Failure Due to Delamination in Composite Structures with Ply-drops
Authors
Andersen, Søren Rindom ; Jensen, Jakob Bisgård
Term
4. term
Education
Publication year
2016
Submitted on
2016-06-01
Abstract
Manglende viden om, hvordan ply-drops påvirker styrken af laminerede kompositter, fører ofte til overdimensionering. Dette speciale har til formål at udvikle en model, der kan forudsige brud i laminater med ply-drops med fokus på delaminering. Arbejdet kombinerer forsøg og modellering: Der fremstilles prøver til enakset træk og tryk under både statiske og udmattelseslaster, og revnevækst monitoreres med et USB-mikroskop. Geometrien omkring ply-droppet undersøges, og det elastiske respons valideres ved hjælp af digital billedkorrelation (DIC). På grund af geometrisk variation anvendes en forsimplet geometri og invers modellering for at matche målingerne acceptabelt. I modellen anvendes lineær elastisk brudmekanik i et finit element-setup: Virtual Crack Closure Technique (VCCT) afprøves, men kan ikke bruges alene på grund af oscillerende spændinger ved bi-materiale-grænsefladen; Crack Closure Method fravælges af samme årsag. I stedet bruges Finite Crack Extension Method (FCEM), som baseres på globale størrelser, kombineret med VCCT for at estimere mode-mix. Sammenligning med forsøg viser, at modellen reproducerer revnevæksten i den tykke ende af ply-droppet, mens afvigelser i den tynde ende tilskrives den forsimplede geometri. Modellen forudsiger desuden brudlasten cirka 10 % lavere end målt. Forsøgene indikerer, at delaminering er styrende for skadesudviklingen, og at initieringen afhænger af, om der belastes statisk eller i udmattelse.
A lack of knowledge about how ply-drops influence the strength of laminated composites often leads to oversizing. This thesis aims to develop a model to predict failure in laminates with ply-drops, focusing on delamination. The work combines experiments and modeling: Specimens are manufactured for uniaxial tension and compression under both static and fatigue loading, and crack growth is monitored using a USB microscope. The ply-drop geometry is characterized, and the elastic response is validated with digital image correlation (DIC). Because of geometric variability, a simplified geometry and inverse modeling are used to achieve acceptable agreement with measurements. Linear elastic fracture mechanics is implemented within a finite element framework: The Virtual Crack Closure Technique (VCCT) is tested but proves unsuitable on its own due to oscillatory stresses at the bi-material interface; the Crack Closure Method is likewise not pursued. Instead, the Finite Crack Extension Method (FCEM), based on global quantities, is employed and combined with VCCT to estimate mode mix. Comparison with experiments shows that the model reproduces crack growth in the thick end of the ply-drop, while discrepancies in the thin end are attributed to the simplified geometry. The model also predicts failure loads about 10% lower than measured. Overall, the tests indicate that delamination governs damage evolution and that initiation depends on whether loading is static or fatigue.
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