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Papers by Thomas Svensson
VMEA-metoden for bedomning av osakerheter i bergmekaniska tillampningar. The VMEA-method for Asse... more VMEA-metoden for bedomning av osakerheter i bergmekaniska tillampningar. The VMEA-method for Assessing Uncertainties in Rock Engineering
We will present an application of the probabilistic branch of Variation Mode and Effect Analysis ... more We will present an application of the probabilistic branch of Variation Mode and Effect Analysis (VMEA) implemented as a first order, second moment reliability method. First order means that the failure function is approximated to be linear with respect to the main influencing variables, while second moment means that only means and variances are taken into account in the statistical procedure. We study the fatigue life of an air engine component and aim at a safety margin that takes all sources of scatter and uncertainty into account. Scatter is defined as random variation due to natural causes, such as non-homogeneous material, geometry variation within tolerances, load variation in usage, and other uncontrolled variation. Uncertainty is defined as unknown systematic errors, such as model errors in the numerical calculation of fatigue life, statistical errors in estimates of parameters, and unknown usage profile. By defining also uncertainties as random variables, the whole safety...
Procedia Engineering, 2013
This paper discusses some basic differences between engineering approaches to fatigue reliability... more This paper discusses some basic differences between engineering approaches to fatigue reliability assessment. The comparison is based on the fact that mechanical fatigue is a highly empirical science with large uncertainties in prior knowledge, both regarding observed data and model accuracy. It is claimed that the amount of available prior physical knowledge should decide the level of complexity in reliability tools, not computer resources nor mathematical theory. As a result the second moment statistics is put forward as a useful tool in fatigue reliability assessment, either in the regulated form like in Eurocode or in a free engineering form like through the Variation Mode and Effect Analysis. The latter method is outlined in certain detail and its practical usefulness demonstrated for some engineering reliability problems.
A new class of methods for propagation of uncertainty through complex models nonlinear-in-paramet... more A new class of methods for propagation of uncertainty through complex models nonlinear-in-parameters is proposed. It is derived from a recent idea of propagating covariance within the unscented Kalman filter. The nonlinearity could be due to a pole-zero parametrization of a dynamic model in the Laplace domain, finite element model (FEM) or other large computer models, models of mechanical fatigue etc. Two approximate methods of this class are evaluated against Monte Carlo simulations and compared to the application of the Gauss approximation formula. Three elementary static models illustrate pros and cons of the methods, while one dynamic model provides a realistic simple example of its use.
E3S Web of Conferences
Mould growth in buildings is a complex process, affected by moisture and temperature, the propert... more Mould growth in buildings is a complex process, affected by moisture and temperature, the properties of the building material as well as characteristics of the mould fungi. The complexity poses challenges when assessing the risk of mould growth in buildings. Mathematical models are often used to predict whether mould will grow in a part of building with expected RH and temperature conditions. The models can be described as static or dynamic. In a previous round-robin study, comparing results from models with observations from field studies, the outcome of the dynamic models evaluated depended on the user of the model. Also, the models often underestimated the risk of mould growth. A better agreement was found for static models, especially for the PJ-model. It is a part of a standardised technical specification (SIS-TS 41:2014) and has not previously been described as a model. The critical moisture level (RHcrit), determined by tests according to the method, is used as input. Thus, t...
A method for the generation of discrete time stochastic processes for fatigue testing was previou... more A method for the generation of discrete time stochastic processes for fatigue testing was previously presented. The generated process is constructed according to user requirements regarding irregularity and level crossings. In the present investigation this type of discrete time stochastic process is further investigated in order to calculate prediction limits for user requirements. These limits can be used to decide the length of sequences that need to be generated in a certain fatigue test. For instance, our results show that irregularity factor errors better than 1% are possible to achieve in fatigue applications.
