Transition crazing - shear deformation

J Craze-shear Deformation Transitions — High Entanglement Density Polymers... 

There has been considerable progress over recent years in the study of molecular mechanisms involved in crazing and shear yielding [1,2]. When sample preparation and test conditions (temperature, stress state, strain, strain rate, and thermal history) are chosen to be the same, the effect of molecular variables can be determined. It is now well established that (network) strand density plays an important role in determining the deformation mechanism and in affecting the craze-shear deformation transition. Polymers are... 

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. 

Above its glass transition temperature, shear processes dominate in sPS as was shown earlier. In Figure 19.15, the TEM images of deformed sPS modified with the core-shell modifier prepared using the microsuspension method are reproduced. At the surface of the specimen highly oriented rubber particle are discernible without voiding. In the inner part of the specimen, however, cavi-tated highly oriented particles have been formed. Crazes are not seen in the... 

Some of the most important early experimental observations were of transitions from the quasi-brittle crazing deformation mode to the ductile shear deformation mechanisms with changes in the experimental conditions, such as temperature and strain rate, as well as in polymer variables, such as polymer backbone architecture, blend composition, crosslinking and physical aging state of the polymer glass. One of the strengths of the model of craze growth outlined above is that it allows one to make sense out of some experimentally observed craze-to-shear transitions that had previously defied explanation . The idea behind this explanation is quite simple One writes an expression for the shear yield stress, viz ... 

One of the great successes of the craze growth model is that it predicts a transition from scission-dominated crazing to shear deformation as the strand density of the network is increased. While the shear yield stress is essentially unaffected by dian ... 

Fig. 9. Plot of the true suain ratio in craze and deformation zones showing the transition from crazing to shear deformation as a function of network strand (entangled + crosslinked) density v. The open squares and open diamonds represent uncrossiinked homopolymers and copolymers, the open triangles and hexagons represent uncrossiinked blends of PS and PPO and the filled triangles and circles represent crosslinked PS (After Ref. courtesy of J. Polym. Sd.-Polym. Phys. (Wiley))...

The effects of temperature on craze growth and the competition between crazing and shear deformation have been known for some time to be very complex. Since above Tg the polymer deforms uniformly by shear, one might expect that as the deformation temperature of the polymer glass is raised toward its glass transition temperature a polymer which crazes at room temperature would make a transition to shear de-... 

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