The continual deterioration of infrastructure has motivated researchers to look for new ways of repairing and monitoring existing structures. A particularly challenging problem confronting engineers in the revival of the infrastructure is the rehabilitation of reinforced concrete (RC) structures. Traditionally, the repair of RC beams has been achieved by bonding steel plates to the structure. Although this technique has proven to be reasonably effective, it has several distinct disadvantages such as susceptibility of the steel plates to corrode and the excessive weight of steel plates when used in long-span beams. Recently, there has been an emergence of structural engineering applications employing fibre reinforced polymer (FRP) composites as an alternative to steel plates. FRP composites are well known for their high strength- and stiffness-to-weight ratios, corrosion resistance, durability, and ease of application. Numerous studies have been conducted to prove the efficiency of bonding FRP on structural elements. In spite of this, industrial practitioners are still concerned about premature debonding of the plates before reaching the desired strength or ductility. Premature debonding initiates from the ends of the plate or from intermediate cracks (IC) in the concrete. While end initiated debonding and peeling mechanisms have been researched extensively, researchers have unanimously recognized the lack of data for the FRP-RC structural members subjected to IC debonding. The scarcity of data compiled exemplifies the need to develop more refined numerical analysis tools to reduce the high cost and significant time required to conduct full-scale physical testing. In this study, the results of a comprehensive numerical investigation are presented to assess the failure mechanisms caused by different types of flexural and shear crack distributions in RC beams strengthened with FRP composites. The model is based on damage mechanics modeling of concrete and a bilinear bond-slip relationship with softening behaviour to represent the FRP-concrete interfacial properties. A discrete crack approach was adopted to simulate crack propagation through a nonlinear fracture mechanics based finite element analysis to investigate the effects of crack spacing and interfacial parameters such as stiffness, local bond strength, and fracture energy on the initiation and propagation of the debonding and structural performance. Results from the analysis reveal that the debonding behaviour and load-carrying capacity are significantly influenced by interfacial fracture energy and crack spacing. The debonding propagation is mainly governed by mode II fracture mechanisms. The results provide an insight on the long-term behaviour of a repair system that is gaining widespread use and will be of interest to researchers and design engineers looking to successfully apply FRP products in civil engineering applications.

This thesis is investigating how the effective notch method can be used for fatigue assessment of welded joints.  The effective notch method is based on a finite element analysis where the joint is modeled with all notches fictitiously rounded with a radius of 1 mm. Analyses are performed on a cruciform fillet welded joint where parameters such as, load case, steel plate thickness and weld size, are varied. The achieved lifetime estimations are then compared to calculations with other fatigue assessment methods, linear elastic fracture mechanics and the nominal method. The goal is to draw conclusions about pros and cons of the effective notch method. The results are also compared to experimental fatigue tests performed on the same geometry. The results indicates that the effective notch method tends overestimating the lifetime, especially when the steel plate thickness is small. This leads to a non conservative method that is dangerous to use as guidance when designing. The estimations are though better when considering a toe crack then when considering a root crack. Due to a large scatter in experimental test results, it is hard to validate a fatigue assessment method in an absolute sense. That is also the case for the effective notch method, and more results from experimental fatigue tests are needed before the effective notch method can be fully used. For relative analysis, when variations of the same design needs to be compared, the effective notch can be a very powerful tool. This is because of the flexibility for different geometries that this method grants.

In this master thesis, work done in the TURBO POWER project High temperature fatigue crack propagation in nickel-based superalloys during spring 2012 will be presented. The overall objective of this project is to develop and evaluate tools for designing against fatigue in gas turbine applications, with special focus on the crack propagation in the nickel-based superalloy Inconel 718. Experiments have been performed to study the effect of initial overloads, and it has been shown that even for small initial overloads a significant reduction of the crack growth rate is received. Furthermore, FE simulations have been carried out in order to describe the local stress state in front of the crack tip since it is believed to control, at least partly the diffusion of oxygen into the crack tip and thus also the hold time crack growth behaviour of the material. Finally, an evaluation method for the stresses is presented, where the results are averaged over an identifiable process/damaged zone in front of the crack tip.

