A simplified method for the determination of damage arrest capabilities in stiffened structures

The recent application of Glass Reinforced Aluminium (Glare) in the Airbus A380 has proven to be a suitable candidate to replace metal structures. The reduction in weight and increased performance results in lower emissions and cost reduction. Therefore research has been performed at the Delft University of Technology, dedicated to the residual strength of fibre metal laminates. This has resulted in reliable prediction methods for flat structures. However in practise these structures are reinforced, to be able to support the loads present in aircraft, while minimising mass. The addition of these local reinforcement features has revealed interesting residual strength characteristics. It was observed that these structures may possess the ability to arrest unstable crack growth, when damaged, which is of great interest for the analysis of damage tolerance aircraft structures. Currently no simple prediction method is available, giving insight in the crack growth characteristics and enabling crack growth predictions in the early design phase. Therefore the objective of this thesis is to predict the damage arrest capabilities of stiffened structures in a simplified manner. Based on the literature study it was observed that the current understanding of fibre metal laminate stiffened structures was insufficient for predicting the fracture mechanics of built-up structures. Therefore the available linear elastic fracture mechanics for metal structures is modified to account for plasticity and stiffeners ahead of the crack-tip, by assuming isostrain conditions. As result, the stress intensity reduction factor is successfully implemented, incorporating the presence of a stiffener in combination with the Irwin plasticity correction method. This configuration results in accurate crack growth predictions for residual strength data obtained from literature. The model verification is expanded with experimental test results to validate the crackarrest capabilities of a stiffened structure. Therefore four test samples were tested and analysed, representing the different stages of the residual strength diagram. Based on the test results some difference were obtained in the model, which can be appointed to the negligence of stiffener yielding and stiffener effect on the plastic zone growth. Nevertheless a residual strength has been predicted within 7.5 % of the experimental data. Finally it is concluded that the model is able to predict the crack growth characteristics of a stiffened structure in a simplified manner for both metal and fibre metal laminates.

Subject

Fibre metal laminate
FML
residual strength
stiffened structure
crack arrest

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Student theses

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master thesis

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Thesis_E_van_Hummel.pdf

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