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Strength of Materials Under Simple Stress 13

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STRENGTH OF MATERIALS UNDER SIMPLE STRESS.

13. Materials in Tension. Practically the only materials used extensively under tension are timber, wrought iron and steel, and to some extent cast iron.

14. Timber. A successful tension test of wood is difficult, as the specimen usually crushes at the ends when held in the test ing machine, splits, or fails otherwise than as desired. Hence the tensile strengths of woods are not well known, but the following may be taken as approximate average values of the ultimate atxengthe of the woods named, when "dry out of doors." Hemlock, 7,000 pounds per square inch.

White pine, 8,000 Yellow pine, long leaf, 12,000 44 " " , short leaf, 10,000 44 Douglas spruce, 10,000 66 White oak, 12,000 " Red oak, 9,000 66 15. Wrought Iron. The process of the manufacture of wrought iron gives it a "grain," and its tensile strengths along and across the grain are unequal, the latter being about three-fourths of the former. The ultimate tensile strength of wrought iron along the grain varies from 45,000 to 55,000 pounds per square inch. Strength along the grain is meant when not otherwise stated.

The strength depends on the size of the piece, it being greater for small than for large rods or bars, and also for thin than for thick plates. The elastic limit varies from 25,000 to 40,000 pounds per square inch, depending on the size of the bar or plate even more than the ultimate strength. Wrought iron is very ductile, a specimen tested in tension to destruction elongating from 5 to 25 per cent of its length.

16. Steel. Steel has more or less of a grain but is practically the same strength in all directions.. To suit different purposes, steel is made of various grades, chief among which may be men tioned rivet steel, sheet steel (for boilers), medium steel (for bridges and buildings), rail steel, tool and spring steel. In general, these grades of steel are hard and strong in the order named, the ultimate tensile strength ranging from about 50,000 to 160,000 pounds per square inch.

There are several grades of structural steel, which may be described as follows:* 1. Rivet steel: Ultimate tensile strength, 48,000 to 58,000 pounds per square inch.

Elastic limit, not less than one-half the ultimate strength. Elongation, 26 per cent.

Bends 180 degrees flat on itself without fracture.

*Taken from "Manufacturer's Standard Specifications." 2. Soft steel: Ultimate tensile strength, 52,000 to 62,000 pounds per square inch. Elastic limit, not less than one-half the ultimate strength. Elongation, 25 per cent.

Bends 180 degrees flat on itself.

3. Medium steel: Ultimate tensile strength, 60,000 to 70,000 pounds per square inch. Elastic limit, not less than one-half the ultimate strength. Elongation, 22 per cent.

Bends 180 degrees to a diameter equal to the thickness of the specimen without fracture.

17. Cast Iron.

As in the case of steel, there are many grades of cast iron. The grades are not the same for all localities or districts, but they are based on the appearance of the fractures, which vary from coarse dark grey to fine silvery white.

The ultimate tensile strength does not vary uniformly with the grades but depends for the most part on the percentage of "combined carbon" present in the iron. This strength varies from 15,000 to 35,000 pounds per square inch, 20,000 being a fair average.

Cast iron has no well-defined elastic limit (see curve for cast iron, Fig. 5). Its ultimate elongation is about one per cent.

1. A steel wire is one-eighth inch in diameter, and the ulti mate tensile strength of the material is 150,000 pounds per square inch. How large is its breaking load ? Ans. 1,840 pounds.

2. A wrought-iron rod (ultimate tensile strength 50,000 pounds per square inch) is 2 inches in diameter. How large a steady pull can it safely bear ? Ans. 39,270 pounds.

18. Materials in Compression.

Unlike the tensile, the compressive strength of a specimen or structural part depends on its dimension in the direction in which the load is applied, for, in compression, a long bar or rod is weaker than a short one. At present we refer only to the strength of short pieces such as do not bend under the load, the longer ones (columns) being dis cussed farther on.

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