Tube bending is a widely used manufacturing process in the aerospace, automotive, and other industries. During tube bending, considerable in-plane distortion and thickness variation occurs. The thickness increases at the intrados (surface of tube in contact with the die) and it reduces at the extrados (outer surface of the tube). In some cases, when the bend die radius is small, wrinkling occurs at the intrados. In industry a mandrel is used to eliminate wrinkling and reduce distortion. However, in the case of a close bend die radius, use of a mandrel should be avoided as bending with the mandrel increases the thinning of the wall at the extrados, which is undesirable in the manufacturing operation. The present research focuses on additional loadings such as axial force and internal pressure which can be used to achieve better shape control and thickness distribution of the tube. Based on plasticity theories, an analytical model is developed to predict cross section distortion and thickness change of tubes under various loading conditions. Results from both the FEA and analytical model indicated that at the intrados the increase in thickness for bending with internal pressure and bending with combined axial pull and internal pressure was nearly the same. But in the case of bending with the combination of axial pull and internal pressure there was a significant reduction of thickness at the extrados. A parametric study was conducted for the case of bending with combined internal pressure and axial pull and it was seen that with proper selection of the pressure and axial pull wrinkling can be eliminated, thickness distribution around the tube can be optimized, and cross section distortion of the tube can be reduced. Predictions of the analytical model are in good agreement with finite element simulations and published experimental results. The model can be used to evaluate tooling and process design in tube bending.

A finite element analysis approach is used to determine the susceptibility to wrinkles for thin sheets with clamped ends when subjected to tensile loading. The model problem chosen to do this analysis is the stretching of a thin sheet with clamped-ends. In the preliminary analysis, a stress analysis of thin sheets is done to study the stresses that develop under these boundary conditions. The analysis shows that there is a stretch-induced compressive stress in the transverse direction to the applied load that causes wrinkles. Then, the parametric study is conducted to determine the effect of aspect ratio and strain on the compressive stress. Based on the results of the parametric study, a critical strain value for each aspect ratio is determined for which the corresponding compressive stress is zero. Further buckling analysis is performed to find the buckling modes of the model problem that shows a limit of aspect ratio below which buckling is not possible under given conditions. Finally, post-buckling analysis shows the nature of wrinkles observed in the model problem for different aspect ratios. / text

Tube bending is a widely used manufacturing process in the aerospace, automotive, and other industries. During tube bending, considerable in-plane distortion and thickness variation occurs. The thickness increases at the intrados (surface of tube in contact with the die) and it reduces at the extrados (outer surface of the tube). In some cases, when the bend die radius is small, wrinkling occurs at the intrados. In industry a mandrel is used to eliminate wrinkling and reduce distortion. However, in the case of a close bend die radius, use of a mandrel should be avoided as bending with the mandrel increases the thinning of the wall at the extrados, which is undesirable in the manufacturing operation. The present research focuses on additional loadings such as axial force and internal pressure which can be used to achieve better shape control and thickness distribution of the tube. Based on plasticity theories, an analytical model is developed to predict cross section distortion and thickness change of tubes under various loading conditions. Results from both the FEA and analytical model indicated that at the intrados the increase in thickness for bending with internal pressure and bending with combined axial pull and internal pressure was nearly the same. But in the case of bending with the combination of axial pull and internal pressure there was a significant reduction of thickness at the extrados. A parametric study was conducted for the case of bending with combined internal pressure and axial pull and it was seen that with proper selection of the pressure and axial pull wrinkling can be eliminated, thickness distribution around the tube can be optimized, and cross section distortion of the tube can be reduced. Predictions of the analytical model are in good agreement with finite element simulations and published experimental results. The model can be used to evaluate tooling and process design in tube bending.

Sandwich panels produced by Isolamin AB are comprised of a core, such as polyurethane or mineral wool that is attached between two steel plates. The problem that the thesis attempts to solve is whether a flat profiled sandwich panel of 1mm can withstand loads from 3-storey modular house. To solve this problem investigation in other forms of buckling behaviour will be done. This investigation will be carried out through experimentation, finite element modelling and calculations to find resistance values. Ultimately, these results will be analysed and examined and prove if the sandwich panel with 1mm steel plate is able to withstand a 3-storey modular house.

