fracture toughness of 1010 ± 65 MPa and 16.3 ± 1.7 MPa m1/2, respectively, were observed for the pulsed-shaped samples illustrating that pulse shaping can be an effective way to avoid cracking in brittle materials processed by SLM.

State-of-the-art review on ultra high performance concrete - Ballistic and blast perspective

The world is experiencing multiple blasts and ballistic attacks in the last few decades leading to huge loss of civilians and warriors in fringe territories. In this context, this paper reviews the state-of-the-art information on ultra-high-performance concrete (UHPC) against blast and ballistic impact. The first part briefly discusses the ingredients with special emphasis on fibres behaviour in UHPC. Secondly, the review of dynamic characterization of UHPC carried with the aid of Split Hopkinson Pressure Bar (SHPB), covering pulse shaper technique, dynamic increase factor (DIF) and high strain behaviour. Thirdly, from ballistic perspective, various prediction models, penetration depth analysis and various research to tackle high-velocity impact are addressed. Further, shock tube studies and in-field blast testing of UHPC is summarized under blast perspective. Performance of reinforcement bars in UHPC for mitigation of ballistic and blast attacks is reviewed. Subsequently, numerical simulation and technology to enhance ballistic and blast resistant efficiency, such as inducing prestressing, use of metallic foam and blast resistant design are discussed. This review endeavours to help researchers identify areas of improvement needed in UHPC and highlight the benefits of UHPC for designers and consultants to use in structures to mitigate the effect of impact and blast.

Time-course quantitative mapping of caffeine within the epidermis, using high-contrast pump-probe stimulated Raman scattering microscopy.

BackgroundAn assessment of the drug penetration and distribution profiles within the skin is essential in dermatology and cosmetology. Recent advances in label‐free imaging technologies have facilitated the direct detection of unlabeled compounds in tissues, with high resolution. However, it remains challenging to provide quantitative time‐course distribution maps of drugs within the complex skin tissue. The present study aims at acquiring the real‐time quantitative skin penetration profiles of topically applied caffeine, by means of a combination of pump–probe phase‐modulated stimulated Raman scattering (PM‐SRS) and confocal reflection microscopy. The recently developed PM‐SRS microscopy is a unique imaging tool that can minimize strong background signals through a pulse‐shaping technique, while providing high‐contrast images of small molecules in tissues.Materials and methodsReconstructed human skin epidermis models were used in order to analyze caffeine penetration in tissues. The penetration profiles of caffeine in an aqueous solution, an oil‐in‐water gel, and a water‐in‐oil gel were examined by combining PM‐SRS and confocal reflection microscopy.ResultsThe characteristic Raman signal of caffeine was directly detected in the skin model using PM‐SRS. Integrating PM‐SRS and confocal reflection microscopy allowed real‐time concentration maps of caffeine to be obtained from formulation samples, within the skin model. Compared with the conventional Raman detection method, PM‐SRS lowered the background tissue‐oriented signals and supplied high‐contrast images of caffeine.ConclusionWe successfully established real‐time skin penetration profiles of caffeine from different formulations. PM‐SRS microscopy proved to be a powerful, non‐invasive, and real‐time depth‐profile imaging technique for use in quantitative studies of topically applied drugs.

Pressure and temperature dependent dynamic flow and failure behavior of PMMA at intermediate strain rates

Polymethylmethacrylate (PMMA) has been widely used as transparency in aerospace and automobile engineering to enhance structural reliability under impact loading such as bird strike events. This study investigates the confined compressive behaviour of PMMA at quasi-static 0.01 s−1, medium rate 1-100 s−1 and intermediate strain rates 100-1000 s−1 from room temperature to elevated temperatures. Constant intermediate strain rate loading is achieved by using pulse shaping technique on a bespoke in-house developed split Hopkinson compression bar equipped with a high-speed camera and an environmental chamber. The material presents significant strain rate, temperature and pressure sensitivities. The failure mode at intermediate strain rate changes from brittle fragmentation without confinement to adiabatic shear banding with medium lateral confinement (65.3 MPa), which can be seen from the post-peak slope in the constitutive behavior, melting flow PMMA filaments observed in high speed photography and microstructural analysis. A series of intermediate strain rate experiments at elevated temperatures indicate that the higher temperature results in ductile failure for corresponding pressures. The temperature and strain rate dependent Drucker-Prager (DP) model is found to describe the response of PMMA. The brittle-ductile transition and pressure dependent model provide a better understanding of PMMA at intermediate strain rates, which is appropriate for engineering applications.

