Unstable crazes

Failure stage Coarse craze Slow crack growth Slow crack growth Fast unstable... 

The relationship between craze initiation and propagation, and crack initiation and propagation, is not well understood (31). Crazing clearly precedes formation of unstable cracking. For example, in tensile tests on Lustran A-21 S/AN, crazing occurred at 1.8-2.0% elongation, whereas... 

Before analysing these results, it is interesting to point out that optical and electron microscopies of the sample region ahead of the crack reveal that a craze develops at the crack tip, whatever the type of fracture regime, i.e. stable or unstable [53]. 

Schematic diagram illustrating brittle fracture. It is typical of the temperature range below 0.87g. A crack nucleates at a pre-existing flow or craze and propagates unstably. 

Craze nucleation appears controlled by the nucleation of voids in localized regions undergoing large unstable plastic deformation. The sensitivity of this process to the nature of the flaw structure of the surface makes a detailed comparison of data between different experimental groups or between experiment and theory very difficult. 

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... 

As we discussed in Sect. 2.3, in aU crazable polymers toughness is limited by fracture which occurs almost exclusively by the breakdown of craze matter under stress. When no extrinsic imperfections are present, craze fibrils fracture as a result of molecular level stress concentrations when load-bearing molecules fracture and set off an unstable cascade of molecular scissions. When large, micron-si d particulate... 

Fig. 3. Craze-crack resistance curves for isothermal conditions with = 30 MPav /s and temperature dependent stress-displacement fields for ki = 300 MPa- /m/s and Ki = 3000 MPa- /m/s. The triangle indicates the initiation of crazing while the square corresponds to the onset of unstable crack propagation, defining...

The load-displacement curves for C(T) tests of the neat EpoxyH were almost linear until the final unstable fracture. The fracture toughness value in 77K-LNj was 210 J/m and that in RT-air was 120 J/m. Thus the toughness increased by 1.8 times by changing the test environment from RT-air to 77K-LN. Brown and co-workers have found that amorphous polymers crazed in 77K-LNj, but not in a helium or vacuum at about 78K [20-22]. They have also reported that the stress-strain behavior of all polymers, amorphous and crystalline, is affected by at low temperatures [22]. Kneifel has reported that the fracture toughness of epoxy in 77K-LNj is higher than that in RT-air and 5K, and that the reason for this is the reduced notch effect by plastic deformation [23]. Then, the increase of the fracture toughness of the neat EpoxyH in this study is probably caused by the similar effect. 

Polyacetal is markedly unstable towards lights because even UV radiation of wavelengths as high as 365 mn may initiate its degradation. Polyacetal cannot therefore be used outdoors if it does not contain any light stabilizers. Even after a short weathering, surface crazes and pronounced chalking are observed. 

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