In general, four forms of material are tested, namely:  (1) large timbers, such as bridge stringers, car sills, large beams, and other pieces five feet or more in length, of actual sizes and grades in common use; (2) built-up structural forms and fastenings, such as built-up beams, trusses, and various kind of joints; (3) small clear pieces, such as are used in compression, shear, cleavage, and small cross-breaking tests; (4) manufactured articles, such as axles, spokes, shafts, wagon-tongues, cross-arms, insulator pins, barrels, and packing boxes.

As the moisture content is of fundamental importance (see WATER CONTENT, above.), all standard tests are usually made in the green condition.  Another series is also usually run in an air-dry condition of about 12 per cent moisture.  In all cases the moisture is very carefully determined and stated with the results in the tables.

SIZE OF TEST SPECIMENS

The size of the test specimen must be governed largely by the purpose for which the test is made.  If the effect of a single factor, such as moisture, is the object of experiment, it is necessary to use small pieces of wood in order to eliminate so far as possible all disturbing factors.  If the specimens are too large, it is impossible to secure enough perfect pieces from one tree to form a series for various tests.  Moreover, the drying process with large timbers is very difficult and irregular, and requires a long period of time, besides causing checks and internal stresses which may obscure the results obtained.

On the other hand, the smaller the dimensions of the test specimen the greater becomes the relative effect of the inherent factors affecting the mechanical properties.  For example, the effect of a knot of given size is more serious in a small stick than in a large one.  Moreover, the smaller the specimen the fewer growth rings it contains, hence there is greater opportunity for variation due to irregularities of grain.

Tests on large timbers are considered necessary to furnish designers data on the probable strength of the different sizes and grades of timber on the market; their coefficients of elasticity under bending (since the stiffness rather than the strength often determines the size of a beam); and the manner of failure, whether in bending fibre stress or horizontal shear.  It is believed that this information can only be obtained by direct tests on the different grades of car sills, stringers, and other material in common use.

When small pieces are selected for test they very often are clear and straight-grained, and thus of so much better grade than the large sticks that tests upon them may not yield unit values applicable to the larger sizes.  Extensive experiments show, however, (1) that the modulus of elasticity is approximately the same for large timbers as for small clear specimens cut from them, and (2) that the fibre stress at elastic limit for large beams is, except in the weakest timbers, practically equal to the crushing strength of small clear pieces of the same material.[57]