Initiation and early crack growth in VHCF of stainless steels : Experimental and theoretical analysis

Mechanical fatigue is a failure phenomenon that occurs due to repeated application of mechanical loads. Very High Cycle Fatigue (VHCF) is considered as the domain of fatigue life greater than 10 million load cycles. Increasing numbers of structural components have service life in the VHCF regime, for instance in automotive and high speed train transportation, gas turbine disks, and components of paper production machinery. Safe and reliable operation of these components depends on the knowledge of their VHCF properties. In this thesis both experimental tools and theoretical modelling were utilized to develop better understanding of the VHCF phenomena. In the experimental part, ultrasonic fatigue testing at 20 kHz of cold rolled and hot rolled stainless steel grades was conducted and fatigue strengths in the VHCF regime were obtained. The mechanisms for fatigue crack initiation and short crack growth were investigated using electron microscopes. For the cold rolled stainless steels crack initiation and early growth occurred through the formation of the Fine Granular Area (FGA) observed on the fracture surface and in TEM observations of cross-sections. The crack growth in the FGA seems to control more than 90% of the total fatigue life. For the hot rolled duplex stainless steels fatigue crack initiation occurred due to accumulation of plastic fatigue damage at the external surface, and early crack growth proceeded through a crystallographic growth mechanism. Theoretical modelling of complex cracks involving kinks and branches in an elastic half-plane under static loading was carried out by using the Distributed Dislocation Dipole Technique (DDDT). The technique was implemented for 2D crack problems. Both fully open and partially closed crack cases were analyzed. The main aim of the development of the DDDT was to compute the stress intensity factors. Accuracy of 2% in the computations was attainable compared to the solutions obtained by the Finite Element Method.

The Bereaved Post-9/11 Orphan Boy : Representing (and Relativizing) Crisis and Healing, Tradition and Invention

This article compares the sorrowing child in Jonathan Safran Foer’s Extremely Loud and Incredibly Close (2005) and Brian Selznick’s The Invention of Hugo Cabret (2007), which break traditional novelistic frames through their use of visual material. Through their employment of the orphan figure and their inventive, experimental formal aspects, both Foer’s and Selznick’s novels work as interventions in the debate about the role of fiction after 9/11. Steering clear of a never-ending state of orphanhood, or a return to the nuclear family ideal of the 1950s, they offer different solutions to the family crisis triggered by the loss of a father in a burning building, and, by extension, to the national crisis triggered by 9/11. The bond between father and son that the novels portray represents an affective masculinity that is in line with the emotional narrative work that the two orphan boys perform in the plot and for the readers, which is similar to that of orphan girls in earlier American fiction. In addition to fulfilling the time-honored function of the orphan healing the adult world in a crisis-laden present, Foer’s Oskar and Selznick’s Hugo are post-9/11 “inventions” that highlight the uses of invention in a post-9/11 world.