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  • 1.
    Amer, Eynas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Laser-ablation-induced refractive index fields studied using pulsed digital holographic interferometry2009Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 47, nr 7-8, s. 793-799Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Pulsed digital holographic interferometry has been used to investigate the plume and the shock wave generated in the ablation process of a Q-switched Nd-YAG (λ=1064 nm and pulse duration=12 ns) laser pulse on a polycrystalline boron nitride (PCBN) target under atmospheric air pressure. A special setup based on two synchronised wavelengths from the same laser for simultaneous processing and measurement has been used. Digital holograms were recorded for different time delays using collimated laser light (λ=532 nm) passed through the volume along the target. Numerical data of the integrated refractive index field were calculated and presented as phase maps showing the propagation of the shock wave and the plume generated by the process. Radon inversion has been used to estimate the 3D refractive index fields measured from the projections assuming rotational symmetry. The shock wave density has been calculated using the point explosion model and the shock wave condition equation and its behaviour with time at different power densities ranging from 1.4 to 9.1 GW/cm2 is presented. Shock front densities have been calculated from the reconstructed refractive index fields using the Gladstone-Dale equation. A comparison of the shock front density calculated from the reconstructed data and that calculated using the point explosion model at different time delays has been done. The comparison shows quite good agreement between the model and the experimental data. Finally the reconstructed refractive index field has been used to estimate the electron number density distribution within the laser-induced plasma. The electron number density behaviour with distance from the target at different power densities and its behaviour with time are shown. The electron number densities are found to be in the order of 1018 cm-3 and decay at a rate of 3×1015 electrons/cm3 ns.

  • 2.
    Bergström, Per
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Rosendahl, Sara
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Shape verification aimed for manufacturing process control2011Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 49, nr 3, s. 403-409Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We describe a method to verify the shape of manufactured objects by using their design model. A non-contact measuring method that consists of a stereo-camera system and a single projected fringe pattern is used. The method acquires one image from each camera. Additional shape information from the design model is also used. This surface-measurement method gives an accuracy of about 45 µm. Deviations from the design model within ±1.6 mm can be correctly detected. The measured surface representation is matched to the design model using the ICP-method. Fast performance has been considered adapting the method for on-line use.

  • 3.
    Bunaziv, Ivan
    et al.
    Norwegian University of Science and Technology, Department of Engineering Design and Materials.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Akselsen, Odd M.
    Norwegian University of Science and Technology, Department of Engineering Design and Materials.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Process stability during fiber laser-arc hybrid welding of thick steel plates2018Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 102, s. 34-44Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    TThick steel plates are frequently used in shipbuilding, pipelines and other related heavy industries, and are usually joined by arc welding. Deep penetration laser-arc hybrid welding could increase productivity but has not been thoroughly investigated, and is therefore usually limited to applications with medium thickness (5-15 mm) sections. A major concern is process stability, especially when using modern welding consumables such as metal-cored wire and advanced welding equipment. High speed imaging allows direct observation of the process so that process behavior and phenomena can be studied. In this paper, 45 mm thick high strength steel was welded (butt joint double-sided) using the fiber laser-MAG hybrid process utilizing a metal-cored wire without pre-heating. Process stability was monitored under a wide range of welding parameters. It was found that the technique can be used successfully to weld thick sections with appropriate quality when the parameters are optimized. When comparing conventional pulsed and the more advanced cold metal transfer pulse (CMT+P) arc modes, it was found that both can provide high quality welds. CMT+P arc mode can provide more stable droplet transfer over a limited range of travel speeds. At higher travel speeds, an unstable metal transfer mechanism was observed. Comparing leading arc and trailing arc arrangements, the leading arc configuration can provide higher quality welds and more stable processing at longer inter-distances between the heat sources.