Structured 3D linear space\u2013time light bullets by nonlocal nanophotonics

We propose the generation of 3D linear light bullets propagating in free space using a single passive nonlocal optical surface. The nonlocal nanophotonics can generate space–time coupling without any need for bulky pulse-shaping and spatial modulation techniques. Our approach provides simultaneous control of various properties of the light bullets, including the external properties such as the group velocity and the propagation distance, and internal degrees of freedom such as the spin angular momentum and the orbital angular momentum.

Independently programmable frequency-multiplexed phase-sensitive optical parametric amplification in the optical telecommunication band.

We experimentally demonstrate programmable multimode phase-sensitive amplification multiplexed in the frequency domain for flexible control of parallelly generated squeezed states. We utilize the unique phase-matching condition of a type-II periodically poled potassium titanyl phosphate (PPKTP) crystal and pulse shaping technique to fully control the frequency-domain parallel generation of squeezed states in the optical telecommunication band. We experimentally verify that the independent programmability of phase-sensitive optical parametric amplification (OPA) for the modes corresponding to different frequency bands can be achieved by shaping the pump laser pulse from optical parametric gain measurements using a coherent probe light generated by a degenerate synchronously pumped optical parametric oscillator.

High strain rate compression behaviour of 3D printed Carbon-PA

In the last few years, Fused Filament Fabrication is growing in the industrial field for the manufacture of final products by using new materials with high mechanical performances. Among those, one of the strongest is Carbon-PA. This is a composite material made by Nylon thermoplastic matrix filled with short carbon fibers reinforces. The aim of this work is to investigate its mechanical properties in static and dynamic conditions. Cylindrical specimens were produced by extruding the material in the three main printing directions. Then, uniaxial quasi-static and dynamic compression tests have been performed to evaluate its strain rate sensitivity. Dynamic tests have been carried out through a direct Split Hopkinson Bar setup with a pulse-shaping technique. The results show a compression behaviour dependent on the printing direction and strain rate. The behaviour of Carbon-PA was different between static and dynamic condition, passing from ductile to brittle. Moreover, a tomography analysis was carried out on the samples to evaluate the voids distribution.Graphic abstract

Dynamic failure behaviour analysis of TiB2-B4C ceramic composites by split Hopkinson pressure bar testing

As a novel composite ceramic, TiB2-B4C has been widely used in the lightweight protective armour due to its excellent physical properties such as high hardness, high fracture toughness, and low density. The failure behaviour of TiB2-B4C composite ceramics under impact loading is particularly important in the protective armour design. In the present paper, the split Hopkinson pressure bar (SHPB) testing of the TiB2-B4C ceramics is performed using the pulse shaping technique and high-speed photography. Two typical failure modes of the TiB2-B4C ceramic under different loading rates are summerized, fragmentation and comminution, featuring single-peak and double-peak waveform curves respectively. From the ceramic fragments collected after the impact, the failure process of TiB2-B4C ceramics during loading is analysed from an energy perspective. The results showed that the first impact of the incident bar has a more significant effect on TiB2-B4C ceramics, and the energy utilisation ratio of the specimen is an important index to determine the degree of failure. Moreover, a calculation method is proposed to quantify the crushing process of ceramics based on the fractal theory.

Comparative study of strength-based damage evolution in ultra-high-performance concrete (UHPC) and conventional concrete (CC) under dynamic loading

The initiation and development of damage inside cementitious materials subjected to dynamic loadings, which is manifested by the cracking and fragmentation process, have a significant contribution to the load-bearing capacity of these materials. However, current popular concrete models hypothesize that damage only initiates after the specimen reaches its peak stress, and due in part to the lack of validation from sufficient experimental data, none of the models entertain the possibility of variation in damage evolution among different types of concretes. In this study, the evolution of damage in an ultra-high-performance concrete (UHPC) and a conventional concrete (CC), characterized by the degree of strength degradation, are comparatively investigated throughout the entire pre-/post-peak deformation process as a function of carefully measured plastic strain. Well-controlled intermittent dynamic loadings are achieved on a modified Kolsky compression bar apparatus to first introduce various degrees of damage in the concrete specimens, and then the carefully preserved specimens are tested for residual strength. Coupling with pulse shaping techniques, the incident wave profiles are tailored for UHPC and CC specimens to acquire dynamic stress equilibrium and constant strain-rate deformation. Further evaluation on the residual strength of damaged specimens reveals that damage initiates prior to the peak stress, and there is a distinct difference in damage evolution between these two types of concrete materials. This investigation demonstrates a more comprehensive view of damage evolution in concrete materials and offers novel experimental data to assist the damage model development.

