The their results revealed that strain energy release

The composite technology is widely used in aerospace applications toharness its specific characteristics. Thin walled composite panels are among the most utilizedstructural elements in aerospace structures. They are made of advancedcomposite parts/components, joined together to form the structure of the requiredgeometry and are subjected to any combination of in-plane, out-of-plane andshear loads during its application. Adhesivejoints are employed in advanced aerospace composite structure assemblies torealize light weight structures. However, composite structures are susceptible to performancereduction in the presence of defects, due to manufacturing issues or damagesduring service loads.

 Debonds occurring in the adhesively bonded compositejoints due to manufacturing issues have great impacton the load carrying capability and they are a major concern in aerospacestructures. Hence it necessitates the assessment of damage tolerance of bondedcomposite joints with inherent flaws (like debonds) in the design of modernaerospace vehicles where adhesive joints are widely used.Earlystudies on composite skin-stiffener debonded configurations are reported byWang et al. 1. Bolotin 2 reviewedand presented the most of the aspects concerning the delaminations and otherinterlaminar crack-like defects in composite structures. Yap et al. 3 have proposed a comprehensivefinite element method to study the effect of skin-to-stiffener debonding.

Pradhanet al. 4 carried out parametricstudies on debonding in adhesively bonded composite joints and their resultsrevealed that strain energy release rate is sensitive to the orientation offibers in the composite adherends. Studieson buckling of composite plates having debonds were reported in literature. Kimand Kwon 5,6 have studied the effect of open disbonds in the compositesandwich panel flange joint under compressive loading environment. Kwon and Kim7 have reported that debond growth initiation was strongly affected byadhesive fracture toughness and flange stiffness. Debond length and flangewidth strongly affect buckling but were found to mildly influence debond growthinitiation.

The significance of adhesive thickness and curing conditions on thecritical energy release rate also envisaged. Mikulik et al.8 employed fracture mechanics based crack tip element methodology to predictthe skin-to-stiffener separation.

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Similar studies on debondingin adhesively bonded composite stiffened panels were reported by variousinvestigators 9-11. Few review articles ondamage tolerant analysis were reported in literature 12-16. da Silva et al. 17, 18 presented an extensivesurvey on the analytical models for adhesively bonded joints both single anddouble lap joints and their review reveals that almost all models for lapjoints are two dimensional and linear elastic for both adherends and adhesive. Damage tolerant design is very challenging and requiresexpertise in damage mechanics, fracture mechanics, structural mechanics,material science, and physics to guide the experimental and analytical work. Toacquire the knowledge on damage tolerant design, it is essential to know theeffect of active debonds on the bonded joints during loading. Though thestudies on the adhesively bonded joints with debonds are reported inliterature, it is evident that literature on onset of growth of closed debonds onthe adhesively bonded joints are limited.

Thismotivated the study on the structural response of adhesively bonded compositejoints containing closed (embedded) active debonds. The finite element (FE)tool has been proved useful in predicting the behavior of composite structuresin the presence of active defects. One of the most popular methods implementedin FE tool to analyze debond/crack propagation is Virtual Crack ClosureTechnique, based on fracture mechanics concepts, which is detailed by Krueger 19,20.The method allows obtaining the strain energy release rates and is based on theassumption that, when a crack grows the energy released in this process isequal to the work necessary to close the crack to its initial length beforepropagation. The objective of the present work is to perform a comprehensive studyon the onset of initiation of debond growth in adhesively bonded compositejoints under compressive load.

The influence of parameters such as laminatesequence, debond location, size and its shapes (square and circular) areinvestigated using VCCT with mixed-mode failure criteria.