Avaliação do potencial dos modelos de zona coesiva e XFEM na descrição da fenomenologia da fratura dúctil de corpos de prova Charpy e DWITT fabricados de aços classe API

Abstract

Gas transportation is usually performed employing pipelines. They are considered components of extreme responsibility, as a failure can cause great financial and even human losses. Considering current high toughness steels, failures are usually originated from cracks that evolve like ductile fractures that propagate along hundreds of meters, often resulting in explosions. To prevent this failure, it is necessary to stop, through the properties of the material, the propagation of a crack, causing the so-called crack arrest. To predict if this will occur, it is necessary to understand how the ductile fracture failure process occurs at high speed in pipelines and how it can be represented in laboratory specimens. As a step in this direction, refined simulations of Charpy V-Notch (CVN) and DWTT geometries were developed using cohesive zone and XFEM damage theories. It was possible to reproduce the experimental loaddisplacement curves with both methods. However, XFEM demonstrated greater ability to describe the phenomenon and allowed the aforementioned curves to be calibrated for X65, X80 and X100 steels. In addition, the use of the exponential damage model for ductile materials proved to be recommended for both methods. Effects of element thickness and triaxiality in the damage parameters were also investigated. Furthermore, statistical analyzes were performed to verify the effects of the parameters of the XFEM model. The energies involved in the damage process could be divided into plastic deformation energy, initiation energy and fracture energy, which outlines the path for the correlation of specimen with pipelines. One of the limitations of CVN geometry is the reduced remaining ligament, which does not favor a steady state ductile crack propagation. Besides that, the damage energy is very small when compared to the plastic deformation energy. The DWTT sample minimizes such limitations but still cannot be considered representative of the failure in pipelines. Only a small part of the absorbed energy is spent to steady-state propagation.

Avaliação numérico-experimental do comportamento mecânico de aços API X80 e investigação das variáveis mecânicas e microestruturais envolvidas no crack arrest

“…Assim, apesar do cunho estruturante desta pesquisa, os resultados das análises experimentais apontaram para um norte no que diz respeito às variáveis que impactam na capacidade de frenagem de trincas destes materiais, à exemplo da ocorrência de delaminações em casos onde o arrest não ocorreu. Portanto, este trabalho estabeleceu um alicerce sólido para continuidade das investigações energéticas envolvendo o fenômeno de propagação de trincas em aços de alta tenacidade, partindo das propostas de Leis (2013), Moço (2017) e Pereira (2017), bem como para a identificação das variáveis mecânicas e microestruturais que impactam no crack arrest.…”