“The further bending of the pieces thus treated is accomplished by placing them endwise upon the anvil-block, as shown in [Fig. 2826]. If the hammer is heavy enough to do it, the samples should be closed down by simple pressure, without any striking.
Fig. 2827.
“[Fig. 2827] is a sketch of a simple contrivance, by means of which a common punching machine may be converted temporarily into an efficient test-bending apparatus. The punch and bolster are removed, and the stepped cast-iron block a fixed in place of the bolster. When a sample is placed endwise upon one of the lower steps of the block a the descending stroke of the machine will bend the specimen sufficiently to allow of its being advanced to the next higher step, while the machine is at the top of its stroke. The next descent will effect still further bending, and so on till the desired curvature is attained. It would seem an easy matter, and well worth attention, to design some form of machine specially for making bending experiments; but with the exception of a small hydraulic machine, the use of which has, I believe, been abandoned on account of its slowness, nothing of the kind has come under the writer’s notice.
“The shape of a sample after it has been bent to pass Lloyds’ or the Admiralty test is that of a simple bend, the sides being brought parallel. While being bent the external surface becomes greatly elongated, especially at and about the point of the convex side, where the extension is as much even as fifty per cent. This extreme elongation corresponds to the breaking elongation of a tensile sample, and can only take place with a very ductile material. While the stretching is going on at the external surface, the interior surface of the bend is being compressed, and the two strains extend into pieces till they meet in a neutral line, which will be nearer to the concave than to the convex curve with a soft specimen. When a sample breaks, the difference between the portions of the fracture which have been subject to tensile and compressive strains can easily be seen.
Fig. 2828.
“[Fig. 2828] shows a piece of plate folded close together; and this can generally be done with mild steel plates, when the thickness does not exceed half an inch.