| 1. |
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| 2. |
- Ashwear, Nasseradeen, 1968-
(author)
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Vibration-based Assessment of Tensegrity Structures
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Vibration structural health monitoring (VHM) uses the vibration properties to evaluate many civil structures during the design steps, building steps and service life.The whole function, expressed by stiffness and frequencies of tensegrity structures are primarily related to the level of pre-stress. The present work investigates the possibilities to use this relation in designing, constructing and evaluating the tensegrity structures.One of the aims of the thesis was to improve the current models for resonance frequency simulation of tensegrities. This has been achieved by introducing the bending behaviour of all components, and by a one-way coupling between the axial force and the stiffness.The environmental temperature effects on vibration properties of tensegrity structures have been also investigated. Changes in dynamic characteristics due to temperature variations were compared with the changes due to decreasing pre-tension in one of the cables. In general, it is shown that the change in structural frequencies coming from temperature changes could of several magnitude as those from damage.Coinciding natural frequencies and low stiffness are known issues of tensegrity structures. The former can be an obstacle in VHM, while the later normally limits their uses in real engineering applications. It has been shown that the optimum self-stress vector of tensegrity structures can be chosen such that their lowest natural frequency is high, and separated from others.The environmental temperature effects on vibration properties of tensegrity structures were revisited to find a solution such that the natural frequencies of the tensegrity structures are not strongly affected by the changes in the environmental temperature. An asymmetric self-stress vector can be chosen so that the criterion is fulfilled as well as possible. The level of pre-stress can also be regulated to achieve the solution. The last part of this thesis, services as a summary of the work.
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| 3. |
- Chen, Yousheng, 1985-, et al.
(author)
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A Pretest Planning Method for Model Calibration for Nonlinear Systems
- 2016
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In: Model Validation and Uncertainty Quantification, Volume 3. - Cham : Springer. - 2191-5644 .- 2191-5652. - 9783319297538 - 9783319297545 ; , s. 371-379
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Conference paper (other academic/artistic)abstract
- With increasing demands on more flexible and lighter engineering structures, it has been more common to take nonlinearity into account. Model calibration is an important procedure for nonlinear analysis in structural dynamics with many industrial applications. Pretest planning plays a key role in the previously proposed calibration method for nonlinear systems, which is based on multi-harmonic excitation and an effective optimization routine. This paper aims to improve the pretest planning strategy for the proposed calibration method. In this study, the Fisher information matrix (FIM), which is calculated from the gradients with respect to the chosen parameters with unknown values, is used for determining the locations, frequency range, and the amplitudes of the excitations as well as the sensor placements. This pretest planning based model calibration method is validated by a structure with clearance nonlinearity. Synthetic test data is used to simulate the test procedure. Model calibration and K-fold cross validation are conducted for the optimum configurations selected from the pretest planning as well as three other configurations. The calibration and cross validation results show that a more accurate estimation of parameters can be obtained by using test data from the optimum configuration.
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| 4. |
- Chen, Yousheng, 1985-, et al.
(author)
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An Efficient Simulation Method for Large-Scale Systems with Local Nonlinearities
- 2016
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In: Special topics in structural dynamics, 34th IMAC. - Cham : Springer. - 2191-5644 .- 2191-5652. - 9783319299105 - 9783319299099
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Conference paper (other academic/artistic)abstract
- In practice, most mechanical systems show nonlinear characteristics within the operational envelope. However, the nonlinearities are often caused by local phenomena and many mechanical systems can be well represented by a linear model enriched with local nonlinearities. Conventional nonlinear response simulations are often computationally intensive; the problem which becomes more severe when large-scale nonlinear systems are concerned. Thus, there is a need to further develop efficient simulation techniques. In this work, an efficient simulation method for large-scale systems with local nonlinearities is proposed. The method is formulated in a state-space form and the simulations are done in the Matlab environment. The nonlinear system is divided into a linearized system and a nonlinear part represented as external nonlinear forces acting on the linear system; thus taking advantage in the computationally superiority in the locally nonlinear system description compared to a generally nonlinear counterpart. The triangular-order hold exponential integrator is used to obtain a discrete state-space form. To shorten the simulation time additionally, auxiliary matrices, similarity transformation and compiled C-codes (mex) to be used for the time integration are studied. Comparisons of the efficiency and accuracy of the proposed method in relation to simulations using the ODE45 solver in Matlab and MSC Nastran are demonstrated on numerical examples of different model sizes.
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| 5. |
- Chen, Yousheng, 1985-, et al.
