Title Promoter Affiliations Abstract
"The acoustic polar scan: a novel sonic probe for the characterization of lightweight wood panels" "Wim Van Paepegem" "Department of Materials, Textiles and Chemical Engineering" "Lightweight wooden panels such as Cross Laminated Timber (CLT) offer an alternative to concrete ormasonry walls, as they present an economical solution with several advantages such as easy andfast assembly and good fire resistance. On the downside, the compilation of individual beams indifferent directions gives rise to CLT panels with an increased complexity in mechanical properties,whereas the low mass results in poor acoustical isolation between rooms. Moreover, due to thecombination of wood and glue, the visco-elastic parameters of these panels are very hard to model,which makes the design of constructions with proper characteristics, both structural and acoustical,a difficult matter.The goal of this project is to develop an Acoustic Polar Scan (APS) method that can be employed forthe anisotropic mechanical parameter inversion and acoustical isolation characterization oflightweight wood panels in construction engineering. The APS design and methodology involves amotorized multiple angle of incidence plane wave excitation using a parametric antenna, dataacquisition using laser Doppler vibrometry, and a multi-parameter visco-elastic tensor inversionanalysis of the data. Experiments on real CLT construction elements will be conducted to validateand optimize the methodology and the set-up. The ultimate goal is to reach a deep theoreticalunderstanding of the mechanical and dynamical behavior of CLT that would permit the optimizationof desired design properties."
"The acoustic polar scan: a novel sonic probe for the characterization of lightweight wood panels" "Koen Van Den Abeele" "Soft Matter and Biophysics, Physics, Kulak Kortrijk Campus" "Lightweight wooden panels such as Cross Laminated Timber (CLT) offer an alternative to concrete or masonry walls, as they present an economical solution with several advantages such as easy and fast assembly and good fire resistance. On the downside, the compilation of individual beams in different directions gives rise to CLT panels with an increased complexity in mechanical properties, whereas the low mass results in poor acoustical isolation between rooms. Moreover, due to the combination of wood and glue, the visco-elastic parameters of these panels are very hard to model, which makes the design of constructions with proper characteristics, both structural and acoustical, a difficult matter. The goal of this project is to develop an Acoustic Polar Scan (APS) method that can be employed for the anisotropic mechanical parameter inversion and acoustical isolation characterization of lightweight wood panels in construction engineering. The APS design and methodology involves a motorized multiple angle of incidence plane wave excitation using a parametric antenna, data acquisition using laser Doppler vibrometry, and a multi-parameter visco-elastic tensor inversion analysis of the data. Experiments on real CLT construction elements will be conducted to validate and optimize the methodology and the set-up. The ultimate goal is to reach a deep theoretical understanding of the mechanical and dynamical behavior of CLT that would permit the optimization of desired design properties."
"The acoustic polar scan: a novel sonic probe for the characterization of lightweight wood panels" "Koen Van Den Abeele" "Physics, Kulak Kortrijk Campus, Soft Matter and Biophysics" "Lightweight wooden panels such as Cross Laminated Timber (CLT) offer an alternative to concrete or masonry walls, as they present an economical solution with several advantages such as easy and fast assembly and good fire resistance. On the downside, the compilation of individual beams in different directions gives rise to CLT panels with an increased complexity in mechanical properties, whereas the low mass results in poor acoustical isolation between rooms. Moreover, due to the combination of wood and glue, the visco-elastic parameters of these panels are very hard to model, which makes the design of constructions with proper characteristics, both structural and acoustical, a difficult matter. The goal of this project is to develop an Acoustic Polar Scan (APS) method that can be employed for the anisotropic mechanical parameter inversion and acoustical isolation characterization of lightweight wood panels in construction engineering. The APS design and methodology involves a motorized multiple angle of incidence plane wave excitation using a parametric antenna, data acquisition using laser Doppler vibrometry, and a multi-parameter visco-elastic tensor inversion analysis of the data. Experiments on real CLT construction elements will be conducted to validate and optimize the methodology and the set-up. The ultimate goal is to reach a deep theoretical understanding of the mechanical and dynamical behavior of CLT that would permit the optimization of desired design properties."
