Optimisation of hybrid composite shields for hypervelocity Micro-Meteoroid and Orbital Debris (MMOD) impact

Advancements in space exploration over the last few decades have led to a sharp increase in Micro-Meteoroid and Orbital Debris (MMOD), with the associated increased risk of hypervelocity impact on satellites and space structures. The Whipple shield can mitigate the effects of such impacts and current research is exploring further developments towards effective lightweight passive shielding technology to counter the damaging effects of MMOD. With the increase in MMOD and the prospect of increased space travel in the coming years and decades, it is vital that more research is conducted and improvements are made in the development and design of efficient, lightweight shielding technology to protect both unmanned and manned spacecraft against hypervelocity impacts (HVI). The optimisation of shield design for HVI is a high dimensionality problem, suited to advanced computational approaches. Key variables in HVI shield design that should inform the focus on optimisation include: impact velocity, rear wall thickness, projectile diameter and bumper thickness. A hybrid shield configuration and numerical model are proposed and validated, with alternating layers of aluminium (AL2024) and carbon fibre composites (CFRP, T300 woven-fabric) to form a 5 mm thick target plate consisting of 5 plies. The adaptive coupled FEM-SPH method is used to model the target plates. The proposed hypervelocity shield design optimisation methodology is based on Direct Simulation-based Genetic Algorithm (DSGA) optimisation and is implemented to optimise a multi-variable shield design space. Objective weightings are used to analyse and discuss results, referring to the ratio of the kinetic energy to the shield areal density objectives. A clear transition in the impact behaviour of the optimised MMOD shields is observed in the transition region from high-velocity to hypervelocity impact, where significant levels of kinetic energy dissipation are observed below the transition region, and lower energy dissipation at hypervelocity. The shield design optimisation results show that with a weighted kinetic energy to mass objective of 90:10, the kinetic energy of the back shield plate decreases significantly (62.7%) and the areal density can be reduced by more than 18%. Alternative configurations displayed sub-optimal results based on a trade-off between objective functions

A Review of Validation Methods for the Intracranial Response of FEHM to Blunt Impacts

The following is a review of the processes currently employed when validating the intracranial response of Finite Element Head Models (FEHM) against blunt impacts. The authors aim to collate existing validation tools, their applications and findings on their effectiveness to aid researchers in the validation of future FEHM and potential efforts in improving procedures. In this vain, publications providing experimental data on the intracranial pressure, relative brain displacement and brain strain responses to impacts in human subjects are surveyed and key data are summarised. This includes cases that have previously been used in FEHM validation and alternatives with similar potential uses. The processes employed to replicate impact conditions and the resulting head motion are reviewed, as are the analytical techniques used to judge the validity of the models. Finally, publications exploring the validation process and factors affecting it are critically discussed. Reviewing FEHM validation in this way highlights the lack of a single best practice, or an obvious solution to create one using the tools currently available. There is clear scope to improve the validation process of FEHM, and the data available to achieve this. By collecting information from existing publications, it is hoped this review can help guide such developments and provide a point of reference for researchers looking to validate or investigate FEHM in the future, enabling them to make informed choices about the simulation of impacts, how they are generated numerically and the factors considered during output assessment, whilst being aware of potential limitations in the process

