Propagation slow crack

Outstanding properties impact toughness processability bimodal PE high tempera-ture/high pressure performance, resistance to rapid crack propagation, slow crack growth, toughness ... 

Ceramic composites, which use ceramic fiber or whisker reinforcement in a ceramic matrix, are less susceptible to brittle failure since the reinforcement intercepts, deflects and slows crack propagation. At the same time, the load is transferred from the matrix to the fibers to be distributed more uniformly. These ceramic composites are characterized by low density, generally good thermal stability, and corrosion resistance. 

All thermoplastics are known as viscoelastic materials and hence exhibit time dependent properties. It is known that during crack propagation the influence of time is also involved such that slow crack speeds correspond to long times and high crack speeds to short times. In the following we will be concerned with this problem in more detail. 

Another aspect of viscoelastic behavior is the influence of temperature and in the slow crack propagation region is has also been well documented. Marshall et al. observed that in PMMA the crack speed curves are shifted to lower Kpvalues with increasing temperature and that also Ki decreases in the temperature range from -60 Cto 80 C. 

Such a curve of K, vs. d is naturally dependent on the material investigated and also on material specific parameters as molecular weight in PMMA this leads to a shift to lower K,-values or equivalently G -values with decreasing molecular weight It should be noted that slow crack propagation curves of quasi-brittle materials have been used by different authors [e.g. predict creep life curves ... 

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... 

It should be noted that slow crack propagation curves of quasi-brittle materials have been used (e.g. to predict life-time curves for loaded structures... 

In the previous sections properties of crack tip crazes in thermoplastics within two different regimes of damage behavior have been described, that of stationary and slowly propagating cracks. During steady state slow crack propagation, as described above, at a particular crack speed the crack tip is preceded by a craze zone of constant size, indicating an equilibrium between fibril formation and fibril failure as demonstrated exemplarily in Fig. 3.27 a by two interference micrographs of the... 

It has been proven by other authors that the P-relaxation is the mechanism governing the craze formation process for stationary as well as for propagating crazes in the slow crack speed region Furthermore, also from crack speed measurements (K, vs a) in the slow propagation regime performed on PMMA in the temperature range of —60 °C to -1-80 an activation energy appropriate for the P-process has been derived... 

Figure 8.44. Crack front propagation slowed down by a pinning mechanism. [Adapted, by permission, from Azimi H R, Pearson R A, Hertzberg R W, J. Appl. Polym. Sci., 58, No.2, 1995,449-63.]...

The value of the dissipative factor tan S=G IG for the four adhesive blends is shown in Fig. 22.21. Clearly the addition of diblock in the blend has no effect on the dissipative properties of the adhesives at high frequencies but it has a significant effect at low frequencies. A relaxation experiment such as that described in Fig. 22.20a and b involves a very slow growth of interfacial cracks, precisely in the regime where linear viscoelastic properties differ. We can therefore propose, at least qualitatively, that the spectacular improvement in adhesive properties observed for the high diblock adhesives on EP surfaces is due to their more dissipative character, which slows crack propagation considerably at the interface, therefore avoiding early coalescence between separate cavities, and favors the formation of a fibrillar stracture with the cavity walls. 

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