Static residual strength analysis of fibre composite bonded joints after impact and fatigue using mesoscale progressive damage analysis

Kurzfassung

Adhesive bonding as joining technology for fibre composite aircraft struc- tures has many advantages over mechanical fasteners. Bonded joints resist cor- rosion, seal, weight less than fastened joins, ensure a homogeneous load intro- duction and do not require any hole drilling. Although bonded fibre composite structures have a high resistance to fatigue damage, they are prone to impact damage. For this reason, in addition to the static strength of the pristine, undamaged structure, the residual static strength with impact and fatigue damages is needed during sizing of the aircraft struc- tures. A fail-safe structure must still have 80 to 100 % limit load capability. To date, this information is obtained by the airframer through physical testing due to a lack of validated analysis tools. This work investigates whether the residual strength as well as the failure mode of a bonded structure with (artificial) pre-damages can be acquired by virtual testing using numerical analyses. To compile a residual strength analysis tool as part of a impact - fatigue - static strength simulation tool chain, a FEM-based mesoscale progressive damage analysis using Abaqus / Explicit is set up. The main feature of the method is a user-defined material model based on the plas- ticity model by Sun and Chen in combination with the Failure Mode Concept by Cuntze to model the behaviour of the fibre composite adherends. A solid- as well as a continuum shell-based modelling of the adherends is investigated. These two adherend modelling strategies are combined with two different mod- elling approaches for the film adhesive. The simpler approach uses only cohesive elements to describe the adhesives behaviour. A more sophisticated one com- bines solid elements using the exponential Drucker-Prager yield model with a cohesive surface in the middle of the adhesive layer to model cohesive failure. The so-compiled four different modelling variants are validated against large, 100 mm wide, single lap joint coupon specimens. The specimens are made from HexPly 8552-IM7 prepreg material and a Henkel Hysol EA9695 0.05 NW film adhesive. One test series is pristine and two other series contain (artificial) pre- damages. Based on the comparison with the physical experiments, a recommendation for a modelling strategy is made. With this information, a validated tool to predict the static residual strength and failure mode by virtual testing is provided.