Disbond Damage in Aircraft Sandwich Structures

Arash Farshidi

Research output: Book/ReportPh.D. thesisResearch

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Sandwich composite structures are widely used for various weight critical applications by many industries due to their long list of advantages including high stiffness and strength to weight ratios, prolonged fatigue life, good thermal and acoustic insulation, high energy absorption capacity and lack of corrosion. One of the most experienced damage modes of sandwich composite structures is face/core debonding (or disbonding), which can be introduced during the manufacturing process or during the maintenance or service. Propagation of disbonds under fatigue loading may lead to complete loss of load carrying capacity of the sandwich structure and consequent catastrophic failure. The assessment of the criticality of disbond damage in sandwich structures is therefore crucial for the further expansion of sandwich structure applications. This PhD thesis facilitates the damage tolerance evaluation of sandwich composite structures by explaining the behaviour of disbond damage more accurately.
Disbond fracture in foam core sandwich composites at low temperature was initially studied. The effects of low temperature on disbond fracture toughness and disbond propagation rate at different mode-mixities were investigated. Mixed mode static and fatigue test series were conducted at different temperatures using Mixed Mode Bending (MMB) sandwich specimens. This study provides input on disbond fracture of foam core sandwich composites; however, the input was not used through the further studies of this PhD project. The numerical and experimental studies of disbond damage in honeycomb core sandwich composites with different levels of the load and geometry complexity were later set as the primary focus of this PhD project.
Primarily, the focus was laid on development of a mature disbond fracture analysis tool to make the disbond fracture of sandwich composites more predictable at different levels of the load and geometry complexity. First of all, the numerical determination of energy release rate and mode-mixity for a disbond fracture in honeycomb core sandwich composite was carried out using a two-dimensional (2D) fracture based model. The numerical model was validated against Single Cantilever Beam (SCB) specimen tests and verified against a closed-form semi-analytical model. The results showed an agreement. An extensive sensitivity analysis was also carried out and the effects of the geometry and the material properties of the SCB specimen on the energy release rate and mode-mixity have been investigated. In order to raise the level of confidence, the 2D model was validated further against Sandwich Tearing Test (STT) specimen tests with a propagating face/core interface crack yielding varying mode-mixities. The numerical results agreed well with the measured values as well as the results reported in the literature. The 2D model was also extended to fatigue loading using the experimentally measured mixed mode Paris law, and was validated against STT fatigue experiments. The numerical predictions and experimental results agreed well, and increased the confidence in the analysis of disbond fractures.
Further extension of the numerical model to three-dimensional (3D) geometry with complex fatigue loading was eventually carried out. A fracture based 3D model of unvented sandwich panels with arbitrarily shaped disbond was developed using the sub-modeling technique. The 3D model was benchmarked against the results reported in two earlier numerical and experimental studies. An extensive parametric study was conducted using the 3D model to investigate the effects of different geometry and loading parameters on the energy release rate and mode-mixity along the disbond front as well as the pattern and speed of the fatigue disbond propagation. The analysis tool developed in this study contributes to the development of design and maintenance guidelines for sandwich composites structure.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages146
ISBN (Electronic)978-87-7475-547-0
Publication statusPublished - 2019
SeriesDCAMM Report


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