  • 4.
    Carlsson, Torgny E.
    et al.
    Swedish Defence Research Agency, FOI.
    Mattsson, Roger
    Luleå tekniska universitet.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Elfsberg, Mattias
    Swedish Defence Research Agency, FOI.
    Tegner, Jon
    Swedish Defence Research Agency, FOI.
    Combination of schlieren and pulsed TV holography in the study of a high-speed flame jet2006Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 44, nr 6, s. 535-554Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper presents studies of the propagation of a high-speed turbulent flame jet of an air/hydrogen gas mixture. The experimental results are recorded with the schlieren and the pulsed TV holography method. These methods are compared and combined to benefit from the advantages of each of them. Abel inversion has been used to achieve three-dimensional information i.e. refractive index distributions. Evaluation, calibration and Abel inversion algorithms are described together with experimental results. The results obtained from the different techniques show remarkable similarities concerning both qualitative and quantitative aspects.

  • 5.
    Casselgren, Johan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Rosendahl, Sara
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jonsson, Patrik
    Combitech AB, Mittuniversitetet, Department of Information Technology and Media, Mid-Sweden University.
    Road condition analysis using NIR illumination and compensating for surrounding light2016Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 77, s. 175-182Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An investigation of a NIR camera system for road surface classification has been conducted for several road conditions. The surfaces were illuminated with three wavelengths, 980 nm, 1310 nm and 1550 nm and a halogen lamp, to simulate a real environment application with surrounding light. A measuring scheme to deal with surrounding light has been implemented enabling road condition classification from NIR images in a real environment. The retrieved camera images have been analyzed and an RGB representation of the different surfaces has been created to classify the different road conditions. The investigation shows that it is possible to distinguish between dry, moist, wet, frosty, icy and snowy road surfaces using a NIR camera system in a disturbed environment.

  • 6.
    Eppanapelli, Lavan Kumar
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Casselgren, Johan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Wåhlin, Johan
    Norwegian University of Science and Technology.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Investigation of snow single scattering properties of snow based on first order Legendre phase function2017Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 91, s. 151-159Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Angularly resolved bidirectional reflectance measurements were modelled by ap- proximating a first order Legendre expanded phase function to retrieve single scattering properties of snow. The measurements from 10 different snow types with known density and specific surface area (SSA) were investigated. A near infrared (NIR) spectrometer was used to measure reflected light above the snow surface over the hemisphere in the wavelength region 900 nm to 1650 nm. A solver based on discrete ordinate radiative transfer (DISORT) model was used to retrieve the estimated Legendre coefficients of the phase function and a cor- relation between the coefficients and physical properties of different snow types is investigated. Results of this study suggest that the first two coefficients of the first order Legendre phase function provide sufficient information about the physical properties of snow where the latter captures the anisotropic behaviour of snow and the former provides a relative estimate of the single scattering albedo of snow. The coefficients of the first order phase function were com- pared with the experimental data and observed that both the coefficients are in good agreement with the experimental data. These findings suggest that our approach can be applied as a qualitative tool to investigate physical properties of snow and also to classify different snow types.

  • 7.
    Eppanapelli, Lavan Kumar
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Friberg, Benjamin
    Casselgren, Johan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Estimation of a low-order Legendre expanded phase function of snow2016Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 78, s. 174-181Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The purpose of this paper is to estimate the scattering phase function of snow from angularly resolved measurements of light intensity in the plane of incidence. A solver is implemented that solves the scattering function for a semi-infinite geometry based on the radiative transfer equation (RTE). Two types of phase functions are considered. The first type is the general phase function based on a low-order series expansion of Legendre polynomials and the other type is the Henyey-Greenstein (HG) phase function. The measurements were performed at a wavelength of 1310 nm and six different snow samples were analysed. It was found that a first order expansion provides sufficient approximation to the measurements. The fit from the first order phase function outperforms that of the HG phase function in terms of accuracy, ease of implementation and computation time. Furthermore, a correlation between the magnitude of the first order component and the age of the snow was found. We believe that these findings may complement present non-contact detection techniques used to determine snow properties.

  • 8.
    Eriksson, Ingemar
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Powell, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Melt behavior on the keyhole front during high speed laser welding2013Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 51, nr 6, s. 735-740Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The flow of molten metal on the front wall of a laser generated welding keyhole has been observed by high speed photography, optically measured by mapping the flow of ripples on the liquid surface and theoretically calculated. A clear downward flow can be observed and measured by a Particle Image Velocimetry algorithm. A theoretical calculation of the melt thickness on the keyhole front is also presented. Results indicate that the thickness of the liquid on the keyhole front is similar to that of the resolidified layer found in micrographs of the front if the laser is suddenly turned off. The measured surface ripple flow speeds are between two and four times as high as the theoretical average fluid flow rate.

