No description available.

Carbon fiber-reinforced plastics (CFRPs) have gained widespread popularity as a lightweight, high-strength alternative to traditional materials. The unique anisotropic properties of CFRP make processing difficult, especially using conventional methods. This study investigates laser cutting by ablation as an alternative by comparing two near-infrared laser systems to a typical mechanical machining process. This research has potential applications in the automotive and aerospace industries, where CFRPs are particularly desirable for weight savings and fuel efficiency. First, a CNC mill was used to study the effects of process parameters and tool design on machining quality. Despite high productivity and flexible tooling, mechanical drilling suffers from machining defects that could compromise structural performance of a CFRP component. Rotational feed rate was shown to be the primary factor in determining the axial thrust force, which correlated with the extent of delamination and peeling. Experimental results concluded that machining quality could be improved using a non-contact laser-based material removal mechanism. Laser machining was investigated first with a Yb:YAG fiber laser system, operated in either continuous wave or pulse-modulated mode, for both cross-ply and woven CFRP. For the first time, energy density was used as a control variable to account for changes in process parameters, predicting a logarithmic relationship with machining results attributable to plasma shielding effects. Relevant process parameters included operation mode, laser power, pulse overlap, and cross-ply surface fiber orientation, all of which showed a significant impact on single-pass machining quality. High pulse frequency was required to successfully ablate woven CFRP at the weave boundaries, possibly due to matrix absorption dynamics. Overall, the Yb:YAG fiber laser system showed improved performance over mechanical machining. However, microsecond pulses cause extensive thermal damage and low ablation rates due to long laser-material interaction time and low power intensity. Next, laser machining was investigated using a high-energy nanosecond-pulsed Nd:YAG NIR laser operating in either Q-Switch or Long Pulse mode. This research demonstrates for the first time that keyhole-mode cutting can be achieved for CFRP materials using a high-energy nanosecond laser with long-duration pulsing. It is also shown that short-duration Q-Switch mode results in an ineffective cutting performance for CFRP, likely due to laser-induced optical breakdown. At sufficiently high power intensity, it is hypothesized that the resulting plasma absorbs a significant portion of the incoming laser energy by the inverse Bremsstrahlung mechanism. In Long Pulse mode, multi-pass line and contour cutting experiments are further performed to investigate the effect of laser processing parameters on thermal damage and machined surface integrity. A logarithmic trend was observed for machining results, attributable to plasma shielding similar to microsecond fiber laser results. Cutting depth data was used to estimate the ablation threshold of Hexcel IM7 and AS4 fiber types. Drilling results show that a 2.2 mm thick cross-ply CFRP panel can be cut through using about 6 laser passes, and a high-quality machined surface can be produced with a limited heat-affected zone and little fiber pull-out using inert assist gas. In general, high-energy Long Pulse laser machining achieved superior performance due to shorter pulse duration and higher power intensity, resulting in significantly higher ablation rates. The successful outcomes from this work provide the key to enable an efficient high-quality laser machining process for CFRP materials.

Ultra-wide Band (UWB) has sparked a lot of interest lately from the industry and academia. The growing capacity of the wireless industry is requires a new communication system that satisfies the high data rate which does not interfere with existing RF systems. UWB promises to be this new technology. UWB also promises low power, low cost and flexibility. The UWB Channel opens up a huge new wireless channel with Giga Hertz Capacities as well as the highest spatial capacities measured in bits per hertz per square meter. When properly implemented UWB channel can share spectrum with traditional radio systems without causing harmful interference. In this thesis we studied and compared several reduced complexity sub-optimal Ultra-Wide Band receivers. These receivers include auto correlation receiver, the square value detector and the absolute value detector are studied. We consider OOK and PPM modulation schemes. We examine these schemes and the receivers on Gaussian and UWB indoor channels. We compare the performance with optimal receivers. A transmitter receiver system using 0.1us pulses implemented using existing hardware. A packet consisting of 24 bits were transmitted and the received signal could be verified in real time using a vector signal analyzer. The results show sub-optimal receivers provide a better trade off between robust, complexity and performance.

No description available.

No description available.

No description available.

No description available.

No description available.

