Z l^{2}
(2) For beams (centre loading)       n = ---------
6h

Z l^{2}
(3) For beams (third-pointloading)   n = ---------
5.4h

Z = rate of fibre strain per inch of fibre length.
l = span of beam or length of compression specimen.
h = height of beam.

The values commonly used for Z are as follows: 

Bending large beams                      Z = 0.0007
Bending small beams                      Z = 0.0015
Endwise compression-large specimens      Z = 0.0015
Endwise compression-small       "        Z = 0.003
Right-angled compression-large  "        Z = 0.007
Right-angled compression-small  "        Z = 0.015
Shearing parallel to the grain           Z = 0.015

Example:  At what speed should the crosshead move to give the
required rate of fibre strain in testing a small beam 2” X 2” X
30”. (Span = 28".) Substituting these values in equation (2)
above: 
          (0.0015 X 28^2)
     n = ----------------- = 0.1 inch per minute.
              (6 X 2)

In order that tests may be intelligently compared, it is important that account be taken of the speed at which the stress was applied.  In determining the basis for a ratio between time and strength the rate of strain, which is controllable, and not the ratio of stress, which is circumstantial, should be used.  In other words, the rate at which the movable head of the testing machine descends and not the rate of increase in the load is to be regulated.  This ratio, to which the name speed-strength modulus has been given, may be expressed as a coefficient which, if multiplied into any proportional change in speed, will give the proportional change in strength.  This ratio is derived from empirical curves. (See Table XVII.)

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