Discontinuous crack growth bands

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... 

Fracture surface morphology varies with stress amplitude and test frequency as well as with composition and molecular weight. Even under fully reversed cycling, discontinuous crack growth bands have been observed on the fatigue fracture surface in PS, SAN and ABS but they have not been seen in HIPS. 

Discontinuous crack growth bands were first observed on the fracture surface of PVC specimens subject to cyclic loading by Elinck et al. The effect of molecular weight on band stability has been studied and some of the test results for three different molecular weights are displayed in Fig. 17. As M increases and the number of entanglements per chain rises, the data show that, for any given AK value, a greater... 

Fig. 53. Influence of AK, on craze lengths s and band width b in PVC during discontinuous crack growth...

Fig. 55. Discontinuous growth bands in PVC Crack propagation direction from left to right normal to the bands. AK, = 17.5 N/mm ...

HIC differs from SSC in two major aspects (1) HIC is observed with lower strength steels, and (2) HIC is associated with inclusions in the steel or with anomalous structures such as midwall banding. Crack initiation results from formation of molecular hydrogen at inclusions and discontinuities in the steels, and crack growth results from applied and residual stresses aided by the embrittling effect of hydrogen. 

Regardless of the water content or AK level, no DGB s have yet been observed in any nylon specimens (either Series A or Series B). By contrast, the discontinuous growth of fatigue cracks has been described in studies of both crystalline and amorphous polymers [for example, in polyacetal (29,30), polyethylene ( ), and a variety of poorly crystalline or amorphous polymers ( ) ]. While the process of DGB formation in poorly crystalline or amorphous polymers is fairly well understood ( 5, 12,29,32) the role of well developed crystallinity is not clear. Hence the reason why DGB bands are not observed in nylon 66 is as yet unknown. 

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