Discontinuous growth bands

Fatigue resistance increases with the [PU] up to 50Z, while energy absorption determined from dynamic properties and pendulum impact tests varies directly with the [PU], The micromechanism of failure Involves the generation of discontinuous growth bands associated with shear yielding rather than crazing. 

With respect to micromechanisms of failure, at low values of Ak, discontinuous growth bands whose spacings correspond to many cycles of loading were observed (26,32). Figure 10 shows the effect of composition on r, the spacing of the bands, the yield stress o (estimated from the Dugdale relationship, r ifK /80 ), and tlJe... 

Figure 11. Comparison of ra the length of discontinuous growth bands as a function of AKa for PU/PMHA SINs with values for several other polymers (26).

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

Fig. 56a and b. Micromorphology in discontinuous growth bands in PVC Details of Fig. 55. a Surface structure around the fatigue line b Structure of fibrils at the position between two fatigue lines... 

All of the SAN samples, whether of 25% or 30% AN tested in fatigue at 21 Hz, displayed a fracture surface morphology that was in some respects akin to that of PS and in other respects quite different. Like PS, fracture developed from a surface source and the region surrounding the source was comparatively smooth but, unlike PS tested at this frequency, discontinuous growth bands were seen in this region. 

FCP resistance for the SINs increases with PU content up to 50% and is better in the prepolymer material than in the one-shot material, since the former always has larger values of percent energy absorption. With respect to micromechanisms of failure, the generation of discontinuous growth bands associated with shear yielding is involved in the SINs from the one-shot procedure. On the other hand, the fracture surfaces of the SINs from the prepolymer procedure show extensive stresswhitening phenomenon which is associated with the cavitation around PU domains and localized shear deformation. 

Classical fatigue striations are observed at intermediate water contents, confirming earlier findings. However, no evidence was found (at any water content or AK level) for discontinuous growth bands, which are found in many other polymers. 

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... 

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. 

Photoelectron spectroscopy is a highly surface sensitive technique because of the inelastic mean free path of the photoelectrons Ae, which depends on the electron kinetic energy Ekin and has typical values of 0.2-3nm [31,37,38]. Determination of Schottky barrier heights b, or valence band discontinuities AEyB, can be performed by following the evolution of the position of the valence band maxima with respect to the Fermi level of substrate and overlayer with increasing thickness of the overlayer. For layer-by-layer growth the attenuation of the substrate intensities is given by the inelastic... 

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

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