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... 

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. 

Figure 9.38 is a BF image of a healed fracture that is about 4 / m wide. The fracture walls are straight and separated by a band of new quartz, which has grown in the open crack. The bands of electron-beam damage centers (Section 8.13.1) parallel to the fracture walls probably mark the positions of trace impurity concentrations formed at incremental growth... 

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

With respect to the cracking process between two successive growth bands it has been suggested by Hertzberg and Manson that cracking occurs by a void... 

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

Inspite of a high band initiation stress Og, the resistance of PS to subsequent crack growth and final fracture is relatively low. The K -values of the crystalline polymers are much higher. A similar connection has also been established recently for various polymers under tensile test conditions... 

In summary, short cracks form epsilon CTPZ development, whereas long cracks usually lack the required shear components to generate the shear-band pair, niis creates the anomalous short crack behavior. When epsilon CTPZs form, the shear bands stabilize the crack, thus allowing the DCG process to continue to much higher apparent AK, values, further differentiating from the non-epsilon CTPZ DCG crack growth behavior. 

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