This thesis investigates failure in sandwich panels due to bending, specifically localised buckling or wrinkling, a predominant failure mechanism for thin gauge honeycomb sandwich panels loaded in bending or compression. Over the past 60 years, considerable work has been devoted to understanding wrinkling and trying to predict failure loads in damaged and undamaged panels accurately. Existing wrinkling expressions were shown to over-estimate failure loads by over 100%. Discrepancies between wrinkling expressions and experimental failure loads were previously attributed to imperfections and irregularities in the structure. The aim of this thesis is to investigate this problem and try to accurately predict failure loads and understand the underlying failure mechanisms in damaged and undamaged panels, using a combination of numerical and analytical techniques. Classical wrinkling models use a continuum core to model complex cellular honeycomb cores. This type of model reduces complex cellular geometry to a series of effective properties that provide constant support to the face sheet. In reality, honeycomb cores provide support around the periphery of the cell walls and not across the entire surface of the face sheet. Due to the nature of wrinkling and the size of the wavelength, incorrect representation of the core could affect the failure loads and model. This study made direct comparisons between linear buckling loads of a discrete-cored sandwich panel and a continuum-cored sandwich panel. Discrete properties were converted to continuum properties within a Finite Element package. The result conclusively showed that both models predict the same linear failure loads, disproving the theory that the core representations contribute to the difference between experimental and analytical models. It was also shown that existing wrinkling models can accurately predict linear wrinkling loads. These linear model loads do not necessarily match the collapse strength of the physical panel and in most cases predict a significantly higher value. The research then moves on to developing expressions to convert cellular geometry into continuum properties accurately. Expressions are developed for honeycomb structures with fillets in their junctions. Both out-of-plane and in-plane modulus properties are reviewed and the models are verified against Finite Elements models and experimental results. Studies showed that the restrained in-plane modulus can be up to ten times stiffer than the commonly used free modulus value. This has a significant effect on the wrinkling stress. By using the correct value, the discrete model and continuum models predict the same loads. The classical wrinkling expressions also predict the same wrinkling stress as the Finite Element models. After establishing that the core representation is not the cause of the prediction error, the thesis turns to non-linear Finite Element models to predict failure loads and failure mechanism of thin-gauge sandwich honeycomb structures loaded in bending. A continuum three-dimensional non-linear Finite Element model, with bilinear plasticity, is compared with a set of experiments that use different types of Nomex cores and face sheets. The models show that the panels fail prematurely due to core crushing because of wrinkles forming in the face sheets. Experimental results indicate similar trends. The final section examines the affect of impact damage in honeycomb sandwich structures. Due to the thin face sheets and thick cores used on many aircraft and marine components, sandwich panels offer little resistance to impact events. Resulting damage usually consists of a layer of crushed core and a shallow dent in the face sheet. This type of damage often leads to a significant reduction in the load-carrying capacity of the panel through a full range of damage sizes. Finite element and analytical models were developed to accurately predict and capture the localised wrinkling failure mechanism which occurs in the impacted area. Models were directly compared to experimental results, with a high degree of correlation. The numerical and analytical models showed that impact damaged panels were failing due to wrinkling instability and not due to premature core crushing, which is the case with undamaged panels. They showed that two factors influence the wrinkling failure load: damage depth and damage diameter.
| Identifer | oai:union.ndltd.org:ADTP/277863 |
| Date | January 2006 |
| Creators | Staal, Remmelt Andrew |
| Publisher | ResearchSpace@Auckland |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated., http://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm, Copyright: The author |
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