Re-entrant comer

LDPE is a highly branched materied, whose flow behavioTir exhibits some peculiarities, which constitute a good test for a numerical simulation. As presented in Section lll-l, birefringence patterns are perturbed around the re-entrant comer, which leads to the appearance of W-shaped fringes at the entry of the die land. It can be seen in Fig. 39 that the mPTT model ilows on to obtain these characteristic shapes (the computation is related to experiments carried out at 175 °C and 21 s, for an estimated value of the stress optical coefficient of... 

Mathematically, the stress field surrounding a geometric comer, such as a crack tip, a re-entrant comer or the edge of a bi-material interface can be given as a series expansion of the form ... 

Sadler DM, Barber M, Lark G, Hill MJ (1986) Twin morphology 2. Measurements of the enhancement in growth due to re-entrant comers. Polymer 27(l) 25-33 Sanchez I, Eby R (1975) Thermodynamics and crystallization of random copolymers. Macromolecules 8(5) 638-641... 

Figures 6 and 7 contain plots of the bond normal and shear stresses, respectively, for t = 50 sec and t = 60 min of loading. These stresses represent the value at the thickness from the upper adhesive-adherend interface. The sharp peak at the left-hand edge is due to the singularity caused by the presence of a re-entrant comer in the vicinity of the edge. These results are in good agreement with those results presented in Reference 49 which uses the linear viscoelastic finite-element code, MARC.

When spiral steps from two dislocations are separated by a distance larger than 9pJ2, two-dimensional re-entrant comers may appear and preferential growth will start after that, i.e. the steps will advance faster at the re-entry comers in comparison with other directions. When the step separation is much wider than a critical value, it becomes possible that the supersaturation between neighboring steps can achieve a level sufficient to overcome the barrier of 2D nucleation and to permit nucleation on the surface of spiral layers. 

These defects are generally detrimental to electronic, thermal and mechanical properties of diamond films, although in some cases they may be neutral or beneficial. The frequently occurring defects appear to be stacking faults (twinned clusters with many re-entrant surfaces/comers), which are deemed to play a major role in enhancing diamond growth rates.t ... 

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