A new insight in stress recovery tecniques and hessian reconstruction methods

Stress recovery techniques have been an active research topic in the last few years since, in 1987, Zienkiewicz and Zhu proposed a procedure called Superconvergent Patch Recovery (SPR). This procedure is a last-squares fit of stresses at super-convergent points over patches of elements and it leads to enhanced stress fields that can be used for evaluating finite element discretization errors. In subsequent years, numerous improved forms of this procedure have been proposed attempting to add equilibrium constraints to improve its performances. Later, another superconvergent technique, called Recovery by Equilibrium in Patches (REP), has been proposed. In this case the idea is to impose equilibrium in a weak form over patches and solve the resultant equations by a last-square scheme. In recent years another procedure, based on minimization of complementary energy, called Recovery by Compatibility in Patches (RCP) has been proposed in. This procedure, in many ways, can be seen as the dual form of REP as it substantially imposes compatibility in a weak form among a set of self-equilibrated stress fields. In this thesis a new insight in RCP is presented and the procedure is improved aiming at obtaining convergent second order derivatives of the stress resultants. In order to achieve this result, two different strategies and their combination have been tested. The first one is to consider larger patches in the spirit of what proposed in [4] and the second one is to perform a second recovery on the recovered stresses. Some numerical tests in plane stress conditions are presented, showing the effectiveness of these procedures. Afterwards, a new recovery technique called Last Square Displacements (LSD) is introduced. This new procedure is based on last square interpolation of nodal displacements resulting from the finite element solution. In fact, it has been observed that the major part of the error affecting stress resultants is introduced when shape functions are derived in order to obtain strains components from displacements. This procedure shows to be ultraconvergent and is extremely cost effective, as it needs in input only nodal displacements directly coming from finite element solution, avoiding any other post-processing in order to obtain stress resultants using the traditional method. Numerical tests in plane stress conditions are than presented showing that the procedure is ultraconvergent and leads to convergent first and second order derivatives of stress resultants. In the end, transverse stress profiles reconstruction using First-order Shear Deformation Theory for laminated plates and three dimensional equilibrium equations is presented. It can be seen that accuracy of this reconstruction depends on accuracy of first and second derivatives of stress resultants, which is not guaranteed by most of available low order plate finite elements. RCP and LSD procedures are than used to compute convergent first and second order derivatives of stress resultants ensuring convergence of reconstructed transverse shear and normal stress profiles respectively. Numerical tests are presented and discussed showing the effectiveness of both procedures.

"Who Fears Death" di Nnedi Okorafor, una proposta di traduzione

This dissertaton deals with the translation of selected chapters from Nnedi Okorafor’s novel Who Fears Death. The novel, set in a post-apocalyptic Africa ravaged by inter-ethnic violence, narrates the tale of Onyesonwu, an Ewu, a half-breed born of rape, facing the rejection of her community. Growing up Onyesonwu realizes that the color of her skin is not the only thing that sets her apart from the other inhabitants of Jwahir, as she starts to manifest magical powers, and during an unintentional visit to the spirit realms she finds out that her biological father, a very powerful sorcerer, wants to kill her. At this point the only option left to her is to learn the secret arts of magic under the guidance of Aro, the sorcerer, and then embark on a journey to put and end to the menace posed by her biological father, stop the massacres between the Okeke and Nuru people, and rewrite history. This work is structured in five chapters. The first presents a brief retelling of the author’s life and works. The second chapter constitutes the theoretical frame according to which the novel will be described, and illustrates an analysis on the function of sci-fi literature. The third chapter introduces the novel itself, dealing with its setting and cultural peculiarities, the literary genre to which it belongs, and analysing the themes deemed most relevant, among which the racial and gender issues. The fourth chapter consists of the translation of some chapters from the novel Who Fears Death, and the fifth of a comment on the translation, presenting both a textual analysis, and notes on the choices deemed most interesting or challenging in a translation process perspective.