Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/370

354 IRON to be simply due to internal adjustment of the strains produced during drawing. It does not necessarily follow that the resistance to per cussive force exhibited by a given sample of metal will be in the ratio of its tensile strength. Thus phosphorus when present together with only minute quantities of carbon (forming the so-called " phosphoric steels ") does not very materially decrease the tensile strength, such steels con taining 3 per cent, of phosphorus being often but little inferior to soft non-phosphorized steels in this respect ; but when tested by a " falling weight " (a mass of known weight falling once or oftener from a known height upon the centre of the bar or rail firmly gripped in supports a known distance apart) the phosphorized metals generally show themselves considerably inferior to the non-phosphorized ones. The same remark applies to silicon. According to BEFORE RUPTURE AETER RUPTURE Fig. 65. Dudley the effect of phosphorus, silicon, and carbon in hardening iron and making it less capable of resisting percussion are nearly in the proportions of 3, 2, and 1 relatively to one another. The presence of manganese diminishes this deleterious effect of non-metals ; whilst, if more than minute quantities of carbon be present, the tensile strength as well as the resistance to percussion is greatly diminished by the additional presence of phosphorus or silicon in proportions beyond certain small limiting amounts. Accordingly it is the usual practice to test rails, bars, &c., not only by the determination of the breaking strain for tensile force (measured by pulling asunder, preferably by hydraulic power, a bar turned to known definite dimensions, and made into the shape of fig. 65), but also by a falling weight, a "monkey" (somewhat like a pile driver) being raised to a known height and let drop upon the rail. The particular tests applied in different instances vary mirch ; for instance, some little while ago the official falling weight test for Bessemer rails at Griitz (Austrian South Railway Company s Works) was to permit a weight of 1000 kilos (about a ton) to fall from a height of 15 feet upon the centre of the rail supported by two rests 3 feet apart, any amount of bending being allowed, but not fracture, whilst a test for elasticity or resistance to permanent deflexion was applied by placing a weight of 17,500 kilos on the middle of the rail similarly supported. The North-Eastern Railway (England) similarly at one time tested rails by allowing a weight of 1800 lt> to fall from 4 feet height, the number of blows requisite to produce rupture and the permanent bending produced by each being noted. In other instances the test applied has been a ton weight falling a greater height, such as 20 feet or even 30 feet, the rail being required to stand one such blow only, or a succession, the particular details of the test to be applied being usually specified in each particular case ; thus the Midland Railway Company has tested steel rails by allowing a weight of 1 ton to fall three times from a height of 12 feet, the supports being 4 feet asunder. In just the same way as regards the determination of tensile strength, the dimensions of the piece to be tested (6 inches, 8 inches, 10 inches long, &c.) are usually specified, and the strain which the metal will just stand without becoming permanently elongated (limit of elasticity) determined, as well as the total strain requisite to produce rupture, together with the "ductility" or amount of permanent extension of the test piece and the diminution in section of the bar at the point of rupture. Thus for instance the following numerical data were obtained by Kirkaldy with a particular specimen of West Cumberland Bessemer steel plate, three pieces being tested each 10 inches long, and respectively |, , and f inch in thickness Thick ness in - Indies. Limit of Elasticity in Tons. Ultimate Breaking Strain per Square Ineh. Percentage diminution of Section at Place of Fracture. Permanent Extension in Percentage of Original Length. With 50.000 tt> per Square Inch. With 60,000 lt per Square Inch. At Rupturing Strain. 0-25 0-49 075 207 16-0 15-0 29-8 277 27-6 54-4 507 49-6 2-60 5-52 6-09 8-02 13-9 15-0 23-2 27-3 30-2 These numbers illustrate, amongst other things, the effect upon the final values produced by variations in the diameter of the test pieces ; the shorter and thicker the piece the greater in amount is the permanent extension. In calculating the rupturing strain per square inch, the dimensions of the metal as originally employed before permanent alteration was brought about are employed ; by taking the diminished area at the point of fracture as the section, a much higher value is obtained as the tensile strength per unit area of the extended metal. Tempering steel greatly increases its break ing strain and limit of elasticity, but decreases the permanent extension ; thus the following values represent certain results obtained with Creuzot steels of the A class, in tons per square inch (see Engineering, 1875, p. 119). No. of Classi fication. Not Tempered. Tempered. Limit of Elasticity. Breaking Strain. Percentage Extension. Limit of Elasticity. Breaking Strain. Percentage Extension. t 2472 48-31 13 45-64 74-16 0-2 3 23-07 44-57 17 4171 66-95 7-2 5 21-04 39-81 21 35-63 56-17 11-1 7 18-25 3372 25 2777 43-49 14-6 9 14-26 28-53 29 21-30 35 63 21-0 In making a contract for the supply of steel of a particular quality, the details of the tests of strength to be applied should consequently be duly set forth ; for instance, a short time ago the French Government required certain steel navy tubes, of which the limit of elasticity and breaking strain tested in a particular way were respectively to be 21 and 38 tons per square inch. Similarly in the construction of the Mississippi great bridge the cast steels used were contracted to be of the following qualities : "To be of crucible cast steel : the staves of the tubes to stand a compressive strain of 60,000 and a tensile strain of 40,000 ft per square inch section without permanent set, and to stand a tensile strain of 100,000 lt> per square inch without fracture. Modulus of elasticity to be between 26 and 30 million ft, preferably nearer the lower limit, and as constant as possible ; bars of the same modulus to be selected for the tubes, so that each side shall have same power of resistance ; each bar to be tested and modulus stamped on it. Steel pins, rods, bolts, eyewashers, rivets, &c., and the inch steel plates for envelop ing the staves to stand a tensile strain of 40, 000 Jb per square inch without permanent set, and an ultimate tensile strain of 100,000 ft without fracture." Practical tests of the capability of metal to stand bending double or through some given angle, or twisting round and round in the cold without fracture are often applied, as are also tests of the capability of being bent hot, forged, welded, &c. A test as to the power of resisting repeated bending strains backward and forward through a given angle is sometimes applied ; a particular mechani cal arrangement for effecting this has been described by Olrick. James Price has constructed a machine for testing rails as to durability under rolling wear and tear, consisting of a pair of metal rollers 5 feet in diameter and 16 inches wide, weighing 45 cwts. each, supporting a frame weighing 6J tons, connected with a centre boss and vertical axle, so that the rollers are driven round in a circle, one bearing with 5 the other with 6 tons pressure ; the rails to be tested are bent into a circle or preferably a polygon, to equalize the wear of the rollers, which are driven over them at a speed of 13 or 14 miles per hour until the rails are broken or wear out. It by no means follows that the rail which possesses the greatest tensile strength will resist wear and tear and rolling friction best, although this might be anticipated if, other things being equal, increased tensile strength corresponds to greater hardness ; on the other hand, experience does not always indicate that the most carbonized rails last the longest, although the superiority of ingot metal (Bessemer steel, &c.) over weld iron (not fused) rails is well demonstrated ; it is probable that the interposed film of cinder between the metallic fibres in the latter case greatly facilitates the destruction and wearing away of the upper surface, just as the accumulation of dust and sand on the rail between the passage