Three-dimensional effect in the prediction of flange delamination in composite skin-stringer pull-off specimens

Three-dimensional finite element modeling was used to analyze the delamination between the stiffener flange and the skin of a typical pull-off specimen representing a section of a stiffened skin panel. The finite element analysis results were used to calculate the total strain energy release rate and its Mode I, Mode II, and Mode III components along the delamination front through the width of the pull-off specimen. Three-dimensional modeling makes it possible to represent ply-by-ply lamina properties in the model. However, the ply-by-ply approach should not be used for highly angle ply laminates because of the strong transverse effect from adjacent plies. A smeared properties approach is more robust for the strain energy release rate analysis of delamination between +/-45-degree angle ply laminates. The three-dimensional analysis indicates that delamination failure between the stiffener flange and the skin is dominated by Mode I and Mode II delamination modes. The three-dimensional analysis predicts delamination growth from the interior of the pull-off specimen width, and higher pull-off load is required to grow the delamination close to the free edges. The delamination initiation load along the interior of the pull-off specimen width does not change with the change of the width of the pull-off specimen. However, the wider specimen has stronger free edge effects than the narrower specimen. The lay-up of the skin and the flange can have significant effect on the delamination initiation load. Stiffening the skin and maintaining the compliant flange will increase the delamination initiation load.