  • 9.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Factors affecting weld root morphology in laser keyhole welding2018Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 101, s. 89-98Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Welding production efficiency is usually optimised if full penetration can be achieved in a single pass. Techniques such as electron and laser beam welding offer deep high speed keyhole welding, especially since multi-kilowatt lasers became available. However, there are limitations for these techniques when considering weld imperfections such as weld cap undercuts, interior porosity or humps at the root. The thickness of sheets during full penetration welding is practically limited by these root humps. The mechanisms behind root morphology formation are not yet satisfactory understood. In this paper root humping is studied by reviewing previous studies and findings and also by sample examination and process observation by high speed imaging. Different process regimes governing root quality are presented, categorized and explained. Even though this study mainly covers laser beam and laser arc hybrid welding, the presented findings can generally be applied full penetration welding in medium to thick sheets, especially the discussion of surface tension effects. As a final result of this analysis, a map of methods to optimise weld root topology is presented.

  • 10. Fällström, Karl-Evert
    et al.
    Olofsson, Kenneth
    Luleå tekniska universitet.
    Saldner, Henrik O.
    Luleå tekniska universitet.
    Schedin, Staffan
    Luleå tekniska universitet.
    Dynamic material parameters in an anisotropic plate estimated by phase-stepped holographic interferometry1996Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 24, nr 5-6, s. 429-454Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Material parameters in an anisotropic plate are determined using two non-destructive measuring techniques: real-time phase-stepped electronic speckle pattern interferometry and dual-reference-beam pulsed holographic interferometry. The first technique is used to measure the lower modes of vibration of the plate with free-free boundary conditions. Finite element analysis is then used to determine two effective Young's moduli and the in-plane shear modulus. The second technique is used to detect transient bending waves propagating in the plate and acoustic waves propagating in the surrounding air. A double-pulsed laser is used both to generate the waves and to make holographic recordings of the wave fields. The stiffness of the plate is estimated using the measured deformation field and an analytical solution to the plate impact problem.

  • 11.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Four-pulse interferometric recordings of transient events by pulsed TV holography2003Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 40, nr 5-6, s. 517-528Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    With traditional double-pulsed holographic interferometry or pulsed TV holography, the experiment usually has to be repeated to allow the recording of a time sequence of interferograms of the event. With the proposed technique a sequence of four interferograms of a solitary transient event is measured. A twin oscillator, injection-seeded, pulsed Nd:YAG laser is incorporated into a pulsed TV holography set-up. With orthogonal polarisation and double pulsing of each of the two channels of the laser, four pulses are recorded on two separate CCD-frames. Four interferograms of a laser-impacted plate obtained from the same experiment, show how the bending waves develop and propagate in the plate.

  • 12.
    Hashemzadeh, M.
    et al.
    University of Notttingham.
    Powell, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Voisey, K.T.
    University of Notttingham.
    Fibre laser piercing of mild steel: The effects of power intensity, gas type and pressure2014Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 55, s. 143-149Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Laser piercing is used to generate a starting point for laser cutting. The pierced hole is normally larger than the kerf width, which means that it cannot lie on the cut line. An experimental programme investigating the piercing process as a function of laser and assist gas parameters is presented. An Nd:YAG fibre laser with a maximum power of 2 kW was used in continuous wave mode to pierce holes in 2 mm thick mild steel. Oxygen and nitrogen were used as assist gases, with pressures ranging from 0.3 to 12 bar. The sizes, geometries and piercing time of the holes produced have been analysed. The pierced hole size decreases with increasing gas pressure and increasing laser power. Oxygen assist gas produced larger diameter holes than nitrogen. A new technique is presented which produces pierced holes no larger than the kerf with and would allow the pierced hole to lie on the cut line of the finished product – allowing better material usage. This uses an inclined jet of nitrogen when piercing prior to oxygen assisted cutting.

