Cracking modes

Tabulated below are the energy changes involved for different cracking modes, with numerical values for the case of copper in contact with liquid... 

The relative reactivity of hexane and 3-methylpentane (about 1000) in the isomerization mode was shown to be the same as found for isomerization in HF-SbF5.102 In the cracking mode, however, the ratio is about 10, resulting from the dramatic acceleration of the reaction of hexane compared to that of 3-methylpentane. Further characteristics of the cracking mode are a large excess of branched isomers in the C4—C5 fractions, the absence of unsaturated cracking products, and formation of... 

This mode of failure is possible in all the copper alloys. The principal environment involved is ammonia. The evidence also exists for other media such as citrates, tartrates, nitrites, sulfur dioxide, carbonates, nitrogen oxides and phosphates to be conducive to a stress-corrosion cracking mode of failure of copper alloys. 

The environments, along with the cracking modes of zirconium and titanium, are given in Table 4.88. It is obvious from the table that zirconium alloys are susceptible to stress-corrosion cracking in a variety of environments. It is necessary to subject the weld to heat treatment in order to lower the stress in the weld. The most serious problem encountered in the nuclear applications is delayed hydride cracking in addition to stress-corrosion cracking, particularly in Zr-2.5% Nb alloy. 

Intergranular and transgranular cracking often occur simultaneously in the same alloy. Such transitions in crack modes are observed in alloys with high amounts of nickel, iron chromium and brasses. In corrosion under tension, ruptures are fragile and are sometimes characterized by the presence of cleavages, notably in the case of hydrogen embrittlement.16... 

It is useful to note that there may be more than one cracking mode that can cause a failure. The following four main environmental cracking modes can result when steels are exposed to wet H2S environments (108, 109). 

In this chapter, brittle fracture is first discussed for the simple case of a uniaxial and homogeneous stress field (as in a tensile test), and for a single crack-like flaw which is oriented perpendicular to the stress field (in so-called mode I loading). These simple conditions are suflficient to explain the basic features of brittle fracture. later, the analysis will be extended to more complex and more general situations, including multiple cracks, the arbitrary orientation of cracks (mode II and mode III loading), and multi-axial stress fields. 

Positive polarities would be measured at all sensors, in the case of an opening crack (mode I). In the case of a shear fracture, the polarity of the P-wave onset changes from positive (upward deflection of waveform) to negative (downward deflection) according to the position of the sensor relative to the source and the shear planes (Fig. 5.17). These two examples assume that the sensors have been calibrated properly, so that a positive deflection of the signal indicates movement away from the source (i.e. compression). 

The deviatoric component can be further decomposed into a variety of eigenvalue combinations that represent simple arrangements of equivalent body forces, adapted from fracture mechanics, like that of an opening crack (mode I) or of shear cracks (mode II or III) (Fig. 5.27),... 

By applying the moment tensor analysis, kinematics of cracks can be analyzed (Ouyang, Landis et al. 1992). In the expansion test, which simulates crack propagation due to corrosion of reinforcing steel-bar, the moment tensor analysis was performed to identify cracking mechanisms. Here, crack modes of micro-cracks are classified into a tensile crack, shear crack and the mix-mode as illustrated in Fig. 10.27. 

Fig. 8.27 SEM micrograph of the fracture surface of the material showing the predominantly transgranular fracture crack mode [55], With kind permission of Elsevier. (ADS96RisaMgO-CaO-Al203 glass matrix)...

The Normal plant start up procedure involved the operation of the plant with minimum load of three furnaces in cracking mode and two on standby mode before Propane Refrigeration Compressor (PRC) and Ethylene Refrigeration Compressor (ERC) Run at normal conditions. 

In order to estimate the flared quantity for the normal case, two main assumptions are made 1) in Normal Startup, it is assumed that the plant is operated with minimum load of two furnaces in cracking mode. The minimum load of the fiimace is 26.5 T/h, which is 80 T/h for the three fiimaces. Thus, the flared quantity in 7-8 days is around 15,360 T of gas. 2) In normal shutdown, the fiimace throughput is reduced to 30% of total plant load, which is equivalent to 96 T/h. Thus, the flared quantity in 7-8 days is around 18,432 T of gas. 

All the cracks found were surface cracks. No crack growth rates have been estimated. Metallographic analysis showed three different crack modes ... 

J. T. Hagan, Deformation and Cracking Modes Around Plastic Indentations in Glasses, Verres Refract., 35 (2) 306 314 (1981). 

An increase in frequency induces a decrease of the crack growth rate and a transition from intergranular to transgianular cracking mode imtil air cyclic behavior becomes predominant. 

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