Micromechanical Fracture Testing of Arctic Steel CGHAZ Cantilevers

Snartland, Brage D. (Norwegian Univ. of Science and Technology (NTNU)) | Kvaal, Aksel L. L. (Norwegian Univ. of Science and Technology (NTNU)) | Thaulow, Christian (Norwegian Univ. of Science and Technology (NTNU))

OnePetro 

ABSTRACT

This paper is devoted to micro-scale fracture testing of Arctic steel, by use of focused ion beam machined notched cantilevers. Bainite packets of a weld-simulated course grained heat-affected zone (CGHAZ) was the targeted microstructural aspect, with reference tests performed in pure iron. Micro-scale fracture testing has been developing in the last decade. The main objectives of micro-scale fracture tests are to obtain relevant toughness values for materials used at this scale, and to evaluate the fracture toughness of local microstructural aspects. The latter is the focus of this paper. Several models, including multiple barrier models, require specific material property inputs that are not obtainable through traditional testing at larger scale. Hence, micro-mechanical fracture has been applied to quantify these properties. Linear-elastic and elastic-plastic fracture mechanics parameters are presented and compared, with respect to testing material and temperature. Additionally, a new analytical tool is utilized to determine the criticality of a growing crack in terms of determining the energy required for further crack growth following initiation of stable crack growth.

INTRODUCTION

The industrial activity in the Arctic is rapidly increasing, where accidents may cause severe ecological ramifications. Rough climate conditions and temperatures as low as -60°C require materials with specialized mechanical properties. The materials must display sufficient fracture and wear resistance at low temperatures, while avoiding excessive maintenance and maintaining lifetime integrity. In order to overcome these challenges, small-scale fracture mechanisms and properties must be understood.

BCC structures typically exhibit a rapid transition from ductile to brittle fracture, due to reduced mobility of screw dislocations and a reduced number of available slip systems, as the temperature is lowered (Brinckmann et al., 2008, Schreijäg et al., 2015). Full understanding of this transition requires a defined transition criterion. The change in fracture mode from ductile to brittle occurs over a temperature range that is closely interconnected with the change in deformation energy. Inside this temperature range, the metal exhibits fracture characteristics from both modes. There will be some ductile fracture near the notch, which changes to cleavage as the crack propagates. This is due to increased hydrostatic stresses as the propagation speed increases (Petch, 1958), implying that fracture will switch from ductile to brittle when the stress ahead of the fracture tip becomes capable of Griffith propagation. Fracture mechanics have gained increased interest due to several incidents where structures fail within the designed region of operation, initiating extensive research on fracture mechanisms, fracture initiation, propagation and arrest, as well as the temperature dependence of these mechanisms. In an attempt to enhance the understanding of the fracture mechanisms small-scale testing has been used to localize testing and to reduce the number of variables tested in each experiment.