My PhD work focuses on developing new methods to create the macroscopic patterns in a simple, robust, and versatile way. For macroscopic pattern formation, we first use flow coating as an assembly technique, uniquely balancing two driving forces: (i) evaporative deposition of nonvolatile solutes at a three-phase contact line and (ii) precision movement of a confined meniscus layer. This balance leads to the formation of line-based patterns that range in height and width from nanometers to microns, with lengths greater than centimeters. Moreover, we couple this deposition methodology with functional ligand chemistry on the nanoparticle surface, which allows us to create complex nanoparticle structures. By lifting crosslinked nanoparticle ribbons and ropes, exceptionally intriguing structures emanate from this process. The nanoparticle ribbons and ropes demonstrate a leap forward in nanomaterials fabrication, since the nanoscale properties are embedded within a macroscale object that can be manipulated with conventional methods and engineered into advanced technologies Using mechanical instability, we fabricate a simple, robust stimuli-responsive surface with periodic structures over a large area based upon osmotically-driven surface wrinkling. Although surface wrinkling has received considerable attention in the scientific literature, only a handful of papers have shown the ability to harness perhaps the greatest potential attribute of surface wrinkles: their active reversible nature. The ability to precisely control surface topographic morphologies in accordance with established scaling relationships opens a wide array of advanced materials applications, which do not rely upon cost-limiting fabrication techniques. Specifically, the surfaces respond to solvent exposure by developing well-defined topographic structures over laterally extensive areas due to osmotically-driven differential strains between a surface layer and underlying soft substrate. The observed wrinkling occurs spontaneously, forming hierarchical morphologies with controlled dimensions, and vanishes upon removal of the solvent driving force. The combined responsiveness and reversibility of wrinkling allow for the realization of functional devices, such as smart windows, smart microlens arrays, reversible channels in microfluidic devices. Moreover, by using thermal and osmotic approaches, we study the influence of geometry and material properties on surface instability such as cracking and wrinkling in a trilayer system consisting of a thin film on a soft foundation supported by a rigid substrate.

The introduction of ionization under ambient conditions has greatly simplified mass spectrometric analysis. Over past decade, ambient ionization mass spectrometry (MS) methods have revolutionized the way complex samples are analyzed under environmental conditions without requiring, in most cases, any sample pretreatment. Ambient ionization MS gained popularity among other analytical techniques due to its simplicity and its suitability for analysis of small and large molecules. However, ambient ionization methods can suffer from low accuracy and sensitivity due to matrix effects and interferences within complex samples, as well as from poor ionization efficiency. Matrix effects in ambient ionization are usually caused by ion suppression and may depend on different factors, e.g. matrix-to-analyte concentration ratios, proton affinities of analyte and matrix species. To overcome these challenges, in this thesis we present a new approach where a probe is used both as a direct sampling device and as an efficient ambient ionization source. This approach leverages high surface area gold electrodes, fabricated through low-cost bench-top fabrication methods and functionalized using self-assembled alkyl thiol monolayers, as functional conductive sampling probes (FCSPs) for the extraction and concentration of analytes from a sample solution. FCSPs loaded with the targeted analytes were then used to demonstrate a new and highly efficient ionization approach, called Primary Ion Mass Spectrometry Source (PIMSS). In this approach, following capture, the bound analytes are directly desorbed into the mass spectrometer, where ionization is achieved solely through the extraction voltage applied to the probe. 3D-printing was used to design an interface to couple FCSPs to the mass spectrometer. In this work, we discuss a detailed method development and optimization stage and present capabilities of the proposed assay. / Thesis / Master of Science (MSc)

The present paper studied the buckling phenomena of membranes under tensile load with and without central crack.  The studies of fracture mechanics are tested within certain conditions of membranes. The tensile test has been performed with Al-foil in different crack lengths, i.e. 0 mm, 12.5 mm, 25 mm and 50 mm. The numerical analysis has been carried out by Finite Element Analysis (FEA) and comparing with the theoretical and experimental results.  In this paper, the critical buckling behavior is tested, validated and compared. Same observation of patterns in experiments and the simulation are found. The influence of scale factor for imperfection setting are tested.

