Issue 50

V. Kytopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 414-422; DOI: 10.3221/IGF-ESIS.50.35 415 ical and tribological properties of the matrix material can be improved by adding various reinforcements ranging from very soft materials, like Graphite, to high hardened ceramic particulates like SiC. In recent years particulate reinforced aluminum-alloy composites have shown significant improvement in tribological properties, including sliding wear, abrasive wear, friction and seizure resistance [1-4]. A problem of major importance, especially in high temperature power and tribological technology, is to record and estimate the development of surface damage in severely mechanically and thermally loaded components made of these materials during service. For instance, in tribological applications [2, 4] due to high level of localization of the micro-cracks or crack-like micro effects mainly at the surface, the ability of a correct recording and estimation of surface damage becomes a challenging task. Such recording of damage would make it possible to retain the components in service until replacement is indicated by the damage level approaching a critical value. In this direction, various techniques, like for example sonic inspection, have a long tradition and constitute a strong base in engineering practice of damage control. However, very often micro-cracks and pores, which constitute basic material damage may not be detected or measured by sonic and other non-destructive techniques. In this context, it is noted that today there are several “common” macroscopic techniques for damage control and evaluation working on the macro-and meso-scale level [5, 6]. At the same time, there also exist a lot of advanced physical techniques for surface microanalysis of solids [7-11]. However, very few references could be found in the literature concerning the application of the present SEM-EPMA technique to mechanical and thermal load-induced damage evaluation and characterization of materials [12-14]. In spite of this scarceness, it is to be accepted that convenient combination of this technique with other similar ones [13,15- 19], could provide valuable relationships between damage and other basic parameters of fracture mechanics such as Crack Opening Displacement (COD), the length of the damage zone etc. Therefore, it would be of importance to establish certain complementary experimental techniques for surface damage evaluation and characterization at microscopic level. Such a technique, called Scanning Electron Microscopy-aided Electron Probe Microanalysis (SEM-EPMA), was used for this purpose in the present study. Earlier attempts in this direction have shown that this technique can be a powerful tool providing valuable information about surface damage at microscopic level [12-14]. Therefore, in the present study a further attempt is made to apply an improved version of this technique for a deeper insight into the damage processes to be obtained. In particular, the proper application of this technique on a tensile loaded edge-cracked (notched) specimen was found to allow the evaluation, in a semi quantitative way, of the continuous distribution of mechanical and thermal load-induced micro-damage and hence the description of the microstructural integrity changes, ahead of the notch root. T HEORETICAL CONSIDERATIONS Basic damage aspects he material damage due to quasi-static increase of loading is brought about by the progressive nucleation and growth of microscopic cavities in the materials as a result of its elastic-plastic deformation, and is either elastic- brittle damage or elastic-plastic damage. The microscopic features of the nucleation and growth of such cavities vary significantly depending on the type of material, its macrostructure, the loading conditions, environmental and other factors. The elastic-plastic damage in composite materials is an extremely complicated process originating from the gen- eration of various faults such as matrix cracking, interface degradation, delamination, fiber breakage, fiber-matrix debonding and other processes. This is in contrast to the damage development in geological materials [20] and structures made of geological materials [21] like and metals, where damage under a given loading scheme develops according to a single or a limited number of damage mechanisms. In general, it can be said that the process of gradual loss of integrity of a solid may be attributed to the increasing concentration of micro-cracks due to: • The existence of initial cracks associated with the manufacturing process or the previous loading histories, • Highly localized deformations and the attendant stress concentrations, • Substantial differences in fracture toughness and elastic-plastic constants of the constituent phases, and • Large fluctuations of the stresses which may be attributed to the inhomogeneity of the meso-scale structure. X-ray generation approaches The X-ray quanta are generated at different depths in the material and theoretically can be described by the so-called mass-depth distribution function φ ( ρz ), where the depth is characterized by the effective variable, ρz, in units of mg/cm 2 . X-rays generated at a depth z and collected at a given take-off angle are absorbed along the path length up to the surface layer [22-23]. The absorption process is characterized by an exponential decrease of the function of the linear-absorption (attenuation) coefficient μ , of the material and, is the average of these coefficients for all the elements of the material. In a T