Quintuple AISI 1010 carbon steel core coil for highly focused transcranial magnetic stimulation in small animals

Transcranial magnetic stimulation (TMS) is a non-invasive neuromodulation technique used to regulate the synaptic activity of neurons in the brain, improving the functionality of connecting regions and bringing effective treatment to different neurological and psychiatric disorders. The TMS induced E-field needs to be focal enough to avoid unwanted side effects caused by stimulation of the regions adjacent to the target. Attempts at TMS in small animals like rodents are highly constrained, since most of these studies use commercial equipment intended for humans, with power and coil geometries not designed for small animals. Using finite element modeling in ANSYS Maxwell, the present work shows the design and evaluation of customized arrays of two and five dual-winding solenoids, including a ferromagnetic core, to restrict the stimulation to areas as small as 1 mm2. Each solenoid is made with 50 turns of a wire with thickness = 1 mm, height = 25.4 mm and elliptical top-view cross section. Ferromagnetic cores with V-shape tip sharpening were included, using AISI 1010 carbon steel of 2 T of saturation flux density (Bsat) at 4×104 A/m, and an initial relative permeability µr=667.75. Electric fields and magnetic flux densities were calculated around 4.00 mm below the coil (vertical distance from the top of the scalp to the cortical layer 5/6 in adult rats) with peak currents of 10kA, in a single non-repetitive pulse at 2.5kHz. The achieved 100V/m in a small area of 1 mm2 suggests the suitability of the coil for in vivo experimentation in rodents. Future works will seek to improve the duration of the pulses for repetitive TMS with pulse shaping techniques and validate the novel coil with in vivo experiments in rat models.

Neutron response of the TLYC scintillator

The response of the recently developed Tl2LiYCl6(Ce) scintillator to neutrons was measured and the 35Cl(n,p) channel observed in the material for the first time. The scintillator was exposed to a 252Cf source mounted within a parallel-plate avalanche counter for fission-fragment coincident measurements. Proton- and t+α-like events are selected with the pulse-shape discrimination technique. Comparison of energy-deposition and time-of-flight allows the quenching of the different channels to be assessed. The 6Li(n,t)α channel is found to have its energy quenched to 37(1)% relative to γ-ray detection, while the proton energy deposition spectrum is quenched to 60(1)%.

Experimental study on pulse shaping techniques of large diameter SHPB apparatus for concrete

The constant strain rate loading and dynamic stress equilibrium are prerequisites for ensuring the experimental data of the Split Hopkinson Pressure Bar (SHPB). In order to achieve this requirement, a proper pulse shaper dimensions were used in the experiment. For this purpose, the pulse shaper diameter, thickness and strength and the strike bar length and its velocity on the incident pulse are studied by ∅80mm SHPB apparatus. Then, the parameters affecting the two key inflection points and three loading areas of the incident pulse are discussed. In addition, the improved methods for two typical non-constant strain rate waveforms for the concrete experiments are obtained based on the regular pulse shaping. Moreover, SHPB experiments are conducted for concrete by employing different pulse shaper dimensions. The results show that more valid data can be obtained by employing the pulse shaper with large thickness when the strain rate is non-constant during the experiment. The conclusions and method provide guidance for selecting pulse shaper for concrete SHPB experiments.

Dynamic Compression–Shear Response and Failure Criterion of Rocks with Hydrostatic Confining Pressure: An Experimental Investigation

