Craze material properties

Schirrer, R. Optical Interferometry Running Crack-Tip Morphologies and Craze Material Properties. Vol. 91/92, pp. 215-262. 

Optical Interferometry Rinming Crack-Tip Morphologies and Craze Material Properties... 

As discussed extensively in the previous sretions, some craze material properties may be inferred from the craze shape. The plots showing these properties are much more meaningful than the direct experimental values plotted previously. Nevertheless, they are calculated by means of models whose validity might be open to discussion. The simplest assumption that can be made is that the craze stress is constant along the craze boundary. This has been shown to be true for PMMA air crazes. Section 4.3 wilt be devoted to stress profile along these crazes in solvent vapors. 

As an example, Fig. 8 shows the fracture toughness for PMMA and Fig. 9 the fringe pattern transition at the critical temperature, whereas Fig. 10 shows the lateral face of the sample with the crack-tip above and below the critical temperature. It has also been shown that neither the bulk modulus nor the craze stress varies near the critical temperature (Fig. 11 and 12). It seems that the local material property varying near that particular temperature is the craze stiffness, as shown in Fig. 13. 

Figure 26 shows the life-time at 3 temperatures (—10, 20 and 60 °C) for Polymethylmethacrylate (PMMA). As many physical properties of bulk polymers, the life-time may be shifted along the time axis to get one single master curve shown in Fig. 27. It should be noted that none of the other craze properties (Ki(Vj), craze length, craze thickness) can be shifted like the life-time to obtain a master curve. This fact tends to prove that the life-time is a more relevant material property than craze length or craze thickness alone.

As usual with optical interferometry, craze length, craze thickness, crack velocity, and fracture toughness are obtained from the experiment, and local material properties are obtained from the preceding results and the use of some models of crack-tip craze micromechanics. 

The crazing process does not induce large-scale energy dissipation in pure Polypropylene so the cavitation / dewetting process is the main toughening mechanism in CaC03 filled Polypropylene. The basic micro-mechanisms depend on the material properties and on the loading conditions. 

There is no firm agreement as to the mechanism of craze initiation but it is known that both initiation and propagation are stress-biased, thermally activated processes, dependent upon the material properties. 

The propagation of crazes can be split into three considerations kinetics, interfacial stress and breakdown. Growth is explained in terms of the Taylor meniscus instability model, where the polymer at the growing craze tip becomes less viscous due to the action of stress. The velocity of the craze tip through the material can then be calculated from material properties, the state of the stress-strain field and envirorunental variables since they affect the viscosity boundary at the tip. Variants of... 

---