Plasticity modelling of nickel based super alloy Alloy 718

University dissertation from Luleå tekniska universitet

Abstract: The ever growing demand on reduced fuel consumption in modern aircrafts puts high requirements on manufacturers to reduce weight in all parts of the aircraft. With a total weight of up to one fifth of an aircraft’s total operating weight, ways to decrease the weight of the engine systems are continuously sought. The containment structure that surrounds the fan and turbine in larger commercial aircrafts is designed to prevent any debris to escape and damage any other systems such as fuel tanks or fuselage in the event that a blade should come off. This structure adds considerable bulk to the engine and because of the importance of the containment structure any redesign needs to be thoroughly tested. The high costs associated with containment testing means industry is looking into the feasibility of substituting parts of the expensive experimental testing with more economical numerical simulations. In this thesis modelling of the plastic behaviour of the nickel based super alloy, called Alloy 718, is investigated in an effort to correctly model the material in numerical simulations. This material is one of the most widely used materials in the parts of an aircraft engine subjected to elevated temperatures due to its retained strength and resistance to corrosion and creep. The material models chosen to model the plastic behaviour were the widely used Johnson-Cook and Zerilli-Armstrong models, because of their proven applicability for wide ranges of strain rates. The models were calibrated using data collected from tensile testing performed in a high speed VHS machine from Instron. Tensile tests were performed at quasi-static conditions and raised strain rates up to 1000 s-1. With an induction coil testing was also performed at temperatures up to 650 o C. Fitting the models to the data gave models valid from quasi-static to high rate conditions. In order to test the accuracy of the models they need to be validated. For this purpose a reverse impact experiment usingfree flying discs impacting a long slender rod was designed. This design enables the force history to be accurately monitored throughout the impact, while still achieving high strain rates. An investigation into producing additional data for use in validation was also performed. This investigation utilized a series of high speed photographs on which shape measurements were carried out in order to find parameters such as plate velocity and average strain without interfering with the experimental results

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