The Engineer – 10 April, 1868
Garrison Point Fort, Sheerness.
Having in our last impression given plans and sections of the floors in Garrison Point Fort, we now complete our illustrations of this important work at page 258, and we supply the tabular statement of armour plate tests referred to in the preceding article. The plate tested, of which we record the particulars was made by Messrs. Cammel and Co., of the Cyclops Works, Sheffield, early in 1867. It was 5in thick. When resting against a backing of sand bags the indent of the 68 lb. service cast iron shot fired with a full charge of 16 lb. of powder at 30 yards range was 2in. When the plate was suspended in a way to be presently explained the indent, with the same charge was 2375in. The indent of a chilled shot was pointed head, weighing 115 ½ lb., fired with a 9 lb. charge from the 7in rifled gun at seventy yards range, was 5.7in; and the figure of merit awarded to the plate was A1, of the highest in the Shoeburyness classification. The annexed cut shows the form of the specimens tested, and their position in the plate.
Armour plates for fortifications are required to comply with the following conditions, and to undergo the tests, particulars of which we subjoin.
They must be of the best soft fibrous iron, perfectly welded throughout, and in every respect of the most suitable make and description for the resistance to cannon shot. They may be either rolled or hammered, but in either case must be manufactured throughout the entire satisfaction of the inspecting officer of the War Department. When finished they must be free from flaws and other defects, except very slight surface blisters.
As soon as possible after a given number of plates have been made, the inspecting officer selects at random, and these are sent to Shoeburyness for trial. The samples are not generally more than about 6ft long and 4ft or 5 ft wide. This trial consists of the following tests:- Each sample is suspended by strong links from an overhead beam, so as to be perfectly free to swing on being struck. If of moderate thickness, that is to say up to about 6in, the plate is struck several times by common cast iron spherical shot, fired with the service charge of 16 lb. of powder at a range of thirty yards, the shot being planted so close to each other that the indents overlap or very nearly so. Plates of greater thickness than 6in, are struck by spherical shot from heavier guns. After this has been done, the plate is struck by a chilled cast iron elongated shot with a pointed head, fired from a rifled gun at seventy yards with such a change as just to effect complete penetration. The force thus employed for penetration as well as the amount of indent caused by the spherical shot are noted. The effects of the repeated blows, especially on the rear side of the plate, are carefully observed, and on the result of this and other general indications the plate is awarded its figure of merit in one of the three classes A. B. C., each class having again three degrees in itself. In addition to the foregoing proof small samples are also sometimes taken for trial in test machines, for resistance to tension, compression, and deflection, and the results observed, especially as regards the elasticity of the iron and its appearance on fracture – an example of the mode of test is given in the tables.
When armour bars or planks are used they are required to be equal in quality of iron to the armour plates, and are manufactured by rolling in grooved rolls. They are usually of sections varying from 12in by 5in. to 20in. by 8in, and in length from 12ft to 24ft.
Samples of the armour bars or planks are selected at random by the inspecting officer of the War Department, and sent to Shoeburyness for trial. The proof consists of the following:- Each bar or plank is fixed in a timber frame in such a way that its ends only are held, the distance between the supports being about 10ft. It is then struck at intervals between the supports by common case iron spherical shot, fired from the 68-pounder gun, at a range of ten yards, with reduced charges, and its capability of bending under these blows is carefully observed. The effects also in rear of the shot marks, as well as the depth of the shot marks, as well as the depth of the indents themselves, are noted, and in case of the bar giving way the nature of the fracture is recorded. On these results and other general indications the figure of merit awarded, the order of classification being the same as for common plates. Small samples are also occasionally taken for trial in test machines. No armour plates or bars can be admitted into any work, unless they have been represented by samples that have successfully passed the above proofs. Armour bolts are made of iron specially prepared for the purpose, and they are tested by means of falling weights, the amount of elongation at the time of fracture, and the contraction of area at the point of fracture being minutely observed. On these and the general appearance of the iron in the fracture, together with results obtained in common test machines, the passing of the iron for use in the works depends.
In conclusion we beg to express our thanks to the authorities for the courtesy with which they have placed valuable information and excellent drawings at our disposal.