The strength of materials is their power to resist fracture, while the stiffness of materials is their capability to resist deflection or sagging. A knowledge of their strength is useful, in order to determine their limits of size to sustain given weights safely; but a knowledge of their stiffness is more important, as in almost all constructions it is desirable not only that the load be safely sustained, but that no appearance of weakness be manifested by any sensible deflection or sagging.
93. - Experiments: Constants. - In the investigation of the laws applicable to the resistance of materials, it is found that the dimensions - length, breadth, and thickness - bear certain relations to the weight or pressure to which the piece is subjected. These relations are general; they exist quite independently of the peculiarities of any specific piece of material. These proportions between the dimensions and the load are found to exist alike in wood, metal, stone, and glass, or other material. One law applies alike to all materials; but the capability of materials to resist differs in accordance with the compactness and cohesion of particles, and the tenacity and adhesion of fibres, those qualities upon which depends the superiority of one kind of material over another. The capability of each particular kind of material is ascertained by experiments, made upon several specimens, and an average of the results thus obtained is taken as an index of the capability of that material, and is introduced in the rules as a constant number, each specific kind of material having its own special constant, obtained by experimenting; on specimens of that peculiar material. The results of experiments made to test the resistance of various materials useful in construction - their capability to resist the three strains before named - will now be introduced.