Title (Original): Inter-laboratory comparison of fatigue test with evaluation of the participatin... more Title (Original): Inter-laboratory comparison of fatigue test with evaluation of the participating laboratories calculations of measurement uncertainty In this paper a fatigue testing inter-laboratory comparison is presented. Six Nordic laboratories performed fatigue tests on steel specimens. The specimens were tested with different stress levels (460 MPa, 430 MPa and 400 MPa) and with the load ratio R=S min/S max =0.1. On each level four specimens were tested. The participating laboratories reported the results together with measurement uncertainty. The results show a significant difference between laboratories. However, when the differences in modelling were taken into consideration no significant differences between the laboratories remained. There are large differences in the way the measurement uncertainties were calculated and reported. No laboratory did take the most influencing uncertainty parameter, bending stress (due to misalignment of the testing machine, “incorrect ” sp...
International Journal of Fatigue, 2012
Fatigue endurance assessments of welded details are normally carried out by calculating the relev... more Fatigue endurance assessments of welded details are normally carried out by calculating the relevant stress acting on the detail and identifying a relevant fatigue class (or detail category) with its associated S-N curve. The fatigue strength of most structural details incorporated in design codes has been obtained from fatigue tests conducted under uni-axial loading conditions, which normally result in a uni-axial stress state in the detail. Many of the structural details that exist in fatigue-loaded structures experience some kind of multi-axial loading condition. The subject of the fatigue strength of welded details under multi-axial loads has been the topic of numerous research projects in recent years. The vast majority of these projects were, however, devoted to cracking in the base metal (i.e. toe cracking). Very little has been done with reference to the cracking of fillet welds in combined loading situations (i.e. root cracking). This paper presents new test results from cruciform specimens, in which weld failure initiated at the root in a multi-axial stress state. The tests have been performed at two different load levels and on three different specimen configurations giving different s/r ratios. This permitted an examination of the effect of the shear to normal stress ratio on the fatigue strength of fillet welds. The results of these tests, together with other relevant tests reported in the literature, are then evaluated in relation to the design models proposed in three design standards: Eurocode, IIW and DNV. No obvious dependence on the s/r ratios could be found. The evaluated models all appear to be able to predict the fatigue life of a cruciform weld failing from the root under combined shear and normal stress.
Procedia Engineering
An experimental mean curve and a design fatigue curve corresponding to 95% survival probability w... more An experimental mean curve and a design fatigue curve corresponding to 95% survival probability were derived from realistic fatigue experiments on a non-welded water pressurized piping component with primarily focus on high cycle fatigue. The components were subjected to a synthetic variable amplitude bending deformation. Comparison with the results obtained for a similar piping component with a circumferential butt weld allowed the determination of an experimental fatigue strength reduction factor. Comparison with the fatigue procedure and design curve in ASME BPVC Section III allowed to quantify its conservatism with regards to accounting for the presence of a weldment and more generally transferability.
International Biodeterioration & Biodegradation, 2012
The susceptibility of building materials to mould growth varies. Some are tolerant to high relati... more The susceptibility of building materials to mould growth varies. Some are tolerant to high relative humidity in the ambient air without mould growth occurring, while others are less tolerant, and mould can grow in relative humidity as low as 75%. Within a building, constructions are exposed to different temperatures and relative humidities. To minimise the risk of microbial growth, building materials should be chosen that are tolerant to the expected conditions. In this study, the critical moisture levels for ten building materials with a range of expected critical moisture levels (wood-based materials, gypsum boards and inorganic boards) were evaluated. Samples of the building materials were inoculated with spores from six species of mould fungi (Eurotium herbariorum, Aspergillus versicolor, Penicillium chrysogenum, Aureobasidium pullulans, Cladosporium sphaerospermum, Stachybotrys chartarum) and incubated in test cabinets at specified temperature (10 C and 22 C) and relative humidity conditions (75e95%); growth of mould was analysed weekly for at least 12 weeks. One of the conclusions is that two similar building materials or products may have considerably different resistance to mould growth, and so the results from one type of building material cannot be applied to the other. Also, in order to compare results from different tests, it is important to use the same test method. It is also important to state the temperature at which the critical moisture level applies and how long the material is exposed to the temperature and relative humidity conditions during the test.
