443 



RELATION OF TESTS. 



The relation between the strength under various kinds of tests, such 

 as shear, bending, etc., and compression parallel to the grain, have not 

 been determined yet by an analysis of the data. It is doubtful if any 

 one test can be used to predict the strength of the material under other 

 forms of tests when conditions vary with respect to previous heat treat- 

 ment, moisture, drying or preservative treatment. For instance, brittle- 

 ness induced by overheating is evident in impact tests, but this will not 

 necessarily be evident from the compression test parallel to the grain. 



An investigation of the effect of speed of test is a part of the general 

 study of behavior of wood under three conditions of loading : 



(a) Dead or constant load. 



(b) Ordinary static test with increasing load. 



(c) Impact test. 



(a) Dead load tests exhibit the plasticity of wood. Nearly all defor- 

 mations increase with duration of load, but the deformed beams subse- 

 quently tested show no loss of ultimate strength. Deflection brought about 

 by humid atmosphere is not recovered by subsequent drying. The ques- 

 tion is often asked : "What per cent of the load, as determined by the 

 ordinary static test, will break a beam if left on indefinitely?" This has 

 no answer. 



(c) Under impact loading, wood will submit to greater elastic defor- 

 mation than under the ordinary static tests. Impact bending tests show 

 elastic deformation largely in excess of those experienced under static 

 load. The impact test is made under increasing height of drop. 5 The order 

 of resistance of air dry woods at the ultimate failure strength, so far 

 obtained is as follows : 



Hickory, Longleaf Pine, Douglas Fir, Loblolly Pine, Chestnut, Spruce, 

 Yellow Poplar, Western Yellow Pine, Western Hemlock, Sugar Pine, and 

 Coast Redwood. 



(d) Abrasion Test. The abrasion test is under study." Wood is 

 worn by sand-paper in the Dorrey Machine. 



5 Circular 38, Revised, Forest Service. "Instructions to Engineers in Timber 

 Tests," by W. K. Matt. 



See American S. for T. M., Vol. 7, 1907. "P. U. Impact Testing Ma?h.," by 

 W. K. Hatt. 



