This section is from the book "Notes On Building Construction", by Henry Fidler. Also available from Amazon: Notes on building construction.
In investigating the properties of a specimen of iron or steel a very important point to be ascertained is its limit of elasticity.
The meaning of this term has been defined in several different ways.
Mr. Stoney's definition is the one perhaps best suited to the engineer. He says - "The limit of elasticity may be defined to be the greatest strain that does not produce a permanent set."
A short explanation will perhaps make the meaning of the term more clear than the definition alone would do.
If a small weight be suspended from a bar so as to cause a tensile stress in the direction of its length, the bar will at once begin to elongate.
It will stretch a certain proportion of its own length. This proportion will vary according to the description and quality of the material, and to the amount of weight applied.
If a weight of 1 ton be hung from the end of a wrought iron bar of average quality, having a sectional area of 1 square inch, the bar will stretch about 1/12000 part of its original length.
If the weight be removed, the bar will soon recover itself - that is, it will return to its original length.2 If measured by any ordinary means of measurement, it will be found to be of the same length that it was before the weight was imposed upon it.
This recovery of the bar occurs, however, only up to a certain point. If the load be increased until it amounts to a considerable proportion of the breaking weight, the result produced is very different.
For example, if, instead of 1 ton, a weight of 12 tons be applied to the bar just mentioned, the iron will stretch about 1/1000 of its length. Upon removal of the weight, however, it will not entirely recover itself, but will bo found, upon measurement, to be a little longer than it originally was.
1 Stonev On Strains.
2 See page 317.
This slight increase upon the original length of the bar is called the permanent set. The greatest stress that can be applied to the bar without causing an appreciable permanent set is called the limit of elasticity, or the elastic limit.
It is evident then, that there is a very important line to be drawn. On one side of it are weights, the application of which will produce no appreciable permanent set; on the other side are the weights which produce an appreciable permanent set.
This line of demarcation is called the Limit of Elasticity, or the Elastic Limit. It is, as before said, a certain proportion of the breaking load for the material, and its value is generally stated in lbs. or tons per square inch.
The proportion which the limit of elasticity bears to the breaking load varies very considerably in cast iron, wrought iron, and steel, and even in different specimens of the same classes.
The above remarks have been made with regard to a tensile stress, but the same thing occurs with a bar under compression. Weights placed upon the end of the bar produce no permanent contraction or set up to a certain point. Weights greater than this permanently shorten the bar. This point is called, as before, the elastic limit, or limit of elasticity.
The exact point at which the permanent set commences varies according to the quality and characteristics of the material. A hard brittle iron has a high limit of elasticity, it will not stretch much before breaking; on the other hand, a soft ductile iron soon takes a slight permanent set, but stretches considerably before breaking. Practically, for ordinary good wrought iron, the limit of elasticity may generally be taken at about 1/2 the breaking stress.
So long as ductility is not sacrificed it is important to have material with a high limit of elasticity for nearly all structures, but especially for those which are subjected to loads constantly repeated, as in the case of railway bridges. The reasons for this are given below.