Small-scale cracking

On a small scale, cracking ammonia can produce hydrogen for reduction. Transport and storage of hydrogen as ammonia is compact, and the cracking procedure involves only a hot pipe packed with catalyst and... 

B. Budiansky, J. C. Amazigo and A. G. Evans, Small-scale crack bridging and the fiacture toughness of particulate -reinforced ceramics. J. Mech. Phys. Solids, 36(2), 167 (1988)... 

In the case of joints composed of bonded brittle materials, the small-scale cracking problem provides insight into how much subcritical cracking can occur without... 

Parameters defining the small-scale cracking solution when the asymptotic solution is a quarter plane with one edge fixed and the other edge stress free (Fig. 3)... 

Fig. 17. Comparison of asymptotic interface-comer and small-scale cracking solutions for interfacial normal stress with finite-element results for the full joint model (

Fig. 18. Comparison of finite-element and asymptotic solutions for energy release rate a = 30.7 MPa). O fea. the finite-element solution Casym. the small-scale cracking solution the long crack, steady-state solution.

Fig. 2. Elastic solution for the crack tip stress field with. small scale yielding [24].

The nano indentation technique can also be used to measure fracture toughness at small scales, by using the radial crack, which occurs when brittle materials are indented by a sharp indenter [72,73]. Lawn et al. [74] have shown that a simple relationship exists between the fracture toughness, Ag, and the lengths of the radial cracks, c, in the form of ... 

Dianor [Diamond Alkali, Oronozio de Nora] A process for cracking naphtha to ethylene, adapted for small-scale operation so that polyvinyl chloride could be made by developing nations. Developed in the 1970s by Chem Systems and the two named companies, but not commercialized. 

Lastly, we studied the effect of 7-stress on the effective time to steady state and the corresponding magnitude of the peak hydrogen concentration. We found that a negative T -stress (which is the case for axial pipeline cracks) reduces both the effective time to steady state and the peak hydrogen concentration relative to the case in which the T -stress effect is omitted in a boundary layer formulation under small scale yielding conditions. This reduction is due to the associated decrease of the hydrostatic stress ahead of the crack tip. It should be noted that the presented effective non-dimensional time to steady state r is independent of the hydrogen diffusion coefficient D 9. Therefore, the actual time to steady state is inversely proportional to the diffusion coefficient (r l/ ). 

M. Dadfamia, B. P. Somerday, P. Sofronis, and I. M. Robertson, On the Small Scale Character of the Stress and Hydrogen Concentration Fields at The Tip of an Axial Crack in Steel Pipeline Effect of Hydrogen-Induced Softening on Void Growth, Int. J. Mater. Res., to appear (2008). 

S. G. Larsson and A. J. Carlsson, Influence of Non-Singular Stress Terms and Specimen Geometry on Small Scale Yielding at Crack Tips in Elastic-Plastic Material, J. Mech. Phys. Solids, 21, 263-278 (1973). 

This is the kinetic equation for a simple A/AX interface model and illustrates the general approach. The critical quantity which will be discussed later in more detail is the disorder relaxation time, rR. Generally, the A/AX interface behaves under steady state conditions similar to electrodes which are studied in electrochemistry. However, in contrast to fluid electrolytes, the reaction steps in solids comprise inhomogeneous distributions of point defects, which build up stresses at the boundary on a small scale. Plastic deformation or even cracking may result, which in turn will influence drastically the further course of any interface reaction. 

Piezoelectric detectors are one of the most promising solutions to this problem. They can be installed at various locations on the aircraft body where they can constantly monitor the integrity of the materials of which the body is made. If cracks begin to develop at any location on the body, they can be detected because they result in the formation of small-scale shock waves as air passes over the aircraft body. These shock waves cause minute distortions of the aircraft s surface. While these distortions are too small to he noticed by most ordinary procedures, they are large enough to create electrical signals in nearby piezoelectric devices that can he picked up and analyzed by processing units attached to them. 

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