Crack directions

The naphtha yield was lower for Catalyst B than for the reference and this illustrates the necessity to have enough zeolite surface area in the catalyst to be able to crack all the components in the feed, both those that can be cracked directly and those that must be precracked on the matrix before they can be cracked by the zeolite. Catalyst C had a slightly higher naphtha maximum than the reference catalyst, despite its high matrix surface area. The high matrix surface area of Catalyst C,... 

The central instability h, referred as zone e, is along the crack direction. It has a fibrillar structure, clearly seen in Fig. 62, which is characteristic of a craze. 

D/ W, ratio of a crack is held constant but the dimensions approach molecular dimensions, the crack becomes more retentive. At room temperature, gaseous molecules can enter such a crack directly and by two-dimensional diffusion processes. The amount of work necessary to remove completely the water from the pores of an artificial zeolite can be as high as 400 kj/mol (95.6 kcal/mol). The reason is that the water molecule can make up to six H-bond attachments to the walls of a pore when the pore size is only slightly larger. In comparison, the heat of vaporization of bulk water is 42 kj/mol (10 kcal/mol), and the heat of desorption of submonolayer water molecules on a plane, solid substrate is up to 59 kj /mol (14.1 kcal/mol). The heat of desorption appears as a exponential in the equation correlating desorption rate and temperature (see Molecularsieves). 

Crack direction Parallel to applied stress Perpendicular to stress... 

A multiaxial analysis of the cyclic stresses experienced by the material in the vicinity of the crack initiation sites. On the basis of a contact mechanics analysis, the amplitude of the average tensile and shear stresses is calculated along the initial crack direction in order to identify the nature and the level of the local stress field associated to crack initiation. 

Fig. 22a—c. Microtome section of an intersperujitic crack path in bulk coarse spherulitic PP 1120 a. Figure b indicates as an SEM-micrograph taken from the surface of the specimen the interspherulitic craze formation prior to the cracking process, c shows a site of shear along a spherulite boundary oriented under an angle of about 60° to the horizontal crack direction... 

The high activity shown by the monometallic impregnated catalysts is mainly due to cracking, directly related to the ruthenium metallic phase content and to the mean cluster size. The higher the ruthenium content in the catalyst, the lower the cracking activation energy, as can be seen in Fig. 4. This fact was also observed by Villamil et al [24] in MCP... 

Half-penny crack of radius a at the edge of a spherical pore of radius r with an applied stress a perpendicular to crack direction ... 

Kinematics of these 46 events are plotted in Fig. 8.16. Here, shear and mixed-mode cracks are indicated with the cross symbol, and tensile cracks are denoted by the arrow symbol. In the all cracks, directions of crack normal and crack motion are illustrated. It is found that AE sources of tensile types are mostly concentrated inside the pipe, where the water pressure was applied. The opening directions of tensile cracks are almost vertical to the slit surface, suggesting that water flows due to leakage open the slit. 

It has to be emphasized, however, that the models are not fully compatible among themselves. The tie molecule model often emphasises that the constitution of the polymer determines the number and strength of the anchorage of the tie molecules. Resistance to cracking directly results from the effect of these tie molecules. In the particle model, however, the constitution of the polymer defines its morphology and type and extent of critical... 

From the preceding description of how fracture surface features are formed, it should be apparent that the presence and orientation of those features on a fracture surface can be used to infer crack direction, crack velocity, and other details of the fracture event at the moment the features were created. With that kind of information available, one can deduce a complete history of the failure process, including magnitude and direction of local stresses. In fact, the wealth of information available on the fracture surface makes fractographic examination a very powerful tool in determining how failure occurred. 

Figure 3.476. Progress of crack (roentgenographic) in function of crack direction in the plaques cast under pressure [1255],...

Table 3.136 Normalised toughness, effective orientation and reinforcing effectiveness factor R for different GF-PP materials and crack directions (Note has to be used when the fibres in the layer are nearly parallel to the crack, and the opposite is true iox [1319]...

For tough polyethylenes, the craze angle remains approximately constant as the damage zone grows so the growth of the craze in the crack direction is linearly related to the COD. Figure 12.26 shows the COD versus time for a polyethylene... 

The degree of crack orientation is determined using the stereological method in which a system of oriented secants is superimposed on the image of dendritic cracks and number of intersections is counted. The results may be presented as a rose of intercepts in Figure 9.6 two such roses are shown one indicates quite uniform distribution of crack directions and in the other the cracks are oriented mostly in one direction. 

Stress corrosion cracking starting at pitting corrosion sites can therefore also occur in less aggressive mediums that do not cause stress corrosion cracking directly. In stainless steels, this means resistance to stress corrosion cracking is coupled to resistance to pitting corrosion. 

In the above equation, K represents the crack driving stress intensity in any general direction, is the driving force in the x direction, and a is the angle of inclination of the crack direction to the x direction. 

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