Thoughts on the durability and damage tolerance assessment of adhesively-bonded joints

Abstract

This paper first discusses the accuracy of the standard adhesively-bonded joint design code, the analytical A4EI code, that is used in the aircraft industry to determine the strain-energy density in a complex step-lap joint under shear loads. This joint geometry is then used to assess the ability of the A4EI code, and a standard finite-element analysis, to meet the static design requirement that there be no yield at 115% of the design limit load. Secondly, the analysis tools needed for the durability and damage tolerance (DADT) assessment of adhesively-bonded airframes are discussed. This study demonstrates the significant advantages of now adopting a Harman-Schijve approach, which considers a fatigue crack growth rate approach to DADT assessment. Indeed, this study reveals how very different approaches are required to assess the static versus the DADT performance of adhesively-bonded joints. A key finding is that, if a valid DADT assessment of fatigue crack growth in adhesively-bonded structures is to be performed, then the aspect of the fatigue crack growth data associated with the growth of small cracks in bonded joints needs to be resolved, together with the question of what is the value of the fatigue threshold associated with such small, naturally-occurring cracks in bonded structures.