Image-Based Characterization of Fracture Surface Roughness
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ABSTRACT Analysis of images from SEM, optical and confocal microscopes provides insight into the roughness of fracture surfaces of cement-based materials. Several techniques including 3D surface reconstruction and 2-D vertical contour analysis are compared. Roughness parameters and fractal dimensions are computed to describe the degree of tortuosity of the fracture surface. The tortuosity of the surface (i.e. crack path) is an aspect that has not been adequately integrated into fracture mechanics models of cement-based materials. Topographic maps from confocal microscopy reveal orientation of the crack front throughout the fracture process, allowing analysis of mixed-mode character of nominally mode I fracture. These techniques allow quantification of microstructural parameters to be linked to material properties. INTRODUCTION Fracture surface characterization is of interest in the relationship of geometry and microstructure of cement-based materials to mechanical properties. Images of surfaces from SEM, optical and confocal microscopes provide qualitative information about the fracture. Images from the confocal microscope provide quantitative data in the form of a topographic map. This data can be analyzed for a roughness number, and profiles and surfaces can be measured with varying "yardsticks" to evaluate a fractal dimension. In addition, this surface data can be used in a mechanics of material model to predict local fracture toughness increases from the tortuosity. Confocal microscopy has been shown to be a convenient way to obtain three-dimensional descriptions of cement-based materials 1 . A series of optical sections taken at different focal planes construct a digital image in which each pixel is assigned a value that represents the z-level. The surface area can be estimated by the geometric construction of the triangular planes defined by the x-y-z coordinates of each pixel. The surface roughness number (RN) can be determined from the calculated surface area. The coordinates can also be used to determine a fractal dimension for the ruggedness of the surface geometry. In addition to providing surface data, the confocal image can be "sliced" vertically to obtain profiles in the x-z or y-z plane. These profiles describe the crack front and crack profile tortuosity. A fractal dimension and the crack path deflection angles can be determined. A mechanics of materials model proposed by K.T. Faber and A.G. Evans 2 predicts fracture toughness increases due to crack deflection around second phase particles. The model predicted the crack deflection and twisting from the particle morphology, aspect ratio, spacing and volume fraction. The confocal image provides an actual description of the crack deflection and twisting angles, and can be used directly in the model to estimate the fracture toughness increases.
107 Mat. Res. Soc. Symp. Proc. Vol. 370 0 1995 Materials Research Society
IMAGE ACQUISITION The images were acquired with a confocal laser-scanning microscope with a 2.5X lense at a magnification of 20 to prod
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