Design and experimental validation of a MDAF bonded structure fail safe damage arrest concept

icaf2023 Tracking Number 96

The need for light-weight and cost-effective fibre reinforced airborne primary structures, drives the industry towards more integral design concepts. Adhesive bonding is increasingly employed as an attractive alternative to mechanical fastening. Aside from the weight reduction due to elimination of fasteners, the high stiffness of bonded joints and the smooth load transfer they provide, are major advantages of this concept. However, the lack of reliable bonded-joint damage-growth analysis combined with the inadequacy of commercially available non-destructive inspection methods to evaluate the strength of bonded joints – has inhibited full adaptation of such joining concepts. This work presents an innovative fail-safe MDAF (Mushroom Damage Arresting Feature) fail-safe crack arrest concept for bonded joints in order to ensure predictable slow damage growth. It was proved both by numerical CZM analysis and mechanical Pull test that by implementing a series of unique geometrical mushrooms and recesses along a stiffened panel's stringer foot, damage arresting capability may be achieved and once an initially dis-bond crack reaches the first mushroom front, after an expected loading event, it will be stopped there and a significantly additional energy will be required to overcome the mushrooms bond-line and to further propagate the dis-bond crack. Subsequently, the load will drop and a rapid unstable crack propagation will take place until the next mushroom is reached, where additional energy will be required again to propagate the crack and so forth. On the other hand, it was shown that for the standard baseline specimen with straight stringer foot, an unstable catastrophically damage propagation is expected once the critical load is reached.