Sandwich Core Structures - Failure Modes

The mode in which a sandwich core structure fails on loading can be dermined by establishing, for a given panel geometry and materials properties, which mechanism has the lowest failure load. This information can be plotted on a failure diagram where regions of the diagram correspond to a given failure mechanism. The regions are separated by boandaries where two (or more) mechanisms have the same failure load. The boandaries can easily be found by equating the failure loads for those mechanisms. In examining the equations themseleves, it becomes apparent very quickly that all the equations contain the terms b (sandwich panel width) and c (core thickness) and that, when equating any two equations, these terms vanish. Thus the only design variables to be examined are the ratios , face thickness to span and the relative density of the core. They are used for the axes of the diagram. Equating the failure equations for the face yield and face wrinkling transition we find that

which is independent of and so plots as a horizontal line, above the line, at high relative core densities, Face Yield is the dominant fracture mechanism, below the line, at low relative core densities, Face Wrinkling is the dominant failure mode. The transition equations for the remaining combinations are summarised below. We can ignore bond failure as for most cases the stress for bond failure is considerably lower than that for face yield.

Face Yield - Face Wrinkling 
Face Yield - Core Shear 
Face Wrinkling - Core Shear  
Face Yield - Core Fracture 
Face Wrinkling - Core Fracture 

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