Stable crack growth

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). 

Quinten, A. and Arnold, W. (1989). Observation of stable crack growth in Al203 ceramics using a scanning acoustic microscope. Mat. Sci. Engng. A122, 15-19. [278]... 

Many studies [40-43] have been performed on the fracture behaviour of PMMA as a function of temperature or cross-head speed. As an illustration, Fig. 30 shows the critical stress intensity factor, K c, in a log-log plot as a function of temperature for various crack speeds [40]. The temperature range is limited to + 80 °C in order to avoid ductile tearing. In the stable crack growth regime of interest here, whatever the crack speed, K c decreases with increasing temperature. 

Fig. 30 Fracture toughness, iCic, as a function of crack speed at various temperatures for stable crack growth (From [40])...

Investigation of the crack tip of PMMA after a stable crack growth reveals that, at room temperature, a single craze is present. 

Conditions for stable/unstable crack growth in a layered structure (a) a range of crack length parameters for stable crack growth in a laminate with a residual compressive stress in a top layer (b) stable/unstable crack growth in a laminate with a residual tensile stress in a top layer. 

Both AC and DC potential-drop methods are well-established techniques for monitoring subcritical crack growth. A combined AC/DC potential-drop measuring technique can, in some cases, help in obtaining more information from a single test, in particular for the onset of stable crack growth.172... 

Dietzel, W., Schwalbe, K.H., Monitoring Stable Crack Growth Using a Combined AC/DC Potential Drop Technique, Material Prufing, Band 28, pp. 368-372, 1986. 

L.-O. Fager and J. L. Bassani, Stable Crack Growth in Rate-Dependent Materials with Damage, Journal of Engineering Materials and Technology, 115, 252-261 (1993). 

Paris [75] showed in 1964 that a linear relationship predicted by a simple power law for a double-logarithmic scale exists between the FCP rate da/dN and the applied AK (Fig. 6). The linear dependence is frequently observed only over an intermediate range of growth rates. On the double-logarithmic scale, the region of stable crack growth is usually satisfactorily described by the Paris-Erdogan equation... 

Fig. 9a and b. Stable and unstable fracture behavior as observed on Instron recorder traces when testing constant compliance fracture specimens, a) trace, stable crack growth b) trace, unstable, stick-slip crack growth... 

Fig. 10 a and b. Fracture toughness versus rate (a) and temperature (b) showing typical thermoset fracture behavior. I = initiation A = arrest E = stable crack growth... 

The effects of moisture on epoxy fracture are not conclusive. Scott et al. reported that an amine cured epoxy, normally displaying stick-slip fracture at room temperature and low rates, exhibited stable behavior when immersed in distilled water. Also, they found that the rate necessary to promote the unstable to stable crack growth transition at room temperature was increased by two orders of magnitude in the presence of the water. Yamini and Young , on the other hand, found that testing in water tended to suppress stable behavior and promote stick-slip fracture in an amine cured epoxy over a wider range... 

Fig. 12. Stoichiometric DGEBA/DDS network fracture toughness versus temperature. Fracture toughness from double torsion specimen at crosshead rate of 0.05 cm/min. Network T, s shown by dashed lines. O Initiation Arrest 9 Stable crack growth. (After LeMay...

Since Wf = U and A = It, Equation (14) is similar to Equation (12). It has therefore been argued [23] that the impact fracture energy at crack initiation in polymers with ductile behavior G is rather the essential fracture work we. Furthermore, the second parameter representing the variation of the impact fracture energy during stable crack growth, T, has been attributed to the work dissipated in the outer plastic zone and is not related to the fracture process [23]. 

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