Reconstruction of surface topographies by scanning electron microscopy for application in fracture research

Stereoscopic scanning electron micrographs can be used to reconstruct the microscopic topography of material surfaces. By applying a system for automatic image processing we can obtain Digital Elevation Models (DEMs) of the investigated surface. These DEMs are used to measure the degree of deformation on metallic fracture surfaces. By modelling the deformation the amount of plastic energy that is necessary to shape the microductile fracture surface can be calculated. These values are compared with experimentally obtained results. PACS: 07.80; 42.30; 62.00 The usability of a certain material in a mechanically loaded structure is limited by its strength. The higher the yield strength the larger are the possible loads. A problem arises when there exist precracks in the structure, e.g. caused by fatigue or corrosion. In this case the stresses concentrate at sharp notches (like cracks are) and they can become large enough to cause final failure although the average stress is below the yield stress. Especially, materials with high yield strengths have the tendency to break at low stress intensities since they are brittle and cannot diminish the stress intensities by blunting of the sharp crack tip. That is why, in addition to the strength, a second mechanical parameter (the fracture toughness) must be introduced to estimate the mechanical reliability of a structure. One established parameter for the determination of fracture toughness is the energy dissipation rate D [1, 2] which is related to the amount of plastic energy that is necessary to propagate a crack. Three mechanisms contribute to the dissipation of energy during the crack extension: A certain fraction of the overall dissipated plastic energy is spent for the lateral necking of the specimen, another fraction is spent for deformations within the plastic zone outside the process zone. The third mechanism which dissipates energy is the formation of voids in the process zone. This energy term will further be called void energy. When the voids in the process zone grow and coalesce, the crack length increases. The surface topography is reconstructed by analyzing stereoscopic Scanning Electron Microscope (SEM) images. The technique [3—8] allows the reconstruction of a 3D-object by measuring the parallaxes in two stereo images. By using a system for image processing [9, 10] we can automatically generate DEMs consisting of about 10 000 points. The aim of this work is to estimate the amount of the void energy from analyzing the plastic deformations that led to the final fracture surface topography. Similar estimations were already done in earlier works [11, 12] by modelling the surface topography with a parabolic or ellipsoidal dimple profile. In this work, due to the use of automatic image processing, the real surface shape can be taken into account for the calculation of the deformation energy and no assumptions about the surface profile have to be made.