MALL BEAMS

_Apparatus_: An ordinary static bending machine, a steel I-beam
bearing two adjustable knife-edge supports to rest on the
platform, and a special deflectometer, are required. (See Fig.
31.)

[Illustration: FIG. 31.--Static bending test on small beam. Note
the use of the deflectometer with indicator and dial for
measuring the deflection; also roller bearings between beam and
supports.]

_Preparing the material_: The specimens may be of any convenient
size, though beams 2" X 2" X 30" tested over a 28-inch span, are
considered best. The beams are surfaced on all four sides, care
being taken that they are not damaged by the rollers of the
surfacing machine. Material for these tests is sometimes cut
from large beams after failure. The specimens are carefully
weighed in grams, and all dimensions measured to the nearest
0.01 inch. If to be tested in a green or fresh condition the
specimens should be kept in a damp box or covered with moist
sawdust until needed. No defects should be allowed in these
specimens.

_Marking and sketching_: Sketches are made of each end of the
specimen to show the character of the growth, and after testing,
the manner of failure is shown for all four sides. In obtaining
data regarding the rate of growth and the proportion of late
wood the same procedure is followed as with large beams.

_Adjusting specimen in machine_: The beam should be correctly
centred in the machine and each end should have a plate with
roller bearings between it and the support. Centre loading is
used. Between the movable head of the machine and the specimen
is placed a bearing block of maple or other hard wood, the lower
surface of which is curved in a direction along the beam, the
curvature of which should be slightly less than that of the beam
at rupture, in order to prevent the edges from crushing into the
fibres of the test piece.

_Measuring the deflection_: The method of measuring deflection
of large beams can be used for small sizes, but because of the
shortness of the span and consequent slight deformation in the
latter, it is hardly accurate enough for good work. The special
deflectometer shown in Fig. 31 allows closer reading, as it
magnifies the deflection ten times. It rests on two small nails
driven in the beam on the neutral plane and vertically above the
supports. The fine wire on the wheel at the base of the
indicator is attached to another small nail driven in the beam
on the neutral