COMPOSITE SKIN-STIFFENER DEBOND ANALYSES USING FRACTURE-MECHANICS APPROACH WITH SHELL ELEMENTS

Fracture mechanics analyses of composite skin-stiffener debond configu rations using shell elements are presented. Two types of debond config urations are studied: a flange skin strip debond configuration and a s kin-stiffener debond configuration. The flange-skin strip configuratio n examines debond growth perpendicular to the stiffener while the skin -stiffener configuration examines debond growth parallel to the stiffe ner. Four-node and 9-node shell elements are used to model both debond configurations. The stiffener flange and skin are modeled as two diff erent layers of elements whose translational degrees-of-freedom, in th e bonded portion, of the corresponding flange and skin nodes are const rained to be identical. Strain energy release rate formulae are presen ted for both 4-node and 9-node element models based on the virtual cra ck closure technique (VCCT). In addition, average values of the strain energy release rates are calculated using a gradient method. The VCCT formulae and the gradient method are used to compute the strain energ y release rates (G-values) for both debond configurations. The G-value s predicted by these methods are compared with those predicted by plan e-strain and 3D finite element analyses. Excellent correlation is obta ined among all the analysis results, thus helping to validate the VCCT formulae derived for the 4- and 9-node shell elements.