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Recent studies have confirmed the widening of the weld pool interface, known as a bulge effect, during deep penetration high power laser beam welding. The link between such geometric particularities of the weld pool shape and the hot cracking phenomena is significant. The present work seeks to extend the level of understanding by investigating their relationship. A coupled multiphysics, multiscale numerical framework is developed, comprising a series of subsequent analyses. The study examines the influences of the bulge on the three most dominant effects causing hot cracking, namely the thermal cycles, the mechanical loading, and the local microstructure. The bulge in the weld pool shape forms approximately in the middle of the plate, thus correlating with the location of hot cracking. It increases the hot cracking susceptibility by enhancing the three dominant effects. The numerical results are backed up by experimental data.
Iterative numerical 2D-modelling for quantification of material defects by pulsed thermography
(2019)
This paper presents a method to quantify the geometry of defects such as flat bottom holes (FBH) and notches in opaque materials by a pulse thermography (PT) experiment and a numerical model. The aim was to precisely describe PT experiments in reflection configuration with a simple and fast numerical model in order to use this model and a fit algorithm to quantify defects within the material. The algorithm minimizes the difference between the time sequence of a line shaped region of interest (ROI) on the surface (above the defect) from the PT experiment and the numerical data. Therefore, the experimental data can be reconstructed with the numerical model. In this way, the defect depth of a notch or FBH and its width or diameter was determined simultaneously. A laser was used for heating which was widened to a top hat spatial profile to ensure homogeneous illumination (rectangular impulse profile in time). The numerical simulation considers heating conditions and takes thermal losses due to convection and radiation into account. We quantified the geometry of FBH and notches in steel and polyvinyl chloride plasticized (PVC-U) materials with an accuracy of < 5 %.
Based on forensic evidence, a smouldering fire was observed to have occurred at a murder scene. Identification of a reasonable timeline – specifically the fire dynamics of the ignition and fire growth that occurred coincident with the death that took place – became an important focus of the criminal investigation that followed. The fire service was called when a neighbour saw a grey smoke escaping through the ventilation system of the bathrooms on the roof of the house. One flat door with elevated temperatures was found. The fire fighter who entered the flat first reported later that the flat was completely filled with smoke and all windows were closed. When the fire fighter opened the balcony door, he saw flames on the sofa that he extinguished. Then he found a body on the floor. The autopsy showed later that the victim was dead before the fire started. The police suspected that the murderer probably had deliberately set the fire to destroy evidence. One suspect had been witnessed to be in the flat approximately 2 h before the fire was detected by the neighbour. The aim of this project was to investigate how the fire most likely started and developed.
A fire in a five-storey apartment building was investigated experimentally and numerically. The room of origin of the fire was a living room in the second floor and the fire was started by a candle on a television set. The fire spread externally over the building faccade and internally along the staircase and affected all the flats above leading to two fatalities. It is estimated that the fire was discovered minutes after ignition and the fire service was called very shortly after the detection and was at the scene 9?min after the call. By this time large sections of the façade were on fire already. The rapid fire that spread over the façade and the staircase necessitated detailed investigations. Compliance of building products with the building regulations was investigated. One conclusion of the investigations was that the person who caused the fire by leaving the candle on the TV set unattended should not be held responsible for the two fatalities in the upper floors.
A simplified model for numerical simulation of laser metal deposition process with beam oscillation
(2018)
A model of laser metal deposition with beam oscillation has been developed. The proposed model consists of two coupled sub-models calculating the heat transfer in the deposited part and the free surface of the molten pool, respectively. The heat transfer simulation of the deposited part solves a three-dimensional quasi-stationary heat conduction problem. The free surface of the molten pool are determined by solving the Laplace-Young equation. The developed model enables the layer-by-layer prediction of the shape of the deposited part and the resulting temperature field. It is shown that for an oscillation amplitude equal to the beam radius the peak value of the heat flux decreases by about 53% and 73% in the case of lateral oscillation and circular oscillation, respectively. Lateral oscillating laser beam results in a higher penetration depth due to the higher thermal efficiency. The amplitude of the laser beam oscillation effects the shape of the deposited wall and the deposition rate. A good correlation between the numerically calculated and experimentally observed results is obtained.
A numerical model is presented which consists of a set of partial differential equations for the transport of heat and mass fractions of eight chemical species to describe the onset of self-ignition and the propagation of smouldering fires in deposits of bulk materials or dust accumulations. The chemical reaction sub-model includes solid fuel decomposition and the combustion of char, carbon monoxide and hydrogen.