  • 13. Huntley, J.M.
    et al.
    Benckert, Lars
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Measurement of dynamic crack tip displacement field by speckle photography and interferometry1993Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 19, nr 4-5, s. 299-312Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Speckle interferometry has been used to measure the transient displacement field around a crack due to stress wave loading. Double exposure interferograms are recorded with a pulsed ruby laser, and correlation fringes formed by spatial filtering of the developed film. A new technique is presented for reducing the noise level in spatially filtered speckle interferograms. Independent fringe patterns, obtained from one interferogram but with the filtering aperture sampling different regions of the Fourier transform plane, are digitised by a CCD camera and combined numerically. It is demonstrated that accurate analysis of fringe patterns by the Fourier transform method can be carried out, even if no carrier fringes are present. The measurements are compared with those obtained by speckle photography. Least-squares fitting of the theoretical displacement field is used to calculate the stress intensity factor at the crack tip as a function of time.

  • 14. Huntley, J.M.
    et al.
    Saldner, Henrik O.
    Luleå tekniska universitet.
    Multi-channel pressure sensor using speckle interferometry1995Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 23, nr 5, s. 262-275Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The performance of two-dimensional pressure sensor arrays, consisting of 13 × 13 deformable steel diaphragms, is investigated. The pressureinduced out-of-plane component of the displacement field of all 169 sensors is measured in parallel using phase-stepping speckle interferometry. Subsequent least-squares fitting of the theoretical field provides a quantitative measure of the average pressure acting on each sensor. With monotonically-increasing load and suitable calibration, each pressure value can be measured to an accuracy of better than 1% of the full scale deflection. The current pressure range of 0-2.74 kPa can be extended by increasing the number of interferograms recorded.

  • 15.
    Johnson, Peder
    Luleå tekniska universitet.
    Strain field measurements in industrial applications using dual-beam digital speckle photography1998Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 30, nr 5, s. 421-431Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Dual-beam digital speckle photography is a non-contacting full-field technique which can measure one strain component at the CCD-camera frequency. The technique uses two symmetrically incident laser beams of different wavelengths and the speckle field for each illumination beam is recorded simultaneously before and after object deformation. The system is designed for strain field measurements in rough environments. In this paper some experiments are performed to investigate the system performance. In a tensile test the measured strain value was found to be in good agreement with the strain given by resistance strain gauges. Experiments at elevated temperature have also been performed, where thermal strain fields were measured at 500°C.

  • 16.
    Johnson, Peder
    Luleå tekniska universitet.
    Strain field measurements with dual-beam digital speckle photography1998Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 30, nr 3-4, s. 315-326Artikel i tidskrift (Refereegranskat)
  • 17.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Local flashing events at the keyhole front in laser welding2015Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 68, s. 35-41Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    For sufficiently high power density, high power lasers induce a vertical boiling front that enables keyhole welding. Waves streaming down the keyhole front were recently observed by ultra-high speed imaging. Although the wave flow appears continuous, deeper analysis has revealed that it is composed of flashing events. The evaluation of eleven events for five parameter cases confirms a strong modulation of the recorded grayscale of bright peaks, achieving up to 50% higher brightness. The flash can even end 50% darker than initially, probably in the shadow of the laser beam. The flashing events took place every 5-15 µs, lasted for typically 10–70 µs and moved at a speed of 10–15 m/s. The flashing events are of fundamental significance to understand the keyhole front. They are associated with temperature peaks and with temporary local boiling action, accompanied by ablation pressure that accelerates the melt.

  • 18.
    Kinell, Lars
    Luleå tekniska universitet.
    Multichannel method for absolute shape measurement using projected fringes2004Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 41, nr 1, s. 57-71Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The multichannel approach is combined with the reduced temporal phase unwrapping scheme for rapid shape measurement. The shape is measured using projected fringes and optical triangulation. Fringes are projected using a colour video projector and recorded by a CCD-camera. Using a colour video projector and a colour video camera makes it possible to use the red, green and blue channels individually. This is the multichannel method. In each channel, the blue, green and red carries fringe maps with different fringe pitch. A short sequence of phase-stepped images is projected and acquired sequentially in time. This reduces measurement time by a factor of three when compared to using one channel. From the acquired images, it is possible to calculate absolute phase using the reduced temporal phase unwrapping analysis scheme. For each channel, it is also possible to calculate fringe contrast and fringe amplitude. Therefore, it is possible to retrieve the colour of an object without acquiring an extra image. The method is demonstrated by measuring the shape of two generally coloured and complex shaped objects.

