In Situ Analysis of Fatigue Crack Propagation in Polymer Foams

This paper presents an in situ SEM experimental study on cyclic crack propagation in closed-cell polymer foams. The microscopic failure mechanisms in precracked PVC and PES specimens of 60 and 90 kg/m ³ densities were examined under low-cycle fatigue loading. In the PVC foam, crack propagation occurred incrementally by successive failure of cell boundaries in front of the crack tip. The crack occasionally jumped to cell boundaries above or below the main crack resulting in non-self similar growth. Crack propagation in the PES foam occurred incrementally by extensive plastic tearing and subsequent tensile failure of the cell edge in front of the crack tip. Crack advance sometimes occurred by coalescence of the main crack with a secondary crack above or below the main crack. Such crack bridging involved extensive shear deformation of the cells bridging the two adjacent cracks. Overall, crack growth in the PES foams occurred through the center of the cells. At a given cycle load level, more loading cycles were required to extend the crack in the PES foam than for the PVC foam as a result of the higher ductility of the PES polymer.