Sheet metal hydroforming(flexforming) is a process generally used in the manufacturing of aerospace parts in which a rubber diaphragm forms the sheet on a die with the help pressurized fluid and by this aspect it is different from the conventional stamping process. Some defects occur in the parts that are manufactured by this method and they are not different from the general sheet metal forming defects. Wrinkling, tearing and springback are among those defects. Variety of parts makes difficult to encounter these defects arising the detailed investigation of this process. In this work, the flexforming process was modeled by finite element method in order to investigate the operation windows of the problem. Various two and three-dimensional models were established with and without diaphragm, using explicit and implicit approach for time integration and using solid and shell elements for the blank. Using the material Aluminum 2024-T3 alclad sheet alloy, three basic experiments were conducted: Bending of a straight flange specimen, bending of a contoured flange specimen and bulging of a circular specimen. By these experiments the effects of blank thickness, die bend radius and forming pressure have been investigated. Experimental results were compared with finite element results to verify the computational models. Then, three selected aerospace sheet parts were analyzed and success of the model in the real life applications is proved.

Different fracture modes have been observed in thin film structures. One common approach used in fracture analysis is based on the principle of linear elastic fracture mechanics (LEFM), which assumes pre-existing cracks and treats the materials as linear elastic except for the damage zone around the crack tip. Alternatively, a nonlinear cohesive zone model (CZM) can be used to simulate both nucleation and growth of cracks. In this dissertation, the approaches of LEFM and CZM are employed to study fracture and delamination of elastic thin films on compliant substrates under various loading conditions. First, compression-induced buckling of elastic thin films on elastic compliant substrates is studied by analytical and numerical methods. The critical condition for onset of buckling instability without and with a pre-existing delamination crack is predicted. By comparing the critical strains, a map for the initial buckling modes is constructed with respect to the film/substrate stiffness ratio and the interfacial defect size. For an elastic film on a highly compliant substrate, nonlinear post-buckling analysis is conducted to simulate concomitant wrinkling and buckle-delamination, with a long-range interaction between the two buckling modes through the compliant substrate. By using a layer of cohesive elements for the interface, progressive co-evolution of wrinkling and delamination is simulated. In particular, the effects of interfacial properties (strength and toughness) on the initiation and propagation of wrinkle-induced interfacial delamination are examined. Next, using a set of finite element models, the effects of interfacial delamination and substrate penetration on channel cracking of brittle thin films are analyzed. It is found that, depending on the elastic mismatch and the toughness of interface and substrate, a channel crack may grow with interfacial delamination and/or substrate cracking. By comparing the effective energy release rates, the competition between the two fracture modes is discussed. Cohesive zone modeling is then employed to simulate nucleation and growth of delamination and penetration from the root of a channel crack. By comparing the results from the approaches of LEFM and CZM, the characteristic fracture resistance from small-scale bridging to large-scale bridging is identified. Finally, to determine the nonlinear traction-separation relation for cohesive zone modeling of a bimaterial interface, a hybrid approach is developed by combining experimental measurements and finite element simulations. In particular, both analytical and numerical models for wedge-loaded double cantilever beam specimens are developed. A two-step fitting procedure is proposed to determine the interface toughness and strength based on the measurements of the steady-state crack length and the local crack opening displacements. / text

Title: Theoretical studies of rolled-up and wrinkled nanomembranes Author: Mgr. Peter Cendula Department: Department of Condensed Matter Physics Thesis Supervisors: Prof. Dr. Oliver G. Schmidt, Prof. RNDr. Václav Holý, CSc. Abstract : The thesis is devoted to three similar topics from the field of rolled-up and wrinkled nanomembranes. We start by recalling classical theory of thin plates, which will be used to describe deformation of nanomembranes. In the first topic, relaxation of internal strain is studied when a flat film is partially released from the substrate by etching the sacrificial layer underneath. Energetic competition of the tube and wrinkle shape is quantitatively investigated. Similar model is used to investigate the limiting maximum value of tube rotations. In the second topic, roll-up of initially wrinkled film is shown to favor tubes forming on the flat edge of rectangular wrinkled pattern, enabling precise control of tube position. Experiment is provided to justify our theoretical predictions. In the third topic, quantum well is assumed inside a wrin- kled nanomembrane. Shift of transition energy induced by lateral modulation due to bending strain is quantified, being of interest for strain-sensitive optical detectors and emitters. In addition, lateral localization of electron and hole due to...