  • 19.
    Larsson, Linda
    et al.
    Luleå tekniska universitet.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Thuvander, Fredrik
    Karlstads Universitet.
    Microscopic 3D displacement field measurements using digital speckle photography2004Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 41, nr 5, s. 767-777Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A technique to measure object shape and 3-D displacement fields in micro-scale is offered by microscopic stereo digital speckle photography. The displacement of the random features that are often present on many engineering surfaces when viewed in a microscope is measured with the system, using image correlation. In this paper the equipment, physical model and calibration routines are described. The technique can be applied for sub-mm sized objects of arbitrary shape for small deformation fields. As a verifying experiment, an in-plane rotation of a flat calibration plate is presented. The expected in-plane errors are shown to be less than 0.1μm and the corresponding out-of-plane errors about three times larger. As a pilot experiment, micro-structural paper expansion is studied, when exposed to humidity. The scaling properties of the microscope as well as the sampling criteria and reliability of the system are discussed in detail

  • 20.
    Lycksam, Henrik
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Öhman, Marcus
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Gebart, Rikard
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    High-speed interferometric measurement and visualization of the conversion of a black liquor droplet during laser heating2012Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 50, nr 11, s. 1654-1661Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Black liquor is a mix of organic and inorganic materials that is left after the kraft pulping process. In a modern pulp mill the pulping chemicals and the energy in the black liquor is recovered and used in the pulping cycle by burning the black liquor in a recovery burner. An alternative to the recovery boiler is to gasify the black liquor to produce an energy rich synthesis gas that can be upgraded into synthetic fuels or chemicals. Characterization of black liquor has mostly been done under conditions that are relevant for recovery boilers but the conditions in a gasifier differ significantly from this. In particular the droplets are much smaller and the heating rates are much higher. This paper presents an optical interferometric technique that has the potential to produce data under relevant conditions for gasification. In the paper, results are measured at atmospheric conditions and with relatively low heating rate. However, the method can be applied also for pressurized conditions and at heating rates that are only limited by the frame rate of the digital camera that is used to capture the transient event when the droplets are heated. In the paper the dynamic properties of the gas ejected from and the swelling during conversion of a single droplet are measured

  • 21.
    Mattsson, Roger
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Bending and acoustic waves in a water-filled box studied by pulsed TV holography and LDV2006Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 44, nr 11, s. 1146-1157Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Pulsed TV holography and laser Doppler vibrometry (LDV) are non-destructive and non-contacting methods useful in the testing of mechanical properties in a wall separating two media. This paper presents comparative studies of transient, propagating bending waves in an impact-loaded thin aluminium wall of a box, and acoustic waves generated in the water inside the box using the two mentioned methods. Besides being evaluated and compared, the results were used to investigate qualities of the methods for the specific applications. The measured wave propagations obtained from the two methods are consistent regarding both amplitude and phase. Further, transient pressure fields in the water have been computed from the pulsed TV holography measurements. Since none of the two methods are capable of providing both temporal and spatial information simultaneously, they complement each other.

  • 22.
    Molin, Nils-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Applications of whole field interferometry in mechanics and acoustics1999Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 31, nr 2, s. 93-111Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A description is given of fringe formation in holographic interferometry, in electronic speckle pattern interferometry, in electro-optic or TV holography and for a newly developed system for pulsed TV-holography. A numerical example, which simulates the equations describing the different techniques, is included. A strain measuring system using defocused digital speckle photography is described. Experiments showing mode shapes of musical instruments, transient bending wave propagation in beams and plates as well as sound pressure fields in air are included

  • 23. Molin, Nils-Erik
    et al.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Svanbro, Angelica
    Speckle photography combined with speckle interferometry2004Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 41, nr 4, s. 673-686Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    By combining speckle interferometry (SI) measurements with speckle photography, the fringe visibility can be kept high despite the presence of a large bulk or rotating motion of the object. This combined technique improves the usability and measuring range of both pulsed and phase-stepped Sl-methods. This paper reviews the theory of fringe formation in Sl and shows some recent applications of this combined technique

