Crack factor

As well as measuring the strain to cause a given failure mode, the relative stress cracking factor may be determined, in which the ratio of the failure strain in the test fluid to that produced by a reference fluid is found. 

The computational process of analysis is hidden from the user, and visually the analysis is conducted in terms of M-02-91 or R6 [6] assessment procedure On the basis of data of stress state and defect configuration the necessary assessment parameters (limit load, stress intensity factor variation along the crack-like defect edge) are determined. Special attention is devoted to realization of sensitivity analysis. Effect of variations in calculated stress distribution and defect configuration are estimated by built-in way. 

For interpretation of measuring results, calibration characteristics obtained on the samples in advance is used in the above instruments. However, if number of impediment factors increases, the interpretation of the signals detected becomes more complicated in many times. This fact causes the position that the object thickness T and crack length I are not taken into consideration in the above-mentioned instruments. It is considered that measuring error in this case is not significant. 

The algebraic form of the expression (9.24) for the enhancement factor is specific to the particular reaction rate expression we have considered, and corresponding results can easily be obtained for other reactions in binary mixtures, for example the irreversible cracking A—2B. ... 

Surface Area. Surface area is the available area of fillers, be it on the surface or in cracks, crevices, and pores. The values obtained from different methods for measuring the surface area of a filler may vary significandy. These variations are because of the nature of the methods and in many instances yield information related to the heterogeneity of the surface. Understanding the surface area is important because many processing factors are dependent on the surface area, eg, ease of filler dispersion, rheology, and optimum filler loading. 

Flexural modulus increases by a factor of five as crystallinity increases from 50 to 90% with a void content of 0.2% however, recovery decreases with increasing crystallinity. Therefore, the balance between stiffness and recovery depends on the appHcation requirements. Crystallinity is reduced by rapid cooling but increased by slow cooling. The stress—crack resistance of various PTFE insulations is correlated with the crystallinity and change in density due to thermal mechanical stress (118). 

Therefore, the magnitude of the stress at small distances from the crack tip is a function of the crack length, a, and the remotely appHed stress. O. Close to the crack tip (r ft) the stress can be scaled usiag a parameter called the stress intensity factor, K (9—11) ... 

Fuel factors which contribute to IVD have been shown to include olefins (53,54), alcohols, and cracked stocks (55). 

Fracture Mechanics. Linear elastic fracture mechanics (qv) (LEFM) can be appHed only to the propagation and fracture stages of fatigue failure. LEFM is based on a definition of the stress close to a crack tip in terms of a stress intensification factor K, for which the simplest general relationship is... 

Lack of accepted stress intensity factors for internally pressurized components has, until recently, limited this appHcation. The factors are a function of the size and shape of both cracks and high pressure components as well as modes of loading (91). Stress intensity factors can be derived analytically for some simple geometries, but most require the appHcation of advanced numerical methods (105—107). Alternatively they may be deterrnined experimentally (108). 

Another important appHcation of LEFM is the rate of growth of a fatigue crack under cycHc loading. This is also controlled by the stress intensity factor through an equation of the following form (110) ... 

One aspect of pressure vessel design which has received considerable attention in recent years is the design of threaded closures where, due to the high stress concentration at the root of the first active thread, a fatigue crack may quickly initiate and propagate in the radial—circumferential plane. Stress intensity factors for this type of crack are difficult to compute (112,113), and more geometries need to be examined before the factors can be used with confidence. 

Fig. 15. Crack growth vs cyclic iatensity factor for particulate MMCs.

Resistance to Chemical Environments and Solubility. As a rule, amorphous plastics are susceptible, to various degrees, to cracking by certain chemical environments when the plastic material is placed under stress. The phenomenon is referred to as environmental stress cracking (ESC) and the resistance of the polymer to failure by this mode is known as environmental stress cracking resistance (ESCR). The tendency of a polymer to undergo ESC depends on several factors, the most important of which are appHed stress, temperature, and the concentration of the aggressive species. 

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