XXI.—Experiments to show how Failure under Stress occurs in Timber, its Cause, and Comparative Values of the Maximum Stresses induced when Timber is fractured in Various Ways
The object of this investigation was not to obtain fresh data on the average strength of timber when subjected to destructive stresses. This has been done in great detail by Bauschinger, Lanza, Johnson, and others, and is still being carried on, in America, on a large scale under the Bureau of Forestry. The intention was rather (a) to find the effect of the medullary rays when timber was stressed in compression, tension, shear, or by cross-bending; and (b) to endeavour to connect up in a satisfactory way the maximum stresses induced in cross-breaking with those obtained by direct compression, tension, or shear. For this purpose, then, it was not necessary that the number of experiments should be excessive, but it was essential that the results obtained should be comparable one with another. To obtain this, a single tree of each variety of wood to be tested was procured, the test pieces were sawn out of the best part of the trunk, carefully stacked, and allowed to season for at least twelve months.