Finite element simulations are carried out to study transient stresses and strains in the pre-cracked (pre-notched) 4340 steel plates impacted by a projectile in the direction of the notch ligament. The computations employed Johnson - Cook model which takes into account strain hardening, strain-rate hardening and thermal softening. An approximate solution of the governing equations is sought by using an explicit finite element code DYNA2D. We analyzed the evolution of the shear and hoop stresses considered to be responsible for two modes of failure: opening crack inclined at 70°, and shear crack inclined at -5° to the notch ligament. At small impact speeds and large notch tip radii failure in the 70° direction is due to the high tensile hoop stress. At high impact speeds and small notch tip radii a failure develops predominantly in the (-5°) -- (-15°) direction, within a zone of the maximum shear stress and compressive hoop stress. / Master of Science

No / The formation of tribo-layers during machining is very common phenomena, especially when machining `free machining¿ steels. Several kinds of tribo-layers formed in metal cutting processes have been reported, layers of inclusions from the workpiece, oxide layers due to chemical reaction, plastic deformation layers, material transfer layers (MTLs) or built-up layers (BULs). A new tool¿chip contact model is proposed to explain the tribo-layer phenomena, which considers the nature of the shear strain rate distribution in the secondary deformation zone. A shear strain rate distribution in this zone having a shape similar to that found in the preliminary zone is proposed. A cutting interface (CI) is defined and this interface is at different location to the material boundary of tool and chip (MBTC). This difference is a key factor in the formation of the tribo-layer in the secondary deformation zone. This model can be used in improving tool wear prediction and the estimation of tool life.

Yes / Constitutive models that conform to separable KBKZ specification have been shown to fit steady-state strain hardening rheological data in planar and uniaxial elongational flows, but with inaccuracy in the rate of strain hardening. The single parameter Molecular Stress Function model of Wagner [Rheol. Acta, 39 (2000), 97-109] has been shown to accurately fit the rise-rate in experimental data for a number of strain hardening and strain softening materials. We study this models accuracy against the well characterised IUPAC LDPE data, and present a method for full implementation of this model for flow solution which is suitable for incorporating into existing separable KBKZ software. A new method for particle tracking in arbitrarily aligned meshes, which is efficient and robust, is given. The Quadratic Molecular Stress Function (QMSF) model is compared to existing separable KBKZ based models, including one which is capable of giving planar strain hardening; the QMSF is shown to fit experimental rheological and contraction flow data more convincingly. The issue of `negative correction pressures¿ notable in some Doi-Edwards based models is addressed. The cause is identified, and leads to a logical method of calculation which does not give these anomalous results.

No / This paper presents the details of a methodology for predicting the thermoelastic properties degradation in general symmetric laminates with uniform ply cracks in some or all of the 90° layers. First, a stress transfer method is derived by using the concept of state space equation. The laminate can be subjected to any combination of in-plane biaxial and shear loading, and the uniform thermal loading is also taken into account. The method takes into account all independent material constants and guarantees continuous fields of all interlaminar stresses across interfaces between material layers. By this method, a laminate may be composed of an arbitrary number of monoclinic layers and each layer may have different material property and thickness. Second, the concept of the effective thermoelastic properties of a cracked laminate is introduced. Based on the numerical solutions of specially designed loading cases, the effective thermoelastic constants of a cracked laminate can be obtained. Finally, the applications of the methodology are shown by numerical examples and compared with numerical results from other models and experiment data in the literature. It is found that the theory provides good predictions of the thermoelastic properties degradation in general symmetric laminates.

Yes / The present study investigated long-term drying shrinkage strains of self-compacting concrete (SCCs). For all SCCs mixes, Portland cement was replaced with 0–60% of fly ash (FA), fine and course aggregates were kept constant with 890 kg/m3 and 780 kg/m3, respectively. Two different water binder ratios of 0.44 and 0.33 were examined for both SCCs and normal concrete (NCs). Fresh properties of SCCs such as filling ability, passing ability, viscosity and resistance to segregation and hardened properties such as compressive and flexural strengths, water absorption and density of SCCs and NCs were also determined. Experimental results of drying shrinkage were compared to five existing models, namely the ACI 209R-92 model, BSEN-92 model, ACI 209R-92 (Huo) model, B3 model, and GL2000. To assess the quality of predictive models, the influence of various parameters (compressive strength, cement content, water content and relative humidity) effecting on the drying shrinkage strain as considered by the models are studied. The results showed that, using up to 60% of FA as cement replacement can produce SCC with a compressive strength as high as 30 MPa and low drying shrinkage strain. SCCs long-term drying shrinkage from 356 to 1000 days was higher than NCs. ACI 209R-92 model provided a better prediction of drying shrinkage compared with the other models. / Financial support of Higher Education of Libya (469/2009).

