STRENGTH OF MATERIALS. The strength of materials upon their physi cal constitution—viz., their form, texture, hardness, elasticity, and ductility.
The resistance of materials in engineering works is tested in reference to various strains; such are-1. Extension or tension; 2. Compression or crushing; 3. Transverse Or cross strain ; 4. Shearing strain; 5. Torsion or twisting strain.
1. Ertenkion.—When a rod is suspended vertically, and a weight attached to its end tending to tear it asunder, all its fibers act equally, and its strength evidently depends on the strength of the individual fibers and their number, that is, the area of cross-section of the rod. The following table gives the resistance to rupture of some of the most common materials: Per Square Inch.
Fine sandstone 200 lbs.
Brick . 300 " Common lime 50 " Portland cement 240 " Deal (timber) 5 tons.
Cast iron (ordinary) 61 " Stirling's toughened 12+ " Wrought iron, boiler-plate......... 20 to 24 " bars 25 " Cast.steel 60 " Ropes (hemp), four-fifths ton per pound weight per fathom.
With regard to the elongation of materials under tensional strain, it has been observed that up to a certain limit, which is different for different substances, the elongation is proportional to the extending force, a physical truth the promulgation of which is due to Hooke (q.v.); up to this limit also the body nearly recovers its original form on the removal of the force : this limit is called the limit of elasticity. When this limit is passed, the permanent elongation or destruction rapidly increases until rupture takes place.
The extension of wrought iron is about of its length per ton of strain per square inch, and that of cast iron The limit of elasticity of wrought iron is attained under a strain of 12 tons per square inch; and in the case of American pine 1+ ton per square inch.
2. Compression or Crushing strength of pieces of stone, wood, or iron, whose height is small in proportion to their area, and which absolutely crush under the strain, is proportional to the area of their horizontal section. The following table gives the resistance to crushing of some of the more common materials: Cast iron .. 50 tons per square inch. Wrought iron 16 " Brickwork ....... 30 tons per square foot.
Sandstone . .... 200 " Limestone 490 " tt Deal.... ........ 450 " Oak..... 650 " Up to a certain strain, which is called the limit of elasticity, the diminutions in length of the body are proportional to the compressing force; and are practically the same in amount as the elongations in the case of tensional forces. In the case of wrought iron, the limit is 12 tons per sq.in.; after that strain, its shape and proportions become permanently altered; and where these are of consequence, as in most practical cases. we come to the limit of its utility, which is reached when the load is about 16 tons per sq. inch. It then oozes away beneath additional strain, as a lump of lead would do in a vise.
The mode of ultimate failure of cast iron is quite distinct from that of wrought iron. It crushes suddenly by the sliding off of the corners in wedge-shaped fragments, being a crystalline mass, without sufficient ductility to allow of its bulging horizontally; the angle of rupture at which these wedges slide off being tolerably constant, and varying from 43' to 58'. The limit of elasticity is attained in cubes of deal under a compression of 100 tons per sq.ft.; and in those of oak, 150 tons per sq. foot.
Pillars, round or square, may be divided into three classes-1. Those whose height is not more than 5 times their 2. Those whose height is between 5 and 25 times their diameter; 3. Those whose height is at least 25 times their diameter. The first follow the same laws as cubes or pieces of small height above discussed, and are absolutely crushed; their strength being proportional to their cross section. The second are broken across, partly by crushing and partly by bending. The third give way purely from bending as with a transverse strain, and their strength_ is found by experiment to be directly proportional to the fourth power of their diameter, and inversely propor tional to the square of their length. Thus, in the case of two long pillars of equal length, but of which one has its diameter double that of the other, the strength of the former will be 16 times that of the latter; from which will be apparent the advantage of the tubular form for pillars, as it gives a large diameter, combined with lightness.