The object of the work described was to obtain information about the stresses in stiffened plastic bars. Tests were carried out under tension on a composite bar with a circular hole, and under bending, on a composite bar with a semicircular notch. The bars were of epoxy resin stiffened with duralumin strips. Graphs were obtained from which the stress-concentration factors of the stiffened models may be deduced from those in similar unstiffened models.