Strong Toughening Mechanisms in an Elastic Plastic Laminate

The fracture process of a laminate is analysed. The laminate is a material used for packaging.It consists of a thin aluminium foil with a polymer coating. In both materials, the fractureprocesses are supposed to be dominantly localized plastic deformation. A Barenblatt regionis supposed to spread ahead of the crack tip. This region is analysed in its cross planeinvoking plane deformation conditions. The fracture process is assumed to be continuousreduction of the cross sectional area in the crack plane until the load carrying capacityvanishes with the vanishing cross sectional area. One case where the interface betweenthe two materials is perfectly bonded and one case with delamination of the interface areexamined. The results are compared with the properties of the individual layers. At fracturemechanical testing of the laminate, it is observed that the load carrying capacity increasesdramatically as compared with that of the individual layers. When peak load is reachedfor the laminate, strains are fairly small and only the aluminium is expected to carry anysubstantial load because of the low stiffness of the polymer. It is therefore surprising thatthe strength of the laminate is almost twice the strength of the aluminium foil. The reasonseems to be that the constraint introduced across the interface, forces the polymer to absorblarge quantities of energy at small nominal strain. The toughness compares well with theaccumulated toughness of all involved layers. Based on the results, a method is suggestedfor designing ultra tough laminates based on careful selection of material combinations and interface properties. The method gives a laminate that produces mutlple necking.