The stresses involved in impact are complicated by the fact that there are various ways in which the energy of the striking body may be spent:
(_a_) It produces a local deformation of both bodies at the surface of contact, within or beyond the elastic limit. In testing wood the compression of the substance of the steel striking-weight may be neglected, since the steel is very hard in comparison with the wood. In addition to the compression of the fibres at the surface of contact resistance is also offered by the inertia of the particles there, the combined effect of which is a stress at the surface of contact often entirely out of proportion to the compression which would result from the action of a static force of the same magnitude. It frequently exceeds the crushing strength at the extreme surface of contact, as in the case of the swaging action of a hammer on the head of an iron spike, or of a locomotive wheel on the steel rail. This is also the case when a bullet is shot through a board or a pane of glass without breaking it as a whole.
(_b_) It may move the struck body as a whole with an accelerated velocity, the resistance consisting of the inertia of the body. This effect is seen when a croquet ball is struck with a mallet.
(_c_) It may deform a fixed body against its external supports and resistances. In making impact tests in the laboratory the test specimen is in reality in the nature of a cushion between two impacting bodies, namely, the striking weight and the base of the machine. It is important that the mass of this base be sufficiently great that its relative velocity to that of the common centre of gravity of itself and the striking weight may be disregarded.
(_d_) It may deform the struck body as a whole against the resisting stresses developed by its own inertia, as, for example, when a baseball bat is broken by striking the ball.
|-------------------------------------------------------| | TABLE X | |-------------------------------------------------------| | RESULTS OF IMPACT BENDING TESTS ON SMALL CLEAR BEAMS | | OF 34 WOODS IN GREEN CONDITION | | (Forest Service Cir. 213) | |-------------------------------------------------------| | | Fibre | | Work in | | COMMON NAME | stress at | Modulus of | bending | | OF SPECIES | elastic | elasticity | to | | | limit | | elastic | | | | | limit | |-------------------+-----------+------------+----------| | | | | In.-lbs. | | | Lbs. per | Lbs. per | per cu. | | | sq. in. | sq. in. | inch | | | | | | | Hardwoods | | | | | | | | | | Ash, black | 7,840 | 955,000 | 3.69 | | white | 11,710 | 1,564,000 | 4.93 | | Basswood | 5,480 | 917,000 | 1.84 | | Beech | 11,760 | 1,501,000 | 5.10 | | Birch, yellow | 11,080 | 1,812,000 | 3.79 | | Elm, rock | 12,090 | 1,367,000 | 6.52 | | slippery | 11,700 | 1,569,000 | 4.86 | | white | 9,910 | 1,138,000 | 4.82 | | Hackberry | 10,420 | 1,398,000 | 4.48 | | Locust, honey | 13,460 | 2,114,000 | 4.76 | | Maple, red | 11,670 | 1,411,000 | 5.45 | | sugar | 11,680 | 1,680,000 | 4.55 | | Oak, post | 11,260 | 1,596,000 | 4.41 | | red | 10,580 | 1,506,000 | 4.16 | | swamp white | 13,280 | 2,048,000 | 4.79 | | white | 9,860 | 1,414,000 | 3.84 | | yellow | 10,840 | 1,479,000 | 4.44 | | Osage orange | 15,520 | 1,498,000 | 8.92 | | Sycamore | 8,180 | 1,165,000 | 3.22 | | Tupelo | 7,650 | 1,310,000 | 2.49 | | | | | | | Conifers | | | | | | | | | | Arborvitæ | 5,290 | 778,000 | 2.04 | | Cypress, bald | 8,290 | 1,431,000 | 2.71 | | Fir, alpine | 5,280 | 980,000 | 1.59 | | Douglas | 8,870 | 1,579,000 | 2.79 | | white | 7,230 | 1,326,000 | 2.21 | | Hemlock | 6,330 | 1,025,000 | 2.19 | | Pine, lodgepole | 6,870 | 1,142,000 | 2.31 | | longleaf | 9,680 | 1,739,000 | 3.02 | | red | 7,480 | 1,438,000 | 2.18 | | sugar | 6,740 | 1,083,000 | 2.34 | | western yellow | 7,070 | 1,115,000 | 2.51 | | white | 6,490 | 1,156,000 | 2.06 | | Spruce, Engelmann | 6,300 | 1,076,000 | 2.09 | | Tamarack | 7,750 | 1,263,000 | 2.67 | |-------------------------------------------------------|