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 structures has many advantages over mechanical fasteners. Bonded joints resist corrosion, seal, weigh less than fastened joins, ensure a homogeneous load introduction 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 siting of the aircraft structures. A damaged fail-safe structure must stall have the capability to bear 80 to 100% of limit load, which describes the maximum load authorized during flight, 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 plasticity model by Sun and Chen In combination with the Failure Mode Concept by Cunze to model the behaviour of the fibre composite adherends. A solid as well as a continuum shell-based modeling of the adherends is investigated. These two adherend modelling strategies are combined with two different modelling approaches for the film adhesive. The simpler approach uses only cohesive elements to describe the adhesives behaviour A more sophisticated one combines 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, 100mm wide, single lap joint coupon specimens tested under tensile load. The specimens are made from HexPly 8S52-IM7 prepreg material and a Henkel Hysol EA969S 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 and used In the joint research project "JoinDT - Joining with predictable Damage Tolerance" funded by the German Federal Ministry tor Economics and Climate Action.