Estudo de Segurança dos Protocolos SSL/TLS

Esta dissertação surge da constante evolução por parte das tecnologias e serviços e a dependência que temos destes sistemas. Com esta evolução, aparecem também novas oportunidades para os atacantes. Inúmeras vezes durante o dia acede-se ao correio eletrónico, navega-se pela Internet e efetuam-se operações sensíveis como transações bancárias com os protocolos SSL/TLS sempre presentes, confiando – pelo menos os utilizadores mais atentos – na sua eficácia na proteção que oferecem. Mas afinal o que são estes protocolos e que segurança oferecem? Nos domínios web que os portugueses frequentam assiduamente, a inclusão destes protocolos é suficiente e confiável ou existe exposição às vulnerabilidades? Para responder a estas perguntas, efetuou-se um estudo sobre os protocolos SSL/TLS, cipher suite associada, certificados digitais e vulnerabilidades relacionadas, tendo como objetivo ilustrar boas práticas ou medidas de prevenção na aplicação destes protocolos. Examinou-se soluções que ajudassem a identificar os problemas já mencionados com o intuito de recolher informações para executar uma análise e apresentar os resultados obtidos. A solução preconizada irá auxiliar fundamentalmente os Administradores de Sistemas para elucidar eventuais ameaças com as configurações atuais e pontos a melhorar. Propôs-se, portanto, um método de seleção, recolha e análise para avaliar a exposição dos portugueses aos domínios web que frequentam assiduamente e chegou-se à conclusão que não é possível garantir a partilha de informação de forma segura entre as partes envolvidas. É importante reforçar e alertar não só os utilizadores portugueses como a população em geral para tomarem consciência e mais cuidado quando navegam na Internet, nunca desprezando as boas práticas tanto para administradores de sistemas (ou quem configura e disponibiliza os serviços) como para os utilizadores.This thesis comes from the constant evolution of technologies and services, as well as from our reliance on those systems. With this evolution, new opportunities are created for attackers as well. We check our e-mail, browse the internet or perform sensitive operations online, like bank payments, several times throughout the day, with SSL/TLS protocols always there, while trusting - at least the savvier users - in their efficiency and protection. But what are these protocols and what safety do they guarantee? On the web domains Portuguese usually browse, is the use of these protocols sufficient and trustable or are they being exposed to vulnerabilities? To answer these questions, the author performed a study on the SSL/TLS protocols, cipher suite, algorithms, digital certificates and related vulnerabilities, having, as the main objective, to highlight good practices or preventive measures in these protocols’ application.The proposed solution will mainly assist System Administrators on what are the main threats with current configurations and where to improve these. It was proposed, as such, a method of collecting, processing and analysis to evaluate the exposure of Portuguese users to the web domains more frequently browsed which permitted to conclude that it is not currently possible to guarantee a safe flow of information between all parties involved. It is very important to improve and to alert not only Portuguese users but also other users in general to have a more cautious approach when browsing the internet, never dismissing on best practices, either for system administrators (or whoever is responsible for configuring and providing those services) or for end users

Crashworthiness study on hybrid energy absorbers as vertical struts in civil aircraft fuselage designs

[Abstract:] This research concerns the crashworthiness study and enhancement of commercial aircraft fuselage structures by incorporating crushable hybrid energy absorbers to work as vertical struts. To assess their contribution on a representative aircraft structure, a numerical simulation of a Boeing 737-200 drop test is developed and validated with experimental data available in the literature. The fuselage section is then simulated both with and without the fuel tank, showing more harmful effects for the latter scenario. The numerical model accurately captures the experiment’s collapse process with low artificial energy ratios. Later, four vertical hybrid energy absorbers designed for programmed and progressive collapse, are added in the cargo compartment, connecting the underfloor beams and the frames. Different designs and positions are studied, combining aluminum tubes with square and circular cross-sections, filled with a core made from a GFRP skeleton and foam extrusions. Acceleration graphs show a reduction in passenger injury levels from severe to moderate according to an Eiband diagram when energy absorbers are fitted. Energy trends from the hybrid absorbers are also monitored, with dissipation of up to 10 kJ of the fuselage’s kinetic energy through plastic deformation and collapse. Results also show a significant improvement on the global crashworthiness of the fuselage, leading to an increase in plastic dissipation by the frames from 76 kJ to 122 kJ and a reduction on the accelerations up to 50% when the energy-absorbing structures are added.Ministerio de Economía, Industria, y Competitividad. MINECO; DPI2016-76934-RMinisterio de Economía, Industria, y Competitividad; DPI2016-76934-