Slow crack growth

Failure stage Coarse craze Slow crack growth Slow crack growth Fast unstable... 

Eracture mechanics concepts can also be appHed to fatigue crack growth under a constant static load, but in this case the material behavior is nonlinear and time-dependent (29,30). Slow, stable crack growth data can be presented in terms of the crack growth rate per unit of time against the appHed R or J, if the nonlinearity is not too great. Eor extensive nonlinearity a viscoelastic analysis can become very complex (11) and a number of schemes based on the time rate of change of/have been proposed (31,32). 

Sintered sihcon carbide retains its strength at elevated temperatures and shows excellent time-dependent properties such as creep and slow crack growth resistance. Reaction-bonded SiC, because of the presence of free sihcon in its microstmcture, exhibits slightly inferior elevated temperature properties as compared to sintered sihcon carbide. Table 2 (11,43) and Table 3 (44) show selected mechanical properties of sihcon carbide at room and elevated temperatures. 

But cracks can form, and grow slowly, at loads lower than this, if either the stress is cycled or if the environment surrounding the structure is corrosive (most are). The first process of slow crack growth - fatigue - is the subject of this chapter. The second -corrosion - is discussed later, in Chapters 21 to 24. 

It is worrying that a vessel which is safe when it enters service may become unsafe by slow crack growth - either by fatigue or by stress corrosion. If the consequences of catastrophic failure are very serious, then additional safety can be gained by designing the vessel so that it will leak before it breaks (like the partly inflated balloon of Chapter 13). Leaks are easy to detect, and a leaking vessel can be taken out of service and repaired. How do we formulate this leak-before-break condition ... 

Using this model, Xu et al. have demonstrated that for sufficiently slow crack growth rate, i.e., ri - 0, the fracture toughness G a) is given by ... 

Fracture Mechanics Tests One problem of both sustained load and slow strain-rate tests is that they do not provide a means of predicting the behaviour of components containing defects (other than the inherent defect associated with the notch in a sustained load test). Fracture mechanics provides a basis for such tests (Section 8.9), and measurements of crack velocity as a function of stress intensity factor, K, are widely used. A typical graph of crack velocity as a function of K is shown in Fig. 8.48. Several regions may be seen on this curve. At low stress intensity factors no crack growth is... 

In TPE, the hard domains can act both as filler and intermolecular tie points thus, the toughness results from the inhibition of catastrophic failure from slow crack growth. Hard domains are effective fillers above a volume fraction of 0.2 and a size <100 nm [200]. The fracture energy of TPE is characteristic of the materials and independent of the test methods as observed for rubbers. It is, however, not a single-valued property and depends on the rate of tearing and test temperature [201]. The stress-strain properties of most TPEs have been described by the empirical Mooney-Rivlin equation... 

ESC, described in Section 4.9.4, is a physical phenomenon and is the acceleration of stress cracking by contact with a fluid, i.e., stress cracking will occur without the fluid at sufficiently long times. Ultimately, the slow crack growth that follows crazing reaches a critical point when fast crack growth and failure occurs. This failure, with or without the accelerating effect of a fluid, is a creep rupture effect (see Section 8.12). 

Now the measured value of KIc or GIc is dependent upon the sharpness of the crack employed and an essential requirement of fracture mechanics testing is that a "naturally sharp crack is used for the experimental determination of K,c or G C. If a relatively blunt crack is used, an optimistically high value of KIc or G,c may be recorded. Suitable techniques include slow, controlled pressure applied to a razor blade, possibly having pre-cooled the specimen in liquid nitrogen and fatigue crack growth. 

Connaliy, J.A. and S.B, Brown Slow Crack Growth in Single Crystal Silicon. Science, 1537 (June 12, 1992). 

Locally Heterogeneous Deformation Crazing and Slow Crack Growth. .. 22... 

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