Crack behavior micro cracking

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In TPs, craze zones can develop that are important microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction... 

Behavior of Cr-18Re alloys was similar to Cr-35Re alloys. Some micro cracking at grain boundaries up to a depth of 200 pm on the thermally... 

Thermomechanlcal Behavior. Requirements for optical performance Impose unprecedented requirements for dimensional stability of polymers used In hlgh-concentratlon reflectors. Requirements for mechanical compatibility are also strict for photovoltaic systems subjected to moisture and thermal stresses. Moisture, temperature, and UV, separately and In combination, can change the volume and thus the stress state of polymers. For example, temperature and humidity cycles alone do not cause surface micro-cracks In polycarbonate. However, In the presence of UV radiation, such cycles cause microcracks, while UV alone does not [32]. An understanding of these relationships Is essential to permit reliable design of equipment that uses polymers. 

It is well known in concrete engineering that the fracture process zone is created ahead of a notch (crack) in concrete, without revealing the notch sensitivity. Nucleation of micro-cracks in the fracture process zone is clarified as shown in Fig. 10.25 (Nomura, Mihashi et al. 1991). The fracture process zone was ideally introduced in order to explain the tension-softening behavior. Then, as shown in the figure, the presence of the zone is physically confirmed. 

It should be noted that although no distinction is made between the endo and exo stereoisomers (Fig. 12.25) of furan-maleimide adduct (assuming that their thermal behavior is at least similar if not equal), differences in thermal reversibility (rDA, DA) can have an important in uence on the material properties, especially in the case of thermoremendable polymers and networks. For instance, by breaking only the covalent bonds of the endo DA-isomers inside the network imder mild conditions and reforming them in a less stressed state, stresses can be relaxed. Important material properties such as durability, adhesion to other materials, appearance of micro-voids and micro-cracks, etc., can be improved (Canadell et al., 2010). 

Figure (1) represents examples of V-Ki curves in a polycristalline zirconia ceramic, which exhibit the typical pattern of SCG due to stress corrosion at the crack tip, including the presence of thresholds under vacuum, water and air conditions. The threshold, Kio, often observed on macro-cracks was questioned because of the differences observed between the behavior of micro- and macro-cracks in polycrystalline ceramics [15]. The results obtained in figure (2) for the same polycrystalline zirconia show that the threshold obtained for macro- (i.e. by Double Torsion) and micro- cracks (indentations) are in agreement. It is of course possible that reinforcement mechanisms, discussed below, can change significantly the value of the threshold if they lead to a R-Curve behavior. 

At a microscopic level, the SCG behavior of a given ceramic is dependent on the amount of reinforcement. Thus, for a same type of ceramic, i.e. for the same V-Kjtip intrinsic response, V-Ki curves can be influenced by the microstructure. Two major toughening mechanisms have been discussed crack bridging and phase transformation toughening. Other mechanisms are postulated to occur micro-cracking and residual stresses for example. They might also influence V-Kj curves. 

From these results, it is possible to conclude that hybrid coatings can protect metal substrates from electrochemical corrosion, qualitatively better than inorganic coatings. The incorporation of organic groups make it possible to increase ductility and thickness, reducing defects and the appearance of micro-cracks and enhancing behavior toward electrolytic corrosion. 

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