Effects of adherend thickness and taper on adhesive bond strength measured by portable pull-off tests

Portable pull-off test using the Pneumatic Adhesion Tensile Testing Instrument (PATTI) has been employed to quantify the residual strength of bonded patch repairs on retired aircraft, with a view towards establishing a reliability-based airworthiness certification methodology for bonded repairs of primary structures. The pull-off strength obtained using this field-testing method may be affected by two major factors: the structureʹs cross-sectional thickness and tapering near repair edges. This paper aims to investigate and quantify the effects of these two parameters on the pull-off strength, through a combination of experimental testing and computational modelling. By subjecting scarf joints of varying angles to tensile loading, film adhesive FM300 was found to obey the modified Drücker–Prager/cap plasticity criterion. With these data as inputs to a finite element model and a failure criterion that defines final fracture as when the maximum strain at a characteristic distance reaches the adhesiveʹs failure strain, strength predictions of PATTI tests were carried out, with results being compared to experimental data. The results show that the modified Drücker–Prager/cap plasticity model gives better prediction than the von Mises yield criterion, with the characteristic distance being determined by an inverse method using test results pertinent to tensile butt joints.