"SUPERPANELS: Strengthening and Upholding the Performances of the new Engineered Research PANELS." "Wim Desmet" "Production Engineering, Machine Design and Automation (PMA) Section" "A great effort has been spent in the mechanical engineering fields in order to conceive new panels of improved static and dynamic performances. This was due to the search of innovative design solutions specifically tailored for facing with the problem of transportation engineering where the vehicle requires high stiffness, low weight, low noise, low cost design. The project will allow the co-operation among five different research groups of different sizes but all active at international scale, in the automotive, aerospace, naval, and railway transportation. The process of exchanging researchers and coordinating activities will allow maximising the specific expertise of each group, and aiming also to a faster selection and developments of the candidate panels, configuration and materials. In fact, the range of such panels is now very large according also to the possibility offered by new technologies, but these last are still to be deeply investigated in order to explore their capabilities. Further, on the researcher shelf there are innovative methods that only a decade ago were at an initial stage; in fact, the increase of the knowledge about standard predictive structural and structural-acoustic tools has been impressive and in the near future some of the methods now used at research level will enter in the common engineering practice. In this project they will be extensively used, tested and verified against the specific measurements that will be needed to verify the predicted and expected performances. Several candidate solutions will be suggested and motivated at the beginning of the project, so defining a sort of current design. The results will qualify how and where the specific performances have been increased without penalties for any other standard requirements. The main attention will be devoted to assure compliance with the static requirements and to improve the dynamic and vibroacoustic performances. Other emerging requirements as the flammability will be also investigated."
"Spectral flowcytometry (Cytek Aurora)" "Susan Schlenner" "Department of Human Genetics, Molecular Digestive Oncology, Laboratory for Tumor Immunology and Immunotherapy, Laboratory of Cell Stress & Immunity, Laboratory of Genetics of Autoimmunity (VIB-KU Leuven Center for Brain & Disease Research), Laboratory for Tumor Microenvironment and Therapeutic Resistance (VIB-KU Leuven Center for Cancer Biology), Translational Research in GastroIntestinal Disorders" "Spectral flow cytometry is greatly outperforming conventional polychromatic flow cytometry and is expected to replace the previous generation technology in the mid-term. The manifold advantages include unprecedented data quality and deep resolution. Here, we wish to implement the Cytek Aurora spectral analyser into the KUL FACS core as the first instrument available in Belgium positioning the KU Leuven as a key reference for cell phenotyping among other applications in biomedical and clinical research. We wish to highlight the urgent need for a high-parameter flow cytometry instrument at the KU Leuven as the single previous generation instrument available represents a serious bottleneck in (a) available analysis bookings and (b) available parameters with the accompanying complications in antibody panel design and downstream analyses. The Cytek Aurora instrument addresses all current shortcomings and will greatly facilitate research in numerous domains at a fraction of the cost of a conventional instrument."
"Bayesian and non-Bayesian methods for the elimination of high-dimensional nuisance parameters" "Geert Dhaene" "Research Centre of Econometrics, Leuven" "Empirical economics makes increasing use of panel data. To control for unobserved heterogeneity (e.g., heterogeneity in technology across firms, heterogeneity in preferences across consumers, heterogeneity across teachers and students in matched student-teacher data), a common practice is to introduce agent-specific parameters (e.g., additive fixed effects) into panel data models. Such parameters are high-dimensional nuisance parameters and, in many models, this leads to an incidental parameter problem: standard estimation methods such as least squares or maximum likelihood fail to consistently estimate the model parameters that are assumed common to all agents. This research proposal seeks to extend recently proposed exact or approximate solutions to incidental parameter problems in the following new and important directions: (i) Bayesian bias corrections of the likelihood function via the posterior predictive density or via refined implementation of bias-reducing priors; (ii) empirical-likelihood based adjustments for incidental parameter bias in the GMM framework; (iii) deriving theoretical bounds on the incidental parameter bias in regression models with censoring (where simulations suggest that the bias is small). To summarize, there are many model and data settings where applied researchers are naturally led to introduce agent-specific parameters. My goal is to provide researchers with reliable methods of inference in those situations."
