The fatigue of metals under repeated strains

“The Fatigue of Metals” is the name which has been given to the effect produced by oft repeated impacts or strains.

Spangenberg's experiments, an account of which, translated for Van Nobtrand's Magazine, is given in the following treatise, were, as will be seen, in continuation of Woehler's. The results of these very important experiments have been before the profession for some years, but, strange to say, seem to have attracted no attention; and tests of iron and steel still go on for the purpose of determining their elasticity, their elongation under strain, their ultimate strength and other qualities, while Wohler and Spangenberg's experiments show that it is very doubtful that these bear any proportion to the durability of the metals.

These experiments prove that there is a limit of strain within which iron is practically indestructible, and that that limit is but little over 30,000 lbs per square inch for the best iron. If, as in some of the braces of the Warren truss, and other forms, there is both tension and compression at different times, the limit is the sum of the two strains. They further show the dangerous character of truss work when there is ambiguity of strains. It is to be hoped that the translation of Spangenberg's book will excite sufficient interest to lead to a continuation of these experiments in this country.


FATIGUE OF METALS.

In vols. X., XIII., XVI. and XX. of the Zeitschrift f. Bauwesen are published the experiments of A. Wohler upon the strength of iron and steel, with a description of the apparatus used, a statement of his views of the laws, and a mathematical comparison of the different kinds of resistance. Wohler, induced by the novelty of the results obtained, requested the Industrial Bureau to authorize the repetition of his experiments. At the suggestion of Prof. Reuleaux, the writer was intrusted with the investigation.

We quote the laws deduced by Wohler, and give a brief account of his processes. He says:

"Rupture of material may be caused by repeated vibrations, none of which attain the absolute breaking limit."

"The differences of the limiting strains are sufficient for the rupture of the material."

Assuming the lower limit of tension at zero, it follows from this law that the number of repeated strains necessary for rupture is inversely proportional to the greatest tension borne by the fibres which are subject to greatest strain.

Wohler's apparatus was of four kinds:

(1) For rupture by repeated load.

(2) For repeated bending in one direction of prismatic rods.

(3) For experiments on loaded rods under constant bending strain.

(4) For torsion by repeated load (strain).

The power was transmitted by a shaft to (1), (2) and (4) by means of an eccentric; to (3) by a drum on a steel shaft having ends with conical bores, into which the piece subjected to torsion was fastened with an apparatus of screws.