Mechanical Engineering

In this study, a three-dimensional (3D) finite element model is developed to investigate thermally induced stress field during hybrid laser–gas tungsten arc welding (GTAW) process. In the hybrid welding case, we focus on the GTAW process sharing common molten pool with laser beam and playing an augment role in the hybrid welding system. An experiment-based thermal analysis is performed to obtain the temperature history, which then is applied to the mechanical (stress) analysis. A modified material model is used to consider the influence of face-to-face contact between the top and bottom metal sheets in the thermo-mechanical analysis of welding lap joints. Results show that the normal stress components prevail in the weld zone during hybrid welding process, and maximum thermal stress exceeding the yield point of material exists at the heat affected zone (HAZ) near the weld pool. Increasing the welding speed causes the penetration and width of weld bead to decrease, and the thermal stress concentration at the welded joint is also reduced accordingly. After welding and cooling down, longitudinal tensile stress (SZ) and transverse compressive stress (SX) are retained in the formed weld, and the higher longitudinal compressive stress exists around the weld bead. In addition, a series of experiments are performed to validate the numerical results, and a qualitative agreement is achieved. Compared to the welded joint obtained by GTAW and laser welding alone, the residual stress concentration in the weld joint obtained by hybrid laser–GTAW is the minimal one.

A stochastic approach that investigates the effects of soil spatial variability on stabilisation of soft clay via prefabricated vertical drains (PVDs) is presented and discussed. The approach integrates the local average subdivision of random field theory with the Monte Carlo finite element (FE) technique. A special feature of the current study is the investigation of impact of spatial variability of soil permeability and volume compressibility in the smear zone as compared to that of the 10 undisturbed zone, in conjunction with uncoupled three-dimensional FE analysis. A sensitivity analysis is also performed to identify the random variable that has the major contribution to the uncertainty of the degree of consolidation achieved via PVDs. The results of this study indicate that the spatial variability of soil properties has a significant impact on soil consolidation by PVDs; however, the spatial variability of soil properties in the smear zone has a dominating impact on soil consolidation by PVDs over that of the undisturbed zone. It is also found that soil volume compressibility has 15 insignificant contribution to the degree of consolidation estimated by uncoupled stochastic analysis.

The current work examines the heat-and-mass transfer process in the laser multilayered cladding of H13 tool steel powder by numerical modeling and experimental validation. A multiphase transient model is developed to investigate the evolution of the temperature field and flow velocity of the liquid phase in the molten pool. The solid region of the substrate and solidified clad, the liquid region of the melted clad material, and the gas region of the surrounding air are included. In this model, a level-set method is used to track the free surface motion of the molten pool with the powder material feeding and scanning of the laser beam. An enthalpy–porosity approach is applied to deal with the solidification and melting that occurs in the cladding process. Moreover, the laser heat input and heat losses from the forced convection and heat radiation that occurs on the top surface of the deposited layer are incorporated into the source term of the governing equations. The effects of the laser power, scanning speed, and powder-feed rate on the dilution and height of the multilayered clad are investigated based on the numerical model and experimental measurements. The results show that an increase of the laser power and powder feed rate, or a reduction of the scanning speed, can increase the clad height and directly influence the remelted depth of each layer of deposition. The numerical results have a qualitative agreement with the experimental measurements.

The temperature field due to heating from a high-powered direct diode laser is calculated first by using an experiment-based finite element model. The Monte Carlo method is applied, which uses the calculated temperature history as thermal loading to obtain the grain growth in the heat-affected zone (HAZ) of dual phase steel DP980. The martensite decomposition in the heat treatment is considered to be a function of the scanning speed. Numerical results demonstrate that the increased scanning speed of the laser beam under laser power of 2 kW decreases the temperature gradient in the HAZ, resulting in finer martensite grains and a smaller percentage of martensite decomposition in the HAZ. This is in good agreement with the experimental data.

A model based on a double-ellipsoidal volume heat source to simulate the gas metal arc welding (GMAW) heat input and a cylindrical volume heat source to simulate the laser beam heat input was developed to predict the temperature field and thermally induced residual stress in the hybrid laser–gas metal arc (GMA) welding process. Numerical simulation shows that higher residual stress is distributed in the weld bead and surrounding heat-affected zone (HAZ). Effects of the welding speed on the isotherms and residual stress of the welded joint are also studied. It is found that an increase in welding speed can reduce the residual stress concentration in the as-weld specimen. A series of experiments has been performed to verify the developed thermo-mechanical finite element model (FEM), and a qualitative agreement of residual stress distribution and weld geometrical size is shown to exist.