"Analysis of time varying relationships in multi-country monetary time series." "Christophe Croux" "Research Centre for Operations Research and Statistics (ORSTAT), Leuven, Research Centre of International Economics, Leuven" "The main goal of empirical macroeconomics is to understand the relationships between macroeconomic variables, such as the gross domestic product (GDP), inflation and interest rates. This allows for better macroeconomic predictions and helps macroeconomic policy makers to set a sound fiscal and monetary policy. For example, understanding the effect of the short term interest rate on inflation is essential for a central bank, which is responsible for price stability. Also, good GDP predictions allow governments to anticipate the future evolution of the economic activity by taking stabilizing actions in advance.Empirical macroeconomic analysis usually starts from a time series dataset in which the variables of interest are observed for subsequent time periods, typically quarterly. Such datasets thus consist of historical data, where the observations are realizations of how the economy has behaved in the past, and which makes it difficult to derive causal relationships between the variables. In contrast, note that in experimental data, used in for instance pharmaceutical studies, cause and effect can be clearly distinguished by exogenously manipulating a variable, for example the intake of a certain drug, and then observing the consequences on another variable, for example the patient’s health. Still, also information on the linkages between macroeconomic variables can be obtained. First, instead of true causality, macroeconomists often study the incremental predictive power between variables instead, which is called ‘Granger causality’. A variable is said to Granger cause another variable if it improves the prediction of the other variable. Second, macroeconomists often make additional ‘identification assumptions’ on the causality between the variables, which allows them to extract exogenous shocks. One popular identification assumption is the ‘recursive ordering scheme’, in which the ordering of the variables determines how rapidly the different variables can react to exogenous shocks in the other variables. For example, it is typically assumed that inflation and GDP are relatively sluggish variables, such that they do not immediately react to an interest rate shock.In order to extract patterns from the macroeconomic dataset, statistical models are used. In this thesis, we mainly use the Vector Autoregression (VAR) model, which is the workhorse in empirical macroeconomics to study linear relationships between multiple time series. Unlike more structural economic models, VAR models are fully data-driven and require no ex-ante expert knowledge on the dependencies between the variables. In the VAR model, each variable depends on both the past values of the variables in the model and a shock. The vector autoregression coefficients thus represent the effects of past values of the variables on the current value of each variable. While the standard VAR model assumes that these coefficients stay the same over time, this thesis focuses on modeling changing macroeconomic relationships. First, Time Varying Parameter Vector Autoregression (TVP VAR) models allow the vector autoregression coefficients to evolve smoothly over time. For example, the effect of an interest rate shock on inflation is found to be different in the 1970s compared to the period afterwards. Second, frequency domain techniques describe the time series as a weighted sum of sinusoidal components with different frequencies, for instance slowly fluctuating and quickly fluctuation. Interestingly, the relationships between the variables can differ across these different frequency components. Third, for a panel dataset in which the variables are observed both for subsequent time periods and different countries, also the cross-country variation in the coefficients can be analyzed.This thesis contains essays on the empirical relationship between macroeconomic time series, often in a multi-country setting. While the standard VAR model is used in Chapter 2, the time variation in the coefficients is studied in Chapters 3 and 5, the frequency domain analysis is performed in Chapter 1 and the cross-country variation is analyzed in Chapter 4.The first chapter studies the predictive power of domestic stock prices for the future domestic economic activity in the frequency domain. We develop a multi-country test for Granger causality for each of the frequency components. Using 1991Q1-2010Q2 quarterly data for the G-7 countries, we report that the slowly fluctuating components of stock prices have large incremental predictive power for the future GDP, while this is not the case for the quickly fluctuating components.The second chapter analyzes impulse response functions of vector autoregression models for variables that are linearly transformed. The impulse response function of a vector autoregression model is an often used tool in