Laser beam welding | EPFL Graph Search

Laser beam welding (LBW) is a welding technique used to join pieces of metal or thermoplastics through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequently used in high volume and precision requiring applications using automation, as in the automotive and aeronautics industries. It is based on keyhole or penetration mode welding. Operation Like electron-beam welding (EBW), laser beam welding has high power density (on the order of 1 MW/cm2) resulting in small heat-affected zones and high heating and cooling rates. The spot size of the laser can vary between 0.2 mm and 13 mm, though only smaller sizes are used for welding. The depth of penetration is proportional to the amount of power supplied, but is also dependent on the location of the focal point: penetration is maximized when the focal point is slightly below the surface of the workpiece A continuous or pulsed

Simulation of the welding process of steel tube joints made of S355 and S690

Janna Krummenacker

Welding is an important joining method for the work with metals. During the welding process, the material is heated and deforms plastically which leads a change in microstructure in the heat affected zone, to complex residual stress distributions, and distortions. The residual stresses can have a major impact, among other phenomena, on the fatigue resistance of a welded structure. The aim of this work was to develop a model with the finite element software Abaqus for the simulation of a welding process for butt welded tubes and the calculation of the resulting welding residual stress field. The residual stress field depends on several input parameters, like the geometry, the heat input, the welding speed, the material data, and the number of weld passes. The chosen dimensions of the tubes are realistic for tubular steel bridge constructions. Since, no experimental welding procedures were conducted, the welding parameters like the heat input and the welding speed were estimated in order to obtain a realistic temperature distribution in the weld and the heat affected zone. The assignment asked to place a main focus on the influence of the material data and the number of weld passes. Within the framework of this work, tensile tests at three different temperatures were conducted in order to find the deformational behavior of the two different steel grades S355 and S690. The results show generally good agreement with the data given by the European Standard for the structural fire design of steel structures [11], that is noted Eurocode 3 (EC3) in the course of this work. According to the test results, the yield strength at room temperature are underestimated by the Eurocode 3. Based on the experiments and the data given in the Eurocode 3, the influence of the mechanical properties were studied in several Abaqus models. For the description of the temperature dependent material behavior, a bilinear stress-strain curve was implemented. The resulting stress distributions show a similar trend, but the offset values between the stress curves of two models are dependent on the position in circumferential direction and the distance in axial direction. During the literature study, different welding simulation programs were compared. With specific welding simulation programs, mostly two-dimensional models can be realised. A major advantage of these programs is, that phase transformations can easily be considered and therefore a more realistic material model can be implemented provided that the temperature dependant behavior of the different microstructural phases are known. The influence of the number of weld passes was studied by comparing a single- and a multi-pass model with the material data from the Eurocode 3. The total heat input and the welding speed was the same for all the models. The tendency of the resulting stress distributions varied greatly and therefore implies that a multi-pass model cannot be approximated by a single-pass model.

2011

Temperature and oxygen-concentration dependence of singlet oxygen production by RuPhen as induced by quasi-continuous excitation

Veronika Huntosova, Jaroslav Varchola, Georges Wagnières

Advanced Search Home > Journals > Photochemical & Photobi... > Temperature and oxygen-... For Authors & Referees | For Librarians | For Members Journal cover: Photochemical & Photobiological Sciences Photochemical & Photobiological Sciences Issue 12, 2014 A society-owned journal publishing high quality research on all aspects of photochemistry and photobiology. Impact Factor 2.939 12 Issues per Year Indexed in Medline Journal Home Previous Article | Next Article Paper Temperature and oxygen-concentration dependence of singlet oxygen production by RuPhen as induced by quasi-continuous excitation Jaroslav Varchola,a Veronika Huntosova,b Daniel Jancura,ab Georges Wagnières,c Pavol Miskovskyab and Gregor Bánó*ab Show Affiliations Photochem. Photobiol. Sci., 2014,13, 1781-1787 DOI: 10.1039/C4PP00202D Received 06 Jun 2014, Accepted 02 Oct 2014 First published online 06 Oct 2014 | | Share on citeulike | Share on facebook | Share on twitter | | More PDF Rich HTML Send PDF to Kindle Download Citation Help Request Permissions Abstract Cited by Related Content Assessment of partial pressure of oxygen (pO2) by luminescence lifetime measurements of ruthenium coordination complexes has been studied intensively during the last few decades. RuPhen (dichlorotris(1,10-phenanthroline) ruthenium(II) hydrate) is a water soluble molecule that has been tested previously for in vivo pO2 detection. In this work we intended to shed light on the production of singlet oxygen by RuPhen. The quantum yield of singlet oxygen production by RuPhen dissolved in 0.9% aqueous NaCl solution (pH = 6) was measured at physiological temperatures (285–310 K) and various concentrations of molecular oxygen. In order to minimize the bleaching of RuPhen, the samples were excited with low power (

2014

Application of Adaptive Optics to Focusing and Imaging

Daniel Iwaniuk

Adaptive optics is used to abate aberrations with a wavefront correction. It is widely used in astronomy and is starting to expand into applications of laser focusing and imaging with high numerical apertures, e.g. microscopy. The physical background of focusing laser light and imaging is the same, so we can use the same methods. In both cases, light is propagating through a lens; either it becomes focused into a small region or light from a small region becomes imaged on a camera. Modulations applied on the lens to implement wavefront corrections behave similarly in both applications. A powerful simulation tool was created to characterize the impact of those modulations. As an example, we validated a design for a Fresnel lens produced on a glass fibre tip to focus its emitting light. We have developed solutions mainly for three different problems. First, a high depth of focus enables keeping a laser beam focused within a larger length or imaging objects from different positions simultaneously. In photography, this can be attained by stopping down the aperture, which introduces a huge loss of light. State-of-the-art for focusing into a line segment also shows an inefficient performance. We present an elegant lens design, which enables highly efficient elongation of the depth of focus. Preliminary studies have shown that it might be a feasible alternative for current intra-ocular lens implants in ophthalmology or for 3D visualization in imaging and microscopy. A high depth of focus also has a large potential in optical lithography and data storage, because the focal position is enlarged and does not have to be adjusted precisely to obtain a useful spot. Second, specimen-induced aberrations can affect even a perfectly adjusted, diffraction limited lens system. A planar refraction index mismatch introduces spherical aberrations, degrading the optical resolution. As a first step to correct them, we were able to characterize simultaneously the refractive index and the thickness of an unknown medium that is placed between the lens and its focal region. This was done by clever manipulation of the beam angles with the same adaptive optics element in the focusing and in the imaging system. Finally, the medium-induced spherical aberrations were corrected based on the characterization results. The point spread function degradation of a focused laser beam was completely removed, which might be useful in optical tweezers or in laser processing of biological samples. While imaging through the planar refractive index mismatch, the bending of the object field was corrected and the diffraction limited performance restored.

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