Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1992. / Vita. Abstract. Includes bibliographical references (leaves 121-124). Also available via the Internet.

Thesis (M.S.)--Missouri University of Science and Technology, 2009. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed August 14, 2009) Includes bibliographical references (p. 16-17).

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1994. / Vita. Abstract. Includes bibliographical references (leaves 121-126). Also available via the Internet.

A numerical model of the temperature field during pulsed laser cutting of thin sheets (approximately 2.5 x l0⁻⁵ m) was developed. Cutting was simulated through removal of nodes from a finite difference scheme based on sensible heating to the phase change temperature and a single value of latent heat (melting or vaporization). The pulsed laser model predicts a heat-affected zone of less than 0.02 mm for pulsed laser cutting. For comparable cutting with a continuous power laser, a heat-affected zone between 0.05 and 0.10 mm is predicted. Thermal stress levels were predicted to be an order of magnitude lower for pulsed laser cutting than for continuous power cutting. The stress levels predicted by the model also increased with cut speed. Experimentally, pulsed laser cutting yielded better cut quality, based on less cracking, than continuous power cutting. In addition, the cut quality deteriorated as the cutting speed was increased for the continuous power laser. Presently, application of pulsed laser cutting is limited by its low cutting speed, which is restricted by the energy density of the laser. The model predicts that increasing energy density will decrease the size of the heat-affected zone and increase the maximum cutting speed. Therefore, pulsed laser cutting at high speeds should be attainable without deterioration in cut quality. / Master of Science

The research presented here investigates the effects of laser trimming on the state of stress in alumina Al₂O₃ hybrid microelectronics substrates. Evaluation of stress was performed using x-ray diffraction residual stress analysis and dynamic strain measurements using strain gages before and after laser trimming. X-ray diffraction measurements were carried out in both the longitudinal and transverse directions on the front and back sides of the substrates. The dynamic strain measurements were performed in situ with strain gages attached to the bottom of the substrates while the substrates were trimmed with a 400 watt YAG laser. The substrates were characterized using optical microscopy, scanning electron microscopy / energy dispersive x-ray analysis (SEM/EDAX), electron probe microanalysis (EPMA) and electron spectroscopy for chemical analysis (ESCA). The results from these characterization steps gave results for fractography (optical), surface and bulk composition (SEM/EDAX), chemical composition (ESCA) and phase analysis (EPMA). Results show that laser trimming produces stress gradients which are generally tensile in nature and could have deleterious effects on the mechanical integrity of the substrates if used in hybrid microelectronic applications. Furthermore the stress distribution across the substrates was found to be uniformly distributed showing no peak stresses near the heat affected zone (HAZ) boundary. Phase analysis determined that the substrates contained a magnesium aluminum spinel phase (MgAl₂O₄) and that the glass and pore phases are randomly distributed in the substrates. This could have some overall effect on the state of residual stress in the substrates after they have been laser trimmed. / Master of Science

Since the development of thick film hybrid microelectronic processing, there has been a need for methods to adjust for tighter tolerances for electrical components through a trimming process. Components/elements, as produced, show a tolerance of the order of ±10% due to the variability of the screen printing process associated with film curing conditions. The methods that have arisen from this need encompass a variety of technologies and techniques. The usefulness of each method is based on its operation, flexibility, repeatability, and post-trim effects on the resistor's reliability. The work in this thesis concerns the laser trimming of resistor components to a tight tolerance. It is the objective of this thesis to address the performance of an Nd:YAG laser operation and interface with a computer. The first task involves a computer hardware system to be interfaced to the laser control system, this task includes both design and implementation. The second task consists of a software operating environment to be flowcharted, written, and tested. The third task involves the computer interface driving the laser in the process of trimming resistor components using different types of cuts. The trimmed resistor performance is evaluated as part of the study. / Master of Science

No description available.