  • 24.
    Moradi, Mahmoud
    et al.
    Faculty of Mechanical Engineering, K.N. Toosi University of Technology, Tehran.
    Ghoreishi, Majid
    Faculty of Mechanical Engineering, K.N. Toosi University of Technology, Tehran.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    An investigation on stability of laser hybrid arc welding2013Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 51, nr 4, s. 481-487Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The stability of the weld surface quality resulting from laser–arc hybrid welding of 4 mm thick steel was studied. The trends of stability in terms of top weld width variation were estimated by using design of experiments, where different types of unstable welds were distinguished. High speed imaging of the process supported the interpretation of the trends. High arc voltage and short distance laser–arc has destabilized the process. For a stable process the applied spray mode has caused a short arc and symmetric central drop transfer while for high voltage the arc became long and wide and the drops traveled sideward. The potential and limits of the design of experiments method for such kinds of applications were discussed.

  • 25.
    Olsson, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Eriksson, Ingemar
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Powell, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Advances in pulsed laser weld monitoring by the statistical analysis of reflected light2011Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 49, nr 11, s. 1352-1359Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper describes two new techniques for monitoring the quality of laser welds by statistical analysis of the reflected light signal from the weld surface. The first technique involves an algorithm that analyses the variance of the peak values of the reflected signal as a measure of the stability of the surface during pulsed Nd:YAG laser welding in the heat conduction mode. Kalman filtering is used to separate a useful signal from the background noise. A good correlation between weld disruption and signal fluctuation has been identified. This technique could be used in tandem with the present practice of simply using the peak values of reflected (or emitted) light as an indicator of weld quality. The second technique investigated involves an assessment of the temporal shape of the power distribution of individual reflected pulses in comparison with an average of the results from a high quality weld. Once again a high correlation between a poor signal match and inferior quality welding was discovered, which may pave the way towards a new generation of optical weld monitoring devices.

  • 26.
    Olsson, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Eriksson, Ingemar
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Powell, John
    Langtry, A.V.
    GE Healthcare, Oxford.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Challenges to the interpretation of the electromagnetic feedback from laser welding2011Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 49, nr 2, s. 188-194Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper considers the point that it is not possible to interpret individual weld perturbations from the raw electromagnetic feedback collected from laser weld zones. The presentation of electromagnetic data as a 3D cloud is presented as a new, useful tool in the analysis of this feedback. It is shown that there is a very low correlation between the plasma or thermal signals and the reflected light signal from the weld zone, and that a strong correlation exists between the plasma and thermal signals. It is also demonstrated that data points from a weld perturbation form a different 3D cluster to those from the stable welding process. A strategy for future real time data analysis is presented utilising a suitably shaped data cloud envelope. The rates of data fit to the various segments of such an envelope could be correlated with specific weld anomalies.

  • 27.
    Pocorni, Jetro
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Powell, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander F. H.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Dynamic laser piercing of thick section metals2018Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 100, s. 82-89Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Before a contour can be laser cut the laser first needs to pierce the material. The time taken to achieve piercing should be minimised to optimise productivity. One important aspect of laser piercing is the reliability of the process because industrial laser cutting machines are programmed for the minimum reliable pierce time. In this work piercing experiments were carried out in 15 mm thick stainless steel sheets comparing a stationary laser and a laser which moves along a circular trajectory with varying processing speeds. Results show that circular piercing can decrease the pierce duration by almost half compared to stationary piercing. High speed imaging (HSI) was employed during the piercing process to understand melt behaviour inside the pierce hole. HSI videos show that circular rotation of the laser beam forces melt to eject in opposite direction of the beam movement, while in stationary piercing the melt ejects less efficiently in random directions out of the hole.

