Refine
Document Type
- Doctoral thesis (5)
Has Fulltext
- yes (5)
Is part of the Bibliography
- no (5)
Language
- English (5) (remove)
Keywords
- Finite-Elemente-Methode (2)
- Sandwichelemente (2)
- Steifigkeit (2)
- ABAQUS AWI (1)
- Belastung (1)
- Biegung (1)
- Blech (1)
- Bonded steel joints (1)
- Feuer (1)
- Finite element method (1)
Institute
- FG Stahl- und Holzbau (5) (remove)
This thesis is dedicated to the structured sheet metals topic. Structured sheet metals are semi-finished products with the honeycomb (cell) shape, made by cold forming of the flat sheets. They represent an innovative technology in the field of lightweight construction. In recent years, this technology has undergone dynamic development what gives to researchers possibilities to improve the level of knowledge about structured sheets metals properties and explore new application fields for them.
The structuring improves basic properties, such as stiffness, compared to the flat sheet material. Due to these advantages, the use of structured sheet metals offers enormous innovation potential for efficient lightweight construction in many industrial sectors such as aerospace, rail transport technology, architectural products. Structured sheets have already been implementing in lighting technology, in the manufacture of household appliances and even in automotive industry. So that, the question arises if it possible to apply this kind of material successfully in other areas such as building industry.
This work contains an overview of the lightweight constructions historical development and stiffening elements in the steel industry, also the creation process of structured sheet metals, manufacturing investigated specimens, further bending tests, numerical simulations, analysis and comparison of resulting data and possible further use of lightweight beams with structured sheet elements as a building construction.
In this work beams compound of structured and flat plates are investigated. Steps of structured plates manufacturing process are described: hydroforming, point- and laser welding and bending. As a result, lightweight beams of two main shapes are manufactured: C- and square sectioned. There are four types of beams for each of the shapes are investigated. Every type has same geometrical dimensions, but thicknesses vary.
The series of laboratory tests with created beams under load is made. Three- and four-points bending tests are chosen for that.
This work also presents numerical analysis based on conducted experiments: buckling and global non-linear behavior of specimens by use of the software package ABAQUS/CAE are obtained. The behavior of the beams with structured and flat sheets under load is analyzed and the comparison of parameters such as load bearing capacity and stiffnesses is made.
Finally, in addition to laboratory and simulations, parametric modelling is done. Also, based on parametric calculations, the proposal for calculation the beams stiffness for with higher thicknesses is given. It allows to predict the behavior of beams with structured sheets with different thicknesses without manufacturing and conducting the expensive and time-taking laboratory experiments.
In conclusion, the recommendations for the simplifying of manufacturing process and for the improving of the beams stiffnesses are given.
Steel cladding structures such as sandwich panels can replace bracing systems to provide further stability to individual structural members such as beams and columns. Previous researches studied the stabilizing effects of sandwich panels on the whole structure at ambient temperatures. It was shown that considerable savings could be achieved in the case of using steel cladding systems. In the STABFI (Steel Cladding Systems for Stabilisation of Steel Buildings in Fire) project, the primary objective was to study the stabilizing behavior of cladding systems in the fire. The current thesis is a part of the STABFI project focusing on the bending and translational stiffness of sandwich panels at ambient and elevated temperatures.
The thesis consists of two separate parts, the bending and translational performance of sandwich panels at ambient and elevated temperatures. Sandwich panels are typically composites of two thin steel sheets and a core of higher thickness and lower density. They are valued for their excellent thermal properties. This research employs two different materials, including mineral wool (MW) and Polyisocyanurate (PIR), as a core.
In the first part of the thesis, the bending tests carried out in Prague are described. The experimental results are presented in the first phase of this part. A finite element (FE) model is developed to validate simulations with experimental results, and then a comprehensive parametric study is carried out. During the parametric study, different factors such as panel thickness, width, span, the thickness of steel sheets, and the fire's influence on panels' mechanical behavior are investigated. Moreover, the analytical solutions obtained from Eurocodes (EN 14509, 2013) at ambient temperature are employed to predict the bending stiffness values. The analytical solutions are then developed to apply at elevated temperatures by incorporating the reduction factors into the equations. Eventually, the accuracy of suggested analytical equations is compared with numerical results.