Laser beam cutting is an attractive and innovative manufacturing process which has many advantages compared to conventional cutting methods. However, with increasing workpiece thickness an increase of the roughness along the kerf surface can be observed, which, in turn, can negatively affect the mechanical properties, in particular the fatigue strength. In this context, the impact of laser cutting on fatigue behavior is discussed in this work. Specimens were cut out by disk laser from sheets with 2 mm, 4 mm and 6 mm thickness made of metastable austenitic stainless steel type AISI 304. Fatigue specimens with different surface conditions were tested in order to separately evaluate the influence of the different kinds of macroscopic defects produced by the cutting process. Additionally, the notch effect sensitivity for different amounts of deformation-induced α'-martensite induced before cyclic tests was evaluated using a specific notch geometry. Furthermore, based on the fact that high frequency testing is performed in the present investigation, the likely influences of test frequency on material response must be considered. For this reason and because metastable austenitic stainless steels are well known for their strain rate sensitivity, the steel AISI 304 and the role of surface micro-defects produced by laser beam cutting were analyzed regarding the influence of load frequency on the cyclic response and fatigue behavior, and the findings of the investigation are thoroughly discussed in this work. Fatigue tests were performed at load frequencies of 100 Hz and 1,000 Hz using two resonance pulsation test systems, as well as by means of a servo-hydraulic test machine at 1 Hz and 50 Hz. Fractographic analyses served to evaluate the failure-relevant characteristics responsible for crack initiation. Moreover, scanning electron microscopy was used to assess the changes caused by laser cutting on the geometrical and microstructural features. The qualitative analyses of the cut kerf characteristics were accomplished by means of optical microscopy. The cyclic deformation behavior was characterized based on the evaluation of stress-strain hysteresis loops and temperature measurements. The deformation-induced phase transformation from γ-austenite to α'-martensite was globally and locally evaluated by means of magneto-inductive measurements and electron backscattered diffraction analysis, respectively. The analyses showed that laser beam cutting creates three kinds of defects, which are a pronounced relief-like structure along the cut surface, burr in the underside of the cut edge and pores in the interface between the recast layer and base material or inside the recast layer. As a consequence, the fatigue strength of parts cut by laser beam is around 40% lower in comparison to specimens in a macroscopically quasi defect-free state. The most significant reduction of fatigue life is attributed to the notch effect of the burr, followed by the notch effect created by pores for 4 mm and 6 mm thick specimens, while surprisingly the influence of the surface relief is of minor significance. An evaluation of the fatigue results based on comprehensive fractographic analyses allows to explain the reasons for distinct differences between the aforementioned influence factors. Furthermore, the notch sensitivity of the steel AISI 304 increases as the amount of α'-martensite formed prior to the cyclic experiments becomes higher. Moreover, the impact of test frequency on the cyclic deformation response, deformation-induced phase transformation and fatigue behavior was characterized as well as the role of surface micro-defects on the fatigue behavior as a function of test frequency was identified. The assessment showed that higher amounts of α'-martensite formation and lower plastic strain amplitudes were observed when the cyclic experiments were carried out at lower frequency, promoting higher fatigue strengths. However, the influence of test frequency for specimens containing surface micro-defects is dominant in the low cycle fatigue to high cycle fatigue regime, whereas in the high cycle fatigue and very high cycle fatigue range the fatigue life determining parameter is the severity of the micro-notches present along the laser cut surface. / Das Laserstrahlschneiden stellt ein innovatives Verfahren dar, welches im Vergleich zu konventionellen Schneid prozessen eine