PERMANENT SET UNDER TENSILE AND COMPRESSIVE STRAINS.

(753.) "Cast Iron.”—Table 115 gives the result of Mr. Hodgkinson's experiments on the set from both strains; those with tensile strain were made on round bars of iron united together at the ends, so that the whole length exclusive of the couplings was 50 feet: the diameter was about 11 inch, equal to one square inch area. There were nine experiments upon four kinds of cast iron namely, Lowmoor No. 2, Blaenavon No. 2, Gartsherrie No. 3, and a mixture of Leeswood No. 3 and Glengarnock No. 3 in equal proportions. There were two experiments upon each of the simple irons, and three upon the mixture:—the mean experimental tensile strains are given in col. 1, and the corresponding sets in col. 4.

The sets under compressive strains were obtained by experiments on the same kinds of iron, the bars being 10 feet long, and 1 inch square nearly, and to prevent lateral flexure they were enclosed in a strong iron frame, &c. The mean experimental compressive strains are given in col. 1, and the corresponding sets in col. 5.

(754.) By plotting these experimental results in diagrams we have obtained Fig. 217, and from that we have found the sets for even tons and half-tons given in col. 3 of the Table. In col. 6 we have given the ratio of the sets under the same amount of tensile and compressive strains :—it will be observed that with small strains, the compressive sets are much greater than the tensile ones, but with heavy strains the tensile sets are much the greater. The fact is that the amount of set is governed, not by the absolute strain alone, but by the relative strain with reference to the ultimate or breaking weight. As the ultimate strain is approached, the set is very rapidly increased; the ultimate strength of cast iron being six times greater for compressive than for tensile strains, defect of elasticity, as manifested by the set, tells more rapidly with the latter, so that while with the small strain of half a ton per square inch, the set with a compressive strain is greater than with a tensile in the ratio of 1.713 to 1, they become about equal with 3 tons, and only about half or 574 to 1 with 7 tons per square inch,

TABLE 115.-Of the PERMANENT SET of CAST IRON under TENSILE and COMPRESSIVE STRAINS: by Direct Experiments.

TABLE 115.-Of the PERMANENT SET of CAST IRON under TENSILE and COMPRESSIVE STRAINS-continued.

that being about the breaking weight by tensile strain, but only one-sixth of the breaking weight by compressive strain.

(755.) The ultimate compressive strength of cast iron is about 42 or 43 tons per square inch (132), but the set was not observed with strains greater than about 19 tons, as in the Table. With 2 tons per square inch Tensile strain, which is about 3rd of the strength of cast iron, the extension by col. 4 of Table 88 is 00044625, and the set with that strain by col. 4 of Table 115 being 0000245, is equal to 0000245 00044625 = .055, or 5 per cent. With 14 tons per square inch crushing strain, which is about 3rd of the ultimate compressive strength of cast iron, the compression by col. 5 of Table 89 is ⚫002731,

and the set by col. 5 of Table 115 being 000443, is equal to 000443002731 ⚫016, or 1.6 per cent. only.

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By cols. 4, 5 the permanent set with any given tensile or compressive strain may be easily calculated by interpolation, remembering that it is proportional to the square of the strain (752). Thus, the set of a cast-iron bar after a tensile strain of 6000 lbs., or 2.7 tons, may be found from col. 4, which gives 0000245 for 2 tons, therefore 0000245 × 2.72 ⚫0000286 for 2.7 tons, which with a length of say 72 870 inches, gives 0000286 × 870 = ·25, or inch permanent set, &c.

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2.52 feet, or

(756.) "Wrought Iron."-The results of Mr. Hodgkinson's experiments on the set of wrought iron under tensile strains are given in cols. 5, 5 of Tables 94, 95, the "sets" being reduced to parts of the length of the bar. The observed sets with very small strains are very anomalous, due no doubt to the difficulty of measuring with accuracy such minute distances as the sets really are with light strains, even when the bars are of great length:-thus the observed set with 7571 lbs. was only 0022, or 4 inch on a length of 50 feet.

1

454

The sets for even tons were obtained by interpolation between the next greater and lesser experimental sets.

By plotting the experimental observations in a large diagram, we have obtained the sets in col. 2 of Table 116; the mean set per ton given in col. 3 will serve to calculate the set for strains intermediate between those given in the Table, by simple interpolation, which will be accurate enough for ordinary purposes. Thus, with a bar 100 feet, or 1200 inches long, with say 12.4 tons per square inch Tensile strain, the set after the strain was taken off would be 00000725 × 12.4 × 1200 108 inch; more accurately it would be 108 × 12.4°÷12° = 115 inch. For strains under 8 tons, it has been assumed in Table 116 that the sets are proportional to W2 (752). Tables 94, 95 show that with heavy strains, time is very influential on the Permanent Set.

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(757.) Cols. 6, 6 in Tables 94 and 95 show that the permanent sets give very clearly and definitely the point at which a wrought-iron bar is overstrained by a tensile strain. Thus, in

TABLE 116.-Of the PERMANENT "SET" of WROUGHT IRON under TENSILE STRAINS.

Table 94, the set with 27,761 lbs. is 9.3 per cent. of the extension, but, with 30,284 lbs. becomes 36.5 per cent.: again, in Table 95, 26,675 lbs. gives a set of 8.2 per cent. of the extension; but 29,343 lbs. gives 20 per cent., &c. With both these bars therefore about 12 tons per square inch will decidedly overstrain the iron with very heavy strains the permanent set is nearly equal to the extension, being as much as 92 per cent. in Table 94, and 94 per cent. in Table 95.

We have unfortunately no experiments on the set of wrought iron under compressive strains; neither have we any experi