The drying sequence in the manufacturing process for paperboard involves evaporation of water, primarily from within the fibres. The vapour is then transported out of the web by pressure or concentration gradients. As the moisture transport from the paper web to the ambient is quicker than the moisture transport within the fibre network to the surfaces of the web, moisture gradients develop through the thickness of the web. This work concerns effects on the mechanics of paper drying from the variation in moisture through the relatively thin structures of paper and paperboard. Distributions of inplane residual stresses through paper materials in the unloaded state after drying are believed to be caused by the varying moisture through the thickness during drying. The distributions in general exhibit compressive stress near the board surfaces and tensile stress in the interior of the board. This may be modified after drying and is also affected by structural variation in the material between different plies of multi-ply paperboards. The stress development during drying is important because it influences the resulting material properties of the paper and because it can lead to curl, which is a quality problem. The residual stresses themselves are an error source in simulation or evaluation of the mechanical behaviour of paper. In this work, residual stress distributions in paperboard were determined experimentally, to clarify the mechanisms of residual stress build-up. An experimental method for such tests was also developed. Based on the experimental findings, the mechanics of paper drying was modelled and the stress build-up simulated. Simulation offers a way of studying how the properties of paper develop during drying of wet paper webs.

In this thesis, the residual stress distribution of metal film sputtered on silicon substrate are studied. The commercial Marc finite element method package is used in this work. The thermal-mechanical model is employed in the residual and thermal stress analysis of thin film during the sputtering process. Two models finite element are used in this study. One is the 2D axial-symmetric model and the other is the 3D. The 2D axial-symmetric model was employed to investigation the residual stress distribution in 4¡¨, 6¡¦¡¦, and 8¡¦¡¦ wafer during the UBM sputtering process. The 3D model was used to study the effects of sputtering parameters, i.e. sputtering temperature and film thickness, on the residual stress distribution. The effect of etching process on the sputtered film has also been studied by using the 3D model. Results indicate the proposed model can simulate the residual stress distribution successfully.

<p>The drying sequence in the manufacturing process for paperboard involves evaporation of water, primarily from within the fibres. The vapour is then transported out of the web by pressure or concentration gradients. As the moisture transport from the paper web to the ambient is quicker than the moisture transport within the fibre network to the surfaces of the web, moisture gradients develop through the thickness of the web. This work concerns effects on the mechanics of paper drying from the variation in moisture through the relatively thin structures of paper and paperboard.</p><p>Distributions of inplane residual stresses through paper materials in the unloaded state after drying are believed to be caused by the varying moisture through the thickness during drying. The distributions in general exhibit compressive stress near the board surfaces and tensile stress in the interior of the board. This may be modified after drying and is also affected by structural variation in the material between different plies of multi-ply paperboards.</p><p>The stress development during drying is important because it influences the resulting material properties of the paper and because it can lead to curl, which is a quality problem. The residual stresses themselves are an error source in simulation or evaluation of the mechanical behaviour of paper.</p><p>In this work, residual stress distributions in paperboard were determined experimentally, to clarify the mechanisms of residual stress build-up. An experimental method for such tests was also developed. Based on the experimental findings, the mechanics of paper drying was modelled and the stress build-up simulated. Simulation offers a way of studying how the properties of paper develop during drying of wet paper webs.</p>

9th German-Japanese Bridge Symposium, September 10-11, 2012, Kyoto, JAPAN (GJBS09)

Thick-section austenitic stainless steels have widespread industrial applications, especially in nuclear power plants. The joining methods used in the nuclear industry are primarily based on arc welding processes. However, it has recently been shown that the Narrow Gap Laser Welding (NGLW) technique can be used to join materials with thicknesses that are well beyond the capabilities of single pass autogenous laser welding. The heat input for NGLW is much lower than that of arc welding, as are the expected levels of residual stress and distortion. The multi-pass laser welding technique, based on the narrow gap approach, is an emerging welding technology which can be applied to thick-section welds using a relatively low-power laser, but the process is more complicated than autogenous laser welding, since it is necessary to introduce filler wire to narrow gap weld configurations. Despite this complexity, the technique is very promising for improving the penetration capabilities of the laser welding process. However a limited amount of research has been conducted on the development of the NGLW technique; the control and optimization of weld bead quality inside the narrow gap is still an area of weakness. The research described in this thesis involves investigations on NGLW of AISI grade 316L austenitic stainless steel, and the performance of the resulting welds. Design-of-experiments and statistical modelling techniques were employed to understand and optimize the welding process. A statistical model was used in order to understand the significant process parameters and their interactions, allowing improved control of the weld quality in ultra-narrow gap (1.5 mm gap width) welds. The results show a significant improvement in weld quality can be achieved through the use of statistical modelling and multi-variable optimisation. The microstructure characteristics and mechanical properties (e.g. tensile strengths, fatigue, bending strength and fracture toughness) of the NGLW samples were examined and compared with those of other welding techniques - autogenous laser welding and gas-tungsten arc welding (GTAW). The work shows that NGLW of 316L steel sheets up to 20 mm thickness have generally better or comparable mechanical properties than those of GTAW but with much higher welding productivity. The results of detailed investigations of the 2D residual stress distributions, material distortions, and plastic strain characteristics of the NGLW technique are described. The contour method was employed for residual stress evaluation of the NGLW technique, and the results were validated using X-Ray and neutron diffraction measurements. The results were compared with those obtained with GTAW. The results suggest that the longitudinal tensile residual stresses in NGLW joints are 30-40% lower than those for GTAW joints. The influence of the laser power and number of passes for the NGLW technique, on the developed residual stress and plastic strain has been investigated, and the influence of welding strategy and the use of restraint during welding were also investigated. To understand the thermal history in NGLW and its effect on residual stress, finite element analysis was carried out using ABAQUS to numerically model the behaviour of residual stress across the multipass NGLW weld joints. The model has been validated with the experiments using temperature measurements and in terms of residual stresses the model is compared with neutron diffraction and the contour method. There is a very good correlation between the model and experimental results. The influence of both the laser power and welding speed on the induced residual stress during the NGLW process was also investigated using the model. The aqueous, pitting and stress corrosion cracking behaviour of the NGLW joints were investigated, and the results compared to those for GTAW joints under the same conditions. The results show that NGLW joints have better resistance to pitting corrosion than the GTA welds. Preliminary results also suggest that NGLW has better resistance to stress corrosion cracking.