Building and Environment, 2013
Materials that are stored or used in damp conditions may be subject to mould growth. However, all... more Materials that are stored or used in damp conditions may be subject to mould growth. However, all materials are not equally susceptible; for each specific material, there is a critical moisture level for mould growth. If this is exceeded, there is a risk that mould fungi will develop on the material. This level can be determined in accelerated laboratory tests, at constant temperatures and relative humidity (RH) favourable to mould growth. Within a building however, these parameters are expected to vary from one part of the construction to another, and are seldom constant; there is fluctuation in temperature and RH due to seasonal or shorter-term variations. In this study, test pieces of the same materials tested in a laboratory environment were placed in three outdoor ventilated crawl spaces and three outdoor ventilated attics, where the temperature and RH varied, and mould growth on the test pieces was studied over 2.5 years. Material-specific mould growth curves were produced based on critical moisture levels, as determined in laboratory experiments under constant temperature and RH. When the actual conditions of RH and temperature exceeded these curves, there was mould growth on the test pieces if the time was sufficiently long. The conclusion from the study is that although conditions in laboratory studies are simplified and accelerated, the results serve well to indicate mould growth within a building construction.
Problems related to metal fatigue modelling and testing are here treated in four different papers... more Problems related to metal fatigue modelling and testing are here treated in four different papers. In the first paper different views of the subject are summarized in a literature survey. In the second paper a new model for fatigue life is investigated. Experimental results are established which are promising for further development of the model. In the third paper a method is presented that generates a stochastic process, suitable to fatigue testing. The process is designed in order to resemble certain fatigue related features in service life processes. In the fourth paper fatigue problems in transport vibrations are treated.
This paper discusses some basic differences between engineering approaches to fatigue reliability... more This paper discusses some basic differences between engineering approaches to fatigue reliability assessment. The comparison is based on the fact that mechanical fatigue is a highly empirical science with large uncertainties in prior knowledge, both regarding observed data and model accuracy. It is claimed that the amount of available prior physical knowledge should decide the level of complexity in reliability tools, not computer resources nor mathematical theory. As a result the second moment statistics is put forward as a useful tool in fatigue reliability assessment, either in the regulated form like in Eurocode or in a free engineering form like through the Variation Mode and Effect Analysis. The latter method is outlined in certain detail and its practical usefulness demonstrated for some engineering reliability problems.
VMEA-metoden for bedomning av osakerheter i bergmekaniska tillampningar. The VMEA-method for Asse... more VMEA-metoden for bedomning av osakerheter i bergmekaniska tillampningar. The VMEA-method for Assessing Uncertainties in Rock Engineering
We will present an application of the probabilistic branch of Variation Mode and Effect Analysis ... more We will present an application of the probabilistic branch of Variation Mode and Effect Analysis (VMEA) implemented as a first order, second moment reliability method. First order means that the failure function is approximated to be linear with respect to the main influencing variables, while second moment means that only means and variances are taken into account in the statistical procedure. We study the fatigue life of an air engine component and aim at a safety margin that takes all sources of scatter and uncertainty into account. Scatter is defined as random variation due to natural causes, such as non-homogeneous material, geometry variation within tolerances, load variation in usage, and other uncontrolled variation. Uncertainty is defined as unknown systematic errors, such as model errors in the numerical calculation of fatigue life, statistical errors in estimates of parameters, and unknown usage profile. By defining also uncertainties as random variables, the whole safety...
Procedia Engineering, 2013
This paper discusses some basic differences between engineering approaches to fatigue reliability... more This paper discusses some basic differences between engineering approaches to fatigue reliability assessment. The comparison is based on the fact that mechanical fatigue is a highly empirical science with large uncertainties in prior knowledge, both regarding observed data and model accuracy. It is claimed that the amount of available prior physical knowledge should decide the level of complexity in reliability tools, not computer resources nor mathematical theory. As a result the second moment statistics is put forward as a useful tool in fatigue reliability assessment, either in the regulated form like in Eurocode or in a free engineering form like through the Variation Mode and Effect Analysis. The latter method is outlined in certain detail and its practical usefulness demonstrated for some engineering reliability problems.
