Craze width

In a thermoplastic material it is, therefore, important to distinguish between the crack opening stretch (= COS) and the maximum craze width 2v. To characterize plastic deformation and fracture behavior of a thermoplastic material the maximum length of stretched fibrils and hence the maximum craze width 2v is a more fundamental parameter than the crack opening stretch. The latter, in addition, depends on the relaxation behavior of the broken remnants on the fracture surface. 

Fig. It a and b. Variation with crack speed of the dimensions of the craze zone at the tip of a propagating crack in PMMA a maximum craze width 2Vj b craze length s...

Figure 11 a shows the maximum craze width 2v as a function of crack speed d and Fig. lib the corresponding craze length s. Initially the results were interpreted as... 

In the context of the results for moving cracks discussed in the previous section it should be noted that the craze sizes reported in this section have been measured from just after onset of slow crack propagation to speeds up to 10 " mm/s Quantitative results of maximum craze width as a function of temperature T are compiled in Fig. 15 for PMMA pc and PVC At a first glance there... 

In PMMA the effects of molecular weight and of molecular weight distribution on craze dimensions and on crack opening have been intensively studied In Fig. 17 the results of the maximum craze width are represented as function... 

Fig. 16. Maximum craze width 2Ve in polycarbonate measured at —30 °C as a function of, molecular weigth M ...

Fig. 17. Maximum craze width 2V(. in PMMA of different molecular weight distributions as a function of molecular weight M at different temperatures...

In all papers mentioned above the characterization of the craze growth has been performed by measuring the craze length. By application of the optical interference method the essential information on the growth of craze width is awiilable. Figure 22 shows the interference fringe patterns and measured craze zone sizes at a stationary crack tip in PMMA loaded in a creep test at a constant K,-value at loading times of 150 s and 2 1(F s... 

Fig. 23a and b. Growth of a primary craze at a stationary crack tip in PMMA in air, K, = 19 N/mm a craze length s as a function of loading time t b maximum craze width 2v as a function of loading time t... 

Fig. 32. Maximum craze width 2v, at the crack tip as a function of crack propagation rate da/dN...

In order to obtain information on the craze growth and on the fibrillation process the maximum craze widths are plotted against craze lengths s for high molecular... 

Further information on craze thickening in fatigue can be obtained by comparing the width of the strined craze with the craze width 2v at the lower fatigue load. 

Fig. 36. Relationship between craze width 2v and length s during crack propagation under fatigue and quasistatic (circular area) loading...

The quantitative development of craze length s and maximum craze width 2v at the crack tip are given in Fig. 50 as functions of the cycle number N for several successive crack jumps. In this experiment (AK, = 15.2 N/mm, R < 0.1) the crack remained stationary up to a critical cycle number Ne which is connected with a critical end length Se or end width 2v respectively. Together with the result of Fig. 49 it is obvious that the crack does not go through the craze all the way up to the craze tip as is usually assumed It stops well ahead of the craze tip, even at this... 

Fig. 52. Influence of AK, on craze widths 2v at the crack tip in PVC during discontinuous crack growth...

The craze just behind the tip, however, is only a few nanometers wide, whereas crazes typically can attain widths of a few micrometers or so before fracture. Clearly, while the craze tip advance mechanism is responsible for initially generating very short lengths of fibrils, most of the fibril structure is generated by the mechanism of craze width growth. Even the stress conditions at the craze tip can be dramatically altered by the widening of the craze well behind the tip if the craze continues to widen without the craze tip advancing, the stress at the craze tip will rise until the craze tip... 

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