In this work, the mesh-free smoothed particle hydrodynamics (SPH) method is applied in the modelling of the direct laser interference patterning (DLIP) of metal surfaces. The DLIP technique allows the fabrication of periodic microstructures on technical surfaces using nanosecond laser pulses. Here, the interference of two coherent partial beams with a sinusoidal energy density distribution of the interference pattern is concerned, which is employed to generate line-like surface structures. However, the mechanisms effective during nanosecond pulsed DLIP of metals are not yet fully understood. The physical phenomena occurring due to the interaction of laser radiation with metallic materials are first considered and the governing differential equations are stated. The fundamentals of the SPH method and the approaches to the numerical treatment of the conservation equations are presented. Physical processes relevant to the modelling of laser material processing are solved by suitable SPH techniques, i.e. the approximations are verified with respect to test problems with analytical or known numerical solutions. Consequently, the SPH method is used to devise a thermal model of the DLIP process, considering the absorption of the laser radiation, the heat conduction into the workpiece and the latent heat of involved phase changes. This model is extended to compute the melt pool convection during DLIP, which is driven by surface tension gradients due to temperature gradients. For this purpose, an incompressible SPH (ISPH) method is used, representing a novel approach to the modelling of the laser-induced melt pool flow. The numerical model is employed to perform simulations of DLIP on metal substrates. Firstly, the thermal simulation of the single pulse patterning of stainless steel is in good agreement with experimental results. The application of DLIP to stainless steel and aluminium is then simulated by the comprehensive model including the melt pool flow. Moreover, this model is further extended to consider the non-linear temperature dependence of surface tension, as in liquid steel in the presence of a surface active element. The simulation results reveal a distinct behaviour of stainless steel and aluminium substrates. A markedly deeper melt pool and considerable velocity magnitudes of the thermocapillary convection at the melt surface are computed for DLIP of aluminium. In contrast, the melt pool flow is less pronounced during DLIP of stainless steel, whereas higher surface temperatures are predicted. Hence the Marangoni convection is a conceivable effective mechanism during the structuring of aluminium at moderate energy density. The different character of the melt pool convection during DLIP of stainless steel and aluminium is corroborated by experimental observations. Furthermore, the simulations for stainless steel with different sulphur content indicate distinct melt pool flow patterns and support the explanation of the microstructures found after DLIP experiments. The role of vapourisation and the induced recoil pressure in the microstructure evolution due to DLIP on metal substrates at elevated fluences could be prospectively investigated. In this regard, the consideration of the melt pool surface deformation in the ISPH algorithm, and particularly a suitable pressure boundary condition, is required.:I The research problem 1 Motivation 2 Modelling of laser material processing 2.1 Interaction of laser radiation with materials 2.1.1 Absorption of laser radiation 2.1.2 Heat conduction and phase change 2.1.3 Molten pool convection 2.1.4 Vapourisation regime 2.2 Mathematical modelling of laser material interaction 2.2.1 Conservation equations in Lagrangian formulation 2.2.2 Influence of surface tension 3 State of the art in laser microprocessing and the SPH method 3.1 Laser microprocessing 3.2 Simulation of direct laser interference patterning 3.3 The mesh-free smoothed particle hydrodynamics method 3.3.1 Fundamental approximations and kernel function 3.3.2 Particle distribution and interaction length 3.3.3 Approximation of derivatives 3.3.4 Treatment of boundaries 3.3.5 Neighbourhood search 