Crack formation mechanics

Thomsen, N.B., Fischer-Cripps, A.C., Swain, M.V., 1998. Crack formation mechanisms during micro and macro indentation of diamond-like carbon coatings on elastic-plastic substrates. Thin Solid Films 332, 180—184. 

Sasai K, Mizukami Y (1994) Crack formation mechanism of gas injection type immersion nozzle. CAMP ISIJ 7 24... 

Where caustic deposits occur, the resultant corrosion of steel by caustic gouging or stress corrosion cracking (SCC) mechanisms produces particulate iron oxides of hematite and magnetite. It is common to see white rings of deposited sodium hydroxide around the area of iron oxide formation. 

Hillerborg, A., Modeer, M. and Petersson, P.E. (1976). Analysis of crack formation and crack growth in concrete by means of fracture mechanics and finite elements. Cement Concrete Res. 6, 773-782. 

Displacement of the crack front—from the start of crack formation until final destruction—takes place at a variable rate. For the crack to overcome impediments (such as macromolecules, chain bundles, super-molecular structure formations, inclusions, and micropores), it needs varying time lengths, and the fissure perimeter takes on a sinuous form. The limit between different formations on the entire perimeter of the crack front is evidently determined by the equilibrium set up between elastic mechanical forces and bond forces displacement of the crack front results from this equilibrium. 

Layered ceramic Si3N4 composites on the basis of TiN or TiCN have been prepared by tape casting and hot pressing and showed highly anisotropic electrical and mechanical characteristics. One idea behind this development is the detection of crack formation under loading conditions by electrical conductivity measures [584, 585],... 

The mechanisms of stress corrosion can be quite different in the case first mentioned the effect can be explained by a chemical reaction, which leads to irreversible mechanical damage when the material is under stress. In the second case we could think of a physical acceleration of a process of crack formation, which, otherwise, would take a much longer time under stress because of the lower interfacial tension... 

General rules for the occurrence of stress corrosion cannot be given, since the phenomenon is very specific for certain combinations of polymer and environment, and is also dependent on the processing conditions in manufacturing the article (such as the occurrence of cooling stresses and orientations). When plastics are used in articles which are subjected to mechanical stress, such as pipes, crates, bottles, screw-caps, etc., the risk of stress corrosion in the presence of fat, soap or organic liquids, should always be taken into account. Proper choice of material and dimensions can minimize the occurrence of crack formation. 

All attempts to carry out marker experiments with Ni-Zn and Co-Zn diffusion couples in the same way as with Ni-Bi ones (see Section 1.8 of Chapter 1) were unsuccessful because of crack formation, not allowing an unambiguous determination of the diffusing species in the growing intermetallic layers. These proved, however, extremely useful for understanding the mechanism of multiple-layer formation. Namely, it became quite clear that this phenomenon is most frequently a result of secondary reactions occurring in cracked couples. 

Most of the technical raw materials used in solid reactions are polycrystalhne, and these - when in solid form - undergo a range of modifications during milling. The first observation is the fracture of the solid into smaller pieces. The fracture mechanics are a topic of discussion, and fracture can be divided into two steps, namely crack formation and crack propagation . Depending on the ability of the material to undergo plastic deformation before fracture, either brittle or ductile fracture can be defined ... 

Fig. 35a. Mechanism of crack I ormalion at intersections of shear bands (i) sequence of shear band formation, jii) chain scission at the intersections (schematic) b Shear crack formation (arrow) in a shear band of type A, leading to a 50 pm wide displacement of type B — shear bands on both sides of the crack...

Highly oriented PP monofilaments (drawn to break at 25 ) are more resistant to photo-oxidation than medium drawn fiber ( x5 orientation at 230 C) as measured both by oxidation product build up and mechanical deterioration (2 ). Both types of PP monofilaments also show crack formation during photooxidation CFlgure 7). The lower orientation material shows quite wide, deep cracks whereas the fully oriented fiber shows many short, narrow cracks for... 

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