Experimental Characterisation and Failure Behaviour of Inserts Embedded in Stiffened Honeycomb Sandwich Panel

In sandwich structures, inserts are used for the assembly of joints and attachment of equipment. The inserts further appropriately distribute the localised load to the sandwich panel. Sandwich panel inserts are prone to pull-out loading during operation. Theoretically, predicting the pull-out strength is challenging due to the multitude of damage mechanisms involved. Whatever the case, any localised load directly applied to a sandwich panel will cause deformations at different levels at the core and face sheet. In a plain sandwich panel by construction, the core does not have the necessary stiffness to distribute the forces effectively. The insert pull-out strength in sandwich structures has been investigated by several experiments. The experimental evidence shows that there are different failure scenarios depending on the insert geometry and components, also the order of appearance of the failure modes is very important. Moreover, the experimental tests show that the insert strength is principally related to the shear strength of the core. The present experimental work deals with the investigation of different failure regimes of inserts embedded in honeycomb sandwich coupons with doublers. The pull-out tensile tests on sandwich coupons with inserts are carried out on three different configurations, namely one insert embedded sandwich coupon, a second insert embedded in sandwich coupon with doubler and a third insert embedded in sandwich coupon with doubler and bolt. The experimental results show that there are five modes of failure regimes, i.e. elastic shear failure to flange debond and its disintegration from sandwich coupon. During the course of failure, how strength and stiffness vary was captured in different regimes. Based on experimental evaluation configuration three gives additional strength and stiffness which is an added advantage for structural design.