  • 28. Powell, John
    et al.
    Al-Mashikhi, S.O.
    Faculty of Engineering, University of Nottingham.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Voisey, K.T.
    Faculty of Engineering, University of Nottingham.
    Fibre laser cutting of thin section mild steel: An explanation of the ‘striation free’ effect2011Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 49, nr 8, s. 1069-1075Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper presents the results of an experimental and theoretical investigation into the phenomenon of 'striation free cutting', which is a feature of fibre laser/oxygen cutting of thin section mild steel. The paper concludes that the creation of very low roughness edges is related to an optimisation of the cut front geometry when the cut front is inclined at angles close to the Brewster angle for the lasermaterial combination

  • 29.
    Samarjy, R.S.M.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling. University of Mosul, College of Engineering, Department of Mechanical Engineering.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Transient interaction of a boiling melt with a pulsed Nd:YAG-laser2017Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 88, s. 28-36Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The boiling front induced by a pulsed Nd:YAG-laser at very slow translation speed was studied. The purpose is to understand fundamental melt movement mechanisms. The melt was observed by high speed imaging, with and without illumination. When switching on the laser beam a hole is drilled through a bulk of melt. The hole expands and the boiling pressure gradually opens the melt bridge, instead developing an interaction front similar to cutting. These conditions remain in quasi-steady state during the pulse. The ablation pressure from boiling shears waves down the front and keeps the melt downwards in a stable position. When switching off, the waves smoothen and in absence of boiling the surface tension drags the melt back upwards, to semi-torus-like Catenoid shape. Evidence on the large melt pool and its shape was achieved by three-dimensional reconstruction from cross section macrographs. The basic findings how melt can move with and without ablation pressure can enable controlled melt dynamics for various laser processing techniques, like remote cutting, ablation, keyhole welding or drilling.

  • 30.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Some recent advances in electronic speckle photography1998Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 29, nr 2-3, s. 125-144Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Major advances in electronic speckle photography are reviewed. Topics include the correlation properties of laser speckles, the principles and expected accuracy of electronic speckle photography, the effect of the digital recording and evaluation, and some recent applications. These applications involve two techniques for the measurement of 3D deformation fields, measurement of the in-plane strain field components using defocused laser speckle, and measurement of object shape.

  • 31.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Larsson, Linda
    Luleå tekniska universitet.
    Monitoring microstructural material changes in paper through microscopic speckle correlation rate measurements2004Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 42, nr 2, s. 193-201Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A system for real-time digital speckle correlation rate measurements is developed and applied on printing paper in the micro-scale. The system consists of a continuous laser and a CCD camera connected to a PC. To measure on objects in the micro-scale the system is connected to a stereomicroscope where one of the channels is used for imaging and the other channel is used for illumination of the object surface. The system is evaluated through measurements on printing paper over an area of 1 mm2 and with a spatial resolution of 30 μm on the object. The results show that the system is able to monitor the dynamics in paper during both wetting and drying in real-time over a time scale ranging from one-tenth of a second to several hours

  • 32.
    Sundqvist, Jesper
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Shachal, L.
    Holo/Or Ltd.
    Brodsky, A.
    Holo/Or Ltd.
    Kong, Choon Yen
    TWI Ltd., Granta Park, Great Abington, Cambridge.
    Blackburn, Jon
    TWI Ltd., Granta Park, Great Abington, Cambridge.
    Assuncao, Eurico
    LAETA, IDMEC, Instituto Superior Técnico, Universidade de Lisboa.
    Quintino, Luisa
    LAETA, IDMEC, Instituto Superior Técnico, Universidade de Lisboa.
    Numerical optimization approaches of single-pulse conduction laser welding by beam shape tailoring2016Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 79, s. 48-54Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    While circular laser beams are usually applied in laser welding, for certain applications tailoring of the laser beam shape, e.g. by diffractive optical elements, can optimize the process. A case where overlap conduction mode welding should be used to produce a C-shaped joint was studied. For the dimensions studied in this paper, the weld joint deviated significantly from the C-shape of the single-pulse laser beam. Because of the complex heat flow interactions, the process requires optimization. Three approaches for extracting quantitative indicators for understanding the essential heat flow contributions process and for optimizing the C-shape of the weld and of the laser beam were studied and compared. While integral energy properties through a control volume and temperature gradients at key locations only partially describe the heat flow behaviour, the geometrical properties of the melt pool isotherm proved to be the most reliable method for optimization. While pronouncing the C-ends was not sufficient, an additional enlargement of the laser beam produced the desired C-shaped weld joint. The approach is analysed and the potential for generalization is discussed.

