A ΔJ approach for nonlinear fatigue crack propagation : Experimental and numerical investigation of a ductile superalloy

• This dissertation is a product of an academic-industrial collaboration between the Division of Solid Mechanics at Linköping University and Siemens Energy AB. The focus is on enhancing the design optimisation procedures for high-temperature components in industrial gas turbines (IGT). The research is centred around the behaviour of the nickelbased high-temperature superalloy Haynes 230 under service-like conditions with predominant thermal loads. The aim is to improve fatigue crack propagation life predictions for nonlinear conditions.The research findings validate the thermal ageing fatigue effects on Haynes 230 as seen in existing literature, especially on constitutive properties and crack initiation. Additionally, it was observed that thermal ageing has a minor effect on the crack growth rate up to 600 °C, which can be managed by updating the crack driving force with thermally aged properties.The project mainly focused on nonlinear crack propagation at isothermal and thermomechanical fatigue conditions. A new method for crack length description, using a modified compliance method, is introduced. This method simplifies and enhances the accuracy of crack length measurements and has become an established method for evaluating the single edge notch specimen used in the project under thermo-mechanical fatigue conditions.The nonlinear fatigue parameter ΔJ was incorporated into both Finite element method (FEM) computations and test evaluations, revealing linear trends with crack growth rates in loglog. The research highlights the crucial role of crack opening in establishing a correlation between ΔJ and crack growth rate. It was also concluded that the linear fatigue fracture parameter ΔK tends to underestimate the crack growth behaviour, resulting in non-conservative outcomes if the elasto-plastic stresses from the tests are considered.Lastly, a constitutive description of Haynes 230, based on the Ohno-Wang theory, under negligible viscoplastic effects, and an extension of the cycle jumping procedure that takes into account the significant hardening between the initial and midlife stages of the material, is presented. By this the notched geometry in 3D could be simulated with satisfying accuracy.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Teknisk mekanik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Applied Mechanics (hsv//eng)

Nyckelord

ΔJ

Cyclic J-integral

Fatigue testing

Nickel-based superalloy

Thermo-mechanical fatigue

Elasto-plastic fracture mechanics

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