M. Tresca has contributed to the _Comptes Rendus_ some observations on the
effect of hammering, and the variation of the limit of elasticity of
metals and materials used in the arts.
He says that hitherto, in considering the deformation of solids under
strain, two distinct periods, relative to their mechanical properties,
have alone been recognized. These periods are of course the elastic limit
and the breaking point. In the course of M. Tresca's own experiments,
however, he has found it necessary to consider, at the end of the period
of alteration of elasticity, a third state, geometrically defined and
describable as a period of fluidity, corresponding to the possibility of a
continuous deformation under the constant action of the same strain. This
particular condition is only realized with very malleable or plastic
bodies; and it may even be regarded as characteristic of such bodies,
since its absence is noticeable in all non-malleable or fragile bodies,
which break without being deformed. It is already known that the period of
altered elasticity for hard or tempered steel is much less than for iron.
In 1871 the author showed that steel or iron rails that had acquired a
permanent set were at the same time perfectly elastic up to the limit of
the load which they had already borne. With certain bars the same result
was renewed five times in succession; and thus their period of perfect
elasticity could be successively extended, while the coefficient of
elasticity did not appear to sustain any appreciable modification. This
process of repeated straining, when there is an absence of a certain
hammering effect, renders malleable bodies somewhat similar to those which
are not malleable and brittle. There is an indication here of another
argument against the testing of steam boilers by exaggerated pressures
before use, which process has the effect of rendering the plates more
brittle and liable to sudden rupture.
M. Tresca also protests against the elongation of metals under breaking
strain tests being stated as a percentage of the length. The elongation is
in all cases, chiefly local; and is therefore the same for a test piece
12 inches or 8 inches long, being confined to the immediate vicinity of
the point of rupture. The indication of elasticity should rather be sought
for in the reduction of the area of the bar at the point of rupture. This
portion of the bar is otherwise remarkable for having lost its original
condition. It is condensed in a remarkable manner, and has almost
completely lost its malleability. The final rupture, therefore, is that of
a brittle zone of the metal, of the same character that may be produced by
hammering. If a test bar, strained almost to the verge of rupture, be
annealed, it will stretch yet further before breaking; and, indeed, by
successive annealings and stretchings, may be excessively modified in its
proportions.
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