422 STRENGTH OF MATERIALS vex toward the base line. The initial portion of the diagram, therefore, determines whether the material tested has been subjected to inter- nal strain, or whether it is homogeneous as to strain. This is exhibited by the direction of this part of the line, as well as by its form. The existence of internal strain causes a loss of stiffness, which is shown by the deviation of this part of the line from the vertical to a degree which becomes observable by compar- ing its inclination with that of the line of elastic resistance. 31. In fig. 4, the strain diagram A is that of zinc. The concave form at the commencement indicates its inelastic nature, its slight altitude shows its weakness, and, breaking at 65, it is shown to lack duc- tility. Tin, T, is vastly more ductile, but is still less tenacious. B and C are the diagrams given by cast and forged copper, the latter twisting 500, and its fibres stretching to three times their original length. Cast copper is compara- tively weak and brittle. Wrought iron gives the strain diagram D. It indicates the elastici- ty of the metal, its ductility, and its strength. The elastic limit is plainly indicated. The con-
4W Ml Ipi M 1 BCCB 30,000 - ~ ft s s i ^ fcs ^ -^ ~^ -- ^ 3 > I =* ^ "^ ^ U ^ U 30.000 f . _,<=i -< | 5 -4 4^: ^*'- s 40 220 200 130 160 HJ 120 130 CD 40 LO FIG. 4. Strain Diagrams. cavity of the initial portion of the line indi- cates some internal strain, and the horizontal portion immediately above the elastic limit shows that the metal was " seamy " and not perfectly homogeneous. The lines e and O are " elasticity lines." They differ slightly in direc- tion from the initial portion of the diagram, confirming the previously indicated presence of internal strain. E is the terminal portion of the diagram of a soft ductile iron. F is that given by a very strong and ductile and excep- tionally homogeneous iron, a very smooth and symmetrical curve. G is a soft Bessemer steel. II is somewhat harder, the one containing - 4 and the other 0'5 per cent, of carbon. I and J are tool steels containing 1 per cent, of carbon. K is medium, L spring, and M double shear steel. N and P are obtained from white and gray cast iron. One is stiff, hard, and brittle, the other weaker, soft, and comparatively tough. O is a malleableized cast iron made from N; it has lost no strength, and has gained considerable ductility. Strain diagrams may be produced by plotting data obtained by observation in the usual manner and simi- larly interpreted. 32. An examination of the fracture in each case assists in determining the character of the material, and in interpret- ing the strain diagram. The following fig- ures exhibit the characteristics of various qualities of iron and steel. Fig. 5 resembles that which gave the diagram marked D. The metal is good and tough, but seamy, and not thoroughly worked, as is shown by the cracks extending around the neck and by the irreg- ularly distributed flaws on its end. Fig. 6 exhibits the appearance of the sample F. The surface of the neck was originally smoothly turned, polished, and fitted to gauge. Under test it be- came curiously altered and assumed a rough, striated appearance. The end has the peculiar appearance character- istic of tough and ductile metals, and the uniformly bright appearance of the fractured section shows that all held together up to the instant of rup- ture, and that fracture finally took place by shearing. Fig. 7 represents the appearance of low steels. The peculiarities of the finest tool steels are exhibited in fig. 8. In this the fracture is ragged and splintery, and the separated surfaces have a beauti- fully fine, even grain, which proves the excel- lence of the material. The surface, which was turned and polished in bringing the metal FIG. 5. FIG. 7- to size, remains as perfect as before the speci- men was broken. By an inspection of the bro- ken test pieces in this manner, the grade of the steel, and by the practised eye the slightest