(author)
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Experimental Validation of a Nonlinear Model Calibration Method Based on Multiharmonic Frequency Responses
- 2017
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In: Journal of Computational and Nonlinear Dynamics. - : ASME Press. - 1555-1415 .- 1555-1423. ; 12:4
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Journal article (peer-reviewed)abstract
- Correlation and calibration using test data are natural ingredients in the process of validating computational models. Model calibration for the important subclass of nonlinear systems which consists of structures dominated by linear behavior with the presence of local nonlinear effects is studied in this work. The experimental validation of a nonlinear model calibration method is conducted using a replica of the École Centrale de Lyon (ECL) nonlinear benchmark test setup. The calibration method is based on the selection of uncertain model parameters and the data that form the calibration metric together with an efficient optimization routine. The parameterization is chosen so that the expected covariances of the parameter estimates are made small. To obtain informative data, the excitation force is designed to be multisinusoidal and the resulting steady-state multiharmonic frequency response data are measured. To shorten the optimization time, plausible starting seed candidates are selected using the Latin hypercube sampling method. The candidate parameter set giving the smallest deviation to the test data is used as a starting point for an iterative search for a calibration solution. The model calibration is conducted by minimizing the deviations between the measured steady-state multiharmonic frequency response data and the analytical counterparts that are calculated using the multiharmonic balance method. The resulting calibrated model's output corresponds well with the measured responses.
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| 6. |
- Chen, Yousheng, 1985-, et al.
(author)
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Informative Data for Model Calibration of Locally Nonlinear Structures Based on Multi-Harmonic Frequency Responses
- 2016
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In: Journal of Computational and Nonlinear Dynamics. - : ASME Press. - 1555-1415 .- 1555-1423. ; 11:5
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Journal article (peer-reviewed)abstract
- In industry, linear FE-models commonly serve as baseline models to represent the global structural dynamics behavior. However, available test data may show evidence of significant nonlinear dynamic characteristics. In such a case, the baseline linear model may be insufficient to represent the dynamics of the structure. The causes of the nonlinear characteristics may be local in nature and the remaining parts of the structure may be satisfactorily represented by linear descriptions. Although the baseline model can then serve as a good foundation, the physical phenomena needed to substantially increase the model's capability of representing the real structure are most likely not modelled in it. Therefore, a set of candidate nonlinear property parameters to control the nonlinear effects have to be added and subjected to calibration to form a credible model. The selection of the calibration parameters and the choice of data for a calibration metric form a coupled problem. An over-parameterized model for calibration may result in parameter value estimates that do not survive a validation test. The parameterization is coupled to the test data and should be chosen so that the expected co-variances of the chosen parameter's estimates are made small. Accurate test data, suitable for calibration, is often obtained from sinusoidal testing. Because a pure mono-sinusoidal excitation is difficult to achieve during a test of a nonlinear structure, the excitation is here designed to contain sub and super harmonics besides the fundamental harmonic. The steady-state responses at the side frequencies are shown to contain valuable information for the calibration process that can improve the accuracy of the parameter estimates. The nonlinear steady-state solutions can be found efficiently using the multi-harmonic balance method. In this paper, synthetic test data from a model of a nonlinear benchmark structure are used for illustration. The model calibration and an associated K-fold cross-validation are based on the Levenberg-Marquardt and the undamped Gauss-Newton algorithm, respectively. Starting seed candidates for calibration are found by the Latin hypercube sampling method. The realization that gives the smallest deviation to test data is selected as a starting point for the iterative search for a calibration solution. The calibration result shows good agreement with the true parameter setting, and the K-fold cross validation result shows that the variance of the estimated parameters shrinks when adding sub and super harmonics to the nonlinear frequency response functions.
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| 7. |
- Chen, Yousheng, et al.
(author)
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Validation of a model calibration method through vibrational testing of a mechanical system with local clearance
- 2016
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In: Proceedings of ISMA2016 International conference on noise and vibration engineering and USD2016 International conference on uncertainty in structural dynamics. - Leuven, Belgium : Katholieke University Leuven. - 9789073802940 ; , s. 2581-2595
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Conference paper (peer-reviewed)abstract
- Nonlinear finite element models are often validated using experimental data. A previously proposed calibration method, which concerns pre-test planning, multi-sinusoidal excitation and an effective optimization routine, is improved with an extended version of the pre-test planning. The improved method is validated using a test structure with a clearance type nonlinearity. From the pretest planning, an optimal configuration for the data acquisition is determined. The multi-harmonic nonlinear frequency response functions (FRFs) of the structure under test are then generated by a multi-sinusoidal excitation. Model calibration is conducted by minimizing the difference between the experimental multi-harmonic nonlinear FRFs and their analytical counterparts. The uncertainties of the estimated parameters are assessed by a k-fold cross validation, which confirm that the uncertainties of the estimated parameters are small when the optimal configuration is applied.