The model has been validated against lab-scale self-ignition and smouldering propagation experiments and then applied to predictions of fire scenarios in a lignite coal silo. Predicted reaction temperatures of 550 K and propagation velocities of the smouldering front of about 6 mm/h are in good agreement with experimental values derived from lab-scale experiments.
Für die zerstörungsfreie Prüfung großer Betonbauwerke unter dem Aspekt der Analyse von Spannkanälen hat sich die Ultraschalltechnik in den letzten Jahren als geeignete Methode herausgestellt, da sowohl die Messtechnik insbesondere die Ultraschallwandler als auch die numerischen Auswerteverfahren in einem Stadium sind, das es erlaubt, größere Messflächen und damit größere Volumina zu beschallen und abzubilden. Mit Hilfe der Modellierung der Ausbreitung elastischer Wellen in komplexen Medien können praxisrelevante Problemstellungen untersucht werden, auch unter dem Aspekt, Testkörper zu planen oder gar zu ersetzen. Durch die Auswertung der Phase der Rekonstruktion eines Streuvorgangs können Fehlstellen klassifiziert und damit Aussagen über die Bauwerkssicherheit signifikant objektiviert werden. Die Methoden werden, im Hinblick auf eine ganzheitliche Sichtweise von skalaren und polarimetrischen Abbildungsverfahren mit Unterstützung der Modellierung und der anschließenden Klassifizierung durch Phasenanalyse, an ausgewählten Beispielen erläutert.
For nondestructive testing of concrete buildings under the aspect of tendon duct analysis, ultrasonic testing has proven to be a suitable method, as both the measuring technique in particular the acoustic sensors and the numerical methods allow the insonification and imaging of larger surfaces and volumes. With the aid of numerical modelling of elastic wave propagation in complex media real life problems can be examined with the goal to design or even to replace test specimen. By determination of the phase of an imaged scattering event, defects can be classified, and the prediction of the integrity of buildings can be significantly objectified. The methods are explained for selected examples in an integral way with respect to scalar and polarimetric imaging algorithms supported by modelling and subsequent classification through phase analysis.
The transport phenomena in the wire feed laser beam welding are investigated numerically. A three-dimensional transient heat transfer and fluid flow model coupled with free surface tracing and element transport is developed. A ray-tracing method with local grid refinement algorithm is used to calculate the multiple reflections and Fresnel absorption on the keyhole wall. The filler material flows backward along the lateral side of the weld pool, and subsequently flows forward along the longitudinal plane. The occurrence of the bulging phenomenon may further prevent the downward transfer of the additional elements to the root of the weld pool.
The current paper focuses on the determination of reliable numerical models of earth block masonry wallettes under different loading conditions. Uniaxial compression and diagonal compression tests were performed. Experimental behaviour was modelled with a non-linear model able to describe the cracking behaviour. The simplified approach based on macro-modelling shows a satisfactory accuracy and low computational costs. The results reproducing the uniaxial compression are in good correspondence with the post-elastic behaviour observed in the experimental campaign. The micro-modelling approach adopted to reproduce the shear behaviour, even with high computational cost, represents a suitable tool to predict the masonry collapse mechanism.
eepage processes through a dike with a fault zone on the landside and on the seaside are numerically and experimentally investigated. The numerical simulation is based on a two-phase (water-gas) flow model concept for porous media. Different discrete model concepts accounting for the fault zones are compared among each other and with the experiments. A good agreement between experimental and numerical results was obtained when the fault zones were discretized with 1D elements or with highly resolved 2D elements. We generally recommend choosing 1D elements. When the fault zone is modeled as an open pipe, the seepage velocity is considerably overestimated. Concerning dike stability, a fault zone on the landside is much worse compared to a fault zone on the seaside.
Experiments were performed to investigate the self-ignition behaviour of accumulations of four different technical dusts at oxygen volume fractions ranging from 1.3 to 21%. For this purpose a laboratory oven used for hot storage testing was modified to allow flushing with the pre-mixed oxygen/nitrogen mixture of the desired composition. It was found that for all sample volumes investigated the self-ignition temperatures were higher the lower was the oxygen volume fraction. In addition, the type of reaction changed obviously, since the apparent activation energy significantly decreased at oxygen volume fractions below 6%. However, it was still possible to observe exothermic effects at oxygen volume fractions as low as 1.3%. A numerical model was established to simulate the process of self-ignition including the coupled heat and mass transfer within the dust accumulation using a finite element solver. The model consists of six balance equations for the heat transfer and the transport of five chemical species. It shows that the model reflects self-ignition in dust accumulations with satisfying accuracy, as long as the input data generated by preceding experiments are reliable.