Welding is an essential process in many industries for both the production and repair of engineering plant, notably pressure vessels and piping. However, welding processes cause large magnitudes of residual stress to be induced within the structure. Residual stress can be defined as a stress that exists in a material when it is under no primary loading. Whilst residual stresses can be reduced by post weld heat treatment, such treatments are not always possible, and so high residual stresses can remain in serviceThe current methodology for evaluating fracture toughness from specimens, particularly if these contain weld residual stresses is presented in BS7448-1997. This method relies on the assumption that the effect of residual stress on fracture toughness measurements can be negated by the application of a local compression, to the ligament ahead of the pre-crack in the test specimen. Recent research has investigated the validity of this assumption. The results suggest that, far from being removed, the residual stresses are modified or even enhanced via local compression. This can reduce the value of measured fracture toughness below its true value. In order to ensure the validity of fracture toughness measurements in materials that contain residual stress, a more robust method for its quantification is developed.The aim of this project was to extend current understanding regarding the magnitude and distribution of residual stresses retained in standard fracture mechanics specimens removed from welds and the consequent effects of these stresses on measured fracture toughness, both in terms of the crack driving force and crack-tip constraint. Furthermore, the project aimed to derive improved methods for the quantification of valid values of fracture toughness from laboratory specimens containing residual stresses. This was achieved via a combination of analytical and experimental work.The effect of specimen extraction on the level of retained residual stress in specimens extracted from non stress-relieved welds was investigated using parametric finite element analyses. Simplified methods to quantify the levels of residual stresses in fracture mechanics specimens removed from welds and their significance, in terms of contribution to crack driving force, are proposed.The influence of residual stresses on the measured fracture toughness properties of ferritic pressure vessel steel, tested in the cleavage fracture regime, has also been studied. A refined method of out-of-plane compression was devised and used to generate significant residual stresses in three-point bend specimens. This method was then used experimentally, alongside supporting elastic-plastic analyses, to quantify the effects of the residual stresses on fracture toughness in terms of both crack driving force and crack-tip constraint in geometrically high and low constraint specimens. A method whereby fracture toughness data, obtained from specimens containing residual stresses, can be corrected to provide valid fracture toughness properties using constraint based fracture mechanics alongside a simple fracture model has been proposed. The main conclusions from the work are as follows. Significant weld residual stresses have been shown to be retained in certain laboratory specimens post extraction from non stress-relieved welds. The magnitude and distribution of retained residual stress has been shown to be dependant on: • Material yield and flow properties • Specimen size; where larger specimens are more likely to retain significant levels of residual stress than smaller specimens • Specimen type; either compact tension (CT) or single edge notched bend (SENB), where there is a tendency for specimens to retain higher relative levels of residual stress in the directions of their largest dimensions; i.e. bend specimens retain more residual stress along their length than CT specimens and CT specimens retain more residual stress across their width than bend specimens • Extraction location, e.g. full thickness, near surface, mid-thickness etc. The stress partitioning method has been shown to provide a useful estimating approach for assessing the levels of residual stress retained in fracture mechanics specimens extracted from non stress-relieved welds in certain orientations.Retained residual stresses have been shown to affect both crack driving force and crack-tip constraint in both low and high geometrically constrained 50mm bend specimens manufactured from A533B ferritic steel. The residual stress has been shown to dominate the level of crack-tip constraint condition over and above the geometric and loading factors during the early stages of loading. The effects of residual stress on crack driving force and crack-tip constraint have been shown to result in fracture loads and, therefore, measured fracture toughness values that vary widely from those to be expected in the material under small-scale yielding conditions; i.e. if a standard specimen were to be tested containing no residual stress. Two-parameter (J-Q) fracture mechanics has been shown to provide a valid approach for quantifying fracture toughness properties from high and low constraint specimens, with and without residual stresses, with all data being shown to be consistent with a J-Q failure locus for a given level of cleavage probability.

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Laser Engineered Net Shaping (LENS™) is a rapid-manufacturing procedure that involves complex thermal, mechanical, and metallurgical interactions. The finite element method (FEM) may be used to accurately model this process, allowing for optimized selection of input parameters, and, hence, the fabrication of components with improved thermo-mechanical properties. In this study the commercial FEM code SYSWELD® is used to predict the thermal histories and residual stresses generated in LENS™-produced thin plates of AISI 410 stainless steel built by varying the process parameters laser power and stage translation speed. The computational results are compared with experimental measurements for validation, and a parametric study is performed to determine how the thermo-mechanical properties vary with these parameters. Thermal calculations are also performed with the code ABAQUS® to evaluate its potential use as a modeling tool for the LENS™ process.

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