Rate of crack propagation

Fig. 22. Nomialized pull-off energy measured for polyethylene-polyethylene contact measured using the SFA. (a) P versus rate of crack propagation for PE-PE contact. Change in the rate of separation does not seem to affect the measured pull-off force, (b) Normalized pull-off energy, Pn as a function of contact time for PE-PE contact. At shorter contact times, P does not significantly depend on contact time. However, as the surfaces remain in contact for long times, the pull-off energy increases with time. In seinicrystalline PE, the crystalline domains act as physical crosslinks for the relatively mobile amorphous domains. These amorphous domains can interdiffuse across the interface and thereby increase the adhesion of the interface. This time dependence of the adhesion strength is different from viscoelastic behavior in the sense that it is independent of rate of crack propagation.

A further estimation of the corrosion resistance of maraging steel can be obtained from data on the rate of crack propagation. Although the rate of crack propagation has been found to be a function of stress intensity in some alloys, for many alloys and heat treatments there is a range of stress... 

When stress corrosion involves very localised dissolution, with the geometrical requirements of a crack to be fulfilled, the rate of anodic dissolution may be expressed as a rate of crack propagation... 

In the high crack velocity regime three different values of Kid can be assigned to one rate of crack propagation depending on the state of crack acceleration. This behaviour was ascribed to inertia effects associated with crack acceleration and deceleration. Such a hypothesis is corroborated by the computed K data (also shown in Fig. 9), which were obtained from a finite element model, taking into consideration the mentioned transient dynamic linear elastic effects [35]. 

Brittle fracture in metals is characterized by a rapid rate of crack propagation, with no gross deformation and very little microdeformation. This is demonstrated by cleavage. Brittle fracture can occur without warning. Cleavage fracture exhibits little or no plastic deformation and occurs along well-defined crystallographic planes. 

Thus, whether a crack will run a great distance or quickly arrest depends on the relative rates of crack propagation and depressurization. Prediction of the outcome requires a computer code that compares the gas dynamics and the fracture dynamics. The framework for such RCP prediction has been developed [1], but an important task remains before RCP management can be implemented A ftindamentally sound, independent test for measuring the material property governing RCP and crack arrest. 

The rate of crack propagation is a strong function of the applied stress. 

Bueche and White15) observed the progress of rupture in a tensile specimen. Because the rate of crack propagation reached a maximum which was equal to the veloc-... 

Stage n is the stress corrosion region where the rate of corrosion is similar to the rate of crack propagation. Therefore, the crack remains sharp and propagates into weakened glass. 

V rate of crack propagation limiting crack velocity... 

Creep life is limited by the rate of crack propagation. Critical crack size development depends on the stress dependence of the failure time. The time-to-failure at stresses under tension below a certain limit (82 MPa for alumina) is a consequence of an increase in failtrre strain at the crack tip. Basically, failure is... 

Figure 11.28 schematically represents a crack in a metal that is in contact with gaseous hydrogen. The crack propagates by successive brittle fractures, which result from the presence of hydrogen in the zone of maximum stress, just in front of the crack tip. Three reaction steps participate in the accumulation of hydrogen in this zone, and therefore determine the rate of crack propagation ... 

The most common form of evaluation is time to failure for stress corrosion specimens. Failure usually means observation of the first crack. In relatively brittle materials, small cracks propagate rapidly causing the material to break into pieces. For specimens with constant loads, the rate of crack propagation increases as cracking proceeds, and failure is usually manifested as breaking of the specimen. Most specimens, however, are stressed by means of a constant strain. In such cases, the rate of propagation decreases after the initial crack is formed and, consequently, the best measure of time to failure is the time to observation of the first crack. 

---