Surface microcracking

On the basis of the energy representations of the process of brittle fracture, Griffith stated that the work of external forces is spent for the volume deformation of a material and the formation of new surfaces—microcracks. Stresses in the top of microcracks of the loaded material are many times greater than the average stresses in a cross section of the sample. If the stress value at the top of the main crack is equal to theoretical strength, the crack quickly grows and the sample collapses. 

Should pitting occur, a heterogeneous mechanism for metal dissolution defines a localized attack that may involve metal penetration in thin stmctural sections, hi the case of massive stmctural sections, pitting is usually of little significance. On the other hand, surface fatigue failures due to pitting mechanism are weU documented in the literature. Hence, pitting develops with surface microcracks. 

Microscopic development of surface microcracks optical and electronic microscopy... 

If aging is limited to the surface, microcracks may form. Crack propagation (under load) can continue into the interior of the material. Figure 1.42. 

Study of photodegradation of tridimensional networks is considerably more difficult, largely due to the lack of solubility. Study of these crosslinked systems is very interesting, from a practical point of view, because processes involved in the photo-oxidative degradation of a network play an important role in surface microcracking, or perhaps indirectly in hydrolytic ageing. 

Assuming that is the depth of the surface microcrack and that is the tensile strength at the crack location, the stress intensity factor K characterizing the stress condition ahead of the crack tip is given according to ... 

Figure 14.4 Fabricated elastomeric coplanar waveguides utilizing 3- am-thick gold layers (a) Electron micrograph of microcracked morphology, (b) atomic force microscope scan showing surface microcracks, and (c) 40-mm-long gold CPW electrodes (with dimensions as shown in Figure 14.3) remain conductive when flexed as shown.

When a specimen includes an elastic discontinuity, such as an edge, a step, a crack, or a joint interface, an acoustic image of the elastically discontinuous and peripheral portions visualized by the SAM shows unique contrast such as fringes or black stripes when the lens is defocused toward the specimen. As a model shown in Fig. 12, this type of contrast appears as an interference effect of surface acoustic waves incident on and reflected from elastic discontinuities. Since the thickness of the film is about 100 nm, the same effect may not be remarkable at the operating frequency ranging from 0.6 to 1 GHz for the discontinuities. However, this technique is useful for enhancing existence of surface microcracks that conventional microscopes may not be able to visualize... 

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