empirical macroeconomics to analyze the response of the variables in the model to different types of shocks. For many empirical applications, it is of interest to know how the impulse response functions would change if one or more variables in the VAR model are replaced by a linear transformation of the original variables. One example of such a transformation is the replacement of a nominal growth rate variable in a VAR model that also includes inflation by its corresponding real growth rate variable, which is the difference between the nominal growth rate variable and inflation. We show that the new impulse response is equal to the linear transformation of the original impulse response if and only if the new shock is equal to the linear transformation of the original shock. Sufficient conditions for this relationship between impulse responses are derived for the setting where the same type of shock is studied in the linearly transformed and original model. In particular, we consider shocks in one error term only, orthogonalized shocks and generalized shocks.The third chapter compares Bayesian estimators with different prior choices for the amount of time variation in the coefficients of time varying parameter vector autoregression models using Monte Carlo simulations. In Bayesian statistics, the posterior estimate of the parameters in the model is a combination of, on the one hand, the prior assumption on the distribution of these parameters and, on the other hand, the information contained in the data. When the sample size is very large, the prior specification is not that important as it is swamped by the large amount of information in the data. However, for typical macroeconomic time series, the prior is very important. Since the commonly used prior only allows for a tiny amount of time variation, less restrictive priors are proposed. Additional empirical evidence on the time varying response of inflation to an interest rate shock is then provided for USA: while a major and statistically significant ‘price puzzle’ is detected for the period 1972-1979, the estimated response of inflation to an interest rate shock is negative for most other time periods.The fourth chapter investigates empirically how the impact of a residential house price shock on household credit and GDP is influenced by the degree of the mortgage market flexibility. Countries with a flexible mortgage market, such as United States and United Kingdom, are characterized by a high loan to value ratio, low transaction costs of mortgage refinancing and easy access to second mortgages and home equity loans. Countries with an inflexible mortgage market, such as France and Italy, are characterized by the opposite. We hypothesize a stronger effect of house price shocks for the former countries because the financial accelerator mechanism for existing home owners is expected to be stronger and because the effect of higher house prices on the required amount of savings of future first time house buyers is expected to be smaller. A panel vector autoregression model is estimated separately for a group of eight countries with a flexible mortgage market and for a group of eight countries with an inflexible mortgage market. While both household credit and GDP increase after a positive house price shock for both groups of countries, we do not find empirical evidence that these responses are stronger for countries with a flexible mortgage market.The fifth chapter investigates the determinants of sovereign credit ratings, which are ordinal measures of the creditworthiness of a sovereign government assigned by a rating agency. We quantify for the three major credit rating agencies how the importance of the different sovereign credit rating determinants changed after the start of the European debt crisis in 2009. For this end, we estimate a multi-year ordered probit model, using a sample of 90 countries for the years 2002-2015. Our model allows for time variation in the importance of the different determinants and it takes into account the ordinal nature of the credit rating. We provide empirical evidence that the credit rating agencies changed their sovereign credit rating assessment after the start of the European debt crisis in 2009. The financial balance, the economic development and the external debt became substantially more important after 2009 and the effect of Eurozone membership switched from positive to negative. In addition, GDP growth gained a lot of importance for highly indebted sovereigns and government debt became much more important for countries with a low GDP growth rate.In the epilogue chapter, I give my personal view on the way statistical inference should be used in business and economic applications. In particular, I call for more focus on the evaluation of ‘economic importance’, i.e. the estimated magnitude of an effect together with its estimation error, and for less focus on the often less relevant and frequently misunderstood concept of ‘statistical significance’, which only informs on the existence of the effect."
