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Military personnel are required to perform effectively in extreme enviroments. Optimal performance in combat enviroments is a complex process and its neural basis is poorly understood. Understanding the factors that influence how an individual is able to perform to a high standard and cope with the demands of the situation while under extreme operational stress is vital. As stressful events can have a lasting impact on soldiers and while for some deployment can lead to positive change for others it can increase the risk of suffering from post-traumatic stress disorder (PTSD). To better understand how soldiers are able to perform effectively, in the first study of the thesis we developed a psychometrically robust measure of mental robustness that was informant rated and relevant to combat operations. The measure assesses a soldier’s ability to make decisions under pressure and their ability to function effectively when faced with emotional challenging situations as two separate dimensions. A second study confirmed the factor structure of the measure and also provided initial evidence for its construct validity. The measure underpinned our final study (Study 3) which combined psychometric measures, behavioural and functional imaging to produce a deeper understanding of the relationship between activity in key brain regions and key components of robustness. Study 3 assessed soldier’s ability to make decisions under pressure when presented with combat relevant stimulus. The study employed two tasks; Task 1 required individuals to attend to emotional aspects of the stimuli as they would do in during combat and Task 2 required soldiers to attend to the non-emotional aspects of the stimuli. Our findings suggest that robustness acts as a resistance resource and although it does not protect against PTSS it does allow a curvilinear relationship between PTSS and performance. The ultimate goal of this thesis is to better understand the critical factors required for optimal military performance during deployment. This will allow more targeted training that will help highly motivated individuals achieve excellence.

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In this research, a systematic and multi-disciplinary study was conducted to understand the fundamental science of moisture-induced degradation of interfacial adhesion. The research is comprised of both experimental and modeling components of analysis and consists of four primary components. First, the moisture transport behavior within underfill adhesives is experimentally characterized and incorporated into a finite element model to depict the moisture ingress and interfacial moisture concentration for each respective level of moisture preconditioning. Second, the effect of moisture on the variation of the underfill elastic modulus is demonstrated and the physical mechanisms for the change identified. Third, the aggregate effect of moisture on the interfacial fracture toughness of underfill to both copper and FR-4 board substrates is determined. This includes the primary effect of moisture being physically present at the interface and the secondary effect of moisture changing the elastic modulus of the adhesive when absorbed. Last, the recovery of both the elastic modulus and interfacial fracture toughness from moisture preconditioning is assessed with reversible and irreversible components identified. Using adsorption theory in conjunction with fracture mechanics, an analytical model is developed that predicts the loss in interfacial fracture toughness as a function of moisture content. The model incorporates key parameters relevant to the problem of moisture in epoxy joints identified from the experimental portion of this research, including the interfacial hydrophobicity, epoxy nanopore density, saturation concentration, and density of water. This research results in a comprehensive understanding of the primary mechanisms responsible for the interfacial degradation due to the presence of moisture. The experimental results obtained through this research provide definitive data for the electronics industry to use in their product design, failure analysis, and reliability modeling. The predictive model developed in this research provides a useful tool for developing new adhesives, innovative surface treatment methods, and effective protection methodologies for enhancing interfacial adhesion.

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The effects of variations in alloying content on the microstructure and mechanical properties of high strength steel weld metals have been studied. Based on neural network modelling, weld metals were produced using shielded metal arc welding with nickel at 7 or 9 wt. %, manganese at 2 or 0.5 wt. % while carbon was varied between 0.03 and 0.11 wt. %. From mechanical testing, it was confirmed that a large gain in impact toughness could be achieved by reducing the manganese content. Carbon additions were found to increase strength with only a minor loss to impact toughness as predicted by the modelling. The highest yield strength (912 MPa) in combination with good impact toughness (over 60 J at -100 °C) was achieved with an alloying content of 7 wt. % nickel, 0.5 wt. % manganese and 0.11 wt. % carbon. Based on thermodynamic calculations and observed segregation behaviour it was concluded that the weld metals solidify as austenite. The microstructure was characterised using optical, transmission electron and high resolution scanning electron microscopy. At interdendritic regions mainly martensite was found. In dendrite core regions of the low carbon weld metals a mixture of upper bainite, lower bainite and a novel constituent - coalesced bainite - formed. Coalesced bainite was characterised by large bainitic ferrite grains with cementite precipitates and is believed to form when the bainite and martensite start temperatures are close to each other. Carbon additions were found to promote a more martensitic microstructure throughout the dendrites. Mechanical properties could be rationalised in terms of microstructural constituents and a constitutional diagram was constructed summarising microstructure as a function of manganese and nickel contents.

