Damage tolerance of composite stiffened structures in post-buckling conditions

Composite stiffened panels have been extensively used in primary aerospace applications over the past decades, due to their high specific strength and stiffness to weight ratios. In spite of their apparent superiority to metals, such as the possibilities to integrate parts and reduce the number of fasteners, the susceptibility of composite stiffened structures to interface delamination has been proven to be critical. Delamination is one of the most important mechanisms of damage in laminated fiber-reinforced composites because their interlaminar strengths are relatively weak. In stiffened aeronautical structures, the main cause of the delamination is usually introduced by foreign object impacts or compressive loading. Under the latter condition, the buckling of the skin and stiffener exhibits opposite buckling mode shapes, and so that the delamination occurs in the interface. Moreover, most of the in-service structures are under cyclic loading, the delamination can appear when the post-buckling has been reached thousands of times. It is therefore important to investigate the onset of the critical delamination under single static loading and develop methods to characterize the onset and propagation of the delamination under cyclic loading. Those methods may assist to understand the damage tolerance, where the growth of sub-critical cracks under repeated loading may cause the structures' failure. Numerical and experimental works on the fatigue delamination of unidirectional double cantilever beams have demonstrated good predictive capabilities in the structural response, in terms of predicting the maximal load that the structure could sustain as well as the crack length versus the number of cycles in a power law relationship. The research presented in this dissertation expands the work on static and fatigue delamination of composite laminates from 2-dimensional unidirectional shells to 3-dimensional stiffened panels. The damage tolerance behaviour of conventional stiffened panel is investigated based on the fracture mechanics and structural analyses carried out with finite element program ABAQUS. Two different approaches to simulate delamination between the skin and stiffener are discussed. The first approach implements VCCT (virtual crack closure technique) in the finite element shell model as pre-defined bonded region where the crack initiates. The second method considers the cohesive zone modelling in the interface, for which there is no need to know the crack initiation locations. The post-buckling strength bearing capability and crack growth characteristic of two types of the stiffened panels having different configurations subjected to distributed compressive load are also examined with respect to the material, dimension, geometry, layups and stiffener shapes. Firstly, a single L-stiffened structure was proposed to investigate the structural response, especially the damage tolerance behaviour under cyclic loading. Secondly, a T-stiffened subcomponent, supposed to be double L-stiffener, was carried out to study further how the delamination propagated under static and fatigue loading. The specimens were manufactured and tested for the experimental characterization of static and fatigue delamination under compression. Contrary to unidirectional DCB under pure fracture mode I, it is established that the fatigue delamination crack of stiffened structures was under mixed-mode fracture type. Hence the fatigue parameters under mixed-mode fracture are required to define the power law between the crack length and the number of cycles. The most important observations resulted from the aid of experimental results and their correlation to the finite element models are described as follows. Experimental results show that delamination propagation appears in the post-buckling regime. Generally, the fatigue delamination propagated in an unstable way and is potentially the most detrimental for structural stability, while the free-edge delamination in the stiffener web affected strongly the load-carrying capacity of the structure. From the experimental results, the maximum and minimum pre-buckling stiffness among the three L-stiffened specimens varied by 7 percent and 17 percent compared to the finite element analysis ones. The finite element model predicts an increase in maximal load of 30 percent compared to the tested one due to the manufacturing defects. Moreover, a comparison of above mentioned two approaches on T-stiffened specimens revealed that the Abaqus VCCT algorithm and cohesive approach properly predict crack propagation onset even under post-buckling deformations, which results in a valuable tool for a preliminary design in terms of ultimate load and damage types. A further discussion of the finite element predictions was achieved by distributing the loads on the skin and stiffener parts separately. The direct cyclic fatigue algorithm was found to be unsuitable for fatigue analyses of panels which present nonlinear displacements and are under mixed-mode loading. However, the idea of using the Abaqus VCCT for calculations of the energy release rate components provided detailed information about the mode mixity.

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