High power direct diode laser (HPDDL) based cladding is found to be an economical process for repairing or building valued components and tools that are used in the automotive, aerospace, nuclear and defense industries. In this study, a 2-kW HPDDL of 808 nm in wavelength, rectangular-shaped laser spot of 12 mm × 1 mm with uniform distribution (top-hat) of laser power is used to carry out the experiments. An off-axis powder injection system is used to deposit tool steel H13 on the AISI 4140 steel substrate. A number of experiments are carried out by changing the laser power and scanning speeds while keeping a constant powder feed rate to produce different sizes of clad. An experimentally based finite element (FE) thermal model is developed to predict the cross-sectional temperature history of the cladding process. The temperature-dependent material properties and phase change kinetics are taken into account in this model. As-used experimental boundary conditions are adopted in this model. The acquired temperature history from the FE model is used to predict the temperature gradient, rates of heating and cooling cycles, and the solidification of the clad to the substrate. The FE thermal model results are coupled with thermo-kinetic (TK) equations to predict the hardness of the clad to the substrate. Metallurgical characterization and hardness measurements are performed to quantify the effect of processing parameters on the variation of clad geometry, microstructure, and the change of hardness of the clad to the substrate. The results show that a good metallurgically bonded clad of hardness uniformity is achieved.

A solid/liquid/gas unified model has been developed to investigate the gradient composition formation during the plasma deposition manufacturing (PDM) composite materials process. In this model, an enthalpy porosity model was applied to deal with the melting and solidification of the deposited layer, and a level-set approach was introduced to track the evolution of the free surface of the molten pool and the deposited layer. Moreover, complicated physical phenomena occurring at the liquid/gas interface, including forced convection heat loss, heat emission and plasma heat source, have been incorporated into the governing equations by source terms. In this study, the numerical experiment of nickel base alloy powder deposited on the medium steel substrate by PDM technique was implemented based on the staggered grid and SIMPLEC algorithm. Concentration gradient distribution of the solute material at the composite material interface, fluid flow and temperature distribution in the molten pool and the deposited layer have been investigated in detail.

A solid/liquid/gas unified model has been developed to investigate the gradient composition formation during the plasma deposition manufacturing (PDM) composite materials process. In this model, an enthalpy porosity model was applied to deal with the melting and solidification of the deposited layer, and a level-set approach was introduced to track the evolution of the free surface of the molten pool and the deposited layer. Moreover, complicated physical phenomena occurring at the liquid/gas interface, including forced convection heat loss, heat emission and plasma heat source, have been incorporated into the governing equations by source terms. In this study, the numerical experiment of nickel base alloy powder deposited on the medium steel substrate by PDM technique was implemented based on the staggered grid and SIMPLEC algorithm. Concentration gradient distribution of the solute material at the composite material interface, fluid flow and temperature distribution in the molten pool and the deposited layer have been investigated in detail.

view of the practical and fundamental importance to heat and mass transfer, we present a two-equation turbulence model for incompressible flow within a fluid saturated and rigid porous medium. The derivation consists of time-averaging the general (macroscopic) transport equations and closing the model with the classical eddy diffusivity concept and the Kolmogorov-Prandtl relation. The transport equations for the turbulence kinetic energy (κ) and its dissipation rate (ε) are attained from the general momentum equations. Analysis of the κ-ε equations proves that for a small permeability medium, small enough to minimize the form drag (Forchheimer term), the effect of a porous matrix is to damp turbulence, as physically expected. For the large permeability case the analysis is inconclusive as the Forchheimer term contribution can be to enhance or to damp turbulence. In addition, the model demonstrates that the only possible solution for steady unidirectional flow is zero macroscopic turbulence kinetic energy. The implications of this conclusion are far reaching. Among them, this conclusion supports the hypothesis of having microscopic turbulence, known to exist at high speed flow, damped by the volume averaging process. Therefore, turbulence models derived directly from the general (macroscopic) equations will inevitably fail to characterize accurately turbulence induced by the porous matrix in a microscopic sense.