The continual deterioration of infrastructure has motivated researchers to look for new ways of repairing and monitoring existing structures. A particularly challenging problem confronting engineers in the revival of the infrastructure is the rehabilitation of reinforced concrete (RC) structures. Traditionally, the repair of RC beams has been achieved by bonding steel plates to the structure. Although this technique has proven to be reasonably effective, it has several distinct disadvantages such as susceptibility of the steel plates to corrode and the excessive weight of steel plates when used in long-span beams. Recently, there has been an emergence of structural engineering applications employing fibre reinforced polymer (FRP) composites as an alternative to steel plates. FRP composites are well known for their high strength- and stiffness-to-weight ratios, corrosion resistance, durability, and ease of application. Numerous studies have been conducted to prove the efficiency of bonding FRP on structural elements. In spite of this, industrial practitioners are still concerned about premature debonding of the plates before reaching the desired strength or ductility. Premature debonding initiates from the ends of the plate or from intermediate cracks (IC) in the concrete. While end initiated debonding and peeling mechanisms have been researched extensively, researchers have unanimously recognized the lack of data for the FRP-RC structural members subjected to IC debonding. The scarcity of data compiled exemplifies the need to develop more refined numerical analysis tools to reduce the high cost and significant time required to conduct full-scale physical testing. In this study, the results of a comprehensive numerical investigation are presented to assess the failure mechanisms caused by different types of flexural and shear crack distributions in RC beams strengthened with FRP composites. The model is based on damage mechanics modeling of concrete and a bilinear bond-slip relationship with softening behaviour to represent the FRP-concrete interfacial properties. A discrete crack approach was adopted to simulate crack propagation through a nonlinear fracture mechanics based finite element analysis to investigate the effects of crack spacing and interfacial parameters such as stiffness, local bond strength, and fracture energy on the initiation and propagation of the debonding and structural performance. Results from the analysis reveal that the debonding behaviour and load-carrying capacity are significantly influenced by interfacial fracture energy and crack spacing. The debonding propagation is mainly governed by mode II fracture mechanisms. The results provide an insight on the long-term behaviour of a repair system that is gaining widespread use and will be of interest to researchers and design engineers looking to successfully apply FRP products in civil engineering applications.

Steel fibers or fibers in general are utilized in concrete to control the tensile cracking and to increase its toughness. In literature, the effects of fiber geometry, mechanical properties, and volume on the properties of fiber reinforced concrete have often been experimentally investigated by numerous studies. Those experiments have shown that useful improvements in the mechanical behavior of brittle concrete are achieved by incorporating steel fibers. This study proposes a simulation platform to determine the influence of fibers on crack propagation and fracture behavior of fiber reinforced concrete. For this purpose, a finite element (FE) simulation tool is developed for the fracture process of fiber reinforced concrete beam specimens subjected to flexural bending test. Within this context, the objective of this study is twofold. The first one is to investigate the effects of finite element mesh size and element type on stress intensity factor (SIF) calculation through finite element analysis. The second objective is to develop a simulation of the fracture process of fiber reinforced concrete beam specimens. The properties of the materials, obtained from literature, and the numerical simulation procedure, will be explained. The effect of fibers on SIF is included by unidirectional elements with nonlinear generalized force-deflection capability. Distributions and orientation of fibers and possibility of anchorage failure are also added to simulation. As a result of this study it was observed that with the adopted simulation tool, the load-deflection relation obtained by experimental studies is predicted reasonably.

A phenomenological cohesive term is proposed and added to an existing cohesive constitutive law (by Roy and Dodds) to model the crack tip high inertia region proposed by Gao. The new term is attributed to fracture mechanisms that result in high energy dissipation around the crack tip and is assumed to be a function of external energy per volume input into the system. Finite element analysis is performed on PMMA with constant velocity boundary conditions and mesh discretization based on the work of Xu and Needleman. The cohesive model with the proposed dissipative term is only applied in the high inertia zone i.e., to cohesive elements very close to the crack tip and the traditional Roy and Dodds model is applied on cohesive elements in the rest of the domain. It was observed that crack propagated in three phases with a speed of 0.35cR before branching, which are in good agreement with experimental observations. Thus, modeling of high inertia zone is one of the key aspects to understanding brittle fracture.

Solar cell business has been very critical and challenging since more efficient and low costs materials are required to decrease the costs and to increase the production yield for the amount of electrical energy converted from the Sun's energy. The silicon-based solar cell has proven to be the most efficient and cost-effective photovoltaic industrial device. However, the production cost of the solar cell increases due to the presence of cracks (internal as well as external) in the silicon wafer. The cracks of the wafer are monitored while fabricating the solar cell but the present monitoring techniques are not sufficient when trying to improve the manufacturing process of the solar cells. Attempts are made to understand the location of the cracks in single crystal and polycrystalline silicon solar cells, and analyze the impact of such cracks in the performance of the cell through Scanning Acoustic Microscopy (SAM) and Photoluminescence (PL) based techniques. The features of the solar cell based on single crystal and polycrystalline silicon through PL and SAM were investigated with focused ion beam (FIB) cross section and scanning electron microscopy (SEM). The results revealed that SAM could be a reliable method for visualization and understanding of cracks in the solar cells. The efficiency of a solar cell was calculated using the current (I) - voltage (V) characteristics before and after cracking of the cell. The efficiency reduction ranging from 3.69% to 14.73% for single crystal, and polycrystalline samples highlighted the importance of the use of crack monitoring techniques as well as imaging techniques. The aims of the research are to improve the manufacturing process of solar cells by locating and understanding the crack in single crystal and polycrystalline silicon based devices.

No description available.

No description available.

No description available.