Design of Sandwich Structures

Failure modes for sandwich beams of GFRP laminate skins and Nomex honeycomb core are investigated Theoretical models using honeycomb mechanics and classical beam theory are described A failure mode map for loading under point bending is constructed showing the dependence of failure mode and load on the ratio of skin thickness to span length and honeycomb relative density Beam specimens are tested in point bending The e ect of honeycomb direction is also examined The experi mental data agree satisfactorily with the theoretical predictions The results reveal the important role of core shear in a sandwich beam s bending behaviour and the need for a better understanding of indentation failure mechanism High order sandwich beam theory HOSBT is implemented to extract useful infor mation about the way that sandwich beams respond to localised loads under point bending High order or localised e ects relate to the non linear patterns of the in plane and vertical displacements elds of the core through its height resulting from the unequal deformations in the loaded and unloaded skins The localised e ects are examined experimentally by Surface Displacement Analysis of video images recorded during point bending tests A new parameter based on the intrinsic material and geometric properties of a sandwich beam is introduced to characterise its susceptibility to localised e ects Skin exural rigidity is shown to play a key role in determining the way that the top skin allows the external load to pass over the core Furthermore the contact stress distribution in the interface between the central roller and the top skin and its importance to an indentation stress analysis are investigated To better model the failure in the core under the vicinity of localised loads an Arcan type test rig is used to test honeycomb cores under simultaneous compression and shear loading The experimental measurements show a linear relationship between the out of plane compression and shear in honeycomb cores This is used to derive a failure criterion for applied shear and compression which is combined with the high order sandwich beam theory to predict failure caused by localised loads in sandwich beams made of GFRP laminate skins and Nomex honeycomb under point bending loading Short beam tests with three di erent indenter s size are performed on appropriately prepared specimens Experiments validate the theoretical approach and reveal the