Propagation, crack and nucleation

Delamination Wear Involves material removal subsequently to plastic deformation, crack nucleation, and propagation in the subsurface (Jahanmir and Suh, 1977 Suh, 1986)... 

The second implication of the mechanism involving crack nucleation and propagation, as so far outlined, is that the presence of any cracks associated with an indent means a lowered hardness value for the material. Furthermore, increased iTO radial cracking that occurs with increased load must mean a bigger error in hardness values as indentation pressure is deflected into the crack nucleation and propagation mechanisms. This supports the intuitive position stated at the opening of this section. 

The high toughness of bone results from the ability of its micro- and nanostructure to absorb the deformation energy without crack nucleation and propagation [15, 16]. Both mineral nanoparticles and the mineralized fibrils initially deform, but to different degrees [17]. 

Figure 35 In situ observation of crack nucleation and propagation in MWNT-PS thin films as induced by thermal stresses. The cracks propagate along weak MWNT-PS interfaces or the regions with relatively low MWNT density. The MWNTs are inclined to align and bridge the crack wake then break and/or pull out of the matrix (see MWNTs A-D). Adapted with permission from Qian, D. Dickey, E. C. Andrews, R. Rantell, T. Appl. Phys. Lett. 2000, 76,2868. ...

Concrete is an inherently brittle material with low tensile strength and strain capacities. Its brittle characteristics lead to easy nucleation and propagation of cracks, thus restricting its range of applications. To address this deficiency, fibers of different materials such as asbestos, glass, metal, and synthetics " are used as additives, with the following results"" ... 

The mechanical interaction between the different epitaxial layers may result in the formation of misfit dislocations. Nucleation and propagation of cracks can ensue if the mismatch in thermal expansion coefficient is relatively large. The defects significantly influence the physical properties of the thin films. Examples from different material combinations and models of how to predict the numbers for critical thicknesses are provided in Section 14.4. 

Fig. IX-37. The molecular dynamic simulation of deformation and fracture of two-dimensional crystal a - plastic deformation and formation of a dislocation at elevated temperature (upper portion) and development of brittle crack at low temperature (lower portion) b - simultaneous processes of crack nucleation and foreign atom propagation at elevated temperature (both upper and lower portions)...

There are also applications of quantum theory for instance in the onset of a failure in a material. The failure starts on the atomic scale when an interatomic bonding is stressed beyond its yield-stress threshold and breaks. The initiation and diffusion of point defects in crystal lattice turn out to be a starting point of many failures. These events occur in a stress field at certain temperatures. The phenomena of strain, fatigue crack initiation and propagation, wear, and high-temperature creep are of particular interest The processes of nucleation and diffusion of vacancies in the crystal lattice determines the material behavior at many operation conditions. 

Following the above analytical conclusions, because the maximum tensile stress occurs at the periphery of the indentation, circle BB, it would be expected that at some critical load a crack would nucleate and propagate around the circumference BB. On a surface of glass or another amorphous material a ring crack is observed, but on highly anisotropic crystalline ceramics a less well traced crack is produced as the ring has a definite... 

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