TY - THES

T1 - Influence of Cold Rolling on the Fatigue Crack Growth Behavior of Tungsten

AU - Pillmeier, Simon

N1 - embargoed until null

PY - 2020

Y1 - 2020

N2 - Tungsten possesses the highest melting point of all metals and has excellent high temperature and creep strength, chemical resistance against molten metals, molten salts and most acids. Therefore, the use as a structural material in high temperature applications (e.g. for cooling pipes in fusion reactors) is desirable. So far the limiting factor was its high ductile to brittle transition temperature paired with poor fracture toughness at ambient temperatures. It was shown that severe plastic deformation such as severe cold rolling could not only increase strength, but also improve ductility and toughness of tungsten. In this work, the influence of cold rolling on the fatigue crack growth (FCG) behavior was investigated. Therefore, tungsten sheets with three different thicknesses (2 mm, 1 mm and 100 µm) corresponding to different degrees of deformation were selected to prepare compact-tension (CT) and single edge notched tension (SENT) specimens. To account for orientation influences the crack propagation direction parallel, perpendicular and under 45° to the rolling direction was selected. It could be shown that a pronounced cyclic R-curve behavior evolves, leading to high threshold stress intensity factor ranges ∆Kth, even at high load ratios. Furthermore, a pronounced anisotropy of the FCG behavior was observed where the crack growth direction perpendicular to the rolling direction exhibits the highest ∆Kth and the lowest crack growth-rates. Especially the tungsten foils with 100 µm thickness showed remarkable FCG behavior with a pronounced Paris-regime. As a consequence tungsten qualifies as a good candidate for the utilization as a laminate material applicable for structural applications.

AB - Tungsten possesses the highest melting point of all metals and has excellent high temperature and creep strength, chemical resistance against molten metals, molten salts and most acids. Therefore, the use as a structural material in high temperature applications (e.g. for cooling pipes in fusion reactors) is desirable. So far the limiting factor was its high ductile to brittle transition temperature paired with poor fracture toughness at ambient temperatures. It was shown that severe plastic deformation such as severe cold rolling could not only increase strength, but also improve ductility and toughness of tungsten. In this work, the influence of cold rolling on the fatigue crack growth (FCG) behavior was investigated. Therefore, tungsten sheets with three different thicknesses (2 mm, 1 mm and 100 µm) corresponding to different degrees of deformation were selected to prepare compact-tension (CT) and single edge notched tension (SENT) specimens. To account for orientation influences the crack propagation direction parallel, perpendicular and under 45° to the rolling direction was selected. It could be shown that a pronounced cyclic R-curve behavior evolves, leading to high threshold stress intensity factor ranges ∆Kth, even at high load ratios. Furthermore, a pronounced anisotropy of the FCG behavior was observed where the crack growth direction perpendicular to the rolling direction exhibits the highest ∆Kth and the lowest crack growth-rates. Especially the tungsten foils with 100 µm thickness showed remarkable FCG behavior with a pronounced Paris-regime. As a consequence tungsten qualifies as a good candidate for the utilization as a laminate material applicable for structural applications.

KW - Wolfram

KW - Bruchmechanik

KW - Ermüdungsrisswachstum

KW - Kaltwalzen

KW - Fusionsreaktor

KW - Tungsten

KW - Fracture Mechanics

KW - Fatigue Crack Propagation

KW - Cold Rolling

KW - Fusion Reactor

M3 - Master's Thesis

ER -