"Exact and approximate solutions to incidental parameter problems." "Geert Dhaene" "Research Centre of Econometrics, Leuven" "Panel data are becoming ever more available in economics and other disciplines. In panel data models, researchers often wish to include agent-specific parameters (often called fixed effects) to control for unobserved heterogeneity. While the fixed-effect approach is attractive, many dynamic or nonlinear fixed-effect models face a so-called incidental parameter problem: standard estimation methods (e.g. least squares, maximum likelihood) fail to consistently estimate the model parameters that are common to all agents. This research seeks to extend recently proposed exact or approximate solutions to incidental parameters problems (it depends on the model whether an exact solution is possible at all) in the following empirically relevant directions: the inclusion of time dummies; non-stationary data settings; higher-order bias corrections targeting at vey short panels; linear models for panel data with spatial and temporal dynamics."
"On the Multi-scale Vibroacoustic Behavior of Multi-layer Core Topology Systems" "Mohamed Ichchou, Wim Desmet" "Mecha(tro)nic System Dynamics (LMSD)" "In this last decades, honeycomb sandwich panels have been the subject of intensive researches. Indeed, their high mechanical performances combined to a low stiffness to weight ratio result in a reduced acoustic efficiency. Therefore, many designs are usually proposed to overcome this issue. Besides, different methods are developed to model more complex structures using the periodic structure theory to study the wave propagation allowing to investigate the vibroacoustic parameters.The main purpose of this thesis is to investigate the vibroacoustic multi-scale behavior of multi-layer core topology systems which consist on stacking layers of honeycomb cores leading to an impedance mismatch between layers. In addition, such structures allow to increase the design space up to now limited to standard sandwich panels made of a single honeycomb core. Therefore, it is possible to obtain many configurations keeping the mass constant with simple shifting process between layers. A parametric model is proposed allowing to extract the unit cell through the thickness of the panel and to apply the periodic structure theory.Modelling multi-layer core topology systems has been performed using the wave finite element method, and an extended method has been proposed to solve the acoustic transmission problem. The study is focused on transition frequencies, the sound transmission loss as well as veering effects and internal resonances, to finally optimize the geometrical parameters and to analyze their influence on the acoustical and mechanical performances of the structure.Although the out-of-plane compression properties of multi-layer core topology systems are reduced, it is possible to strongly improve the in-plane compression properties. These later are studied by comparing a multi-layer hexagonal core and a standard single hexagonal core.Finally, using multi-layer core topology systems and a perforated upper skin, it is possible to increase the energy dissipation occurring inside the core and thus, improve the sound absorption coefficient. Therefore, the thermo-viscous effect is considered. The acoustic behavior is similar to porous media and the Johnson-Champoux-Allard parameters are retrieved to characterize the acoustic fluid flow.An improvement of the sound transmission loss and the sound absorption coefficient is obtained in a broadband frequency and the obtained resonance frequencies can be modified. However, this leads to lower mechanical properties especially the compression modulus and the dynamic rigidity.Keywords: multi-layer core systems, vibroacoustic, transmission loss and sound absorption coefficient, transition frequency, veering, optimization."
