Bridging the gap between theory and operational practice
Due to their superior mechanical properties and low densities, aluminium alloys have an edge over other structural materials for aircrafts. The airframe of most modern aircrafts consists of approximately 80% aluminium in terms of weight. The high strength aluminium alloy 2024, specifically, remains the preferred material for damage critical areas due to its resistance to crack propagation. On the other hand, medium strength aluminium alloys are used in those areas where it is important to increase strength without excess material. The greatest potential for commercial exploitation is offered by aluminium alloys such as high purity aluminium-zinc-magnesium (Al-Zn-Mg) alloys. The ultimate objective of the IDA project was to confirm that the theoretical properties of aluminium alloy 2024 verified by experience are transferable to other alloys. Project partners at the Institute of Structures and Advanced Materials in Greece proposed a new model to predict the rate with which fatigue cracks grow. They took into consideration that the crack growth rate depends not only on the magnitude of applied stress, but also on the morphology of the crack. Furthermore, crack growth was assumed to correspond to the growth of the plastic deformation zone. For assessing the lifetime of aircraft structural components, localised plastic deformation was attributed to residual stresses developed in the material ahead of the crack after overloads. The stress intensity was calculated numerically using finite elements. On the other hand, because materials behave differently when subjected to cyclic and monotonic loads, their mechanical properties obtained from tests where the load applied is steadily increased and then reversed were used. The validity of the proposed model was verified on aluminium alloy specimens and the obtained analytical results were in good agreement with actual test data from fatigue investigations. In addition, information on the evolution of fatigue damage could be provided for service conditions that are difficult to reproduce in the laboratory because of the complexity of the load spectrum.