A new class of methods for propagation of uncertainty through complex models nonlinear-in-paramet... more A new class of methods for propagation of uncertainty through complex models nonlinear-in-parameters is proposed. It is derived from a recent idea of propagating covariance within the unscented Kalman filter. The nonlinearity could be due to a pole-zero parametrization of a dynamic model in the Laplace domain, finite element model (FEM) or other large computer models, models of mechanical fatigue etc. Two approximate methods of this class are evaluated against Monte Carlo simulations and compared to the application of the Gauss approximation formula. Three elementary static models illustrate pros and cons of the methods, while one dynamic model provides a realistic simple example of its use.
E3S Web of Conferences
Mould growth in buildings is a complex process, affected by moisture and temperature, the propert... more Mould growth in buildings is a complex process, affected by moisture and temperature, the properties of the building material as well as characteristics of the mould fungi. The complexity poses challenges when assessing the risk of mould growth in buildings. Mathematical models are often used to predict whether mould will grow in a part of building with expected RH and temperature conditions. The models can be described as static or dynamic. In a previous round-robin study, comparing results from models with observations from field studies, the outcome of the dynamic models evaluated depended on the user of the model. Also, the models often underestimated the risk of mould growth. A better agreement was found for static models, especially for the PJ-model. It is a part of a standardised technical specification (SIS-TS 41:2014) and has not previously been described as a model. The critical moisture level (RHcrit), determined by tests according to the method, is used as input. Thus, t...
A method for the generation of discrete time stochastic processes for fatigue testing was previou... more A method for the generation of discrete time stochastic processes for fatigue testing was previously presented. The generated process is constructed according to user requirements regarding irregularity and level crossings. In the present investigation this type of discrete time stochastic process is further investigated in order to calculate prediction limits for user requirements. These limits can be used to decide the length of sequences that need to be generated in a certain fatigue test. For instance, our results show that irregularity factor errors better than 1% are possible to achieve in fatigue applications.
Title (Original): Inter-laboratory comparison of fatigue test with evaluation of the participatin... more Title (Original): Inter-laboratory comparison of fatigue test with evaluation of the participating laboratories calculations of measurement uncertainty In this paper a fatigue testing inter-laboratory comparison is presented. Six Nordic laboratories performed fatigue tests on steel specimens. The specimens were tested with different stress levels (460 MPa, 430 MPa and 400 MPa) and with the load ratio R=S min/S max =0.1. On each level four specimens were tested. The participating laboratories reported the results together with measurement uncertainty. The results show a significant difference between laboratories. However, when the differences in modelling were taken into consideration no significant differences between the laboratories remained. There are large differences in the way the measurement uncertainties were calculated and reported. No laboratory did take the most influencing uncertainty parameter, bending stress (due to misalignment of the testing machine, “incorrect ” sp...
International Journal of Fatigue, 2012
Fatigue endurance assessments of welded details are normally carried out by calculating the relev... more Fatigue endurance assessments of welded details are normally carried out by calculating the relevant stress acting on the detail and identifying a relevant fatigue class (or detail category) with its associated S-N curve. The fatigue strength of most structural details incorporated in design codes has been obtained from fatigue tests conducted under uni-axial loading conditions, which normally result in a uni-axial stress state in the detail. Many of the structural details that exist in fatigue-loaded structures experience some kind of multi-axial loading condition. The subject of the fatigue strength of welded details under multi-axial loads has been the topic of numerous research projects in recent years. The vast majority of these projects were, however, devoted to cracking in the base metal (i.e. toe cracking). Very little has been done with reference to the cracking of fillet welds in combined loading situations (i.e. root cracking). This paper presents new test results from cruciform specimens, in which weld failure initiated at the root in a multi-axial stress state. The tests have been performed at two different load levels and on three different specimen configurations giving different s/r ratios. This permitted an examination of the effect of the shear to normal stress ratio on the fatigue strength of fillet welds. The results of these tests, together with other relevant tests reported in the literature, are then evaluated in relation to the design models proposed in three design standards: Eurocode, IIW and DNV. No obvious dependence on the s/r ratios could be found. The evaluated models all appear to be able to predict the fatigue life of a cruciform weld failing from the root under combined shear and normal stress.