Ink-jet microdispensing technology was used to develop an instrument for the quantitative determination of the olfactory threshold. An electrical pulse applied to the piezoelectric element produces a deformation that is transmitted to the fluid which results in a drop of fluid being ejected through the orifice mounted at one end of a piezoelectric tube. An electronic console actuates the piezoelectric dispensing elements and controls the number of drops that are dispensed and evaporated to create a fragrance cloud. The number of drops that are generated, evaporated and presented to the patient's nose for detection is adjusted according to a preset algorithm until the patient's threshold is discovered. Neurodegenerative disease patients tested with the developed olfactometer showed a significant elevation of their olfactory threshold as compared to normal controls. This result agrees with literature studies that indicate the sense of smell is one of the first affected by neurodegenerative disease. Through its precise control and detection capability, the digital olfactometer described in this paper can be used as an early screening tool for neurodegenerative disease through olfactory threshold determination.

A pplications of microfluidics and MEMS (micro-electromechanical systems) technology are emerging in many areas of biological and life sciences. Non-contact microdispensing systems for accurate, high-throughput deposition of bioactive fluids can be an enabling technology for these applications. In addition to bioactive fluid dispensing, ink-jet based microdispensing allows integration of features (electronic, photonic, sensing, structural, etc.) that are not possible, or very difficult, with traditional photolithographic-based MEMS fabrication methods.

The combination of drugs with devices, where locally delivered drugs elute from the device, has demonstrated distinct advantages over therapies involving systemic or local drugs and devices administered separately. Drugeluting stents are most notable. Ink jet technology offers unique advantages for the coating of very small medical devices with drugs and drug-coating combinations, especially in cases where the active pharmaceutical agent is very expensive to produce and wastage is to be minimized. For medical devices such as drug-containing stents, the advantages of ink-jet technology result from the controllable and reproducible nature of the droplets in the jet stream and the ability to direct the stream to exact locations on the device surfaces. Programmed target deliveries of 100 lg drug, a typical dose for a small stent, into cuvettes gave a standard deviation (SD) of dose of 0.6 lg. Jetting on coated, uncut stent tubes exhibited 100% capture efficiency with a 1.8 lg SD for a 137 lg dose. In preliminary studies, continuous jetting on stents can yield efficiencies up to 91% and coefficients of variation as low as 2%. These results indicate that ink-jet technology may provide significant improvement in drug loading efficiency over conventional coating methods.

The explosive vapor trace detectors deployed in the field require frequent verification and calibration to guarantee their accurate operation. In this paper we describe the latest advancements in the use of precision micro-dispensing technology for explosive vapor generation. The portable explosive vapor generator uses digitally controlled ink-jet dispensing to precisely eject minute amounts of dilute explosive solutions and convert them into vapor by placing them on a heater. The amount of explosive delivered to the detectors can be controlled by the number of drops (dose mode -specified number of drops is generated) and the frequency of the droplet generation (continuous mode -droplets are generated continuously at fixed frequency. The control for the heater temperature allows setting specific temperatures, but also permits specification of temperature profiles. By using a small thermal capacitance heater it is possible to achieve very sharp temperature increases. The ability to precisely control the heater temperature over a wide range permits the generation of vapors for all common explosives.

Heat and momentum transport is investigated theoretically and numerically considering a rectangular enclosure filled with clear fluid or with fully saturated porous medium, under time-periodic horizontal heating. Numerical simulations, of various configurations, indicate that the natural convection activity within the enclosure peaks at several discrete frequencies, with the climax attained at a heating frequency referred to as resonance frequency. A general theory for predicting this resonance frequency is developed from the natural frequency of the flow circulating inside the enclosure. The resonance frequency can be calculated by solving a system of non linear equations, function of the averaged Rayleigh number, the Prandtl number, the enclosure aspect ratio, the heating amplitude, and the Darcy number for the porous medium case. Theoretical predictions agree well with numerical results. It is shown that the convection intensity within the enclosure increases linearly with heating amplitude for a wide range of parameters. Moreover, the flow response to pulsating heat is continuously enhanced as the system becomes more permeable. Time evolution graphs, phase-plane portraits, and streamlines highlight several distinct phases of the periodic heating process.

This theoretical and numerical study investigates the natural convection flow within a fluid saturated porous medium enclosure subjected to intermittent heating from the side (hot wall). A theory for predicting the natural convection frequency of the flow wheel circulating inside the enclosure is developed from the general porous medium equations on a scaling basis. Physical insight indicates that heat flow resonance might occur when the input heat frequency matches the flow wheel circulating frequency. Numerical simulations confirm the existence of a preferred (resonance) input heat pulsating frequency. They also reveal that at the resonance frequency the fluid saturated porous medium system behaves as a dynamic thermal insulator in which the strong natural convection activity within the system, characterized by high amplitude heat flow oscillations, coexists with a damped oscillatory heat flux at the isothermal (cold) wall.