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| 8. |
- Forsberg, Anna, 1985-
(author)
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Immunomodulatory effects of probiotic supplementation during pregnancy and infancy in allergy prevention studies
- 2017
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Doctoral thesis (other academic/artistic)abstract
- The incidence of allergic diseases is increasing, possibly due to a reduced intensity and diversity of microbial stimulation. More knowledge is needed on the immunological mechanisms underlying the eczema preventive effect of pre- and postnatal probiotic supplementation. The pregnancy period seems to be of essential importance, since both epidemiological and experimental animal studies show the importance of microbial exposure during gestation on allergy prevention.We have performed a study where the probiotic lactic acid producing bacteria Lactobacillus reuteri was supplemented to pregnant women, at risk of having an allergic infant. The pregnant mothers received the study product from gestational week 36 until delivery, and the infants then continued with the same product until one year of age. The probiotic, as compared with placebo, supplemented infants had less IgE-associated eczema at two years of age.In order to investigate how the supplementation affected the immune system peripheral blood was collected and immune cells were stimulated with common allergens and TLR ligands. The probiotic treated group responded with a more regulated response to allergens and TLR2 ligands in comparison to the placebo supplemented group. We also investigated how the probiotic supplementation affected the epigenetic methylation pattern in circulating T helper cells during infancy, observing the most pronounced effects at birth.In a follow up study, supplementation was started earlier to possibly gain a stronger allergy preventive effect via changes in maternal immune regulation. Supplementation with Lactobacillus reuteri and ω-3 fatty acids started at gestational week 20 and throughout pregnancy. After 20 weeks of supplementation, some immunomodulatory effects among circulating activated regulatory T cells and a subpopulation of monocytes were noted. Several systemic immune modifying effects of pregnancy were observed.In summary, probiotics show several immunomodulatory effects in infants and pregnant women. However, more research is needed to better understand the effects of the probiotic supplementation to aid future identification of more efficacious allergy preventive strategies.
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| 9. |
- Jacobsson, Bo, et al.
(author)
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Förtidsbörd största perinatala problemet [Preterm delivery an overview on epidemiology, pathophysiology and consequences for the individual and the society]
- 2019
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In: Läkartidningen. - : Läkartidningen Förlag AB. - 0023-7205 .- 1652-7518. ; 116
-
Research review (peer-reviewed)abstract
- Preterm delivery in Sweden constitutes 5.7 % of all deliveries, which is among the lowest rates in the world. There has not been any increase in the proportion of iatrogenic preterm deliveries during the last decades.The main hypothesis concerning the causality of preterm delivery is still that of the ascending infection from the vagina to the uterus and inflammation resulting in contractions, rupture of membranes and delivery. The mechanisms behind parturition at term are still elusive and this is also true for preterm delivery. The genetic contribution to preterm delivery is about 25-30 %. The first genes that are associated with preterm delivery and gestational duration have recently been published. Huge progress has been made in care of preterm born infants. Sweden has among the lowest rates of mortality and morbidity in the world, especially in the lowest gestational weeks. New modes of care, family-centered care and hospital-assisted home care, have empowered the parents and reduced the cost for care.
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| 10. |
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| 11. |
- Linderholt, Andreas, 1968-, et al.
(author)
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A Comparison of the Dynamic Behavior of Three Sets of the Ampair 600 Wind Turbine
- 2015
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In: Dynamics of Coupled Structures. - Cham : Springer. - 2191-5644 .- 2191-5652. - 9783319152080 ; , s. 99-110
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Conference paper (other academic/artistic)abstract
- The Ampair 600 wind turbine assembly has been modified to suit as a benchmark structure in the pursuit of finding best practices for experimental substructure coupling of structural dynamic systems. Seven such systems have been assembled in test laboratories in Europe and in the USA. We scrutinize the dynamic behavior of the total assembly of three of those by vibration testing and compare the test outcome from seemingly identical assemblies. The aim is to support future component synthesis activities with high fidelity data and support future model validation. Comparisons are made by evaluating deviations of measured frequency response functions, the differences of identified structural eigenfrequencies and the correlation between eigenvector estimates. The testing is made in two parts. First, a partly assembled system, not including the hub and blades, is tested. This constitutes one possible and logical subsystem splitting that is likely to appear in future substructure synthesis efforts. In the second part, the full system assembly is tested. The test procedure, the test setup, the obtained test data and test data statistics are presented.
