Most studies that have examined chloride-induced corrosion of steel in concrete have focused on sound concrete. However, reinforced concrete is seldom uncracked and very few studies have investigated the influence of cracked concrete on rebar corrosion. Furthermore, the studies that have examined the relationship between cracks and corrosion have focused on unloaded or statically loaded cracks. However, in practice, reinforced concrete structures (e.g. bridges) are often dynamically loaded. Hence, the cracks in such structures open and close which could influence the corrosion of the reinforcing steel. Consequently, the objectives of this project were (i) to examine the effect of different types of loading on the corrosion of reinforcing steel, (ii) the influence of concrete mixture design on the corrosion behaviour and (iii) to provide data that can be used in service-life modelling of cracked reinforced concretes. In this project, cracked reinforced concrete beams made with ordinary Portland cement concrete (OPCC) and high performance concrete (HPC) were subjected to no load, static loading and dynamic loading. They were immersed in salt solution to just above the crack level at their mid-point for two weeks out of every four (wet cycle) and, for the remaining two weeks, were left in ambient laboratory conditions to dry (dry cycle). The wet cycle led to three conditions of exposure for each beam: (i) the non-submerged region, (ii) the sound, submerged region and (ii) the cracked mid-section, which was also immersed in the solution. Linear polarization resistance and galvanostatic pulse techniques were used to monitor the corrosion in the three regions. Potentiodynamic polarization, electrochemical current noise and concrete electrical resistance measurements were also performed. These measurements illustrated that (i) rebar corroded faster at cracks than in sound concrete, (ii) HPC was more protective towards the rebar than OPCC even at cracks and (iii) there was a minor effect of the type of loading on rebar corrosion within the period of the project. These measurements also highlighted the problems associated with corrosion measurements, for example, identifying the actual corroding area and the influence of the length of rebar. The numbers of cracks and crack-widths in each beam were measured after the beam’s initial exposure to salt solution and, again, after the final corrosion measurements. HPC beams had more cracks than the OPCC. Also, final measurements illustrated increased crack-widths in dynamically loaded beams, regardless of the concrete type. The cracks in both statically and dynamically loaded OPCC and HPC beams bifurcated at the rebar level and propagated parallel to the rebar. This project also examined the extent of corrosion on the rebars and the distribution of corrosion products in the concrete and on the concrete walls of the cracks. Corrosion occurred only at cracks in the concrete and was spread over a larger area on the rebars in HPC than those in OPCC. The damage due to corrosion was superficial in HPC and crater-like in OPCC. Regardless of the concrete type, there was a larger distribution of corrosion products on the crack walls of the dynamically loaded beams. Corrosion products diffused into the cement paste and the paste-aggregate interface in OPCC but remained in the crack in HPC. The most voluminous corrosion product identified was ferric hydroxide. Elemental analysis of mill-scale on rebar which was not embedded in concrete or exposed to chlorides was compared to that of the bars that had been embedded in uncontaminated concrete and in cracked concrete exposed to chlorides. In uncontaminated concrete, mill-scale absorbed calcium and silicon. At a crack, a layer, composed of a mixture of cement paste and corrosion products, developed between the mill-scale and the substrate steel. Based on the results, it was concluded that (i) corrosion occurred on the rebar only at cracks in the concrete, (ii) corrosion was initiated at the cracks immediately upon exposure to salt solution, (ii) the type of loading had a minor influence on the corrosion rates of reinforcing steel and (iv) the use of polarized area led to a significant underestimation of the current density at the crack. It is recommended that the effect of cover-depth on (i) the time to initiation of corrosion and (ii) the corrosion current density in cracked concrete be investigated.

