Tomas Månsson

Growth of surface cracks under constant and variable amplitude loading

Fatigue crack growth experiments were performed on surface cracked tensile specimens of Inconel 718 at 400 °C. The loading was carried out at constant as well as at variable amplitude. The experimental results for the mean growth rate were compared with predictions based on data obtained from testing of compact tension specimens. Both nominal data as well as data corrected from measured crack closure were used in the predictions. The corrected data provided much better predictions than the nominal ones indicating that the level of crack closure during the testing of the surface cracked specimens was much lower than in compact tension specimens.

Modelling of fatigue crack growth in Inconel 718 under hold time conditions - application to a flight spectrum

Gas turbine operating cycles at high temperatures often consist of load reversals mixed with hold times; the latter occurring either as cruise for aero engines or at continuous power output for land based turbines, but also at low frequency loading conditions, e.g. slow “ramp up” of engine thrust. The hold time conditions cause the crack to grow by intergranular fracture due to material damage near the crack tip, thus rapidly increasing the crack growth rate. Since the damaged zone will affect the crack propagation rate due to cyclic loadings as well, the complete load history of a component therefore has to be considered. The crack propagation model presented in this paper is based on the damaged zone concept, and considers the history effect in the form of damaged zone build up during hold times, and subsequent destruction as the crack propagates onwards by rapidly applied load reversals. By incorporating crack closure for handling different R-values, an aero engine component spectrum is evaluated for a surface crack at 550 °C. The result shows a good correlation to model simulation, despite the complexity of the load spectrum.

Scatter in Dwell Time Cracking for a Ni-Based Superalloy in Combination With Overloads

Safe life of gas turbines is always of major concern for manufacturers in order to ensure passenger safety and stable continuous power output. An increasing amount of resources have been put into research and development to assure that all safety aspects are covered in the design of new turbines and to ensure that enough frequent service intervals are scheduled to avoid complications. Many of these issues require good knowledge of material properties and of how to use these in the design process. Some of these relate to fatigue which is of major concern in all parts of a development programme. However, while some fatigue problems have been extensively studied, some have not. One example is crack growth with influence of dwell times at elevated temperature in combination with cyclic loading. Such loading conditions have been shown to give a different cracking behaviour compared to rapid cyclic loading, increasing the growth rate significantly with respect to the number of load cycles. Improved models for predicting this behaviour is therefore of major interest for gas turbine manufacturers, and could substantially increase the reliability. As a result, more research is needed in order solve these problems.The work presented in this dissertation has focused on how to predict life under the above-mentioned circumstances. The materials used in high temperature gas turbine applications are often nickel-based superalloys, and in this work the most common one, Inconel 718, has been studied. Mechanical experiments have been performed under operation like conditions in order to receive material data for the subsequent modelling work. The modelling approach was chosen such that the underlying physics of the dwell time cracking have been incorporated on a phenomenological basis, creating a model which can be physically motivated as well as used for industrial applications. The main feature of the modelling work has been to track material damage which is received from dwell times, how this interacts with cyclic loading and how it affects the crack growth rate, thus creating a load history dependent model.The outcome of this work has resulted in a model which is both easy to use and which has shown to give good correlation to available experimental data. Key components such as calibration for cheap and easy parameter determination, validation on complex engine spectra loadings, three dimensional crack growth, overload influences, material scatter, thermo-mechanical fatigue crack growth and the impact of high cycle fatigue loadings, are all covered in the presented work, both as experimental findings and as continuous development of the modelling concept.The dissertation consists of two parts. In the first an introduction with the theory and background to crack growth with dwell times is given, while the second part consists of 10 papers.

Quantitative Assessment of the Impact of Alternative Manufacturing Methods on Aeroengine Component Lifing Decisions

Static structural aeroengine components are typically designed for full lifetime operation. Under this assumption, efforts to reduce weight in order to improve performance result in structural designs that necessitate expensive manufacturing solutions to ensure high reliability. However, there exist repair techniques that enable the consideration of alternative manufacturing and/or maintenance strategies, in which case different component lifing decisions may be preferable. The research presented in this paper proposes a value-maximizing design framework that models and optimizes component lifing decisions in an aeroengine product-service system context by considering manufacturing and maintenance alternatives. To that end, a lifecycle cost model is developed as a proxy of value creation. Component lifing decisions are made to minimize net present value of lifecycle costs. The impact of manufacturing (represented by associated intial defects) and maintenance strategies (repair and/or replace) on lifing design decisions is quantified by means of failure models whose output is an input to the lifecycle cost model. It is shown that, under different conditions, it may not be prudent to design for full life but rather accept shorter life and then repair or replace the component. This is especially evident if volumetric effects on low cycle fatigue life are taken into account. It is possible that failure rates based on legacy engines do not translate necessarily to weight-optimized components. Such an analysis can play a significant supporting role in engine component design in a product-service system context.

Component Lifing Decisions and Maintenance Strategies in the Context of Aeroengine Product-Service Systems Design

Static structural engine components are typically designed for full lifetime operation. Efforts to reduce weight in order to improve performance result in structural designs associated with higher lifing uncertainty: Maintaining reliability levels may necessitate expensive manufacturing and maintenance solutions. In practice, repair techniques for such structures are available; however, they are not planned for during the design process. The objective of the research presented in this paper is to model and optimize component lifecycle costs with respect to lifing decisions, demonstrated by means of an aeroengine component design example. Both technical (failure) and legislative (certification) implications are considered. The impact of maintenance strategies (repair and/or replace) on lifing design decisions is quantified. It is shown that, under different conditions, it may not be prudent to design for full life but rather accept shorter life and then repair or replace the component. This is especially evident if volumetric effects on low cycle fatigue life are taken into account. It is possible that failure rates based on legacy engines do not translate necessarily to weight-optimized components. Such an analysis can play a significant supporting role in engine component design in a product-service system context.

Modeling of Crack Growth With Dwell Time for Aero-Engine Spectra Loadings in a Ni-Based Superalloy

Testing and simulation of aero-engine spectra with dwell times are reported in this paper. The modeling concept used is built on linear elastic fracture mechanics (LEFM) and provides a history-depe ...

Modelling of Crack Growth With Dwell Time for Aero Engine Spectra Loadings in a Ni-Based Superalloy

Testing and simulation of aero engine spectra with dwell times are reported in this paper. The modelling concept used is built on LEFM and provides a history dependent evolution description of dwell damage and its interaction with cyclic load. The simulations have been carried out for three spectra, 1) cyclic loads, 2) combined sustained load and cyclic loads and 3) slow load ramps and cyclic loads, all for surface cracks at 550°C for Inconel 718. All simulations show reasonable good agreement with experimental results. Prediction of multiple tests of several batches is also provided to show statistical scatter.Copyright