Patching is a high-tech repair procedure that is very adequate for compressor blisks with larger damages. This repair concept has the advantage that the added patch provides the same mechanical strength as the parent material of the blade and the initial aerodynamic contour of the blade is fully restored. However, the welding process locally induces stresses in the heat affected zone at the patch-to-blisk interface. These welding residual stresses influence the fatigue life of the repaired blade and have to be considered during the design phase of patch repairs. In this work, we contribute to the design of patch repairs by introducing a numerical simulation to predict weld-induced stresses in repaired compressor blades. Therefore, a finite element model is developed that includes sequential thermal and mechanical analyses of blisk blades. The temperature field caused by the welding torch is determined by performing a transient heat transfer analysis. The model also reflects the changes in the geometry due to the additional patch material and subsequent re-contoured patch. Different patch geometries are evaluated and compared in terms of their resulting stress levels. Basically, two kinds of patch geometries with long and short welding seams are studied. The stationary stress distribution of the repaired blade results from the superposition of residual stresses with steady stresses due to rotational and pressure forces. Thus, we provide the basis for a new fatigue assessment of the repaired blade considering the residual stress level in the patch-to-blisk interface.

Advanced repair techniques intended for jet engine parts are continuously under development and improvement. Patching is a high-tech approach towards reduced scrap rates and an extended life of high pressure compressor blisks. In this work, we contribute to the structural design of patches for compressor blisks with improved high cycle fatigue behaviour. A fully parameterised patch model is developed, which allows the accurate description of the patch geometry. High cycle fatigue is assessed for welding seam positions specified by the patch model. On the basis of this automated process, a multi-objective optimisation is carried out. The fatigue strength and the length of the welding seam are defined as conflicting targets. Pareto-optimal solutions are calculated using a generalised pattern search algorithm. The engineer’s decision for a specific patch geometry can thus be made based on the optimisation results. The application of the new approach to a compressor blisk demonstrates the influence of vibration modes on fatigue strength. We identify sets of optimal suited patch geometries in accordance to the specified damage pattern.

In order to reduce the “cost of energy” for wind turbines it is an ongoing trend to increase the rotor diameter, which increases fatigue loads in the blade root area. Thus, a critical prerequisite for increased rotor diameter is the reduction of loads, which can be utilized by passive and active measures. This paper is giving an overview of current research work towards the use of a flexible trailing edge for load reduction as it is being pursued in the German national SmartBlades project. The active trailing edge is designed to change the lift of the outer blade in a way to counteract sudden changes caused by gusts or wind shear. Areas that are covered include the simulation towards the load reduction potential of such flexible trailing edges, the structural design of the trailing edge itself as a compliant mechanism, its experimental validation and fatigue investigation as well as multistable approaches for the design of such trailing edge flaps.