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The nucleation rate was measured by directly counting the number of nuclei, which were developed while an isotactic polypropylene melt was flowing under shear in a thin film. The nucleation rate was enhanced with an increased rate of shear, e.g., by a factor of 10 larger at the rate of shear of 14 s 21 compared with the quiescent state, at 1348C. The ratio of the shear-enhanced nucleation rate to the nucleation rate in the quiescent state was larger at a higher temperature of crystallization, i.e., about 10 times at 1348C to 590 times at 1408C. The increase of the nucleation rate under shear flow was explained by a reduction of the lateral and end (fold) surface free energies; the product s s 2 s e decreased to 3.2 Â 10 27 for the sheared melt, from 6.0 Â 10 27 (J m 22 ) 3 for the isotropic state. The free energy reduction was caused by transition of the nucleus formation mode from three-dimensional folded chain nuclei to two-dimensional bundle nuclei, in which chains lie down on the glass substrate, aligning parallel to the flow direction.

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The present authors have assumed that the sensitivity of steel to reheat cracking can be most appropriately evaluated by its own plastic deformability.Based on this assumption, the authors undertook a study of reheat cracking testing of steel by means of the stress relieving test, and of the method for evaluating the sensitivity of steel to reheat cracking.The results of the study obtained are summaraized as follows:(1) The reheat cracking test method using the weld thermal-restraint stress and the straincycle simulator has the advantage in that it allows the test to be made continuously during the heating and holding processes.