Composite skin-stiffener debond analyses using fracture mechanics approach with shell elements

Fracture mechanics analyses of composite skin-stiffener debond configurations using shell elements are presented. Two types of debond configurations are studied: a flange skin strip debond configuration and a skin-stiffener debond configuration. The flange-skin strip configuration examines debond growth perpendicular to the stiffener while the skin-stiffener configuration examines debond growth parallel to the stiffener. Four-node and 9-node shell elements are used to model both debond configurations. The stiffener flange and skin are modeled as two different layers of elements whose translational degrees-of-freedom, in the bonded portion, of the corresponding flange and skin nodes are constrained to be identical. Strain energy release rate formulae are presented for both 4-node and 9-node element models based on the virtual crack 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 energy release rates (G-values) for both debond configurations. The G-values predicted by these methods are compared with those predicted by plane-strain and 3D finite element analyses. Excellent correlation is obtained among all the analysis results, thus helping to validate the VCCT formulae derived for the 4- and 9-node shell elements.