Use of an atomic force microscope to measure surface deformations in polymeric systems

Abstract

An atomic force microscope is used to image an unstressed sample which is subsequently stressed and re-imaged. An image analysis program is described which allows mapping of surface deformations from the images. In order to show the scope of the present analysis of deformation in plastics, three load conditions showing deformation in a glassy plastic have been selected. In the first a simple tensile load is applied and the image is obtained at low magnification. These images show the general, homogeneous deformation expected with a general elongation in the load direction, an obvious Poisson contraction in the surface plane normal to the load direction and some slight suggestion of Poisson contraction in the through-thickness direction. This loading configuration is intended to reproduce the predicted deformations to illustrate the applicability of the method under simple load conditions. The second loading condition, of a tensile specimen containing a hole, is intended to illustrate the applicability of the system to more complex loading conditions where stress concentrations modify the deformation pattern. The third loading condition is around the tip of a crack propagating in a predictable way in a standard fracture toughness specimen. These images are intended to show the usefulness of the method in characterizing deformations in a complex stress field during failure of the material.