Strength of Materials
The Encyclopedia of Founding (1892)
The strength of materials is governed by their physical constitution, or by their form, texture, hardness, elasticity, and ductility; and they are tested in reference to various strains--as tensile, crushing, transverse, shearing, and torsional strains. Tensile is represented by suspending a rod vertically and attaching weights at the other end, which will tend to tear it asunder. Its strength depends on the strength of each individual fibre, and upon their number. Crushing.--The strength of individual pieces of material whose height is small in proportion to their area, and which are absolutely crushed under the strain, is proportionate to the area of their horizontal section. Transverse.--A beam is bent from its horizontal shape if one end be fixed and the other loaded. It is supposed that the compressions and extensions for the given strain are about equal if the beam be not strained beyond the limit of its elasticity; we approach the breaking-strain if we go beyond this limit. Shearing.--The shearing force is exerted when the particles in one plane are caused to slide over those in another, as in cutting plate-iron with the shears. Torsion.--If we make one end of a shaft fast, and exert a force to cause it to rotate, we may twist the shaft asunder at its weakest part. In this case the fibres farthest from the centre will resist the most, diminishing in proportion as the centre is reached. STRENGTH OF MATERIALS. TENSILE STRENGTH. CRUSHING STRENGTH. TRANSVERSE STRENGTH. TORSION. Substances. Weight required to pull a bar 1 inch square asunder. Weight required to crush 1 inch square. Bar 1 inch square and 1 foot long. Weight suspended at one end. Relative strength to resist torsion Lead being 1. Cast iron..... 25,000 120,000 600 9 Wrought iron..... 60,000 50,000 900 10.1 Steel..... 90,000 125,000 1,500 16.6 Gold, cast..... 20,000 35,000 Silver, cast..... 40,000 500 Copper, cast..... 20,000 100,000 4.3 Tin, block..... 4,000 15,500 50 1.4 Zinc, cast..... 2,500 30 Brass, yellow..... 28,000 4.6 Bronze Copper 10. Tin 1..... 36,000 5 Lead, cast..... 1,800 7,000 20 1 --See WEIGHT OF METALS.
What Has Changed in the Last Century
The 1892 Encyclopedia of Founding lists the five classic loading cases transtransverse, shearing and torsional still the framework taught today, and its tabulated figures (60,000 lb per square inch to pull apart a one-inch wrought-iron bar, 90,000 for steel) are of the right order for the metals then available. What has gone is the underlying picture: the account speaks of a bar's strength depending on "the strength of each individual fibre, and upon their number," whereas metals are now understood as crystals whose strength is governed by the movement of dislocations, a mechanism proposed independently by Taylor, Orowan and Polanyi in 1934. Two other gaps loom larger still holdfatigue under repeated loading and the growth of cracks from tiny flaws, the subject of A. A. Griffith's 1920 paper and of the fracture mechanics built on it from the 1950s, which explains why structures fail far below the table's figures. Modern high-strength alloy steels reach several times the 90,000 lb quoted, and design now proceeds by computed stress fields and code-set limit states rather than by a single breaking load divided by a factor of safety.
Written for this edition, 2026.