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THE METAL-BENDERS” by PROFESSOR JOHN B. HASTED

Extensions and contractions

In the last chapter I attempted to interpret synchronous strains by invoking a physical model of a moving ‘surface of action’ at which strains are experienced. This might be regarded as some sort of field of dynamic strain or of stress, but it is a field with unusual properties; we imagine the strain to be zero or very small at points in space which do not lie on the surface; but at points which do lie on the surface, possibly at all these points, bending strains or stresses are experienced. The surface moves slowly in space, possibly under weak control of the subject.
However, everybody knows that to bend a strip of metal it takes the action not of one force but of at least three, arranged thus: /\ \/ /\
This action is known as a ‘three-point load’. It might be supplemented by other forces acting at neighbouring points, or even by a continuous array of forces; yet always there must be two opposed torques, centring on different points within the specimen (a shear). How can such torques be produced at a ‘surface of action’?
In the Nicholas Williams data there are strain gauge signals which indicate a permanent deformation of a metal specimen, without any actual bending being visible. Examples (not illustrated) are signals A 4(2), B 2(2), C 1(2). I did not enter such events on the sensitivity graph (Figure 4.3) since, being drawn fully logarithmically, it cannot display zero bend angles.
The signals indicate either a permanent extension or a permanent bend (since the strain gauge is not on the neutral axis). But since no bend is visible, a permanent extension must be indicated. Possibly many of the elastic deformation signals are also extensions rather than bends. For the bending of a strip of metal there is extension on the convex side and contraction on the concave side, so that a single strain gauge would be inadequate to distinguish a bend from an extension.