Crack critical velocity

Diffusion Flame. When a slow stream of fuel g s flows from a tube into the atmosphere, air diffuses across the boundary of the stream and Brms an envelope of expl mixture around a core of gas. The core decreases in height until it disappears at some distance above the tube. It thus assumes the shape of a cone. On ignition, a flame front spreads thru the mixture and stabilizes itself around the cooe of fuel gas. The hydrocarbons in common fuel gases crack to form free C H. The shell of carbon-bearing gas so formed gives such flames their luminosity Turbulent Jet Flame. When a gas stream issues from an orifice above a certain critical velocity, it breaks up into a turbulent jet that entrains the surrounding air. The flame of such a jet consists of random patches of combustion and no cohesive combustion surface exists... 

The first three mechanisms of surfactant adsorption have been criticized mainly because crack propagation velocities in comminution devices are often much more than the rate of adsorption of the surfactants normally used as grinding aids (20). There is considerable evidence and concurrence of opinion on the prevention of agglomeration as the mechanism of grinding aid action (21). 

Crack branching is a common feature in failure of ceramic components. Cracks branch at a critical velocity, which is of the order of half the speed of sound in the specific glass under study. The acceleration of crack initiation to the critical velocity depends on the energy dissipation available from the release of stored energy. The energy source can be applied stress, prestressing or residual stress. 

Some typical features of fracture origin such as internal or surface crack are shown in Figure 2.32. The fracture mirror is a smooth surface formed when an internal crack travels radially outwards upon acceleration. Then it starts to deviate from the original plane upon reaching a critical velocity, and intersects with the microstructural features such as inclusion or precipitate. This may also occur as the direction of principal tensile... 

Although much work has been done on the dynamic crack propagation in brittle materials for many years, the mechanism that governs the dynamics of a crack is not well understood [1,2]. Recently, the problems associated with the dynamic brittle fracture have attracted much attention in physics community. Fineberg, Gross, Marder and Swinney [3] performed a refined experiment of mode-I dynamic crack propagation in brittle material, PMMA (polymethyl-methacrylate). Improving the resolution of the crack velocity measurements, they have discovered the existence of a critical velocity, = 0.36c, beyond which the velocity of... 

The critical velocity was found to depend on the temperature and the impactor size it increases with decreasing temperature and impactor size. These dependencies are analyzed using a theoiy of contact mechanics that includes a dynamic effect [15]. They observed that the rebound velocity dropped suddenly at the impact velocity when macroscopic cracks began to appear at the impact point, so they thought that the impact condition for crack initiation was related to the critical velocity. Their analysis concluded that the critical velocity was determined by the ice strength, which was known to depend on temperature and strain rate. The unified equation of restitution coefficient that they derived is... 

Mist. The crack deviates either because it reaches a critical velocity, intersects an inclusion, or there is a change in the internal stresses in the glass. The fracture surface is rougher and less reflective. This region is often difficult to see in polycrystalline ceramics. 

A precondition of toughness enhancement is prevention of premature crack propagation there are some crack stop mechanisms including effects of crack tip blunting, reduction of critical stress concentrations, or reduction of crack length and crack propagation velocity (2). 

In equation (5.103), V is the crack velocity, Ki the stress intensity coefficient, and B and n are constants dependent on the material and its environment. The constant B is sometimes written as V to signify that it represents the critical velocity of crack growth at failure. The exponent n is an important parameter known as the stress-corrosion susceptibility coefficient of the ceramic. 

Upon reaching its critical velocity, the crack begins to branch—that is, the crack surface changes propagation direction. At this time there is a roughening of the crack interface on a microscopic scale and the formation of two more surface features—misf and hackle these are also noted in Figmes 12.28 and 12.29. The mist is a faint annular region just outside the mirror it is often not discernible for polycrystalline ceramic... 

Qualitative information regarding the magnitude of the fracture-producing stress is available from measurement of the mirror radius (r in Figure 12.28). This radius is a function of the acceleration rate of a newly formed crack—that is, the greater this acceleration rate, the sooner the crack reaches its critical velocity, and the smaller the mirror radius. Furthermore, the acceleration rate increases with stress level. Thus, as fracture stress level increases, the mirror radius decreases experimentally it has been observed that... 

The double torsion test specimen has many advantages over other fracture toughness specimen geometries. Since it is a linear compliance test piece, the crack length is not required in the calculation. The crack propagates at constant velocity which is determined by the crosshead displacement rate. Several readings of the critical load required for crack propagation can be made on each specimen. 

When the load has reached a critical plateau value, the crack continues to propagate at constant load. Crack propagation can be stopped by removing the load, with the implication that several readings can be made on one test specimen. Crack velocity is determined by the crosshead speed, modulus of the material and specimen dimensions. 

In materials of lower velocity the situation becomes simpler. If the shear critical angle is outside the lens angle, then there will be no Rayleigh phenomena. Such materials would include many polymers and composites. There may still be longitudinal head waves, and these can reveal some of the features associated with Rayleigh wave imaging, such as surface cracks. But two further factors enhance subsurface imaging. First, because the velocity is lower, the... 

Fig. 6.2.13. Dependence of crack velocity on applied stress intensity factor. Schematic diagram of crack velocity data indicating three regions of crack growth. Boundaries of these three regions are given by specific values of K, (K0, Kt, K2, KIC) and by a critical value of the crack velocity (Flc). (After Fuller et a/., 1980)...

Exposure to seawater results in decrease in critical stress intensity factor and the susceptibility to SCC68 0.2% Fe improves the resistance to SCC presence of >5 wt percent of A1 increases the velocity of cracking Sn in the alloy decreases SCC resistance chloride bromide and iodide induce or accelerate SCC69 Occurs by trangranular cleavage of a-phase in which a-phase controls the crack propagation rate Intergranular corrosion due to formation of titanium methoxide... 

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