Craze dimensions

Besides, analysing the effect of MW of PMMA on the increase in craze dimensions with time [44], leads to the conclusion that the molecules in the craze fibrils are not fully stretched, retaining folds and thus keeping entangled. So, it supports the consideration of r(crc) as a disentanglement time. 

In PMMA of high molecular weight the craze dimensions have been measured at moving crack tips in the speed range of 10 mm/s room temperature using an experimental set-up especially developed to apply the... 

The effect of molecular weight on craze dimensions has been investigated in more detail in PC andPMMA The results of Pitman and Ward in PC measured... 

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... 

From the reported craze dimensions, the tensile creep moduli E and craze stress have been derived by the aid of the Dugdale model. For PMMA the thus evaluated creep moduli are shown as a function of temperature T in Fig. 18 together... 

At the lower end of the crack speed range the craze dimensions increase the slowlier the crack moves. This is to be seen as a reflection of the time dependence of craze growth discussed in Sect. 3.2.2 (slower speeds = longer times). At the other end of the speed range, at speeds rising to the transition speed ( 10 mm/s), the increasing build-up of heat in and around the craze leads to a rise in temperature. For example, using the data for PMMA (1) in Fig. 15, it can be estimated that there... 

For craze zones at the tips of static and of moving cracks under quasistatic loading conditions it has been shown in Sect. 3 that the normal stress acting on the craze zone and the modulus E can be derived from the measured craze dimensions using the Dugdale model. 

It can thus be concluded that during the so-called continuous mode of fatigue crack propagation there is no simple correlation between fatigue striation spacing and craze dimensions. 

A third method which recently provided considerable insight into the role of crazes in deformation and fracture of amorphous polymers is the optical interference measurement of crazes (preceding a crack). Since the pioneer work of Kambour, this method has been widely used to determine characteristic craze dimensions and critical displacements. W. Doll gives an overview on recent results and on their interpretation in terms of fracture mechanics parameters (stress intensity factor, plastic zone sizes, fracture surface morphology, fracture energy). 

The influence of molecular weight distribution has been examined in that region of molecular weight showing a marked increa in craze dimensions at constant... 

From the reported craze dimensions, the tensile creep moduli E and craze stress... 

Fig. 3.22a, b. Speed dependent variation of the craze dimensions at the tips of propagating cracks in PMMA of different molecular weights a craze kngth s and b maximum craze width 2v ... 

Continuing the discussion of the results in Fig. 3.22, for crack speeds above about 10 mm/s it may be observed that craze dimensions are nearly constant in PMMA of LMW and increase in HMW PMMA with crack speed up to a = 1(F mm/s. The reason for this constant or increasing craze size is to be seen in the following Due to the fibrillation process which is associated with internal friction, heat is produced in front of the propagating crack tip The local heat output due to the... 

In the context of the above paragraph it is worthwile to re-examine the data of HMW PMMA in Fig. 3.22 showing the measured craze dimensions as functions of crack speed. The time dependence of these data is shown together with the respective craze growth data from Fig. 3.6 schematically in Fig. 3.28. Crack speed has been converted into time using Eq. (16) with the correlation length 1 = 300 nm determined empirically in Fig. 3.26. 

In the interval between two successive crack jumps the crack length remains unchanged and hence the stress intensity factor range determined macroscopkally is constant. In order to investigate the influence of the stress intensity factor on the craze dimensions, crazes under different loads have been compared. 

Fig. 4.26. Influence of AK, on critical craze dimensions during RCG in PVC a maximum craze widths at lower load (2vo) and at upper load immediately after (2vi) and just before (2vJ crack jump, b craze lengths (s,) and before (Sj) crack jump and band width (b)...

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