"Evaluation and Optimisation of Adhesive Point Fixings in Structural Glass" "Jan Belis" "Department of Structural Engineering and Building Materials" "Its main property, i.e. transparency, has ensured that structural glass is used extensively throughout the world and has become an integral part of the skyline of major cities and constructions in general. Nonetheless, connections in structural glass members still represent a field of research of structural glass engineering which is developing and expanding rapidly. Traditional systems to connect glass to the supporting substructure and to improve the overall transparency, often consist of so-called point-fixings. With the current state of technology, it is common to use bolted solutions for point-fixings in glass. However, adhesive point-fixings have several advantages compared to bolted point-fixings, such as the stress redistributing ability of the adhesive, no weakening or residual stresses in the glass due to glass perforation, prevention of a thermal bridge, etc. These advantages, together with promising strength values are important reasons why adhesive bonding is currently considered a very promising alternative for structural glass point-connections. In this work a design method for glass panels supported by adhesive point-fixings is proposed by means of a flowchart. The design is based on the time-efficient SLG-method (Superposition of Local and Global components), developed by Beyer for the design of bolted point-fixings. In the flowchart, the design is divided in two parts, i.e. the glass design and the adhesive design. In the former, the glass panel is separated in a global component and a local component. Due to the separation into one global component that can be built up with a less dense mesh pattern and one local component that is built up with a more complex and dense mesh pattern, the stress distribution can be determined in a very time-efficient manner. The stress and deformation in the field are examined by means of the global model. The stress in the vicinity of the connection is examined by the sum of the global stress and the maximum principal stress in the local models. For the adhesive design, only one local model can be considered, i.e. the multi-axial model. By applying the obtained failure criteria the adhesive layer can be examined for the considered loads. The occurring stress in the glass panel and in the adhesive layer can be altered by chancing geometrical parameters and material parameters. When both designs are satisfied, the design of glass panels supported by adhesive pointfixings for the considered configuration is completed. The suitability of the SLG-method for adhesive point-fixings is examined by a FEA comparison between the stress distributions conducted on the one hand by the SLG-method and on the other hand by a FEA model in which the total glass plate with the adhesive connections is built up in detail with volumetric elements. Despite small deviations, the SLG-method predicts the occurring stresses in a glass panel supported by adhesive point-fixings conservatively and accurately. The stress distribution in the adhesive layer can also be determined by the SLG-method. As the stress distribution consists of only the sum of the local stresses in the SLGmethod, these stresses are derived from the multi-axial model. The local model is numerically validated for different geometrical properties (three glass thicknesses and three diameter connectors), different material properties (two adhesive types) and three different load conditions (tension, shear and multiaxial load). By supporting the glass panel along a circumference with a diameter equal to six times the connector diameter, the deformation of the glass panel is also taken into account. This deformation causes important stress concentrations in the adhesive layer. The multi-axial local model can be used to directly determine the sum of the local stresses. The material models in the local model are obtained for a rubber-like adhesive (MS-polymer Soudaseal 270 HS) and a glassy adhesive (two-component epoxy 3MU+2122 Scotch-WeldU+2122 9323). Two-component epoxies and acrylates are thermosetting adhesives. The behaviour of these materials is typically elastic until failure and will fail at relatively small strains by the initiation and propagation of a crack. However, many adhesives are rubber-like materials, such as silicones and MS-polymers. Local deformations of the small-scale test specimens were measured through 3D-DIC. The comparison between the data from the test machine and the DIC-output revealed major differences between the measured deformations. This confirms that the use of DIC is needed to accurately measure the occurring strains during the small-scale tests. The total model for the validation of the SLG-method is experimentally validated. The experimental results demonstrate that the highest stresses are reached with the smallest edge distances. As expected the deformations are significantly larger with small edge distances. The numerical analyses show stress peaks which are not visible in the experiments. This highlights the benefits and necessity of numerical investigation. To reduce the stress in the glass panel or adhesive layer following actions can be taken, in order of decreasing influence: - Increase the number of connections; - Decrease the panel size; - Increase the edge distance; - Decrease the eccentricity; - Increase the connector diameter; - Increase the glass thickness; - Decrease the YoungU+2019s modulus of the adhesive; - Change the boundary condition to hinged; - Increase the adhesive thickness; - Decrease the Poisson ratio of the adhesive. Furthermore, the failure criteria for the two selected adhesives were determined. The failure load obtained from the failure criterion maximum shear strain predicts the experimental failure load with a mere difference of 2% for the MS-polymer Soudaseal 270 HS. Due to the stress singularities in the adhesive layer with the 2cepoxy 3MU+2122 Scotch-WeldU+2122 9323 B/A the ""stress at a distance"" approach must be applied for this adhesive. Only the failure criterion maximum shear stress with the stress considered at a distance equal to the full adhesive thickness from the stress singularity predicts the experimental failure load. These failure criteria predict the experimental failure loads of the local models conservatively and accurately. For adhesives between flexible adhesives and stiff adhesives, both failure criteria must be applied, the lowest failure load will be the actual failure load."