In the ordinary type of Wöhler machine used for testing materials in fatigue under reversed bending stresses, the load system is stationary in space, and variation of the stress system with respect to the test piece is obtained by rotating the test piece. It is, of course, essential to the success of the test that the system of displacements caused by the application of the load system to the test piece should remain stationary in space; but, since the test piece rotates, this requirement can only be fulfilled if the material of the test piece is isotropic. Thus, if an attempt were made to test a single crystal in a Wöhler machine it might be anticipated that either actual elastic antisotropy or the virtual anisotropy due to restricted slip movement would cause the deformation to vary with the orientation of the stress system relative to the axes of the crystal and that "whipping" of the specimen would occur. Three such attempts have indeed been made: but in spite of great care exercised in setting up the specimens and in applying the loads, only in one case, in which the orientation of the crystal was such as to provide effective symmetry about the axis of the specimen, was the test successful. A new type of testing machine recently developed at the N. P. L. for testing specimens in fatigue under systems of combined bending and torsional stresses, differs in principle from the Wöhler machine in that the variation of stress is produced by actual variation of load. In this machine both me test piece and the orientation of the stress system remain stationary, only the magnitude of the stresses being varied. The deformation of the test piece is therefore only that due to one type of stress system fixed in relation to the orientation of the test piece and varying only in magnitude. Moreover, the construction of the machine is such that the strain of the test piece is not required to be of the same type as the stress system applied,
e. g
., the application of pure bending moment does not restrict the test piece to pure bending strain and the test piece remains free to twist also if necessary. These conditions render this type of machine perfectly suitable for test on single crystals. Accordingly, tests have been carried out in this machine on three single crystals of aluminium; the first was tested under reversed flexural stresses, the second under reversed torsional stresses and the third under a combination of reversed flexural and reversed torsional stresses.
The behaviour of three single crystals of aluminium in fatigue under complex stresses
