Study of the fracture behavior of flexible polypropylene foams using the Essential Work of Fracture (EWF)

Two types of commercial low density polypropylene based flexible foams produced by extrusion foaming were characterized in terms of their fracture behaviour using the concept of the Essential Work of Fracture (EWF), focusing on the influence of the foam's relative density and cellular structure on the values of the fracture parameters. With that in mind, correction procedures based on the expansion ratio of the foams and their cellular structure were proposed, with the objective of taking the complexity of these materials into account in the obtained fracture parameters. Significant differences were found between the fracture parameters of the two foams related to differences in their cellular structure, particularly cell size, cell aspect ratio and preferential cell orientation. Generally speaking, the specific fracture elastic contribution in the two considered extrusion directions increased with increase of the cell aspect ratio, especially in the case of the foams with a marked cell orientation in the direction of the extrusion flow. In any case, the fracture parameters for all foams were considerably lower in the direction perpendicular to the extrusion flow, hence demonstrating the highly anisotropic fracture behaviour of these foams due to the anisotropic cellular structure induced during foaming.