Stick-slip fracture mechanism

A number of other mechanisms [53-65] have been suggested for melt fracture. Based on a stick-slip mechanism, it is purported [53] that, above a critical shear stress, die polymer experiences intermittent slipping due to a lack of adhesion between itself and die wall, in order to relieve the excessive deformation energy adsorbed during the flow. The stick-slip mechanism has attracted a lot of attention [53-63], both theoretically and experimentally. The other school of drought [64,65] is based on thermodynamic argument, according to which, melt fracture can initiate anywhere in the flow field when reduction in the fluid entropy due to molecular orientation reaches a critical value beyond which the second law of thermodynamics is violated and flow instability is induced [64]. 

In conventional extrusion the maximum pressure that can be applied without stick-slip is around 0.23 GPa and so this provides a "process dependent" restriction on extrusion rate and the maximum draw ratio available. In push-pull extrusion, forces up to the tensile fracture of the material can be applied so the true maximum draw ratio for the material at a given temperature can be obtained. The limit of temperature and draw ratio detected in these studies has been added to the extrudability map (Figure 6) showing the maximum regions of the temperature/draw ratio map that may be studied. Clearly the push-pull process greatly extends the range of the extrusion technique. A series of samples prepared at the same draw ratio and temperature, but with different combinations of push and pull, have the same modulus, i.e. mechanical properties are independent of the applied pressure. 

It has been proposed that polymers such as HDPE, which is considered a linear polymer, as well as other linear polymers such as PP and PS have a similar mechanism for fracture. In fact, the molecular weight dependence of the critical shear stress (tor) for fracture was found to be similar for linear polymers (Middleman, 1977). A statistical fit of the data for fracture gave the following relation for the critical shear stress for the onset of slip-stick melt fracture, Zcr, for linear polymers ... 

The aspect of the flow curves in Fig. 9.6 indicates the predominant condition of the test. For instance. Fig. 9.6a shows uniform serrations (stick-shp) associated with a continuous, step-by-step process of fiber slipping away off the polymeric block. This exclusive pullout mechanism occurred only in the first part of the curve depicted in Fig. 9.5c. In this case, the fiber was intact after the test and, in some cases, covered with a layer of polyester. By contrast. Fig. 9.6d shows a smooth curve up to fracture. In this case, no pullout process occurred, and the fiber had undergone tensile rupture in association with the last horizontal part of the curve. The intermediate condition of both pullout and fiber rupture are shown in Fig. 9.6b, c. 

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