Vielzahl an Vorteilen aufweist, aber mit zunehmender Blechdicke auch den entscheidenden Nachteil der Zunahme der Oberflächenrauheit der Schnittkanten, was sich negativ auf die mechanische Festigkeit, insbesondere die Schwingfestigkeit, auswirken kann. Deswegen ist die Anwendung dieser Technik zur Herstellung von strukturellen Bauteilen aufgrund der Bildung ausgeprägter Oberflächenreliefs und dem Fehlen zuverlässiger Schwingfestigkeitsdaten begrenzt. In diesem Zusammenhang werden in dieser Arbeit die Auswirkungen des Laserschneidens auf das Ermüdungsverhalten diskutiert. Aus Blechen des metastabilen Austenitstahls AISI 304 mit Dicken von 2 mm, 4 mm und 6 mm wurden Proben mit Hilfe eines Scheibenlasers herausgeschnitten. Ermüdungsproben unterschiedlicher Oberflächenqualität wurden getestet, um den Einfluss der verschiedenen Arten von makroskopischen Defekten, die durch den Schneidprozess erzeugt wurden, separat zu bewerten. Zusätzlich wurde die Empfindlichkeit der Kerbwirkung bei unterschiedlichen Gehalten an verformungsinduziertem α'-Martensit, welcher vor den zyklischen Versuche erzeugt wurde, unter Verwendung einer spezifischen Kerbgeometrie ausgewertet. Außerdem, basierend auf der Tatsache, dass in der vorliegenden Untersuchung eine Hochfrequenzprüfung durchgeführt wurde, wurden der Stahl AISI 304 und die Rolle von Oberflächenmikrodefekten durch das Laserstrahlschneiden hinsichtlich des Frequenzeinflusses auf das zyklische Ansprechverhalten und Ermüdungsverhalten analysiert. Ermüdungsprüfungen wurden bei Frequenzen von 100 Hz und 1000 Hz unter Verwendung von zwei ähnlichen Resonanzpulsationstestsystemen, sowie mittels einer servohydraulischen Prüfmaschine bei 1 Hz und 50 Hz durchgeführt. Fraktographische Analysen dienten dazu, die für die Rissinitiierung verantwortlichen fehlerrelevanten Eigenschaften zu bewerten. Darüber hinaus wurde die Rasterelektronenmikroskopie verwendet, um die durch das Laserschneiden verursachten Veränderungen der geometrischen und mikrostrukturellen Eigenschaften zu bewerten. Die qualitativen Analysen der Schnittkanteneigenschaften wurden mittels optischer Mikroskopie durchgeführt. Das zyklische Verformungsverhalten wurde anhand der Auswertung von Spannungs-Dehnungs-Hystereseschleifen und Temperaturmessungen charakterisiert. Die verformungsinduzierte Phasenumwandlung von γ-Austenit zu α'-Martensit wurde global und lokal mittels magneto-induktiven Messungen bzw. Elektronenrückstreubeugungsanalyse ausgewertet. Die Analysen zeigen, dass ein ausgeprägtes Relief entlang der Schnittfläche sowie ein Grat an der Unterseite der Schnittkante bei 2 mm, 4 mm und 6 mm dicken Platten gebildet wurden. Zusätzlich haben sich Poren in der Grenzfläche zwischen dem umgeschmolzenen Material und dem Grundmaterial oder innerhalb des umgeschmolzenen Materials bei den 4 mm und 6 mm dicken Platten gebildet. Infolgedessen zeigen die laserstrahlgeschnittenen Proben eine 40% niedrigere Dauerfestigkeit im Vergleich zu den Proben, welche in einem makroskopisch quasi defektfreien Zustand sind. Obwohl beim Schneiden dickerer Platten gröbere Oberflächen entstehen, ist die Verringerung der Dauerfestigkeit entgegen den Erwartungen unabhängig von der Probendicke. Die größte Verringerung der Ermüdungslebensdauer ist auf die Kerbwirkung des Grates zurückzuführen, gefolgt von der Kerbwirkung des Oberflächenreliefs für 2 mm dicke Platten bzw. des Einflusses von Poren im Fall der 4 mm und 6 mm dicken Platten. Ein Zusammenhang zwischen der Phasenumwandlung von γ-Austenit zu α'-Martensit, die durch zyklische Verformung entsteht, und der Lastfrequenz wurde nachgewiesen. Eine niedrigere Prüffrequenz ruft für den untersuchten unsymmetrischen Belastungsverlauf einen signifikanten zyklischen Kriecheffekt, höhere mittlere Dehnungsniveaus, höhere Mengen an α'-Martensitbildung, geringere plastische Dehnungsamplituden und daher höhere Ermüdungsfestigkeiten hervor. Bei Proben mit Oberflächenmikrodefekten, die durch das Laserstrahlschneiden erzeugt werden, hängt der Einfluss der Prüffrequenz auf das Ermüdungsverhalten vom aufgebrachten Belastungsniveau ab. Dieser Einfluss dominiert im LCF zu HCF Bereich, während im Übergang vom HCF in den VHCF Bereich die Ermüdungslebensdauer von der Schwere des Schadens abhängig ist, welcher in der Probe durch lebensdauerbestimmende, zufällig entlang der lasergeschnittenen Reliefoberfläche eingebrachten Mikrokerben hervorgerufen wird.