Most studies that have examined chloride-induced corrosion of steel in concrete have focused on sound concrete. However, reinforced concrete is seldom uncracked and very few studies have investigated the influence of cracked concrete on rebar corrosion. Furthermore, the studies that have examined the relationship between cracks and corrosion have focused on unloaded or statically loaded cracks. However, in practice, reinforced concrete structures (e.g. bridges) are often dynamically loaded. Hence, the cracks in such structures open and close which could influence the corrosion of the reinforcing steel. Consequently, the objectives of this project were (i) to examine the effect of different types of loading on the corrosion of reinforcing steel, (ii) the influence of concrete mixture design on the corrosion behaviour and (iii) to provide data that can be used in service-life modelling of cracked reinforced concretes. In this project, cracked reinforced concrete beams made with ordinary Portland cement concrete (OPCC) and high performance concrete (HPC) were subjected to no load, static loading and dynamic loading. They were immersed in salt solution to just above the crack level at their mid-point for two weeks out of every four (wet cycle) and, for the remaining two weeks, were left in ambient laboratory conditions to dry (dry cycle). The wet cycle led to three conditions of exposure for each beam: (i) the non-submerged region, (ii) the sound, submerged region and (ii) the cracked mid-section, which was also immersed in the solution. Linear polarization resistance and galvanostatic pulse techniques were used to monitor the corrosion in the three regions. Potentiodynamic polarization, electrochemical current noise and concrete electrical resistance measurements were also performed. These measurements illustrated that (i) rebar corroded faster at cracks than in sound concrete, (ii) HPC was more protective towards the rebar than OPCC even at cracks and (iii) there was a minor effect of the type of loading on rebar corrosion within the period of the project. These measurements also highlighted the problems associated with corrosion measurements, for example, identifying the actual corroding area and the influence of the length of rebar. The numbers of cracks and crack-widths in each beam were measured after the beam’s initial exposure to salt solution and, again, after the final corrosion measurements. HPC beams had more cracks than the OPCC. Also, final measurements illustrated increased crack-widths in dynamically loaded beams, regardless of the concrete type. The cracks in both statically and dynamically loaded OPCC and HPC beams bifurcated at the rebar level and propagated parallel to the rebar. This project also examined the extent of corrosion on the rebars and the distribution of corrosion products in the concrete and on the concrete walls of the cracks. Corrosion occurred only at cracks in the concrete and was spread over a larger area on the rebars in HPC than those in OPCC. The damage due to corrosion was superficial in HPC and crater-like in OPCC. Regardless of the concrete type, there was a larger distribution of corrosion products on the crack walls of the dynamically loaded beams. Corrosion products diffused into the cement paste and the paste-aggregate interface in OPCC but remained in the crack in HPC. The most voluminous corrosion product identified was ferric hydroxide. Elemental analysis of mill-scale on rebar which was not embedded in concrete or exposed to chlorides was compared to that of the bars that had been embedded in uncontaminated concrete and in cracked concrete exposed to chlorides. In uncontaminated concrete, mill-scale absorbed calcium and silicon. At a crack, a layer, composed of a mixture of cement paste and corrosion products, developed between the mill-scale and the substrate steel. Based on the results, it was concluded that (i) corrosion occurred on the rebar only at cracks in the concrete, (ii) corrosion was initiated at the cracks immediately upon exposure to salt solution, (ii) the type of loading had a minor influence on the corrosion rates of reinforcing steel and (iv) the use of polarized area led to a significant underestimation of the current density at the crack. It is recommended that the effect of cover-depth on (i) the time to initiation of corrosion and (ii) the corrosion current density in cracked concrete be investigated.

Inspection of solder joint interconnection has been a crucial process in the electronics manufacturing industry to reduce manufacturing cost, improve yield, and ensure product quality and reliability. New inspection techniques are urgently needed to fill in the gap between available inspection capabilities and industry requirement of low-cost, fast-speed, and highly reliable inspection systems. The laser ultrasound inspection system under development aims to provide a solution that can overcome some of the limitations of current inspection techniques. Specifically, the fully developed system will be an automated system that is capable of inspecting hidden solder joints with multiple defect types. This research work includes the following aspects: 1) Inspection system integration and automation to improve system throughput and capability, system performance characterization by stability study and gage repeatability and reproducibility study , 2) Development and implementation of signal processing methods, including time-domain correlation coefficient analysis, auto-comparison method, and frequency-domain spectral estimation, to allow for fast and accurate interpretation of vibration signals, 3) Development of a finite element modal model followed by experimental validation. The modal analysis results indicate there are unique mode frequencies and mode shapes associated with certain solder joint defects, and 4) Study of the systems unique capability in detecting solder joint fatigue cracks.

In the design of cryogenic fuel tanks for the next generation Reusable Launch Vehicles (RLVs), the permeability of liquid hydrogen (LH2) across the thickness of the tank is a critical issue. The rate of permeation of LH2 is largely dependent on the internal damage state of the composite tank wall. Damage in the form of matrix cracks in the composite material of the tank is responsible for the through-the-thickness permeation of LH2. In this context, the detection of matrix cracks takes on an unprecedented significance. In this work, an ultrasonic technique for the ply-by-ply detection of matrix cracks in laminated composites is developed. Experimental results are presented for graphite/epoxy laminates with different lay-ups and laminate thicknesses. Matrix cracks in each of the plies of the laminated composites were detected even when there was a rather high density of cracks in all of the plies. The ultrasonic data were calibrated by comparing them with the corresponding results obtained by using the traditional methods of optical microscopy and penetrant enhanced X-radiography. Excellent quantitative correlation was observed between the results obtained with ultrasonics and the traditional methods.

