Steel.
The greater proportion of the various fittings employed in the construction of the aeroplane are built up from sheet nickel steel, usually of a low tensile strength, to permit of working in a cold state, as, with a higher grade steel, the process of bending to template by hand, in many cases a none too careful procedure, would result in a considerable weakening of the material at the bend. In addition, the operation of welding, which now enters into the construction of a number of fittings, also necessitates a moderate grade of steel. A higher class of sheet steel, from 35 to 50 tons tensile, is used for parts subject to stress, such as interplane strut-fittings, wiring-lugs, etc. As a higher grade of steel is better from a strength-for-weight point of view, its employment for bent-up clips is desirable, although where such a steel is used it is almost necessary, if the original strength of the material is to be retained in the finished fitting, to effect the various bends in a machine, in conjunction with bending jigs. Careful heat-treatment after bending to shape is an important factor in removing the stresses set up by working, and in rendering the structure of the material more uniform.