Given the shape of a bag, it seems pretty difficult to put shear force on the bag. Since the sealed layer deforms and crumples, if for example you grabbed onto both sides of the middle of the top edge and tried shearing them apart, they would just reorient so that the forces are now more or less normal to the glue...
Apply thumb and forefinger on either side of the seal. Press together and slide. Coefficient of friction between finger and seal might need to be increased somehow (perhaps by texturing the material differently at that point).
Comments
Given the shape of a bag, it seems pretty difficult to put shear force on the bag. Since the sealed layer deforms and crumples, if for example you grabbed onto both sides of the middle of the top edge and tried shearing them apart, they would just reorient so that the forces are now more or less normal to the glue...
Apply thumb and forefinger on either side of the seal. Press together and slide. Coefficient of friction between finger and seal might need to be increased somehow (perhaps by texturing the material differently at that point).