The analysis of fracture toughness test data from standard specimens is often based upon the assumptions of planar crack fronts and homogenous material properties. However, these assumptions do not hold true for all test geometries or real components. The overall objective of this EngD was therefore to develop the methodologies used in fracture assessment of steel components, by incorporating a reduction in the conservatisms inherent in the assessment procedures. These conservatisms are associated with applying a ‘lower bound’ treatment to steel components, which in reality contain significant variability in effective fracture toughness, due to either material considerations (macroscopic or microstructural), or geometrical considerations including the effect of crack tip constraint.The first method developed allows a comparison of a variation of fracture toughness values throughout a component, to a variation of the localised effective crack driving force. The main feature of this method takes advantage of the nature of the ductile-to-brittle transition regime of fracture toughness, where there is significant scatter. This leads to a probabilistic prediction of the location of fracture initiation, and a less conservative estimate of failure load, used to derive enhanced fracture toughness for the component. The second method calculates less conservative fracture toughness values for steels where there is significant heterogeneity in the dataset. The effects of measurement uncertainty on derived fracture toughness values can be monitored to improve probabilistic estimates of the heterogeneous fracture toughness values. These methods have been developed into predictive software tools, validated against data from the literature.Finite element analysis of various configurations of compact tension and bend specimen, under different constraint conditions, was used to identify fracture mechanics parameters and constraint factors that will be of use in deriving accurate fracture toughness relationships from future testing programmes. The viability of low constraint specimens for accurately characterising increases in fracture toughness has been assessed. These recommendations enhance the relationships and advice suggested in the testing standards and literature. Loss of constraint in thin components can be quantified by a triaxiality parameter, which can be used to predict an increase in fracture toughness through use of a damage model, in this case developed based on a ductility exhaustion approach. This model can be used to predict initiation of ductile fracture in configurations with low constraint, leading to less conservative fracture toughness values, enhancing the guidance in the various defect tolerance assessment procedures.

In nuclear environments, irradiation hardening and damage have a detrimental effect on materials performance. Among others, fracture toughness of austenitic stainless steels decreases under neutron irradiation. Helium arising from transmutation reactions is one source of embrittlement leading to that decrement and it is here assumed as a case study, austenitic steel 316L being the material under investigation. The experimental reproduction of irradiation hardening effect on yield stress is attempted here by pre-strain under tensile loading at room temperature. The experimental production of porosity is attempted by inducing ductile damage, creep damage or a combination of them. Damage at the microstructural level is analyzed by metallography, fractography, X-ray tomography and quantified by image processing.After calibrating the elastic, the plastic and the porous plastic constitutive equations by the means of tensile tests on smooth and notched specimens, results from damaging experiments are validated by finite element analysis using the Gurson-Tvergaard-Needleman model. The numerical models obtained represent different levels of damage into the material, as induced by the experiments.Material presenting different levels of damage is then machined for fracture toughness evaluation in the shape of sharp-notched round bars. Fracture toughness initiation is inferred from the load vs. displacement plots applying an opportune fracture criterion. In order to test the suitability of the Gurson-Tvergaard-Needleman model, the load vs. displacement results are validated by retrofitting opportune constitutive laws for each “damaged” state. Retrofitting is discussed in relation to the type of damage produced.Results show that the reproduction of the macroscopic effect of irradiation hardening on yield stress may be attempted for 316L by a pre-strain tensile loading at room temperature for levels up to 1.5 dpa or slightly more. These interrupted tensile tests did not give evidence of void volume fraction production. Creep tests at 650 °C showed sensitization at the grain boundaries but not porosity into the matrix. Creep tests at 1000 °C created 1.2% to 1.8% void volume fraction from grain boundary sliding. Finally, one 7% pre-strained specimen was subjected to creep test at 900 °C and stopped at 5% creep strain, without evidence of porosity into the matrix.Fracture toughness tests on the “damaged” states obtained before showed a decrement of fracture toughness initiation when compared with “undamaged” 316L. Specimens with 30% and 40% eng. strain presented a sensible decrement and exhibited a brittle-like behaviour. The differences in porosity size and physical processes involved suggest not stating that a correlation exists with the helium embrittlement effect on the same property. The Gurson-Tvergaard-Needleman model worked for the “undamaged” material. It proved to be not suited for the brittle-like 30% and 40% eng. strain “damaged” materials because it did not capture the experimental progression of damage.In the end, fracture toughness numerical predictions were made using different values of initial void volume fraction. It was argued that, starting from a threshold value, the brittle-like 30% and 40% eng. strain “damaged” materials revert to a ductile behaviour.