on method of making tests upon the resistance of wood to
shock is to support a small beam at the ends and drop a heavy
weight upon it in the middle. (See Fig. 40.) The height of the
weight is increased after each drop and records of the
deflection taken until failure. The total work done upon the
specimen is equal to the area of the stress-strain diagram plus
the effect of local inertia of the molecules at point of
contact.

The stresses involved in impact are complicated by the fact that
there are various ways in which the energy of the striking body
may be spent:

(_a_) It produces a local deformation of both bodies at the
surface of contact, within or beyond the elastic limit. In
testing wood the compression of the substance of the steel
striking-weight may be neglected, since the steel is very hard
in comparison with the wood. In addition to the compression of
the fibres at the surface of contact resistance is also offered
by the inertia of the particles there, the combined effect of
which is a stress at the surface of contact often entirely out
of proportion to the compression which would result from the
action of a static force of the same magnitude. It frequently
exceeds the crushing strength at the extreme surface of contact,
as in the case of the swaging action of a hammer on the head of
an iron spike, or of a locomotive wheel on the steel rail. This
is also the case when a bullet is shot through a board or a pane
of glass without breaking it as a whole.

(_b_) It may move the struck body as a whole with an accelerated
velocity, the resistance consisting of the inertia of the body.
This effect is seen when a croquet ball is struck with a mallet.

(_c_) It may deform a fixed body against its external supports
and resistances. In making impact tests in the laboratory the
test specimen is in reality in the nature of a cushion between
two impacting bodies, namely, the striking weight and the base
of the machine. It is important that the mass of this base be
sufficiently great that its relative velocity to that of the
common centre of gravity of itself and the striking weight may
be disregarded.

(_d_) It may deform the struck body as a whole against the
resisting stresses developed by its own inertia, as, for
example, when a baseball bat is broken by striking the ball.

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