The purpose of this paper is to describe a method for making repeated load tests and to illustrate several applications of the method.

The general characteristics of fatigue testing machines operating on the resonance principle are described, with particular reference to the mode of excitation. A method of excitation different from that in common use is described, and its advantages and disadvantages are explored. Four applications of the new method are instanced.

The new method proves particularly applicable to the repeated loading of complete structures and components, because it is readily adaptable to parts of awkward shapes loaded in special ways. Moreover, application of the method is not seriously restricted by large strains of the component under test, nor by moderately heavy absorption of energy by the component.

Several further applications of the method to the testing of complete structures are under consideration, and the development of types of fatigue testing machines based on the same method is being explored.

The principal materials employed in modern aircraft structures are aluminium-rich alloys which the aircraft manufacturer will normally buy in the semi-finished (cast or wrought) state. Wrought materials fall into the four main classes of tubes, sheets, extrusions, and forgings; eacri of these forms inherently exhibits distinctive mechanical strength characteristics. In this paper the relation of the mechanical properties of medium and high strength wrought light alloy materials to the grain direction, and the implications of this relation in aircraft design, will be discussed.

It is odd to think that the resistance-wire strain gauge could have been put to use some 40 years ago, about the time of the first filament-wire lamp and the thermionic valve. The physical principles involved were already known. Kelvin had measured the resistance of wires in tension, there were galvanometers in plenty, and even crude valve amplifiers existed. It only really remained for someone to put two and two together. Instead we had to wait until the necessity of stressed-skin construction forced the hand of the engineer into seeking a more effective method of strain analysis than the old empirical ways.