Quantitative metallography in three dimensions

Thanks to the development of metallographic and optical techniques the quan titative characterisation of the geometry of rough surfaces has become very useful in practical applications. Today, numerous different methods are av ailable for determining three dimensional co-ordinates, linear profiles or height contours to a reasonable degree of accuracy and within an acceptable time frame, and for evaluating such data. In this article, the use of such techniques in the field of fractography Is demonstrated with reference to a number of case studies dealt with by the author's own team, supplemented by several examples from other research groups. The following examples are discussed: In a ceramic material different fract ure paths were discerned corresponding to different test temperatures. In h ard metals, it was shown that both the sub-critical and super-critical prop agation of cracks occurred along the same path. From measurements of the pr oportion of fracture surfaces occurring along and in the various different phases present and the depth of dimples occurring during ductile fracture i n the binder phase, the fracture energy of WC-Co hard alloys with different cobalt contents and tungsten carbide grain sizes was calculated and found to be in excellent agreement with actual experimental results. In the case of ceramic fibre/aluminium matrix composite materials, by measuring the hei ght distribution of the positions of fracture of the fibres, the load trans fer in the region of the fracture surface was determined. With this informa tion a model was developed which explains the significant variation in stre ngth of these materials. Finally, a model was developed for metallic materi als using which, from the experimentally determined distributions of the he ight and inclination of the fracture facets, the stress intensity of roughn ess induced crack closing could be accurately predicted.