Aspects of Grain Size Strengthening in Polycrystals

ABSTRACTThe strengthening effect of grain boundaries is well established and observed experimentally as the Hall-Petch relationship. In this paper different mechanisms proposed in the literature to explain the observed Hall-Petch effect are reviewed critically. The fundamental implications of the different approaches are discussed with reference to experimental data for two different classes of materials;-Materials with locked dislocations, i.e. with a sharp yield point behaviour.-Materials without locked dislocations, i.e. with a smooth yielding behaviour.It is shown that a simple model (Bergström) can be used to understand the grain size strengthening in the latter class of materials while more work is needed to quantitatively understand the behaviour of materials showing a sharp yield point.

The Physical Properties of the Pedal Mucus of the Terrestrial Slug, Ariolimax Columbianus

The pedal mucus of gastropods functions in locomotion by coupling the movements of the foot to the substratum. The pedal mucus of the terrestrial slug, Ariolimax columbianus, is suited to this role by the following unusual physical properties. 1. At small deformations the mucus is a viscoelastic solid with a shear modulus of 100–300 Pa. 2. The mucus shows a sharp yield point at a strain of 5–6, the yield stress increasing with increasing strain rate. 3. At strains greater than 6 the mucus is a viscous liquid (η = 30–50 poise). 4. The mucus recovers its solidity if allowed to ‘heal’ for a period of time, the amount of solidity recovered increasing with increasing time.