Rocks in the deep underground are likely subjected to both hydrostatic confining pressure and dynamic compression–shear load. Thus, accurately characterizing the dynamic properties and failure mechanism of hydrostatically confined rocks under combined compression–shear impacting is crucial for the stability assessment of deep underground rock structures. In this study, on the basis of an improved split Hopkinson pressure bar (SHPB) apparatus, the combined dynamic compression–shear tests are performed on inclined cylindrical sandstone specimens with hydrostatic confining pressures. During the test, the dynamic force balance of rock specimens can be well satisfied using the pulse shaping technique. Our results show that the hydrostatic confining pressure and dynamic loading rate help strengthen the load-carrying capacity of rocks. In contrast, the shear component in the dynamic load limits the dynamic peak stress of rocks. As hydrostatic confining pressure increases, the failure surface based on the Drucker–Prager criterion gradually expands outward. Under dynamic loading, the compressive deformation modulus of rocks decreases with increasing shear component in the dynamic load, contrary to its response to hydrostatic confining pressure. Fragmentation analysis indicates that the hydrostatic confining pressure and the shear component of dynamic loading restrict the fracture behavior of rocks. Besides, as the specimen inclination angle and the hydrostatic confining pressure increase, the failure pattern of rock specimens changes from the tensile-dominated failure with a truncated conical surface to the shear-dominated failure with a single shear plane along its short diagonal.

Wave Attenuation and Dispersion in a 6 mm Diameter Viscoelastic Split Hopkinson Pressure Bar and Its Correction Method

The propagation characteristics of viscoelastic waves have been investigated with a 6 mm diameter split Hopkinson pressure bar (SHPB) made of polymethyl methacrylate (PMMA). The strain signals in SHPB tests were improved by the pulse shaping technique. Based on the experimentally determined propagation coefficients, the amplitude attenuation and wave dispersion induced by viscoelastic effects at different impact velocities were quantitatively analyzed. The results indicate that the high-frequency harmonics attenuate faster in a higher phase velocity. With an increase in the impact velocity, the amplitude attenuation of the viscoelastic wave changes slightly during propagation, while the waveform dispersion gradually intensifies. A feasible method by waveform prediction was proposed to verify the validity and applicability of the propagation coefficient. The results indicate that the strain obtained from the small diameter viscoelastic SHPB can be effectively modified by utilizing the propagation coefficient. Furthermore, it is preferred to adopt the propagation coefficient obtained at low impact velocity for correction when the impact velocity varies. Moreover, the PMMA-steel bar impact test was performed to further illustrate the accuracy of the propagation coefficient and the effectiveness of the correction method.

A Wide Dynamic Range Laser Radar Receiver Based on Input Pulse-Shaping Techniques

This paper presents a Time-of-Flight laser radar receiver based on pulse-shaping at the input to the receiver channel, in which the first zero-crossing point of the converted pulse is marked as the timing moment. In this technique, an LC resonator is combined with a nonlinear feedback TIA to achieve high accuracy and high precision within a wide dynamic range. The key advantage is that the receiver does not require any post compensation or gain control techniques so that the total complexity of the TOF radar is reduced considerably. Measurements made in a $0.35~\mu m$ standard CMOS process show a bandwidth of $230MHz$ and an input-referred noise of $70nA$ RMS. The receiver chip consumes $155mW$ power from a $3.3V$ supply. The single-shot precision and accuracy of the receiver within a dynamic range of 1:50,000 are $ \sim 15mm(SNR=12)$ and $\sim \pm 15mm$ respectively. A wider dynamic range can be achieved with a larger accuracy tolerance. The functionality of the proposed receiver channel is also verified over an input pulse variation and temperature range of $0~^{\mathrm {o}}\text{C}$ to $50~^{\mathrm {o}}\text{C}$ .

Performance analysis of discrete wavelet transform for downlink non‐orthogonal multiple access in 5G networks

Non-orthogonal multiple access (NOMA) has been considered a key technology to address the increasing traffic demands for fifth-generation (5G) cellular wireless communication networks and internet of things. The NOMA technique offers efficient bandwidth utilisation, low latency, and support for the massive connectivity of devices in 5G networks. In the conventional NOMA system, orthogonal frequency division multiplexing (OFDM) is used as a pulse-shaping technique that causes high peak to average power ratio (PAPR) due to the superposition of signals of multiple users after passing through inverse fast Fourier transform and spectral inefficiency due to the cyclic prefix insertion. Therefore, an alternate methodology using wavelet OFDM is proposed in this research work, as a pulse-shaping technique for NOMA-based communication systems to improve spectral efficiency and system capacity The main contribution of this study is the analysis of the bit error rate (BER) performance and the effect of carrier frequency offset on system performance of wavelet NOMA (W-NOMA) technique in 5G networks. The simulation results demonstrate that W-NOMA outperforms the conventional NOMA with reference to PAPR and BER.