3.4 Numerical modelling of laser material processing by SPH II SPH model development for direct laser interference patterning 4 SPH modelling of heat transfer and fluid flow 4.1 Solution of the heat diffusion equation 4.2 Formulation of equations governing fluid flow 4.2.1 Equation of continuity 4.2.2 Approximation of pressure gradient term 4.2.3 Treatment of viscosity 4.3 Weakly compressible SPH method for solving fluid flow 4.3.1 Particle motion 4.3.2 Time integration 4.3.3 Time step criteria 4.4 Incompressible SPH method for solving fluid flow 4.4.1 Time integration 4.4.2 Discrete incompressible SPH algorithm 4.4.3 Time step criteria 4.5 Simulation of thermal fluid flow using ISPH 4.5.1 Semi-implicit time integration 4.5.2 Solution of the pressure Poisson equation 5 Verification of the SPH implementation 5.1 Transient heat conduction in laser-irradiated plate 5.1.1 Problem description 5.1.2 Dimensionless formulation 5.1.3 Numerical solution and results 5.2 Viscous flow 5.2.1 Couette flow 5.2.2 Poiseuille flow 5.3 Thermal convection 5.3.1 Natural convection in a square cavity 5.3.2 Rayleigh--Marangoni--Bénard convection in liquid aluminium 6 SPH model of direct laser interference patterning 6.1 Characteristics of the process 6.2 Thermal model 6.2.1 Non-dimensionalisation 6.2.2 Numerical solution of governing equation 6.2.3 Verification of the computation 6.2.4 Numerical test 6.3 Thermofluiddynamic model 6.3.1 Non-dimensionalisation 6.3.2 Numerical solution of governing equations 6.3.3 Discretisation 6.3.4 Resolution independence study 7 SPH simulation of direct laser interference patterning 7.1 Thermal model 7.1.1 DLIP experiments on stainless steel substrates 7.1.2 Thermal simulation of DLIP on steel substrate 7.2 Thermofluiddynamic model 7.2.1 Material properties and simulation parameters 7.2.2 Numerical results for steel substrate 7.2.3 Numerical results for aluminium substrate 7.2.4 Discussion and comparison with experiments 7.3 Extended thermofluiddynamic model 7.3.1 Model parameters 7.3.2 Influence of sulphur content on DLIP of stainless steel 8 Conclusions and outlook Bibliography / In dieser Arbeit wird die direkte Laserinterferenzstrukturierung (Direct Laser Interference Patterning, DLIP) von Metallen mit der netzfreien Smoothed Particle Hydrodynamics (SPH) Methode modelliert. Das DLIP-Verfahren ermöglicht die Fertigung periodischer Mikrostrukturen auf technischen Oberflächen mit Nanosekunden-Laserpulsen. Hier wird die Zweistrahlinterferenz mit einer sinusförmigen Energiedichteverteilung des Interferenzmusters behandelt, die linienförmige Oberflächenstrukturen erzeugt. Die bei der direkten Interferenzstrukturierung von Metallen mit Nanosekunden-Laserpuls wirksamen Mechanismen sind jedoch noch nicht verstanden. Die aufgrund der Wechselwirkung von Laserstrahlung mit metallischen Werkstoffen auftretenden physikalischen Phänomene werden zuerst betrachtet und die sie bestimmenden Differentialgleichungen angegeben. Die Grundlagen der SPH-Methode sowie deren Herangehensweisen an die numerische Behandlung der Erhaltungsgleichungen werden vorgestellt. Für die Modellierung der Lasermaterialbearbeitung relevante physikalische Vorgänge werden mittels geeigneter SPH-Ansätze gelöst, d. h. anhand von Testproblemen mit bekannter Lösung verifiziert. Das mit SPH zunächst erstellte thermische Modell des DLIP-Prozesses berücksichtigt die Absorption der Laserstrahlung, die Wärmeleitung im Werkstück und die Enthalpien der Phasenübergänge. Das Modell wird zur Berechnung der Schmelzbadströmung bei der DLIP-Anwendung, angetrieben von Oberflächenspannungsgradienten verursacht durch Temperaturgradienten, erweitert. Hierbei wird eine inkompressible SPH (ISPH) Methode eingesetzt, in der Simulation laserinduzierter Schmelzbäder ein neuartiger Ansatz. Mit dem numerischen Modell werden Simulationen des DLIP-Verfahrens für metallische Substrate durchgeführt. Die thermische Simulation der Strukturierung von Edelstahl stimmt gut mit einem Experiment überein. Weiterhin wird die Anwendung von DLIP auf Edelstahl und Aluminium mit dem thermofluiddynamischen Modell simuliert. Außerdem wird das Modell um eine nichtlinear