Yes / Objectives Preterm birth (PTB) and small for gestational age (SGA) are major causes of perinatal mortality and morbidity. Previous studies indicated a range of risk factors associated with these poor outcomes, including maternal psychosocial and economic wellbeing. This paper will explore a range of psycho-social and economic factors in an ethnically diverse population. Methods The UK’s Born in Bradford cohort study recruited pregnant women attending a routine antenatal appointment at 26–28 weeks’ gestation at the Bradford Royal Infirmary (2007–2010). This analysis includes 9680 women with singleton live births who completed the baseline questionnaire. Data regarding maternal socio-demographic and mental health were recorded. Outcome data were collected prospectively, and analysed using multivariate regression models. The primary outcomes measured were: PTB (<37 weeks’ gestation) and SGA (<10th customised centile). Results After adjustment for socio-demographic and medical factors, financial strain was associated with a 45 % increase in PTB (OR 1.45: 95 % CI 1.06–1.98). Contrary to expectation, maternal distress in Pakistani women was negatively associated with SGA (OR 0.65: CI 0.48–0.88). Obesity in White British women was protective for PTB (OR 0.67: CI 0.45–0.98). Previously recognized risk factors, such as smoking in pregnancy and hypertension, were confirmed. Conclusions This study confirms known risk factors for PTB and SGA, along with a new variable of interest, financial strain. It also reveals a difference in the risk factors between ethnicities. In order to develop appropriate targeted preventative strategies to improve perinatal outcome in disadvantaged groups, a greater understanding of ethno-specific risk factors is required

This master's thesis is focused on the structural response due to blast loading, where the geometry used was arbitrary but heavily inspired by Siemens Energy. The aim of the thesis is to gain a better understanding on how to model the blast load and how it affects the structure, as well as to study the modeling of bolts with both pre-tension and a damage criteria in an explicit analysis. Lastly, the importance of strain rate dependent material models was studied. Other aspects such as mass scaling and Rayleigh damping were also investigated. The software used to solve these tasks were Hypermesh, Abaqus and Python. To conclude, the conclusions drawn from this thesis was that bolts should be modeled using connector elements, and including pre-tension is more conservative than not using it for the case studied. However, for the material modeling it gives more conservative results when using a strain rate independent material model compared to the strain rate dependent model, and is advised to be used in the future.

In the last decades, ferroelectric properties have been observed in thin films based on hafnia and zirconia. These fluorite-structured thin films have attracted significant interest due to their excellent compatibility with complementary metal-oxide-semiconductor technology. Undoped zirconia has a lower crystallization temperature compared to hafnia-based or HfxZr1-xO2 thin films, making it more attractive for back-end-of-line processes. However, it is commonly reported that undoped zirconia thin films exhibit antiferroelectric properties. When ferroelectric properties are observed in undoped zirconia thin films, they are not as remarkable as those of hafnia-based or HfxZr1-xO2 thin films. In this work, in order to better understand the crucial factors that stabilize the ferroelectric properties in zirconia-based thin films, various process parameters were varied, and physical and electrical characterizations were carried out. The results indicate that the in-plane tensile strain is a crucial factor in stabilizing the ferroelectric orthorhombic phase of these films. The ferroelectric properties of zirconia films are lost when a non-polar monoclinic or tetragonal phase is stabilized due to an in-plane strain that is too small or too large, respectively. The best ferroelectric behavior is achieved when the polar orthorhombic phase can be stabilized with an in-plane tensile strain of 0.4-0.6%. After optimizing the interfaces between the ZrO2 and electrodes, the ferroelectric properties can be further improved in the ZrO2 thin films to be comparable to the HfO2-based ferroelectric thin films.:1 Motivation 2 Fundamentals 3 Experimental Setup 4 Effect of Process Parameters 5 Crucial Factors for Ferroelectric Zirconia 6 Conclusion

京都大学 / 新制・課程博士 / 博士(工学) / 甲第25303号 / 工博第5262号 / 新制||工||2001(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 陰山 洋, 教授 作花 哲夫, 教授 田中 勝久 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DGAM

This work presents a systematic study of the mechanochemical processes leading to chemical reactions occurring due to effects of high-strain-rate deformation associated with uniaxial strain and uniaxial stress impact loading in highly heterogeneous metal powder-based reactive materials, specifically compacted mixtures of Ti/Al/B powders. This system was selected because of the large exothermic heat of reaction in the Ti+2B reaction, which can support the subsequent Al-combustion reaction. The unique deformation state achievable by such high-pressure loading methods can drive chemical reactions, mediated by microstructure-dependent meso-scale phenomena. Design of the next generation of multifunctional energetic structural materials (MESMs) consisting of metal-metal mixtures requires an understanding of the mechanochemical processes leading to chemical reactions under dynamic loading to properly engineer the materials. The highly heterogeneous and hierarchical microstructures inherent in compacted powder mixtures further complicate understanding of the mechanochemical origins of shock-induced reaction events due to the disparate length and time scales involved. A two-pronged approach is taken where impact experiments in both the uniaxial stress (rod-on-anvil Taylor impact experiments) and uniaxial strain (instrumented parallel-plate gas-gun experiments) load configurations are performed in conjunction with highly-resolved microstructure-based simulations replicating the experimental setup. The simulations capture the bulk response of the powder to the loading, and provide a look at the meso-scale deformation features observed under conditions of uniaxial stress or strain. Experiments under uniaxial stress loading reveal an optimal stoichiometry for Ti+2B mixtures containing up to 50% Al by volume, based on a reduced impact velocity threshold required for impact-induced reaction initiation as evidenced by observation of light emission. Uniaxial strain experiments on the Ti+2B binary mixture show possible expanded states in the powder at pressures greater than 6 GPa, consistent with the Ballotechnic hypothesis for shock-induced chemical reactions. Rise-time dispersive signatures are consistently observed under uniaxial strain loading, indicating complex compaction phenomena, which are reproducible by the meso-scale simulations. The simulations show the prevalence of shear banding and particle agglomeration in the uniaxial stress case, providing a possible rationale for the lower observed reaction threshold. Bulk shock response is captured by the uniaxial strain meso-scale simulations and is compared with PVDF stress gauge and VISAR traces to validate the simulation scheme. The simulations also reveal the meso-mechanical origins of the wave dispersion experimentally recorded by PVDF stress gauges.