506 The Philippine Journal of Science 1921 
STRENGTH AS AFFECTED BY MOISTURE CONTENT 
A comparison of the results of tests on air-seasoned material 
with those on green material shows that, in general, all of the 
mechanical properties are improved by seasoning.® Increase in 
strength is especially marked on small pieces free from defects. 
Increase in strength of wood fiber due to drying is, in the case 
of large timbers, largely offset by a weakening of the timber due 
to the formation of checks.° Hatt * says that if the moisture 
content of a seasoned timber is increased it loses strength 
rapidly and if thoroughly soaked with water it will become 
slightly weaker than when green. On this account it is not 
safe in practice to depend upon an increase in strength of timbers 
due to seasoning. When, however, large beams are seasoned 
with ordinary care, it is safe to assume that they will not at 
any time be weaker than they were. when green. 
The time factor in timber tests.2—Variations in the rate of 
application of load have a very pronounced effect upon strength 
and stiffness, as was shown by a specimen under test. If a tim- 
ber compression block or beam is loaded rapidly, it will appear 
to have a higher elastic limit and ultimate strength, and will 
also appear to be stiffer, than when loaded less rapidly. Mills * 
says that this behavior is due to the fact that the deformation 
lags far behind the load, and if a load is permitted to remain 
upon a specimen for a perceptible interval of time the defor- 
mation increases, the amount of increase becoming greater for 
heavier loads. Actual failure appears to be consequent upon 
the attainment of a certain limiting amount of deformation or 
strain, rather than a limiting load or stress. This condition of. 
affairs makes it necessary to standardize tests by adopting cer- 
tain speeds of loading for each class of specimens Or, rather, 
° Tests of Structural Timbers, Bull. U. S. Forest Service 108 (1912). 
” Materials of Engineering Construction, Trans. International Engineer- 
ing Congress 5 (1915). é 
Marks, L. S., Mechanical Engineer’s Handbook. McGraw Hill Book 
Co. (1916). 
2 The first mechanical tests of Philippine woods published were made 
by the Bureau of Forestry of the Philippines. Those tests were made on 
small specimens only, and the testing machine was operated at the fol- 
lowing speeds: For eross-bending, 0.3 inch per minute; for compression along 
the grain, 0.06 inch per minute. See Bull. Philip. Bur. Forestry 4 aaah 
® Mills, A. P., Materials of Construction. New York, John Wiley ¢ 
Sons Inc. (1915) 647. 
