Crack growth-rate

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). 

The use of fatigue data and crack length measurements to predict the remaining service life of a stmcture under cyclic loading is possibly the most common application of fracture mechanics for performance prediction. In complex stmctures the growth of cracks is routinely monitored at intervals, and from data about crack growth rates and the applied loadings at that point in the stmcture, a decision is made about whether the stmcmre can continue to operate safely until the next scheduled inspection. 

ASTM E647-93, "Measurement of Fatigue Crack Growth Rates," Annual Book of ASTM Standards, ASTM Puhhcations, Philadelphia, 1993. 

Fig. 5. Comparison of crack growth rates of titanium aluminides, composites, and IN-100 tests at 650°C, R = 0.1, v = 0.2 Hz except for the composite...

Stress corrosion cracking, prevalent where boiling occurs, concentrates corrosion products and impurity chemicals, namely in the deep tubesheet crevices on the hot side of the steam generator and under deposits above the tubesheet. The cracking growth rates increase rapidly at both high and low pH. Either of these environments can exist depending on the type of chemical species present. 

The level of stress may be, and generally is, much less than the yield strength of the metal. However, in general, higher stresses increase crack growth rate and the number of cracks initiated. 

In the steady-state regime, the crack growth rate is described by... 

Fig. 15.8. Fatigue crack-growth rates for pre-cracked material.

A component is made of a steel for which K. = 54 MN m. Non-destructive testing by ultrasonic methods shows that the component contains cracks of up to 2a = 0.2 mm in length. Laboratory tests show that the crack-growth rate under cyclic loading is given by... 

Nominal stress Crack depth Crack growth rate... 

To discover the effective potential ranges for electrochemical protection, the dependence of the relevant corrosion quantities on the potential is ascertained in the laboratory. These include not only weight loss, but also the number and depth of pits, the penetration rate in selective corrosion, and service life as well as crack growth rate in mechanically stressed specimens, etc. Section 2.4 contains a summarized survey of the potential ranges for different systems and types of corrosion. Four groups can be distinguished ... 

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 ... 

Micro-mechanical processes that control the adhesion and fracture of elastomeric polymers occur at two different size scales. On the size scale of the chain the failure is by breakage of Van der Waals attraction, chain pull-out or by chain scission. The viscoelastic deformation in which most of the energy is dissipated occurs at a larger size scale but is controlled by the processes that occur on the scale of a chain. The situation is, in principle, very similar to that of glassy polymers except that crack growth rate and temperature dependence of the micromechanical processes are very important. 

Fracture mechanics has also been used to predict failure under static stresses. The basis of this is that observed crack growth rates have been found to be... 

Fig. 2.77 Crack growth rate as a function of stress intensity factor...

It would seem easy in principle to separate cracking that proceeds by anodic dissolution from hydrogen-assisted cracking by investigating the effects of polarisation on the crack growth rate, time to failure or some... 

MPa>/m quoted by Speidel and much lower crack growth rates than 10 m/s should be expected even at high K values for properly tempered material. 

Fig. 8.63 Effect of environment on fatigue crack growth rate in 1% Cr-Mo-V steel at 550°C (after Tomkins and Wareing )...

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