Craze micromechanics

The stress profile S(x) along the interface of an isolated craze (one that is not grown from a crack tip) can be considered to be the sum of two terms 

Here E is an effective Young s modulus of the polymer (E = E for plane stress and E/(l — f) for plane strain v = Poisson s ratio) and a is the half length of the craze. The craze displacement profile is computed from 

By requiring that the stress singularity at the stationary crack tip must vanish the level of applied stress can be determined to be 

The dislocation method of stress analysis is also useful for determining craze stress fields in anisotropic (e.g., oriented) polymers . All one needs here is the stress field of a single dislocation in a single crystal with the same symmetry as the oriented polymer (the text by Hirth and Lothe provides a number of simple cases plus copious references to more complete treatments in the literature) the craze stress field can be generated by superposition of the stress fields of an array of these dislocations of density a(x). Dislocations may also be used to represent the self-stress fields of curvilinear crazes (produced by craze growth in a non-homogeneous stress field for example). Such a method has been developed by Mills 

From the surface stress profile S(x) and fibril volume fraction profile Vf(x) it is also possible to find the true stress in the craze fibrils Of(x) which is 

---

Craze Micromechanics at the Onset of Slow Crack Growth. . . 166... 

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. 

Doll, W. and Konczol, L. Micromechanics of Fracture under Static and Fatigue Loading Optical Interferometry of Crack Tip Craze Zones. Vol. 91/92, pp. 137 — 214. 

Fatigue resistance increases with the [PU] up to 50Z, while energy absorption determined from dynamic properties and pendulum impact tests varies directly with the [PU], The micromechanism of failure Involves the generation of discontinuous growth bands associated with shear yielding rather than crazing. 

In order to understand fracture behaviour, it is important to analyse the types of deformation micromechanisms undergone under strain chain scission craze (CSC), shear deformation zone (SDZ), chain disentanglement craze (CDC) and the temperature range over which each one occurs. Furthermore, it is worth wondering whether these micromechanisms are related to /i transition motions. 

As above mentioned, the presence of CMI units within the PMMA chain backbone makes the sample considerably more brittle. Such an effect is already reflected in the type of deformation micromechanisms more and more crazes when the CMI content increases. 

In the case of BPA-PC, the thin film investigation of deformation micromechanisms (Sect. 4.2) shows that CDCs occur around 60 °C. So, it is unlikely that the craze at the crack tip occurring at - 20 °C, or above, could be a CDC. The observed MW dependence of failure originates from the above described mechanism with CSCs. 

Before discussing specific aspects of micro deformation and fracture in bulk polyolefins, some basic notions of microdeformation and the micromechanics of fracture mediated by generation and breakdown of cavitated or fibrillar deformation zones or crazes are introduced. SCG in PE and rate-depen-dent fracture in iPP are then considered in more detail. 

Important current concepts of the micromechanisms involved in crazing and environmental aspects of this subject have been developed by Kramer and coworkers they are described in Chapter 1 of this volume. 

Here, possible micromechanisms of ionomers are discussed. At low ion content (less than 5 mol %), only crazes are noted and these appear to become unstable with increasing ion content. These ionomer samples tend to be more brittle compared with the PS precursor. This is probably due to the aosslinking effect produced by the ionic multiplets that are present. As mentioned before, it is known that small ionic aggregates (multiplets) are produced due to ionic dipole attractions and that... 

In this section the kinetics of craze growth in air will be considered in unnotched specimens as well as at crack tips. We shall not be concerned with the initiation phase and any micromechanism (e.g. leading to craze initiation. 

Unfortunately, the initiation and evolution of crazes do not concern only the majority of thermoplastic glassy polymers, which exhibit brittle behavior. Crazes usually also constitute the dominant micromechanism for failure when many polymers generally considered tough are subjected... 

Kim and Michler have observed the relationship between morphology and strain micromechanisms in cases of both rigid and elastomeric filler growth of voids, by cavitation or debonding [7,31]. Oshyman has reported a transition, at a certain fraction of filler, correlated to the evolution from macroscopic homogeneous strain to micromechanisms such as crazes. It is in fact a transition between independent mode and correlated mode of strain micromechanims [32]. 

---