temperaturabhängige Oberflächenspannung, wie sie für Stahlschmelze in Anwesenheit eines oberflächenaktiven Elements vorliegt, ergänzt. Die Simulationen zeigen ein verschiedenes Verhalten von Edelstahl und Aluminium. Bei der Strukturierung von Aluminium treten ein deutlich tieferes Schmelzbad und erhebliche Geschwindigkeitsbeträge der thermokapillaren Konvektion an der Schmelzeoberfläche auf. Hingegen ist die Strömung bei der DLIP-Anwendung auf Edelstahl schwächer ausgeprägt und höhere Oberflächentemperaturen werden erreicht. Die Marangoni-Konvektion ist daher ein wirksamer Schmelzeverdrängungsmechanismus bei der Strukturierung von Aluminium mit moderater Energiedichte. Die unterschiedliche Schmelzbadströmung für die beiden Werkstoffe wird durch experimentelle Beobachtungen bestätigt. In Abhängigkeit des Schwefelgehalts von Edelstahl zeigen Simulationen verschiedene Strömungsmuster im Schmelzbad und unterstützen die Erklärung experimentell festgestellter Mikrostrukturen. Die Untersuchung der Wirkung der Verdampfung und des induzierten Rückstoßdruckes auf die Strukturausbildung bei höheren Fluenzen erfordert die Berücksichtigung der Oberflächendeformation sowie eine geeignete Druckrandbedingung im ISPH-Algorithmus.:I The research problem 1 Motivation 2 Modelling of laser material processing 2.1 Interaction of laser radiation with materials 2.1.1 Absorption of laser radiation 2.1.2 Heat conduction and phase change 2.1.3 Molten pool convection 2.1.4 Vapourisation regime 2.2 Mathematical modelling of laser material interaction 2.2.1 Conservation equations in Lagrangian formulation 2.2.2 Influence of surface tension 3 State of the art in laser microprocessing and the SPH method 3.1 Laser microprocessing 3.2 Simulation of direct laser interference patterning 3.3 The mesh-free smoothed particle hydrodynamics method 3.3.1 Fundamental approximations and kernel function 3.3.2 Particle distribution and interaction length 3.3.3 Approximation of derivatives 3.3.4 Treatment of boundaries 3.3.5 Neighbourhood search 3.4 Numerical modelling of laser material processing by SPH II SPH model development for direct laser interference patterning 4 SPH modelling of heat transfer and fluid flow 4.1 Solution of the heat diffusion equation 4.2 Formulation of equations governing fluid flow 4.2.1 Equation of continuity 4.2.2 Approximation of pressure gradient term 4.2.3 Treatment of viscosity 4.3 Weakly compressible SPH method for solving fluid flow 4.3.1 Particle motion 4.3.2 Time integration 4.3.3 Time step criteria 4.4 Incompressible SPH method for solving fluid flow 4.4.1 Time integration 4.4.2 Discrete incompressible SPH algorithm 4.4.3 Time step criteria 4.5 Simulation of thermal fluid flow using ISPH 4.5.1 Semi-implicit time integration 4.5.2 Solution of the pressure Poisson equation 5 Verification of the SPH implementation 5.1 Transient heat conduction in laser-irradiated plate 5.1.1 Problem description 5.1.2 Dimensionless formulation 5.1.3 Numerical solution and results 5.2 Viscous flow 5.2.1 Couette flow 5.2.2 Poiseuille flow 5.3 Thermal convection 5.3.1 Natural convection in a square cavity 5.3.2 Rayleigh--Marangoni--Bénard convection in liquid aluminium 6 SPH model of direct laser interference patterning 6.1 Characteristics of the process 6.2 Thermal model 6.2.1 Non-dimensionalisation 6.2.2 Numerical solution of governing equation 6.2.3 Verification of the computation 6.2.4 Numerical test 6.3 Thermofluiddynamic model 6.3.1 Non-dimensionalisation 6.3.2 Numerical solution of governing equations 6.3.3 Discretisation 6.3.4 Resolution independence study 7 SPH simulation of direct laser interference patterning 7.1 Thermal model 7.1.1 DLIP experiments on stainless steel substrates 7.1.2 Thermal simulation of DLIP on steel substrate 7.2 Thermofluiddynamic model 7.2.1 Material properties and simulation parameters 7.2.2 Numerical results for steel substrate 7.2.3 Numerical results for aluminium substrate 7.2.4 Discussion and comparison with experiments 7.3 Extended thermofluiddynamic model 7.3.1 Model parameters 7.3.2 Influence of sulphur content on DLIP of stainless steel 8 Conclusions and outlook Bibliography