In the second part of the thesis, after presenting the translational tests which also conducted in Prague and validation of FE models, an extensive parametric study on the decisive factors such as the steel sheet thicknesses, screw diameters and temperature effects on the sandwich panel connections behavior is performed. The parametric study shows how each parameter affects the shear resistance and stiffness of sandwich panel connections. Furthermore, the deterioration of shear performance at elevated temperatures is evaluated. The analytical solutions achieved from the ECCS manual are used to estimate the shear stiffness and resistance of connections at ambient temperatures. At elevated temperatures, the equations are developed to anticipate the abovementioned values in the fire case. Finally, the safety and accuracy of proposed analytical solutions are assessed.
Statistical size effect in steel structure and corresponding influence on structural reliability
(2018)
This thesis aims to investigate the statistical size effect in the elasto-plastic material and the corresponding reliability of steel structures. The core idea is that the stochastic material properties are directly embedded in mechanical calculations to develop a more accurate and economical design method for steel structure. Moreover, the results of the experimental investigation with different specimen sizes, whose diameter is limit up to 32 mm, show that the statistical size effect exists in steel structures. This thesis demonstrates finally that the structural reliability is affected by the statistical size effect and the structural safety can be optimized by considering this effect.
Because of the uncertainty and non-uniformity of the microscopic imperfection distribution, the material strength in macroscale presents complex randomness. This study described the randomness of material properties through two different ways: developing a stochastic material model for elasto-plastic material and establishing a discrete random field with a general mathematical program. The proposed stochastic material model is extended to analyze the steel structure with multiaxial stress and is integrated into the commercial FEM software for analysis of the complex structures with stress gradient. The stochastic finite element method is implemented to analyze the response of the 3D structures by a general-purpose FEM program when the random field file is imported into the finite element model.
The uniaxial tensile tests with different specimen sizes and different material are carried out to demonstrate the statistical size effect in steel structures. The results show that the variations of the yield and tensile strength increase with the decreasing specimen volume. Moreover, according to the bending tests, it is obvious that the structural component strength is not only related to the specimen volume, but also the stress distribution. These two proposed simulation methods, which are an extension and supplement to traditional simulation methods, can effectively simulate the statistical size effect for the tensile and flexural components in steel structures.
Finally, it is found by studying the influence of statistical size effect on structural reliability that the strength, which is obtained by small specimens through statistical analysis in the laboratory, is no more accurately applicable to large construction. The reliability theory for the structural safety which exists over the decades can be compared and validated or improved through the embedding the stochastic material properties in the numerical simulation.
From modelling point of view, it will be very useful if the parameters of interest which contribute to the residual stresses and distortions in various types of welded joint and structure application can be simulated numerically so that welding performance with respect to the various aspects could be assessed and evaluated in an efficient manner.
The present study focuses on numerical simulation of welding sequence effects on temperature distribution, residual stresses and distortions and load bearing capacity of a T-joint made by square hollow sections. The aim of this work is to investigate the influence of various welding sequence patterns and welding imperfections on the load bearing capacity of a S355 steel square hollow section (SHS) T-Joint. Two different FE-based programs are used to simulate the welding process: the ABAQUS Welding Interface (AWI) and SYSWELD v2014. The type of welding process used is metal active gas (MAG) welding. A fillet weld with a throat thickness of 5mm is performed. Temperature distribution has been measured at different distance with respect to the weld bead for better validation of results from the simulation. Due to the geometrical characteristics of this type of joint, a compressive test is performed. A displacement with a magnitude of -110 mm is applied on the upper surface of the vertical member (brace). The displacement of several points while the loading process has been measured by inductive displacement transducers. Total number of six specimens were tested. Three joints were welded the same for checking the test set-up (statically checks). The ratio of the width of the branch member to the chord member β=0.5 was kept the same for all specimens. So, the influence of welding schemes could be investigated clearer.
