Influence of specimen geometry on fatigue crack growth rates in cast and forged Ti- and Ni-based alloys

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Examensarbete för masterexamen
Master Thesis
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2018
Författare
Tobias, Mattsson
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Assuming linear elastic fracture mechanicsl two cracks with the same stress intensity factor will grow with the same rate, regardless of the geometry of the body. However, at GKN Aerospace test results from fatigue crack growth testing of C(T) and Kb specimens made from castings have shown distinct differences in fatigue crack growth rate where cracks in Kb specimens are growing faster than cracks in C(T) specimens. These observed differences have been studied and quantified. For fatigue crack growth test, cracked Kb specimens loaded in pulsating tension with R = 0 were growing 3.6 times faster than cracks in C(T) specimen, averaged over all test temperatures and the materials Ti-64, Ti-6242 and IN-718. New fatigue crack growth testing using C(T) specimens made from forged Ti-64 and IN-718 has been performed and compared to fatigue crack growth rates in Kb specimens made from forgings. The difference in fatigue crack growth rates between Kb and C(T) specimens made from forgings were found to be very small. In order to investigate why cracks in Kb specimens made from castings are growing faster than cracks in C(T) specimens, the plastic zone size in Kb and C(T) specimens was evaluated through numerical simulations using the finite element method. Further, the grain sizes in cast Ti-64 and IN-718 specimens were measured. The measured grain sizes were found to be approximately 100 times greater in castings than in forgings. The investigations indicate that cracks in castings may be subjected to crack closure and further work aimed to measure the closure levels in Kb specimens compared to C(T) specimens is proposed. Keywords: Fatigue crack growth, FCGR, T-stress, plastic zone size, LEFM, crack closure, Kb specimen, C(T) specimen
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Teknisk mekanik, Applied Mechanics
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