Kink band formation during micro-indentation of oriented polypropylene

Displacement controlled deep penetration micro-indentation testing has been applied to uniaxially oriented polypropylene. The load–displacement measurements recorded showed differences from those reported in earlier micro-indentation experiments on isotropic polymers. Examination of the microstructure ahead of the tip face showed that the deformation mechanisms were different from those observed in isotropic material, and varied with orientation of the tip axis relative to the molecular axis of the material. In the case of penetration parallel to the orientation direction, distinct kink bands were observed ahead of the tip face, localized along the axis of penetration. The deformation could be characterized with a modified version of a kink band formation model used for the axial compression of unidirectional fiber composites.