  • 33. Synnergren, Per
    et al.
    Berglund, Tomas
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, EISLAB.
    Söderkvist, Inge
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Estimation of curl in paper using a combination of shape measurement and least-squares modelling2001Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 35, nr 2, s. 105-120Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Curl is a quality problem that makes paper less suitable for printing. A paper sheet that has structural variations in its thickness such as gradients of fibre orientation, density and filler content, will curl and hence assume a cylindrical shape when its humidity content is changed. We propose a method to measure curl that can be used for automated analysis of the paper quality. The shape of the curled paper is measured using a stereoscopic camera system, which is capable of viewing a random pattern that is projected onto the specimen. The shape of the object is found by calculating the perspective difference in the camera set-up using digital image correlation. The quality parameters that are searched for are the magnitude of curl, which is defined as the inverse of the radius of curvature and the orientation of the curled paper. These parameters are estimated by performing a least-squares fit of a cylindrical shape to the three-dimensional measurement

  • 34.
    Synnergren, Per
    et al.
    Luleå tekniska universitet.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Stereoscopic digital speckle photography system for 3-D displacement field measurements1999Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 31, nr 6, s. 425-443Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Stereoscopic digital speckle photography offers a technique to measure object shapes and 3-D displacement fields in experimental mechanics. The system measures the displacement of a random white light speckle pattern, which somehow is present on the object surface, using digital correlation. This paper describes a general physical model for stereo imaging systems. A camera calibration algorithm, which takes the distortion in the lenses into account, is also presented and evaluated by real experiments. Standard deviations of small deformations as low as 1% of the pixel size for in-plane deformations and 6% of the pixel size for the out-of-plane component are reported. Using the calibration algorithm described, the main source of errors is random errors originating from the correlation algorithm.

  • 35.
    Torkamany, Mohammad
    et al.
    Department of Materials Engineering, Tarbiat Modares University, Department of Materials Science and Engineering,Tarbiat Modares University.
    Ghaini, Malek
    Department of Materials Engineering, Tarbiat Modares University, Department of Materials Science and Engineering,Tarbiat Modares University.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Combination of laser keyhole and conduction welding: Dissimilar laser welding of niobium and Ti-6Al-4V2016Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 79, s. 9-15Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Pulsed Nd:YAG laser welding of pure niobium plate to titanium alloy Ti-6Al-4V sheet in butt joint is studied regarding the laser/metal interaction modes. To obtain the optimized process parameters in dissimilar welding of Ti-6Al-4V/Nb, the melting ratio of laser beam energy for each weld counterpart is evaluated experimentally. Different laser welding modes of keyhole and conduction are predicted regarding the absorbed energy from the similar laser pulses on each weld counterpart. Laser keyhole and conduction welding were observed simultaneously through direct visualization of laser interaction with dissimilar metals using High Speed Imaging (HSI) system.

  • 36.
    Wahba, H.H.
    et al.
    Department of Physics, Faculty of Science, Damietta University, New Damietta City.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Olsson, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    High resolution digital holographic microscopy for the study of aggregated natural cellulose nanowhisker fibers2015Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 73, s. 69-74Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper, digital holographic (DH) microscopy demonstrates its ability to perform a full characterization of nanofibers. The high resolution and magnification of the presented method to study the nanofibers are tested using standard MIL-STD-150A 1951 USAF resolution test target. In this investigation, aggregated natural cellulose nanowhisker fibers are positioned in the front of the microscopic objective using a 3D translation stage in the object arm of DH setup. The recorded off-axis holograms are refocused using the angular spectrum method. The reconstructed complex field is used to calculate optical phase and intensity distributions of the object at different reconstruction depths. A simple algorithm is used to define the focused image with suitable accuracy. The dimensions and orientation of the fibers can be evaluated from the optical field at different depths. Then, the shape and textures along the aggregated natural cellulose nanowhisker fiber can be presented in a 3D space.

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