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| 12. |
- Linderholt, Andreas, 1968-, et al.
(author)
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The Dynamic Behavior of Three Sets of the Ampair 600 Wind Turbine
- 2016
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In: Proceedings of the International Modal Analysis Conference (IMAC XXXIV), 2016. - : SEM Society of Experimental Mechanics.
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Conference paper (other academic/artistic)abstract
- A modified version of the Ampair 600 wind turbine was defined by the SEM substructuring focus group to be a benchmark for use in the pursuit of developing methods for high precision experimental substructuring. Today, seven such, intended to be identical, systems have been assembled at test laboratories within Europe and the USA. This paper reports the experimentally found dynamic behavior of five out of the seven systems. The aim of the study is to provide high fidelity data to support future component synthesis and model validation activities. Comparisons are made by evaluating deviations of measured frequency response functions and identified structural eigenfrequencies together with the correlation between eigenvector estimates. The testing is made in two parts. First, partly assembled systems, not including the rotors or the blades, are tested. This constitutes one possible and logical subsystem splitting that is likely to appear in future substructure synthesis efforts. In the second part, the fully assembled benchmark systems are tested. The test procedure, the test setup, the obtained test data and test data statistics are presented.
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| 13. |
- Linderholt, Andreas, 1968-, et al.
(author)
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Time Domain Dynamic Simulations of Locally Nonlinear Large-Scale Systems
- 2016
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In: Presented at Aerospace technology congress.
-
Conference paper (other academic/artistic)abstract
- In practice, most mechanical systems show nonlinear characteristics within the operational envelope. However, the nonlinearities are often caused by local phenomena and many mechanical systems can be well represented by a linear model enriched with local nonlinearities. Conventional nonlinear response simulations are often computationally intensive; the problem which becomes more severe when large-scale nonlinear systems are concerned. Thus, there is a need to further develop efficient simulation techniques. In this work, an efficient simulation method for large-scale systems with local nonlinearities is proposed. The method is formulated in a state-space form and the simulations are done in the Matlab environment. The nonlinear system is divided into a linearized system and a nonlinear part represented as external nonlinear forces acting on the linear system; thus taking advantage in the computationally superiority in the locally nonlinear system description compared to a generally nonlinear counterpart. The triangular-order hold exponential integrator is used to obtain a discrete state-space form. To shorten the simulation time additionally, auxiliary matrices, similarity transformation and compiled C-codes (mex) to be used for the time integration are studied. Comparisons of the efficiency and accuracy of the proposed method in relation to simulations using the ODE45 solver in Matlab and MSC Nastran are demonstrated on numerical examples of different model sizes.
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| 14. |
- Zamir, Itay, et al.
(author)
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Hyperglycemia in Extremely Preterm Infants Insulin Treatment, Mortality and Nutrient Intakes
- 2018
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In: Journal of Pediatrics. - : Elsevier BV. - 0022-3476 .- 1097-6833. ; 200
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Journal article (peer-reviewed)abstract
- Objective To explore the prevalence of hyperglycemia and the associations between nutritional intakes, hyperglycemia, insulin treatment, and mortality in extremely preterm infants. Study design Prospectively collected data from the Extremely Preterm Infants in Sweden Study (EXPRESS) was used in this study and included 580 infants born <27 gestational weeks during 2004-2007. Available glucose measurements (n = 9850) as well as insulin treatment and nutritional data were obtained retrospectively from hospital records for the first 28 postnatal days as well as 28- and 70-day mortality data. Results Daily prevalence of hyperglycemia >180 mg/dL (10 mmol/L) of up to 30% was observed during the first 2 postnatal weeks, followed by a slow decrease in its occurrence thereafter. Generalized additive model analysis showed that increasing parenteral carbohydrate supply with 1 g/kg/day was associated with a 1.6% increase in glucose concentration (P < .001). Hyperglycemia was associated with more than double the 28-day mortality risk (P< .01). In a logistic regression model, insulin treatment was associated with lower 28- and 70-day mortality when given to infants with hyperglycemia irrespective of the duration of the hyperglycemic episode (P< .05). Conclusions Hyperglycemia is common in extremely preterm infants throughout the first postnatal month. Glucose infusions seem to have only a minimal impact on glucose concentrations. In the EXPRESS cohort, insulin treatment was associated with lower mortality in infants with hyperglycemia. Current practices of hyperglycemia treatment in extremely preterm infants should be reevaluated and assessed in randomized controlled clinical trials.
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