Adhesive Bond Stresses and Strains at Discontinuities and Cracks in Bonded Structures

Abstract

This paper presents analysis procedures to be used for analyzing adhesively bonded structures containing cracks or discontinuities in the metal elements. The analyses are performed for a range of disbonds in each of four basic problems for stiffened structures: (1) stiffener broken at one station or with a finite length removed, sheet intact, (2) stiffener intact, sheet completely broken, (3) one-bay sheet crack, stiffeners intact, and (4) two-bay sheet crack, stiffener intact. Separate failure modes of disbond under shear loads, stiffener yield, and sheet fast fracture are investigated. The solutions are essentially planar (two-dimensional) and such three-dimensional effects as stiffener or sheet bending and peel stresses in the adhesives are not accounted for. The paper contains parametric studies for the governing variables and excellent agreement with test is demonstrated for the available test data (one-bay sheet crack). The analyses are approximate and not of universal applicability, but are simple to use with either pocket electronic calculators or digital computers. Known limitations of the theory are confined to situations in which the adhesive stresses are small and widespread rather than high and concentrated in a small identifiable zone adjacent to the discontinuity. The conconclusion drawn from the examples investigated is that disbonds can be initiated relatively easily at discontinuities in the metal structural elements, because the bond is very stiff, and that care is needed in proportioning the structural elements to control this potential problem. The analyses indicate also that the initial disbond is usually self-arresting and is not catastrophic. Higher loads are usually needed to propagate the disbond and, eventually, induce complete failure which is triggered by stiffener yield or fast fracture of the sheet at the crack tips. The sample cases point to the need to account for adhesive plasticity, stiffener yielding, and changes in load path as disbonds propagate.