The goal of this research was to extend the investigation into a method to represent and analyze the stress field around two parallel edge cracks in a finite body. The Westergaard-Schwarz method combined with the local collocation method was used to analyze different cases of two parallel edge cracks in a finite body. Using this method a determination of when two parallel edge cracks could be analyzed as isolated single edge cracks was determined Numerical experimentation was conducted using ABAQUS. It was used to obtain the coordinate and stress information required in the local collocation method. The numerical models were created by maintaining one crack at a fixed length while varying the length of the second crack as well as the separation distance of the two cracks. The results obtained through the local collocation method were compared with the finite element obtained J-Integrals to verify the accuracy of the results. The results obtained in the analysis showed that the major factor in determining when the second crack’s stress field has to be considered was the crack separation distance. It was found that a reduction in the second crack’s length did not have a significant effect on overall stress intensity factors of the fixed crack. A larger change in the opening mode stress intensity factor can be seen by varying the crack separation distance. As well as seeing a steady reduction in shear mode stress intensity factors as the crack separation was increased. The results showed that after a certain crack separation distance the two cracks could be analyzed separately without introducing significant error into the stress field calculations.

This thesis describes a theoretical and experimental investigation of electromagnetic methods for the detection and measurement of metal fatigue cracks. The available methods are reviewed, with particular attention being paid to mathematical models, and a new model of the electromagnetic field near a metal fatigue crack for small skin-depths is presented which uses a surface impedance boundary condition with the addition of a line source to represent the crack. This leads to a coupled system of two magnetic scalar potentials, one on the crack face which obeys the two-dimensional Laplace equation and one outside the test-piece which obeys the three-dimensional Laplace equation. The behaviour of the field is governed by a parameter m =l/(μ, δ), where l is the size of the field perturbation, μ, is the relative permeability and δ is the skin-depth. When m is small, almost all the flux is concentrated inside the metal and the exterior potential also obeys the two-dimensional Laplace equation, on the test-piece surface. When m is large, the perturbation part of the exterior field has a negligible effect on the field inside the crack so that the crack-face potential may be found by the Born approximation. The general m problem is solved for rectangular and semi-elliptical cracks in flat plates, interrogated by uniform fields, and the solution is verified experimentally. A method for calculating the crack depth from the magnetic field is given, with descriptions of industrial applications. The theory is further developed to find the impedance change in an air-cored circular coil caused by a crack, to find the field near overlapping cracks and to find the field near a crack in an interior corner. Finally, a semi-empirical analysis is presented for a ferrite-cored measuring coil.

Fatigue of metals is a problem that affects almost all sectors of industry, from energy to transportation, and failures to account for fatigue or incorrect estimations of service life have cost many lives. To mitigate such fatigue failures, engineers must be able to reliably predict the fatigue life of components under service conditions. Great progress has been made in this regard in the past 40 years; however one aspect of fatigue that is still being actively researched is the behavior of microstructurally small cracks (MSCs), which can diverge significantly from that of long cracks. The portion of life spent nucleating and growing a MSC over the first few grains/phases can consume over 90% of the total fatigue life under High Cycle Fatigue (HCF) conditions and is the primary source of the scatter in fatigue lives. Therefore, the development of robust fatigue design methodologies requires that the MSC regime of crack growth can be adequately modeled. The growth of microstructurally small cracks is dominated by influence of the local heterogeneity of the microstructure and is a highly complex process. In order to successfully model the growth of these microstructurally small cracks (MSCs), two computational frameworks are necessary. First, the local behavior of the material must be modeled, necessitating a constitutive relation with resolution on the scale of grain size. Second, a physically based model for the nucleation and growth of microstructurally small fatigue cracks is needed. The overall objective of this thesis is best summarized as the introduction these two computational frameworks, a crystal plasticity constitutive model and fatigue model, specifically for aluminum alloy 7075-T6, a high-strength, low density, precipitation hardened alloy used extensively in aerospace applications. Results are presented from simulations conducted to study the predicted crack growth under a variety of loading conditions and applied strain ratios, including uniaxial tension-compression and simple shear at a range of applied strain amplitudes. Results from the model are compared to experimental results obtained by other researchers under similar loading conditions. A modified fatigue crack growth algorithm that captures the early transition to Stage II growth in this alloy will also be presented.

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Submitted by Claudinei Pracidelli (cpracide@ipen.br) on 2014-11-10T11:55:57Z No. of bitstreams: 0 / Made available in DSpace on 2014-11-10T11:55:57Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado em Tecnologia Nuclear) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP