The fatigue life and damage tolerance of composite stiffened panels with indentation damage are investigated experimentally using single-stringer compression specimens. The indentation damage was induced to one of the two flanges of the stringer of every panel. The advantages of indentation compared to impact are the simplicity of application, less dependence on boundary conditions, better controllability, and repeatability of the imparted damage. The tests were conducted using advanced instrumentation, including digital image correlation, passive thermography, and in situ ultrasonic scanning. Specimens with initial indentation damage ranging between 32 and 56 mm in length were tested quasi-statically and in fatigue, and the effects of cyclic load amplitude and damage size were studied. A means of comparison of the damage propagation rates and collapse loads based on a stress intensity measure and the Paris law is proposed. The stress intensity measure provides the means to compare the collapse loads of specimens with different damage types and damage sizes, while the Paris law is used to compare the damage propagation rates in specimens subjected to different cyclic loads. This approach enables a comparison of different tests and the potential identification of the effects that influence the fatigue lives and damage tolerance of postbuckled structures with defects.

Aeronautical composite stiffened structures have the capability to carry loads deep into postbuckling, yet they are typically designed to operate below the buckling load to avoid potential issues with durability and structural integrity. Large out-of-plane postbuckling deformation of the skin can result in the opening of the skin-stringer interfaces, especially in the presence of defects, such as impact damage. To ensure that skin-stringer separation does not propagate in an unstable mode that can cause a complete collapse of the structure, a deeper understanding of the interaction between the postbuckling deformation and the development of damage is required. The present study represents a first step towards a methodology based on analysis and experiments to assess and improve the strength, life, and damage tolerance of stiffened composite structures subjected to postbuckling deformations. Two regions were identified in a four-stringer panel in which skin-stringer separation can occur, namely the region of maximum deflection and the region of maximum twisting. Both regions have been studied using a finite element model of a representative single-stringer specimen. For the region of maximum deflection, a seven-point bending configuration was used, in which five supports and two loading points induce buckling waves to the specimen. The region of maximum twisting was studied using an edge crack torsion configuration, with two supports and two loading points. These two configurations were studied by changing the positions of the supports and the loading points. An optimization procedure was carried out to minimize the error between the out-of-plane deformation of the representative single-stringer specimen and the corresponding region of the four-stringer panel.

The durability and damage tolerance of postbuckled composite structures are issues that are not completely understood and remain difficult to predict due to the nonlinearity of the geometric response and its interaction with local damage modes. A research effort was undertaken to investigate experimentally the quasi-static and fatigue damage progression in single-stringer compression specimens. Three specimens were manufactured with a co-cured hat stringer, and an initial defect was introduced with a Teflon film inserted between one flange of the stringer and the skin. Pre-test finite element analyses were conducted using the virtual crack closure technique to select the range of defect sizes to be considered and the load levels to be applied during the fatigue tests. The tests were monitored with digital image correlation, passive thermography, and ultrasound systems. After an initial opening and extension of the Teflon-induced embedded defect, the specimens sustained a high number of cycles. It was observed that when the skin/stringer separation develops in the opposite flange, it propagates rapidly within a small number of cycles and causes the collapse of the specimen. These test results contribute to a better understanding of the complex response phenomena exhibited by postbuckled stiffened structures subjected to fatigue loads in the postbuckling range.