Procedia Engineering
An experimental mean curve and a design fatigue curve corresponding to 95% survival probability w... more An experimental mean curve and a design fatigue curve corresponding to 95% survival probability were derived from realistic fatigue experiments on a non-welded water pressurized piping component with primarily focus on high cycle fatigue. The components were subjected to a synthetic variable amplitude bending deformation. Comparison with the results obtained for a similar piping component with a circumferential butt weld allowed the determination of an experimental fatigue strength reduction factor. Comparison with the fatigue procedure and design curve in ASME BPVC Section III allowed to quantify its conservatism with regards to accounting for the presence of a weldment and more generally transferability.
International Biodeterioration & Biodegradation, 2012
The susceptibility of building materials to mould growth varies. Some are tolerant to high relati... more The susceptibility of building materials to mould growth varies. Some are tolerant to high relative humidity in the ambient air without mould growth occurring, while others are less tolerant, and mould can grow in relative humidity as low as 75%. Within a building, constructions are exposed to different temperatures and relative humidities. To minimise the risk of microbial growth, building materials should be chosen that are tolerant to the expected conditions. In this study, the critical moisture levels for ten building materials with a range of expected critical moisture levels (wood-based materials, gypsum boards and inorganic boards) were evaluated. Samples of the building materials were inoculated with spores from six species of mould fungi (Eurotium herbariorum, Aspergillus versicolor, Penicillium chrysogenum, Aureobasidium pullulans, Cladosporium sphaerospermum, Stachybotrys chartarum) and incubated in test cabinets at specified temperature (10 C and 22 C) and relative humidity conditions (75e95%); growth of mould was analysed weekly for at least 12 weeks. One of the conclusions is that two similar building materials or products may have considerably different resistance to mould growth, and so the results from one type of building material cannot be applied to the other. Also, in order to compare results from different tests, it is important to use the same test method. It is also important to state the temperature at which the critical moisture level applies and how long the material is exposed to the temperature and relative humidity conditions during the test.
Building and Environment, 2013
Materials that are stored or used in damp conditions may be subject to mould growth. However, all... more Materials that are stored or used in damp conditions may be subject to mould growth. However, all materials are not equally susceptible; for each specific material, there is a critical moisture level for mould growth. If this is exceeded, there is a risk that mould fungi will develop on the material. This level can be determined in accelerated laboratory tests, at constant temperatures and relative humidity (RH) favourable to mould growth. Within a building however, these parameters are expected to vary from one part of the construction to another, and are seldom constant; there is fluctuation in temperature and RH due to seasonal or shorter-term variations. In this study, test pieces of the same materials tested in a laboratory environment were placed in three outdoor ventilated crawl spaces and three outdoor ventilated attics, where the temperature and RH varied, and mould growth on the test pieces was studied over 2.5 years. Material-specific mould growth curves were produced based on critical moisture levels, as determined in laboratory experiments under constant temperature and RH. When the actual conditions of RH and temperature exceeded these curves, there was mould growth on the test pieces if the time was sufficiently long. The conclusion from the study is that although conditions in laboratory studies are simplified and accelerated, the results serve well to indicate mould growth within a building construction.
Problems related to metal fatigue modelling and testing are here treated in four different papers... more Problems related to metal fatigue modelling and testing are here treated in four different papers. In the first paper different views of the subject are summarized in a literature survey. In the second paper a new model for fatigue life is investigated. Experimental results are established which are promising for further development of the model. In the third paper a method is presented that generates a stochastic process, suitable to fatigue testing. The process is designed in order to resemble certain fatigue related features in service life processes. In the fourth paper fatigue problems in transport vibrations are treated.
This paper discusses some basic differences between engineering approaches to fatigue reliability... more This paper discusses some basic differences between engineering approaches to fatigue reliability assessment. The comparison is based on the fact that mechanical fatigue is a highly empirical science with large uncertainties in prior knowledge, both regarding observed data and model accuracy. It is claimed that the amount of available prior physical knowledge should decide the level of complexity in reliability tools, not computer resources nor mathematical theory. As a result the second moment statistics is put forward as a useful tool in fatigue reliability assessment, either in the regulated form like in Eurocode or in a free engineering form like through the Variation Mode and Effect Analysis. The latter method is outlined in certain detail and its practical usefulness demonstrated for some engineering reliability problems.