Green and sustainable engines require accurate and well documented material data for safe operation of the engines at optimum efficiency. The SAGE project aimed at demonstrating open rotor engines and technologies to reduce fuel consumption, weight and increased efficiency of engine components. Structural integrity and safety of engine critical parts had considered with regard to design, manufacturing aspects and in-service maintenance and overhaul.
The engine operating conditions, thermal and mechanical loads, material properties and other influencing parameters were affecting the Approved Life of the component. Extensive analysis, component & engine tests, and inspections during both component manufacturing and in-service had to be performed for verification. In particular, the regulations required for critical parts to fulfil appropriate damage tolerance criteria had to be considered, and the potential for failure from material, manufacturing and service induced anomalies within the Approved Life of the part. This meant that the potential existence of various imperfections, defects and flaws in the component were recognised and are due to material issues, component design and manufacturing. This situation was handled through the incorporation of fracture resistant design, process control and Non-destructive Testing (NDT).
In fabricated components and structures different visual inspection and NDT methods were being used for weld inspection. The quality of the welds determined the fatigue life of a component. This project focused on welds made in IN718, both laser welds and TIG welds. Before testing the specimens were NDT tested (WP2), then high cycle fatigue tested (WP4), creep fatigue tested (WP5), fracture surfaces examined (WP3), statistical analysis performed (WP6) and finally the lifing model developed (WP7). The model was then put to use by the topic manager for design of green and sustainable engines.
The LIFEMOD project aimed at the study of LASER and EB welds in IN718 sheet material.
The project was focused on two research questions:
- Impact of defects on fatigue life originating from the welding process
- Impact on the crack growth rate due to sustained load (hold time effects) at elevated temperature (T=550 °C)
The design problem can be divided into two main categories:
- Lack of information
- Accuracy of the fatigue model
For an accurate analysis of the fatigue life of a component to be possible very accurate information about loads and geometry was needed. In the case of welding additional information about various defects in the weld was needed. The problem of welding is that many different types of defect are possible (pores, cracks, lack of fusion, inclusions, high stress concentrations, alignment, chain porosities etc). The effect of the defects on fatigue life is very difficult to quantify and therefore to predict by some model.
The main concept of LIFEMOD had been to produce welds in IN718 and to use these for study. After manufacture, these welds had been tested by both standard and cutting edge NDT methods to quantify the defect density and morphology. Test specimens for fatigue testing had been produced from the welds and had been tested by both traditional HCF (High Cycle Fatigue) methods and creep-fatigue methods with extended hold times at maximum load. The fracture surfaces have after testing all been studied by fractography and the defect status verified in relation to the fracture. A mass of test data has thus been generated. By statistical analysis of the test data the basis, a lifing model had been proposed. The proposed model was based on accounting for competing failure mechanisms in the fatigue analysis.
Based on the results from the study, valuable results had been obtained that can be used to design new and more efficient gas turbines. A database of the results had been collated and thoroughly analysed. Based on the findings from both testing and post-testing analysis a very flexible probabilistic lifing model for welded structures was developed which can take into account sources of scatter for all parameters in the deterministic model. The probabilistic part was formed by a generic probabilistic tool RAP++. The deterministic model consisted of a crack growth tool (NASGRO) and a fatigue analysis tool (Cragro++) and can be easily extended with other tools.