In fact, no two moving bodies can come together without impact stress.

Impact is therefore the commonest form of applied stress, although the most difficult to measure.
_Failures in Timber Beams_ If a beam is loaded too heavily it will break or fail in some characteristic manner.

These failures may be classified according to the way in which they develop, as tension, compression, and horizontal shear; and according to the appearance of the broken surface, as brash, and fibrous.

A number of forms may develop if the beam is completely ruptured.
Since the tensile strength of wood is on the average about three times as great as the compressive strength, a beam should, therefore, be expected to fail by the formation in the first place of a fold on the compression side due to the crushing action, followed by failure on the tension side.

This is usually the case in green or moist wood.