The failure and the joint behaviour under loading condition are analysed. For better evaluation of results apart from the main cases, two extra numerical simulations are performed which help making a sufficient conclusion. The results for stresses in the x, y and z directions, after and before loading, are plotted and discussed. Furthermore it has been clearly explained if lower magnitude of residual stresses could lead to a positive gain in strength or not. The differences and influencing parameters on the load bearing capacity are identified. Additionally, some recommendations for AWI users are given. Stress concentration and how the number of start and end points could influence the overall behaviour of the joints are recognized.
Study on the temperature effect on lap shear adhesive joints in lightweight steel construction
(2015)
In line with the developments in steel industry, the methods of joining steel members have been developed; therefore, the configuration of functional connections with economic and partly-aesthetic advantages has become possible by the use of the known joining methods, which are bolts, rivets and welding. However, these joining methods do not accompany the further developments and requirements needed to construct lightweight connections or to join dissimilar materials or composite constructions. Moreover, the traditional joining methods do not fulfill the increased requirements of the aesthetics of the joints. In the field of steel constructions, structural engineers might use the bonding technique as an alternative method to join the lightweight steel members or as a helpful mean in the bolted or riveted joints in heavyweight steel structures. Despite the advantages of the adhesive bonding technique, the structural designers in the field of steel constructions are still not able to use it in their practical applications because of the doubts regarding the verifiability of bonded steel joints. This is mainly because of the lack of standards for verifying such joints in steel constructions. To facilitate using this technique in steel constructions, hard efforts have to be performed in order to find out the methods of verifications of bonded steel joints. This starts with understanding the behaviour of the adhesive materials as well as their cohesion ability to the steel surfaces over the whole lifetime of the structure and under all possible loading and environmental conditions. Afterward, the mechanical properties of the adhesives have to be presented by their reliable values that take into account all factors and conditions to which the bonded joint is subjected. These values have to be based on the reliability methods and consequently they are guaranteed for the intended lifetime of the designed structure. It is well known that the adhesives, being viscoelastic materials, are very sensitive to several factors such as the environmental effects, mainly temperature and humidity, and the long-term loading. The loss of strength and durability of adhesives materials, due to the mentioned factors, is an essential aspect that has to be determined and to be taken into account of the structural designers during the design process. For example, it is generally proven that the increase of temperature causes a decrease in the elastic (E) and (G) moduli, cohesive and adhesive forces within the joint and maximum stresses which can be carried by the joint. However, there is still a huge lack in describing the degradations of the mechanical properties quantitatively.
Similarly, the failure in the adhesives, loaded for long time by a constant stress even less than their short-term strengths, is probable due to the well-known rheological phenomenon of viscoelastic materials which is the creep phenomenon. Moreover, the adhesives will creep at high temperatures faster; hence the failure will happen in a shorter time. Describing the long-term behaviour of the structural adhesives is still modest; therefore, the time-to-failure of bonded steel joints under long-term loading cannot be exactly predicted. This is an essential
issue has to be dealt with to fulfill the requirements of employing the adhesive bonding technique in the structural fields including the steel constructions. The efficiency of using adhesive-bonded joints in steel constructions is higher when the adhesives in these joints are loaded in shear. In such shear joints, the lightweight steel members (adherends) are likely to yield before the break within the adhesive layer happens, especially when large bonded areas are used because the developed shear stresses over the most of these areas will be very small. This thesis deals with the temperature influence on the behaviour of two adhesive systems (acrylic and epoxy) and on the capacities of adhesively bonded lap shear joints. The temperature influence is quantitatively described for short-term loading over a service range of temperature from -20 °C to +40 °C. The quantitative description is done by proposing the partial factors and the conversion factors that take the temperature effect into account. This influence is also dealt with for long-term loading to describe the shear creep behaviour of the adhesive materials used. Consequently, the time-to-failure of the bonded lap shear joints due to the creep phenomenon of the adhesives under three applied stresses at room temperature is predicted. Moreover, the estimation of time-to-failure is extended to be used for other shear stress levels. The temperature influence as well as the efficiency of using adhesive-bonded joints in lightweight galvanized steel constructions is also illustrated by giving a practical example of strengthening cold-formed “C” section girders. Comparisons between the two adhesive systems for all cases are given.