Smooth crack growth

Crack propagation observed in epoxies has also been described as smooth crack growth and slip stick growth. Smooth crack growth is more often observed at high strain rates and lower temperatures. Slip stick crack propagation is indicative of... 

When smooth specimens of PS are cycled at 26.7 Hz in a tensile mode at a maximum stress of 17.2 MPa discontinuous crack growth bands similar to those shown in Fig. 8 are seen in the mirror region near the fracture source. These bands are also similar in morphology to those observed in testing notched PS specimens at a minimum stress that is maintained at one-tenth the maximum stress However, DCG bands were not seen when tests are carried out under a fully reversed stress of 17.2 MPa at... 

The failure time, however, incorporates both the time required for crack initiation and a period of slow crack growth so that the separate effect of the environment on each of these stages cannot be ascertained. (Some of the difficulty stems from the lack of a precise definition for crack initiation.) This difficulty is underscored by the results of Brown and Beachem [1] on SCC of titanium alloys. They showed that certain of the alloys that appeared to be immune to stress corrosion cracking in the traditional (smooth specimen) tests are, in fact, highly susceptible to environment-enhanced crack growth. The apparent immunity was explained by the fact that these alloys were nearly immune to pitting corrosion, which was required for crack nucleation in the same environment [1]. 

The number of fatigue cycles associated with pit growth (A pit) and for fatigue crack growth (A feg), and the overall fatigue life Np) in a smooth specimen are as follows ... 

Weld Metals. The weld metal -curves indicated very high crack growth resistance in all filler metals. All curves were smooth, resembling the 13Cr-19Mn base metal. This result confirms the COD evaluation for weld metals. 

---